Organic Spintronics

1.05 Organic Spintronics ¨ sterbacka, A˚bo Akademi University, Turku, Finland S Majumdar, H S Majumdar, and R O ª 2011 Elsevier B.V. All rights reserved.

1.05.1 1.05.2 1.05.2.1 1.05.2.2 1.05.2.2.1 1.05.2.2.2 1.05.2.3 1.05.2.4 1.05.3 1.05.3.1 1.05.3.1.1 1.05.3.1.2 1.05.3.2 1.05.3.3 1.05.4 1.05.4.1 1.05.4.2 1.05.4.2.1 1.05.4.3 1.05.4.4 1.05.4.5 1.05.5 1.05.5.1 1.05.5.2 1.05.6 References

Introduction Basic Concepts of Spintronic Devices Spin Injection Spin Transport and Relaxation Spin–orbit coupling Hyperfine interaction Spin Detection Different Magnetoresistive Effects: GMR and TMR Organics in Electronics Organic Semiconductors Conjugated polymers Small molecules Charge Transport in OSs Advantages of Organics in Spintronics Organic Spin Valves Spin Injection and Detection: Role of Interface Spin Transport and Relaxation Effect of impurity inclusion on the spin-transport property of OS spacers Organic MTJs OLEDs with SP Electrode CNT and Graphene-Based Spin Valves Organic Magnetoresistance Experimental Observations Theoretical Models – Physical Understanding Conclusions and Open Questions

1.05.1 Introduction Until recently, the spin of the electron was ignored in mainstream charge-based electronics. The technology of spintronics (or spin-based electronics), where the electron spin is used as the information carrier in addition to the charge, offers opportunities for a new generation of electronic devices combining standard microelectronics with spin-dependent effects that arise from the interaction between the carrier spin and externally applied magnetic fields. Adding the spin degree of freedom to conventional semiconductor charge-based electronics substantially increases the functionality and performance of electronic products. The advantages of these new devices are increased data processing speed, decreased electric power consumption, and increased integration densities compared to conventional semiconductor

109 111 111 112 112 113 113 113 115 115 115 116 116 117 117 121 125 127 129 131 131 131 132 134 137 139

electronic devices – which are nearly at their physical limits nowadays. The discovery of the giant magnetoresistance (GMR) effect in 1988 is considered as the beginning of the new generation of spin-based electronics [1]; this discovery has led to the Nobel prize in physics (2008). Since then, the role of electron spin in solid-state devices and possible technology that specifically exploits spin rather than, or in addition to, charge properties have been studied extensively [2]. For example, spin relaxation and spin transport in metals and in semiconductors are of fundamental interest from both a physical as well as a technological point of view. A good example of rapid transition from discovery to commercialization for spintronics is the application of GMR and tunneling magnetoresistance (TMR) [3] in magnetic information storage. Since the first laboratory demonstration of GMR 109

110 Organic Spintronics

in 1988, the first GMR device as a magnetic field sensor was commercialized in 1994; and read-heads for magnetic hard disk drives were announced in 1997 by International Business Machines Corporation (IBM). Major challenges in the field of spintronics are the optimization of electron spin lifetimes, the detection of spin coherence in nanoscale structures, transport of spin-polarized (SP) carriers across relevant length scales and heterointerfaces, and the manipulation of both electron and nuclear spins on sufficiently fast time scales [4]. It is envisioned that the merging of electronics, photonics, and magnetics will ultimately lead to new spinbased multifunctional devices. The success of these ventures depends on a deeper understanding of fundamental spin interactions in solid-state materials as well as the roles of dimensionality, defects, and semiconductor band structure in modifying the spin properties. With proper understanding and control of the spin degrees of freedom in semiconductors and heterostructures, the potential for realization of high-performance spintronic devices is excellent. The research in this field so far has led to the understanding that the future of spintronics relies mainly on successful spin injection into multilayer devices and optimization of spin lifetimes in these structures. Hence, for obtaining multifunctional spintronic devices operating at room temperature, different materials suitable for efficient spin injection and spin transport have to be studied thoroughly. In recent years, spintronics has benefited hugely from the class of emerging materials, mainly semiconductors. The III–V and II–VI systems and also magnetic-atom-doped III–V and II–VI systems (dilute magnetic semiconductors) are studied extensively either promising as spin-transport materials or as spin-injecting electrodes [4,5]. Little attention has been paid so far to the use of organic semiconductors (OSs) such as small molecules or -conjugated polymers (PCPs) as spintransporting materials. The conducting properties of the PCPs were discovered in the late 1970s and later on the semiconducting properties of the OS and PCPs have given birth to a completely new field of electronics, namely ‘organic or plastic electronics’. OSs and PCPs are mainly composed of light atoms such as carbon and hydrogen, which leads to large spin-correlation length due to weak spin–orbit coupling and hyperfine interaction. This makes the small

molecules and PCPs more promising materials for transporting spins than their inorganic counterparts [6,7]. The ability to manipulate the electron spin in organic molecules offers an alternative route to spintronics. Recently, different experimental observations of spin response in OS-based devices have underlined the prospect of this research field. First, GMR was demonstrated in planar organic devices [8] and vertical spin valves (SV) [9] where an organic material acted as the nonmagnetic (NM) spacer between two ferromagnetic (FM) contacts. The spin-valve response in organic spin-valve devices is often maintained up to room temperature [10,11]. Second, organic diodes made with NM electrodes exhibit changes in resistance under a magnetic field (magnetoresistance, MR) [12,13] at room temperature – paving way for new application of the well-established organic light-emitting diode (OLED) technology. Shortly, after the demonstration of these two spin-based phenomena in OS devices, organic magnetic tunnel junctions (MTJs) capable of producing 4% MR at room temperature were demonstrated [14a]. The use of organic materials in spintronics has thus led to the birth of a new field in solid-state physics dubbed ‘organic spintronics’. Organic spintronics has been developed fast; novel experimental results and theoretical interpretations emerge quite regularly. This chapter briefly reviews the main experimental results for different organic spintronic devices obtained so far, and also the theoretical understanding of the spin-based phenomena in these devices. In this chapter we also discuss the problems and open questions, and conclude with a preview of the future prospects of this field. The present chapter is organized in the following manner. Section 1.05.2 discusses the basic concepts of the spintronic devices together with the first demonstration of the GMR effect. Section 1.05.3 briefly discusses the use of OSs in electronics, focusing on small molecules and polymers, charge transport, and the advantages of these materials for spintronic applications. Section 1.05.4 describes different organic spintronic devices including organic spin valves, MTJs, and OLEDs with SP electrodes, as well as different aspects of SP injection, transport, and detection. Section 1.05.5 deals with the organic MR effect, the experimental observations, and theoretical explanations reported so far. The chapter ends with conclusions and open questions.

Organic Spintronics

1.05.2 Basic Concepts of Spintronic Devices The three most important aspects of spintronics are (1) injection of SP carriers into the spin-transporting layer, (2) transport of spins in the spin-transporting medium either by diffusion or tunneling, and (3) detection of SP carriers. SP carriers can be generated in the spin-transporting layer by electrical, optical, and other methods as described by Zutic et al. [4]. The three possible schemes for spin injection and detection are shown in Figure 1 [14b] Electrical injection and detection of SP carriers are the method of choice for solid-state device fabrication, as realized in spin valves showing GMR. In this chapter, we mostly discuss such devices. Optical SP injection and detection techniques like the magneto-optical Kerr effect (MOKE) and two-photon absorption measurements are discussed later as experimental techniques for fundamental understanding of spin injection and transport. Spin valves are structures with two FM metals separated by a NM metal or insulator. The role of the FM electrodes is to inject and detect SP carriers, whereas the NM spacer decouples the two FM electrodes in order to enable them to switch their magnetization direction when the external magnetic field is reversed. In the presence of an electrical current, SP carriers are generated in one of the FM electrodes and are subsequently injected into the NM spacer. The SP carriers are then transported through the NM spacer, either through tunneling, drift, or diffusion; and are detected by the other FM electrode. Depending on the relative orientation of the two FM electrodes the device reaches either a high- or low-resistance state marking the off or on state.

1.05.2.1

Spin Injection

The basis for our understanding of SP carrier generation and transport in FM materials and systems was put forward by Mott [15a–b] in 1936. He observed that the resistance of an FM metal changes in the presence of a magnetic field, and explained that at sufficiently low temperatures, where magnon (quantum of a spin wave) scattering becomes very small, electrons of majority and minority spin with magnetic moment parallel and antiparallel to the magnetization direction do not mix in the scattering process. The conductivity can then be expressed as

111

(a)

Detection Injection

(b) Injection polarizer Detection analyzer

(c) Injection polarizer

Detection

Figure 1 Different experimental protocols for studying spin dynamics in semiconductors. (a) In inorganic semiconductors, spin-polarized wave packets can be both created and subsequently detected optically, by using, respectively, circularly polarized light and Kerr (or Faraday) rotation. (b) In the case of spin valves, both detection and injection use ferromagnetic metals. (c) Finally, the injection is through ferromagnets but the detection is done either optically or by muon resonance. From Sanvito S (2007) Spintronics goes plastic. Nature Materials 6: 803–804.

the sum of two independent and unequal parts for two different spin projections, that is, the currents in FM are SP. This model is called the two-current

112 Organic Spintronics

model and has been thoroughly studied and further developed by Fert and Campbell [16]. The degree of SP electrical current injected into the NM region from the FM electrode is the measure of electrical spin-injection efficiency. A theory of spin injection across an FM/NM interface was first offered by Aronov and coworkers [17], and spin injection into a semiconductor was also studied [18a–b]. Subsequent detailed studies were made by Johnson and Silsbee [19a–b], van Son et al. [20], Valet and Fert [21], Hershfield and Zhao [22], and others. Rashba [23a–b] suggested a steady-state flow of electrons along the x direction in a three-dimensional geometry consisting of a metallic ferromagnet (region x < 0) and a NM metal or semiconductor (region x > 0). The two regions, FM and NM (depicted as F and N in Figure 2), form a contact at x ¼ 0, as depicted in Figure 2 as well. The degree of polarization in the current injected into the NM layer is measured by the relative magnitudes of three characteristic resistances. These are the contact resistance rc and the two characteristic resistances rNM and rFM, each given by the ratio of the spin-diffusion length and the effective bulk conductivity in the corresponding region. Two limiting cases correspond to the transparent limit where rc ! 0, and the low-transmission limit, where rc >> rNM, rFM. In other words, the spin polarization of the FM injector, the interfacial characteristics of the FM/NM junction, and the conductivity of the NM spacer are the most important parameters for having efficient spin injection. μ , F

N Δμ LsF

LsN 0

x

Figure 2 Spatial variation of the electrochemical potentials near a spin-selective resistive interface at an F/N junction. At the interface x¼0 both the spin-resolved electrochemical potentials (, ¼".#, denoted with solid lines) and the average electrochemical potential (F, N, dashed lines) are discontinuous. The spin diffusion lengths LsF and LsN characterize the decay of s ¼ "  # (or equivalently the decay of spin accumulation and the nonequilibrium magnetization) away from the interface and into the bulk F and N regions, respectively. From Zutic I, Fabian J, and Sarma SD (2004) Spintronics: Fundamentals and applications. Review of Modern Physics 76: 323–410.

1.05.2.2

Spin Transport and Relaxation

Following successful injection, SP carriers travel through the NM spacer to reach the counter electrode. While traveling through the NM spacers, these carriers lose their original spin-sense direction, that is, the spins relax by various processes as discussed below. The main spin-relaxation mechanisms in solids are the spin–orbit coupling and hyperfine interactions. 1.05.2.2.1

Spin–orbit coupling The spin–orbit coupling is the interaction between the electron’s spin and its orbital motion around the nucleus. When an electron moves in the finite electric field of the nucleus, the spin–orbit coupling causes a shift in the electron’s atomic energy levels due to the electromagnetic interaction between the spin of the electron and the electric field. In the rest frame of the electron, there exists a magnetic field created by the interaction of the angular momentum of the electron and the electric field of the nucleus. The electrical field in this case can have various physical origins, such as the electric field of an atomic nucleus or the band structure of a solid [24]. The spin–orbit coupling increases with the atomic number Z of the atom as Z4 in the case of a hydrogen-like atom [25]. The general derivation of spin–orbit coupling from the Dirac equation for an electron of mass m and charge –e < 0 in an external electrical field E(r) ¼ r (r) yields HSO ¼

  eh ˆ ? Eð! r Þ! p 4m2 c 2

ð1Þ

where ! p is the momentum operator and ˆ the Pauli spin matrices. There are two main contributions to spin–orbit coupling in most inorganic solids, namely the Dresselhaus contribution and the Rashba contribution. The Dresselhaus contribution occurs in crystals with bulk inversion asymmetry, implying that there is a net electric field for certain crystal directions [26,27], whereas the Rashba contribution occurs in systems with net electric field due to structural inversion asymmetry [28,29]. There are several spin– orbit-coupling-related spin-relaxation mechanisms in NM solids among which we discuss the three main mechanisms in details, that is, the Elliot–Yafet (EY), D’yakonov–Perel (DP), and Bir–Aronov–Pikus (BAP) mechanisms. The EY mechanism [30] deals with the relaxation of conduction electron spins through momentum

Organic Spintronics

scattering (such as by phonons or impurities) if the lattice ions induce spin–orbit coupling on the electron wave function. Any momentum scattering event has a finite probability to flip the spin. The EY mechanism leads to a spin-relaxation time proportional to the momentum-scattering time. Momentum scattering is generally caused by defects or impurities at lower temperature region and by phonons at higher temperature regions [4]. EY is the dominant mechanism in metals; however, some recent results [31,32] suggested that it could be dominant in OSs also. The DP [27] mechanism arises when there is no center of symmetry in the solid and is therefore directly related to the Dresselhaus contribution. Spin dephasing occurs because electrons feel an effective magnetic field resulting from the spin– orbit interaction, which changes in random directions every time the electron scatters to a different momentum state. This results in a loss of initial spin memory. In the case of frequent scattering events, the spin relaxation slows down as the spin cannot follow the internal magnetic field when it changes too rapidly. Therefore, the spin-relaxation time is inversely proportional to the scattering time. The BAP [33] mechanism is caused by the electron–hole (e–v) exchange interaction, and therefore only plays a role in systems where there is a large overlap between the electron and hole wave functions. This is an important mechanism for p-doped semiconductors, in which spin relaxation of conduction electrons can proceed through scattering, accompanied by spin exchange with holes. 1.05.2.2.2

Hyperfine interaction Another source for spin relaxation is the hyperfine interaction. Hyperfine interaction originates from the interaction between a nucleus and its surrounding environment. The hyperfine interactions may shift energy levels or lift their degeneracy. Generally, the electron spins interact with the nuclear spins. The electron–nuclear coupling Hamiltonian is given by Hhyp

N ! X ! ¼ S ? Ai Ii

ð2Þ

i

where Ii is the spin operator for nucleus i, S is the electron spin, and Ai is the coupling strength between them. The nuclear spins affect both the spin-relaxation time, T1 and spin-dephasing time, T2. For an electron spin interacting with N nuclear p spins, the statistical fluctuation varies as 1/ N

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[34,35]. Hence for more delocalized electron wave functions, the influence of the nuclei will be less.

1.05.2.3

Spin Detection

Spin detection is the process of collecting or detecting the number of SP carriers reaching the second FM electrode with their original spin direction. For an efficient spin collection either ballistic or tunneling transport between the NM spacer and the second FM electrode is needed. Hence, it is very important to have a well-defined interface between the NM spacer and the FM electrode. Recent experiments [36] using Fe/GaAs Schottky tunnel barrier showed that both the magnitude and sign of the spin-detection sensitivity can be widely tuned with voltage bias applied across the interface. Experiments and theory suggest that this tuneability comes from the interplay between two physical processes, that is, the bias dependence of the tunneling spin polarization and the bias dependence of spin transport in the semiconductor which can hugely enhance or suppress the spin-detection sensitivities.

1.05.2.4 Different Magnetoresistive Effects: GMR and TMR In 1975, Jullie`re measured tunneling conductance in FM/insulator (I)/FM junctions, where the insulating layer was amorphous Ge. Based on this measurement, he formulated a model for a change of conductance between the parallel and antiparallel magnetization in the two FM layers [37]. The corresponding TMR in the MTJ is defined as TMR ¼


R R"# – R"" ¼ R"" R

ð3Þ

Jullie`re’s model assumed that electrons tunnel without a spin flip, and hence equation 1 yields TMR ¼

2P1 P2 1 – P1 P2

ð4Þ

where spin polarization of the two FM electrodes is P1 and P2, respectively, and are defined as Pi ¼

N"i – N#i N"i þ N#i

ð5Þ

where N" is the Fermi level density of states (DOS) of the up-spin carriers and N# is the DOS of the downspin carriers. Jullie`re’s result was confirmed in 1982

114 Organic Spintronics

when Maekawa and Ga¨fvert [38] observed TMR at 4.2 K using NiO as the tunnel barrier. The spintronics era was boosted by the first experimental realization of the GMR effect. In 1988, Albert Fert’s group in France [1] and in 1989 Peter Gru¨nberg’s group in Germany [39] first demonstrated GMR in magnetic multilayers, where layers of FM and NM metals were stacked on each other (Figure 3). The widths of the individual layers are in the orders of nanometer – that is, consisting of only a few atomic layers. In the original experiments leading to the discovery of GMR, Peter Gru¨nberg’s group [39] used a trilayer system Fe/Cr/Fe, while Albert Fert’s group used multilayers of the form (Fe/Cr)n where n could be as high as 60. The GMR multilayer has the general structure of FM/NM/FM as shown in Figure 3, together with the corresponding electron DOS of the two FM sides. In the absence of a magnetic field (at the top), the two FM layers are decoupled in such a way that they

have opposite magnetization directions. In the presence of a magnetic field, the magnetizations of the two FM layers are parallel (at the bottom). An electrical current was sent through the system for both configurations. Following the two-current model mentioned above, the current through the FM layer is composed of one spin-up current and one spindown current – the resistance for these two currents differ substantially. When an electron leaves the first FM layer and enters the NM metal, additional scattering processes give rise to extra resistance. Since the spin-up and spin-down electrons have different DOS at the Fermi level (EF) (or rather, they originate from energy levels having different characteristics), the resistance not only within the FM layers, but also that originating from the FM/NM interface, would be different for the two spins. Inside the NM layer the up and down spins experience the same resistance; but this resistance is generally low compared to those in the FM layers and FM/NM interfaces, and can be, therefore, neglected.

R/R (H = 0)

FM metal NM spacer (Fe 30 Å /Cr 18 Å )30

FM metal 0.8

NM spacer FM metal

HS

0.7

(Fe 30 Å /Cr 12 Å )35

NM spacer 0.6

FM metal

(Fe 30 Å /Cr 9 Å )60

NM spacer

0.5

FM metal

–40

–30

–20

0

E

E

EF

Barrier

FM1

–10

HS 10

20 30 40 Magnetic field (kG)

Antiparallel

Parallel

E

HS

FM2

E

EF

Barrier

FM1

FM2

Figure 3 (Top left) Schematic diagram of a typical spin-valve structure of ferromagnetic metals separated by thin layers of nonmagnetic spacers showing giant MR effect. (Top right) MR of 3 Fe/Cr superlattices at 4.2 K. The current and the field are along the same [40] axis in the plane of the layes. (Bottom) Spin resolved density of states in FM metals. Arrows in the two FM regions are determined by the majority spin subband. From Baibich M, Broto JM, Fert A, et al. (1988) Giant magnetoresistance of (001)Fe/(001)Cr magnetic superlattices. Physical Review Letters 61: 2472–2475.

Organic Spintronics

–H

+H

–H

+H

Figure 4 A typical tunnel junction resistance hysteresis showing that when the two FM electrodes are in parallel configuration, the device attains a low-resistance state; whereas when the electrodes are in antiparallel configuration, the device exhibits a high-resistance state. The blue line indicates down field scan (–field to þfield), and red line indicates up field scan.

Another modification of this structure is the use of two different FM metals as injector and detector in the spin-valve structure separated by an NM spacer. In this structure, when the applied magnetic field is in between the coercive field of the two electrodes, one of the electrodes reverses its magnetization direction, so that the two FM layers attain antiparallel configuration; consequently, the device resistance attains the high state. When the applied magnetic field is above the coercive field of both the FM electrodes, two FM layers attain parallel configuration and the device resistance attains the low state. In this way, the device operates as a bistable resistive element as shown in Figure 4. From Jullie`re’s model it became evident that highly SP materials are needed for spintronic applications. One prerequisite for optimal performance of solidstate spintronics systems and devices is high spinpolarization, preferably complete polarization even in the absence of an external magnetic field. Numerical support for the existence of such materials – the half-metallic ferromagnets – was provided in 1983 by de Groot and Mueller. [41]. For spin-injection purposes, these inorganic half-metallic manganites are very important as they have very high net spin-polarization even up to high temperatures, and are thus able to inject SP carriers into an NM interlayer [42]. Additionally, they are very stable oxides, able to form a good interface with different materials with matching work functions and have significantly smaller conductivity mismatch with semiconductors compared to transition metals.

materials are used as (semi-)conductors in bulk or as thin films in electrical components like diodes, transistors, switches, and so on. The field of singlemolecule electronics or molecular electronics [44] is not touched further upon in this chapter. The advantages of organic materials include chemical tuning of electronic functionality, structural modifications, ability to form self-assembled structures, and mechanical flexibility. These characteristics are exploited for large-area and low-cost electronic applications. In this section, we briefly discuss the main developments in organic electronics involving bulk or thin films in the context of spintronics.

1.05.3.1

Organic Semiconductors

OSs are different from conventional semiconductors in terms of electronic-based understanding. The most important electronic energy levels are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). These states mostly form an energy gap similar to conventional semiconductors [45]. Present-day OSs have band gaps of 1.5–3.5 eV. OS films and bulk can be disordered (amorphous), polycrystalline, or crystalline in nature depending on the material used and the fabrication process. Devices that have been fabricated using these thin films include OLEDs [46,47], photovoltaic cells [48,49], and field-effect transistors (FETs) [50,51]. Significant improvements in the performance of these devices were observed in the last few years; in addition, new device applications have emerged. In the majority of the semiconducting organic materials, the hole mobility is higher than the electron mobility. Most of the devices that are fabricated are therefore of p-type. The n-type systems are also of interest because they enable the fabrication of p–n junctions, and complementary logic. Some examples have recently been reported in the literature [52a–c]. This is discussed later in this section. First, we discuss the two types of materials that are called OSs, namely the PCPs and small molecules. 1.05.3.1.1

1.05.3 Organics in Electronics Organic materials are mostly electrical insulators. The idea of organic electronics arose after the discovery of highly conducting PCPs [43]. Organic

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Conjugated polymers In carbon-based polymers, the valence electrons of the carbon are bound in sp3-hybridized covalent bonds. The PCPs have a backbone of contiguous sp2-hybridized carbon centers. One valence electron on each center resides in a pz orbital, which is orthogonal to the other three sigma bonds. The

116 Organic Spintronics

electrons in these delocalized orbitals give rise to most electronic properties of these materials. Research on improving the conductance of organic polymers was initiated in the 1960s. High conductivity in iodine-oxidized polypyrrole was already reported in the 1960s [53]. The major breakthrough was the discovery of metal-like conductivities in highly oxidized (iodine-doped) polyacetylene [43,54]. Though the term high conductivity is used here, it has to be noted that almost all known conductive polymers are disordered materials leading to low electronic mobilities compared to their inorganic counterparts. The usual mobilities of disordered polymer films are typically 0.1 cm2 V1 s1, although there are some reports of polymers with large crystalline regions having a relatively high mobility of 3 cm2 V1 s1 [55]. The morphology of PCP films has a significant role in carrier dynamics. Films made from PCPs are typically highly disordered, and this causes a typically Gaussian density of localized states that limits carrier mobility in films. However, the advantage of polymer films is the solution processability that allows mass fabrication possibilities [56].

1.05.3.1.2

Small molecules While thin films of polymers result in amorphous or polycrystalline films, more ordered, occasionally crystalline, films can be fabricated with small molecules, resulting in relatively high mobilities (1 cm2 V1 s1). Small molecules are currently used in the fabrication of most commercial OLEDs. Both the polymers and small molecules are disordered systems in terms of carrier transport but the primary difference is in the fabrication process. Most thin films of small molecules are grown by vapor deposition and are therefore limited by the area of the fabricated devices. The most commonly used small molecule OSs are (8-hydroxyquinoline) aluminum (Alq3), rubrene, and pentacene, which have a field-effect mobility of 6 cm2 V1 s1 [57]. Another way of using the small molecules in organic electronics is in the form of single crystals. Ultrapure organic single crystals have very good reproducible electronic properties [58]. In single crystals, grain boundaries are eliminated and the concentration of charge traps is minimized [59], making them suitable for studying the intrinsic electronic properties of organic materials. The mobilities that can be obtained from these single crystals have reached room-temperature values of

35 cm2 V1 s1 in pentacene [60] and 20 cm2 V1 s1 in rubrene [61].

1.05.3.2

Charge Transport in OSs

Charge transport in organic materials is explained in terms of hopping between localized states. In organic systems the DOS involved in the hopping transport of charge carriers is not exponential, like their disordered inorganic counterparts [62], but rather described by a Gaussian density of localized states. Two models of disordered organic materials are considered: the Gaussian disorder model (GDM) suggested by Ba¨ssler [63] and the correlated disorder model (CDM) considered by Garstein and Conwell [64], Dunlap et al. [65], and Novikov et al. [66]. In both models, the field dependence of carrier mobility has been studied by numerical methods. While analytical calculations have been carried out in order to justify the CDM [64–66], a consistent analytical theory for the nonlinear field effects of the organic disordered solids is still missing. From an experimental point of view, various electrical methods like the time-of-flight (TOF) method, the xerographic discharge method, the equilibrium charge carrier extraction method, the drift current methods under limited range conditions, the space-charge-limited-current (SCLC) method, the conductivity/concentration (/n) method, the FET method, the surface acousto-electric traveling wave method, and magnetic-interaction methods such as Hall effect, MR and cyclotron resonance are used to determine mobility values [67]. A propagating charge carrier in an OS is able to locally distort its host material due to the weak van der Waals force that binds the material together. The charge carrier combined with the accompanying deformation can be treated as a quasi-particle called a polaron [68]. A polaron carries spin-half, whereas two nearby polarons (referred to as a bipolaron) are spinless [69]. The polarons and the bipolarons will be of importance in understanding spin transport in organics discussed in later sections. Band-like conduction in organic materials is only expected at low temperatures for highly ordered systems like single crystals where the carrier mean free path exceeds the intermolecular distance. The valence band then generally originates from the overlap of the HOMO levels, and the conduction band from the overlap of the LUMO levels of the molecules, as described in the band diagram later in Figures 6 and 12 [70–72].

Organic Spintronics

Charge injection into the OS from various metal electrodes is a nontrivial phenomenon. The charge injection mechanism is governed by the interface between the metal electrode and the organic material. Some of the parameters that affect this contact are impurities, structural defects, charging, interface dipoles, chemical moieties, and other effects including the ones arising from the fabrication process. Carrier injection across the metal–organic interface is determined by the energy barrier height and the DOS at the Fermi level of the metal contact [73]. Low-work-function metals such as calcium are used to inject electrons into the LUMO and high-workfunction metals such as gold or indium tin oxide (ITO) are used to inject holes into the HOMO of the OS. The resulting Schottky barrier gives rise to nonlinear (diode-like) behavior. Severe contact resistances can be observed as the result of a mismatch of the HOMO or LUMO with respect to the work function of the electrode metal. The interface resistance depends exponentially on the barrier height and linearly on the DOS of the metal contact at EF. The complete understanding of the electronic properties of the metal/organic interface is yet to be achieved and is an active field of research [74,75]. Due to the softness of the organic materials, the deposition of metal electrodes on top of the organics for device purposes is an important experimental issue. Uncontrolled metal deposition can easily damage the organic material, causing a poor interface between the material and the electrode. The interface properties are especially important for spin injection, as discussed in more detail in later sections. 1.05.3.3 Advantages of Organics in Spintronics The advantages of organics in spintronics appear mainly in terms of spin transport. Spin–orbit coupling is considered small in OSs as they consist mainly of low-Z materials like carbon (C) and hydrogen (H). In materials such as polythiophene sulfur (S) atoms could provide a considerable spin– orbit coupling, but these atoms normally play a marginal role in carrier transport [76]. However, the atomic carbon spin–orbit coupling strength equals 6 meV for p states [77], which is actually quite strong compared to the small interchain hopping matrix elements in organic thin-film devices (typical mobilities are 105 cm2 V1 s1 or less).

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Therefore, the spin–orbit coupling could be a significant spin-scattering mechanism in organic spintronic devices [78a–b]. Moreover, spin–orbit coupling in excitons appears to be quite strong because relatively large intersystem crossing rates between singlet and triplet states have been observed in PCPs. The impact of spin–orbit interaction on spin transport in organics is discussed in more detail in Section 1.05.4.2. The nuclear spins in organic materials originate mainly from the 1H (I ¼ 1/2), 13C (I ¼ 1/2), and 14 N (I ¼ 2) atoms. OSs being mainly hydrocarbons possess a large number of hydrogen atoms in their side chains. The hyperfine fields arising from these hydrogen nuclei often play a very important role in spin-transport phenomenon through the OSs. Earlier studies show that the hyperfine interaction in organic molecules has a strength of 1 meV. When an SP carrier is injected into an OS, it hops between localized states where the site energy is random and distributed as a Gaussian DOS. Now, in the presence of a large number of randomly placed hydrogen atoms in the OSs, each having a random hyperfine field, the SP carriers face spin precession around these local fields, as suggested by Bobbert et al. [79], and the initial spin polarization is lost. Hence, it is concluded that for minimum spin decoherence and better spin transport, molecules having lesser hydrogen atoms, such as fullerenes and carbon nanotubes (CNTs), should be more appropriate candidates for organic spintronics.

1.05.4 Organic Spin Valves The first experimental demonstration of SP injection and transport in OS was reported by Dediu et al. [8] in 2002 in a -conjugated oligomer sexithienyl (T6), having a HOMO level at 4.8 eV and the mobility ranging from 102 to 104 cm2 V1 s1 depending on its morphology. A planar spin-valve-like structure was used where the half-metallic La0.67Sr0.33MnO3 (LSMO) acted as both the spin-injecting and detecting electrode. The electrode separation was through a channel of 100 nm fabricated using electron-beam lithography. A thin (100–150 nm) film of T6 was deposited on the substrate by molecular beam evaporation (Figure 5). The observed linear I–V characteristics were suggested to be a consequence of the proximity of work function of LSMO, estimated to be around 5 eV, and the HOMO level of T6. The spin-penetration depth in T6 was estimated to

118 Organic Spintronics

I V 3.5 20 μm LSMO

T6 w

LSMO

R (GΩ)

100 × 50 μm2

3

H=0

2.5 μm 1.8 × 1.6 mm2

LSMO

4

H = 0 (start) 1

MR = 30% 3.0

H = 3.4 kOe 2

2.5 2.0 × 105

2.5 × 105 Electric field (V cm–1)

3.0 × 105

Substrate Figure 5 (Left) The schematic view of the hybrid junction (drawing not to scale) and DC four probe electrical scheme. The cross-sectional view indicates a region near the spin transport channel. (Right) In time dynamic of the measured magnetoresistance for a 120 nm channel length La0.7Sr0.3MnO3/T6/La0.7Sr0.3MnO3 as a function of magnetic field. La0.7Sr0.3MnO3 film thickness is 100 nm, and T6 film thickness is 100 nm. From Dediu VA, Murgia M, Matacotta FC, Taliani C, and Barbanera S (2002) Room temperature spin polarized injection in organic semiconductor. Solid State Communication 122: 181–184.

be about 250–300 nm at room temperature. A maximum resistance decrease of <30% from the random to the parallel configuration was observed for a 140 nm channel at room temperature. The MR was independent of field orientation (perpendicular or parallel) and no MR effect was observed for channels larger than 200 nm. The first vertical organic spin valve was reported in 2004 by Xiong et al. [9] when the authors observed 40% GMR at low temperatures in spin valves made with small molecule 8-hydroxy-quinoline aluminum (Alq3) as a spacer. The Alq3 was vacuum-deposited on top of the LSMO (with coercive field, Hc < 3 mT) as the bottom electrode and the device was completed with the evaporation of a top electrode of cobalt with Hc < 15 mT. Since the electrodes have different coercive fields, unlike the previous example of the planar configuration described above, both parallel (P) and antiparallel (AP) magnetization of the contacts are readily achieved. A schematic band diagram is given in Figure 6, indicating the HOMO and LUMO levels of Alq3 and the (nearly equal) work functions of LSMO and Co. At low bias voltages, holes are injected from the anode into the HOMO level. The evaporation of the top Co electrode causes pinholes and Co inclusions in the Alq3 layer over a distance of <100 nm. The Co/Alq3 interface was therefore poorly defined. A negative

MR of 40% was observed at 11 K for a 130 nm thick Alq3 layer. The first observation of room-temperature operation of a vertical SV (Figure 7) was reported in 2006 in LSMO/RRP3HT/Co [10] and LSMO/P3OT/ LSMO [11] devices, where the NM material was a polymer, namely regio-regular poly(3-hexyl thiophene) (RRP3HT) and poly(3-octylthiophene) (P3OT). In the RRP3HT-based spin valves, 80% MR was observed at 5 K and 1.5% MR was observed at room temperature. In the P3OT SVs, highest MR response was observed in the temperature range of 210–320 K. This is similar to the trend followed by the variation of MR of LSMO. However, there was a distinct difference in the MR magnitudes for LSMO and SVs. Furthermore, the observation of normal (i.e., positive) MR in the polymeric SVs (PSVs) instead of the inverse MR observed in the Alq3 SVs is also of importance. Pramanik et al. [32] reported the first organic nanowire spin valve, 50 nm in diameter, consisting of a trilayer of FM cobalt, an organic, Alq3, and FM nickel. The nanowires were produced within the 50 nm diameter pores of an anodic alumina film by selective electrodeposition from a solution of NiSO4:6H2O. Alq3 was evaporated on the porous film and cobalt was evaporated on the top (Figure 8). Typical MR traces of the nanowire spin

Organic Spintronics

119

25.0

25.5

40

P

30

P

20 20.0

IΔR/RI (%)

Resistance (kΩ)

11 K

10

17.5

0

–1500

AP

AP

–750

750 0 Magnetic field (Oe)

(a)

(b)

V i

O

Vacuum level

Co/AI

2.8 eV

EF

Alq3

M

FM1

LS

EF

HOMO LSMO

H

Alq3

Co

40 IΔR/RI (%)

7.5 6.0 IΔR/RI (%)

–10

I p = 5.7 eV φ LSMO = 4.8 eV LUMO φ Co = 4.9 eV

FM2 Organic

1500

4.5 3.0

30

11 K

20 10 0 120

1.5

160 200 240 Thickness (nm)

0.0 0

60

120 180 Temperature (K)

240

300

Figure 6 (Top) GMR loop of a LSMO (100 nm)/Alq3 (130 nm)/Co (3.5 nm) spin-valve device measured at 11 K. The blue (red) curve denotes GMR measurements made while increasing (decreasing) H. The antiparallel (AP) and parallel (P) configurations of the FM magnetization orientations are shown in the insets at low and high H, respectively. The electrical resistance of the device is higher when the magnetization directions in FM1 and FM2 films are parallel to each other. (Middle) (a) Schematic representation of a typical device that consists of two FM electrodes (FM1 and FM2) and an OSE spacer. Spin-polarized electrical current I flows from FM1 (LSMO), through the OSE spacer (Alq3), to FM2 (Co) when a positive bias V is applied. An in-plane magnetic field. H, is swept to switch the magnetization directions of the two FM electrodes separately. (b), Schematic band diagram of the OSE device in the rigid band approximation showing the Fermi levels and the work functions of the two FM electrodes, LSMO and Co, respectively, and the HOMO–LUMO levels of Alq3. (Bottom) j
R/Rj measured at V¼ 2.5 mV as a function of temperature for a typical device. The inset shows the GMR value of a series of LSMO/Alq3/Co devices with different thickness, d. The line fit through the data points was obtained using the spin-diffusion model equation, with three adjustable parameters, as explained in the text. All devices were fabricated on the same LSMO film. From Xiong ZH, Wu D, Valy Vardeny Z, and Shi J (2004) Giant magnetoresistance in organic spin-valves. Nature (London) 427: 821–824.

120 Organic Spintronics

110.4

T = 300 K

1.5

110.0

R (kΩ)

109.2

0.5

MR(% )

1.0

109.6

108.8 0.0 108.4 108.0 –300

–200

–100

0 B (mT)

100

200

–0.5 300

Figure 7 Magnetoresistance response of a typical LSMO/RRP3HT/Co device at 300 K as an average of 10 scans showing consistent switching at room temperature. The red line is a guide for the eyes. From Majumdar S, Laiho R, Laukkanen P, Va¨yrynen IJ, Majumdar HS, and ¨ sterbacka R (2006) Application of regioregular O polythiophene in spintronic devices: Effect of interface. Applied Physics Letters 89: 122114.1–122114.3.

valves where the magnetic field is parallel to the axis of the wires at three different temperatures show that there is a background positive MR (Figures 8(a) and 8(b), right panel). Superimposed on this background were nonmonotonic MR peaks located between fields of 80 and 180 mT, which are approximately the coercive fields of the nickel and cobalt layers. These peaks are the signatures of the spinvalve effect. The heights of these peaks decreased with increasing temperature and were barely visible at 100 K. From the relative height of the spin-valve peak
R/R, they extracted the spin-diffusion length in the Alq3 layer and concluded that spin-relaxation length in OS is extremely long (discussed in detail in Section 1.05.4.2). Apart from the reports above, there have been reports on fabrication of SV using CNTs and graphene

(a)

Au wire

–6

–4

Magnetic field (koe) –2 –0 –2

–4

6

4

6

1584 100 K

Cobalt

Resistance (Ω)

Alq3

1 μm

Alq3

1582

1580

1578 (b) 1530

50 K

Nickel

50 nm

Aluminum substrate

Resistance (Ω)

Alumina

1527

1524

1.9 K

1521

1518 –6

–4

2 –2 0 Magnetic field (kOe)

Figure 8 (Left) Schematic picture of Alq3 nanowires connected to Co and Ni electrodes. The nanowires with a diameter of 50 nm are synthesized in a porous AI membrane with a well-ordered hexagonal close-packed arrangement of 50 nm pores. (Right) (a) and (b) Forward and backward scan showing MR for two different temperatures. The magnetic field points parallel to the axis of the wires. From Pramanik S, Bandyopadhyay S, Garre K, and Cahay M (2007) Observation of extremely long spin relaxation times in an organic nanowire spin valve. Nature Nanotechnology 2: 216–219.

Organic Spintronics

In 2004, Ruden and Smith [6] proposed a theoretical model for describing electrical spin injection from an FM contact into a conjugated OS. It was suggested that though the magnetic contact is SP in thermal equilibrium, the OS is unpolarized. To achieve significant spin-current injection, the OS must be driven far out of local thermal equilibrium by an electric current. However, if the injecting contact has metallic conductivity, its electron distribution cannot be driven far from thermal equilibrium by practical current densities. Quasi-equilibration between the conjugated OS and the metallic contact must be suppressed to achieve effective spin injection. This requires a spin-dependent barrier to electrical injection that may be either due to tunneling through the depletion region of a large Schottky barrier or due to tunneling through a thin, insulating, interface layer. The energy barrier for electrical injection in the OS can be varied over a wide range by using metals with different work functions. In addition, insulating tunnel barriers to OSs based on organic molecules can be conveniently fabricated using self-assembly techniques. In 2006, the effect of engineering of the interface between inorganic FM electrode and OS and its effect on the spin injection were demonstrated experimentally [10]. For a RRP3HT spin valve, sandwiched between LSMO and Co electrode, an interfacial chemical reaction between LSMO and RRP3HT was observed, which assists in spin injection. Upon introduction of monolayer of two organic insulators both in the tunneling limit, which destroy the chemical bonding between RRP3HT and LSMO, the spin injection is hindered. Different devices with varying degrees of chemical bonding between LSMO and RRP3HT were studied and it was shown that with decreasing chemical bonding (Figures 9 and 10) the spin injection from the FM half-metal to the OS is systematically decreased. Later, similar results showing chemical reaction between LSMO and organic molecules were also reported in Ref. [81].

AI Co P3HT LSMO MgO (substrate) 0.5

M /MS

1.0

0.0 –0.5 –1.0

T = 5K LSMO Co

LSMO left LSMO covered uncovered with a very thin layer of P3HT

(b)

100 8 7

Down sweep Up sweep Device A Device B Device C

T=5K

6

80 60 40

5

20

4

0

3

MR(%)

1.05.4.1 Spin Injection and Detection: Role of Interface

(a)

R (MΩ)

as the semiconducting spacer. A review on these devices is available in Ref. [80]. We also discuss some recent graphene-based spin-valve results in Section 1.05.4.5. In the next part of Section 1.05.4, we concentrate on the results and understanding of the three important aspects of SV operation – injection, transport, and detection, in terms of the organic spin valves.

121

–20 –300 –200 –100

0 100 B (mT)

200

300

Figure 9 (a) Magnetic hysteresis loop of the two ferromagnetic electrodes LSMO and Co at 5 K. The inset is a microscope image of the dead layer formed on LSMO after washing off the RRP3HT and a cross section of the SV structure. (b) Magnetoresistance (MR) of three different SVs as a function of magnetic field B at 5 K showing 80% MR for device A, 20% for device B, and no MR in device C. The red line indicates average of 10 scans taken at the same time. The dotted lines are a guide to the eyes showing sharp switching and matching of the lower switching field exactly with the coercive field of the FM electrode Co. From Majumdar S, Laiho R, Laukkanen P, Va¨yrynen IJ, Majumdar ¨ sterbacka R (2006) Application of regioregular HS, and O polythiophene in spintronic devices: Effect of interface. Applied Physics Letters 89: 122114.1–122114.3.

In 2007, Zhan et al. [82] studied the spin injection and the role of the interface using photoelectron spectroscopy in LSMO/Alq3/Co SVs. In an unperturbed band diagram shown in Figure 11 of this device structure, the injection of holes is more favorable than the injection of electrons in Alq3. However, inverted MR in these structures is mostly observed. It was found that the introduction of Alq3 on LSMO creates a strong interface dipole of 0.9 eV that shifts the energy levels of Alq3 with respect to the vacuum level. From this interface modification, seen as a modified energy-level diagram in Figure 11, it became evident that the shift makes electron injection into Alq3 more favorable than hole injection. The dipole moment of the Alq3 molecule itself was suggested to be the origin of the interface dipole.

122 Organic Spintronics

(a)

(b) s2

S 2p

Device A

Device B Device C

160

175 170 165 Binding energy (eV)

Br 3d Photoelectron intensity (a.u.)

Photoelectron intensity (a.u.)

s1

Device A

Device B

Device C

62 64 66 68 70 72 74 76 Binding energy (eV)

Figure 10 XPS spectrum of (a) sulfur and (b) bromine on LSMO films covered with RRP3HT, HMDS/RRP3HT, and ODTS/ RRP3HT as done in devices A, B, and C. From Majumdar S, Laiho R, Laukkanen P, Va¨yrynen IJ, Majumdar HS, and O¨sterbacka R (2006) Application of regioregular polythiophene in spintronic devices: Effect of interface. Applied Physics Letters 89: 122114.1–122114.3.

0.9 eV

1.4 eV

Vacuum 0.9 eV

4.9 eV

5.0 eV

4.9 eV – – –

LUMO 2.74 ± 0.13 eV

EF

2.8 eV 1.7 eV HOMO 5.7 eV

LSMO

Alq3

Figure 11 Schematic energy band diagram of the Alq3/LSMO interface. The values directly measured in the experiments of Ref.[82] are evidenced by circles, the optical gap (2.8 eV) between the dashed line and HOMO is taken from literature mentioned in Ref.[82] and the LUMO level is calculated from STS data. From Zhan Y, Bergenti I, Hueso LE, Dediu V, de Jong MP, and Li ZS (2007) Alignment of energy levels at the Alq3/ La0.7Sr0.3MnO3 interface for organic spintronic devices. Physical Review B 76: 045406-1–045406-5.

The same technique was used to identify the interface of Alq3/Co on the detector side of the SV. It was shown that a fully ordered Alq3 layer could result in a shift of about 1 eV, arising from the

Co

+ + +

1.7 eV

+ +

– –

2.1 eV

Alq3

LSMO

Figure 12 Schematic energy band of the standard LSMO/Alq3/Co spin-valve devices. From Zhan Y, Bergenti I, Hueso LE, Dediu V, de Jong MP, and Li ZS (2007) Alignment of energy levels at the Alq3/La0.7Sr0.3MnO3 interface for organic spintronic devices. Physical Review B 76: 0454061–045406-5.

intrinsic dipoles of Alq3 molecules. In the case of Alq3/Co, an additional lowering of the work function by 0.5 eV results from Pauli repulsion (Figure 12). Another important observation is that the shift is similar for both Alq3 /Co (i.e., Co deposited on top of Alq3) and Co/Alq3 (i.e, Alq3 deposited on top of Co) interfaces [82]. The authors suggested that an ordering of molecular dipoles must come about through a strong interaction between Co atoms and Alq3 molecules, since no pre-existing dipole ordering exists at the Alq3 surface prior to deposition of Co atoms. The Co was found to chemically react with

Organic Spintronics

1.0

50

SV on LSMO/MGO SV on LSMO/STO

40

0.6

MR(%)

M/M s

0.8

0.4 LSMO/MGO LSMO/STO LSMO/NGO

0.2 0.0 0

75

123

30 20 10

150 225 300 Temperature (K)

375

0

50

100 150 200 Temperature (K)

250

300

Figure 13 (Left) Normalized field-cooled magnetization (measured at B ¼ 100 mT ) as a function of temperature for the LSMO films made on MGO, STO, and NGO substrates with the optimized parameters. (Right) Temperature dependence of MR response of the LSMO/RRP3HT/Co SVs made on MGO and STO substrates. From Majumdar S, Huhtinen H, Majumdar ¨ sterbacka R (2008) Effect of La0.67Sr0.33MnO3 electrodes on organic spin valves. Journal of Applied HS, Laiho R, and O Physics 104: 033910-1–033910-6.

the phenoxide part of the Alq3 molecules and contribute to this process. Another important aspect of spin injection into the organic spacer is the spin polarization of the injecting electrode. It was shown by studying the temperature dependence of the MR of various LSMO/polymer/cobalt spin valves [83] that even with better LSMO films having close to 70% spin polarization at room temperature (Figure 13) it was not possible to inject spins more efficiently into the OS. It was concluded that the lack of spin polarization in bulk LSMO films is less critical for SV operation compared to other losses. The loss of the SP carriers at the LSMO/OS interface at room temperature is a more dominant effect and drastically reduces the MR signal. This also justifies the importance of the interface on the efficiency of spin injection.

15

MR (%)

MR (%)

–8 –4 –8

0.00

10 5 0

–6

–0.05

100 200 300 Alq3 thickness (nm)

–4

–0.10

MR (%)

MR (%)

–10

0

Dediu et al. [84] showed that although the MR response decreases monotonically with increasing temperature, it is still possible to detect roomtemperature response in Alq3/LSMO devices by improving the FM–OS interfaces. They also showed that even though the surface spin polarization of LSMO is decreased with increasing temperature, it is still possible to inject SP carriers from LSMO into the OS layer (Figure 14). It is to be noted that there are also reports which expressed doubts about spin injection from an FM electrode into OS. In 2007, Xu et al. [85] showed that both in LSMO/Alq3/Co and LSMO/TPP/Co spin valves, an MR response only exists for spacer thickness of approximately 10–15 nm. For devices with higher spacer thickness, the MR response becomes independent of thickness and only 10% of the devices show MR response. With the atomic force

–2 –0.15

0

–12 –400 –200 0 200 400 Magnetic field (Oe)

0

100 200 300 Temperature (K)

–75–50 –25 0 25 50 75 Magnetic field (Oe)

Figure 14 (a) Inverse spin-valve effect at 20 K showing a maximum value of 11%. (b) MR values as a function of temperature. The MR decreases with increasing temperature but persists up to room temperature. (c) Room temperature inverse spin-valve effect. The MR of each individual electrode was carefully studied, enabling us to rule out anisotropic MR as the origin of our findings. A small background nonhysteretic signal, probably intrinsic to the organic semiconductor layer, was subtracted in every case to clearly show the hysteretic spin-valve effect. Form Dediu V, Hueso LE, Bergenth I, et al. (2008) Room-temperature spintronic effects in Alq3-based hybrid devices. Physical Review B 78: 115203-1–115203-6.

124 Organic Spintronics

microscopy pictures, they showed that for spacer thickness of 10–15 nm, the films are not smooth and grow in islands. Thus, they concluded that spin transport in these devices is actually by tunneling and take place through these thinner portions of the spacer. In 2008, Jiang et al. [86] reported absence of MR in the Fe/Alq3/Co spin valves, where the spacer thickness varied from 25 nm to 150 nm. They showed that charge transport in Fe/Alq3/Co spin valves is by holes only, and that the current is injection limited. The transport of hole alone in Alq3 is stable only at low current densities and the authors supported the tunneling interpretation of the earlier reported MR. They concluded that similar to inorganic semiconductors, the large conductivity mismatch between the metal electrodes and the OS prevents spin injection. Inserting a tunnel barrier between the magnetic electrode and the OS did not improve spin injection. The spin injection in OS spacer remained debated due to the lack of enough spectroscopic evidence of SP carrier injection from the FM to the OS. The drawback with the standard spectroscopic techniques for detection of SP carrier injection into OS was the absence of a sizable spin–orbit coupling in the OS. Thus, the spin–orbit coupling property which makes (a)

OS promising for spintronic applications, rendered itself as a hindrance for spectroscopic determination of spin injection. However, two reports by Drew et al. [87] and Cinchetti et al. [88] gave strong experimental evidence for high-efficiency spin injection from an FM electrode into OS layer by using two different spectroscopic techniques specially designed for probing SP carriers in OS. Cinchetti et al. [88] used spin-resolved twophoton photoemission on Co–copper phthalocyanine (CuPC) structures to directly and in situ measure the efficiency of spin injection at the Co–CuPc interface (Figure 15). They reported a spin-injection efficiency of 85–90% for injection into unoccupied molecular orbitals of CuPc and estimated an electron inelastic mean free path in CuPc in the range of 1 nm and a quasi-elastic spin-flip length which is 10–30 times higher. The quasi-elastic spin-flip process with energy loss  200 meV was found to be the dominant microscopic mechanism limiting the spin-diffusion length in CuPc. The measurement of the spin polarization of the injected electrons as a function of CuPc layer thickness showed that spin polarization decreased with increasing CuPC layer thickness. The spindiffusion length at the injection energy 2.4 eV above (b)

Evac

Laser penetration depth

LUMO

CuPc Cobalt

EF HOMO Cobalt

Space

CuPc Energy

Spin polarization (%)

Electron mean free path

Energy

Sample normal direction

Energy-and spin detector Laser pulse

40

Efinal

Ei

2.2 eV 30

2.0 eV 1.8 eV

20

1.6 eV 1.4 eV

10 0 0

2

4 6 8 10 12 14 16 CuPc thickness (ML)

Figure 15 (a) Conceptual principle of the experiments. The sample, constituted of a cobalt thin film covered with a homogeneous CuPc film of variable thickness, is illuminated with pulsed laser light with photon energy h ¼ 3.1 eV. The laser penetration depth (blue-shaded area) is much larger (at least 10 orders of magnitude) than the inelastic mean free path of the electrons excited by the laser (violet-shaded area). As a consequence, a first photon generates spin-polarized electrons in the cobalt film, and only those spin-polarized electrons that reach the surface region in CuPc are subsequently photoemitted by absorbing a second photon from the laser pulse. The energy and the spin component along the cobalt magnetization direction of the photoemitted electrons are analyzed. Energetically, the electrons are excited by the first pulse in intermediate states Iying between the Fermi and the vacuum level of the heterojunction. A second photon gives to some of those excited electrons enough energy to be photoemitted (2PPE process). Before being photoemitted, the electrons must travel from cobalt into CuPc, or in other words they must be injected from cobalt into the molecular orbitals of CuPc Iying above the LUMO level. (b) The spin polarization P ¼ ((Nup – Ndown)/(Nup þ Ndown þ NCuPc tot)) measured at different energies E i between 1.4 and 2.2 eV as a function of CuPc coverage. From Cinchetti M, Heimer K, Wu¨stenberg J-P, et al. (2009) Determination of spin injection and transport in a ferromagnet/organic semiconductor heterojunction by two-photon photoemission. Nature Materials 8: 115–119.

Organic Spintronics

the HOMO level of the OS was determined to be 13 nm in CuPc at room temperature. Drew et al. [87a] presented a direct and depthresolved measurement of the spin penetration away from the injecting interface in a fully functional organic SV. For this, they used the low-energy muon spin rotation (LE-mSR) technique. The technique is based on the fact that the SP current from an FM–OS junction creates a magnetic field, B, in the OS layer, where B decays at an average distance from the FM–OS junction that is related to the spin-diffusion length, S. To measure the decay of B, LE-mSR is used. The muons carry spin (S0) and upon implantation into the OS their stopping depth (z0) is directly determined by their kinetic energy, E0, as shown schematically in Figure 16 [87b]. After implantation, the 100% SP muons decay with a lifetime of 2.2 ms, during which their spins precess according to the local field B (z0) in the OS layer. The measurements yielded a spin-diffusion length of S 10 nm in Alq3 at 90 K. Drew et al. also performed control experiments for validating the technique in a working organic spin-valve device, and from their measurements concluded that the mSR and MR results are in perfect agreement. The mSR and MR experiments were also carried out at various temperatures and showed that S in Alq3 decreases substantially with temperature. The stopping distribution of the fully SP muons can be varied on the scale of about 3–200 nm through the control of the muon implantation energy. The obtained mSR

(a)

Fμ 1

Fμ 2

OSEC

spectra yield the probability distribution of the local magnetic field at the muon sites, often referred to as a mSR line shape, which contains direct information on the local spin polarization of the charge carriers.

1.05.4.2

Spin Transport and Relaxation

Once the SP carriers are injected into the OS they travel through the semiconductor mainly by drift and diffusion under the influence of an electric field. During the transport the SP carriers interact with their environment (trapping, spin precession around a local hyperfine field) and their initial spin direction is lost. This loss of original spin direction is called spin relaxation. For the lateral GMR geometry, it is essential that the injected spin current can be transferred over a length L, which should be smaller than the spin-relaxation length. Besides the spinrelaxation time, the conductivity of the organic conductor therefore needs to be sufficiently large. Whereas the long spin-relaxation time is a clear advantage of organic materials, the relatively low conductivity of most organic conductors is a serious point of concern. However, important progress has been made in recent years. In organic materials, the dominant spin-relaxation mechanisms, spin–orbit coupling and hyperfine interaction are expected to be small, but are not completely negligible for most materials. The dominant relaxation mechanisms in organic materials are still rather unclear. There are a few reports where the

(b)

Δ

ΔP

Δ ~ δB

ΔB SP muons S0;E0

125

z0

Precession

B0 B

Positrons1

Positrons2 S(t)

Detector 1

Detector 2

Spin injection

δB

B0

λ5 Figure 16 (a) Scheme of the experiment (description in Ref. [17]). (b) The field B is formed by the injected spins into the OSEC and is obtained from the probability difference, P, between the measured field distribution with the device current turned on and off, using the skewness parameter,
. In the case shown here, the field B is parallel to B0, and therefore the field distribution has a positive, which corresponds toR an increase of the local field caused by the injected spins into the OSEC. Note that P is conserved, since the integral,
P dB ¼ 0. From Vardeny, V (2009) Organics strike back. Nature Materials 8: 91–93.

126 Organic Spintronics

spin-relaxation length is determined from fitting to Jullie`re formula, but it is hard to distinguish between spin relaxation at the interfaces and within the organic material itself [9]. Further, the simple Jullie`re formula [37] is not always very appropriate for the applied device configurations. Broadly speaking, the spin relaxation can be related to spin–orbit coupling, hyperfine interaction, and scattering due to impurities in the bulk organic material. In the very first report of vertical organic spin valves, Xiong et al. [9] analyzed the obtained GMR effect and its dependence on spacer thickness using an injection and diffusion model. The NM–OS layer was divided into two parts, the total thickness (d) of the OS layer and the part of it d0 with Co pinholes during top electrode evaporation. The neatly deposited OS sublayer with thickness d–d0 was thicker (30 nm) for simple quantum mechanical tunneling through it. The authors assumed that there exists a potential barrier for spin injection at the Co/OS interface, which may be self-adjusted. Once carriers are injected through this interface, they easily reach the neat sublayer. Then, they drift under the influence of the electric field toward the other interface, from where they can be extracted. As the injected carriers reach the end of the ill-defined sublayer, the spin polarization p1 decays in the remaining clean sublayer with a surviving probability exp[–(z – d0)/ S], where z is the drift/diffusion distance along the normal direction to the interface, and S is the spindiffusion length in the neatly deposited OS sublayer. The thickness dependence of the GMR magnitude,


R/R, that is, the maximum relative change in electrical resistance R within the spin-valve hysteresis loop, assuming no loss of spin memory at the interfaces owing to the self-adjusting capability of the OS, is then calculated as
R RAP – RP 2p1 p2 e – ðd – d0 Þ=S ¼ ¼ RAP R 1 þ p1 p2 e – ðd – d0 Þ=S

where RAP and RP denote R in the antiparallel and parallel magnetization configurations, respectively. For inverse MR, RAP < RP and therefore
R/R is negative. With the help of equation 6, p1p2, d0, and S were calculated and the following values of the parameters were obtained: p1p2 ¼  0.32; d0 ¼ 87 nm; and S ¼ 45 nm. However, I–V curves of a diode of Alq3 with a thickness corresponding to the clean film should show a strong T-dependence, inconsistent with the electrical data presented in Ref. [9] In 2007, Wang et al. [89] reported spin-valve devices made of three different organic molecules, namely N,N9-bis(1-naphtalenyl)-N,N9-bis(phenyl)benzidiane ( -NPD), a common hole transporter, Alq3 and 4,49-bis(9-ethyl-3-carbazovinylene)-1,19biphenyl (CVB), a light-emitting -conjugated oligomer sandwiched between LSMO and cobalt electrodes. Subsequently, the spin injection and transport properties were studied by measuring the device MR response at various biasing voltages V and temperatures T (Figure 17). They found that the spin-valve MR response in all devices monotonically decreases with V and is asymmetric with respect to

1.2

3

CVB Alq3

0.6

104 (δPL/PL)

α -NPD 104 (δPL/PL)

Normalized MR

1.0 0.8

0.4 0.2

ð6Þ

2

220 K

0.8 0.4 0.0 950

975 1000 1025 1050 Magnetic field

1

0.0 0

50

100 150 Temperature (K )

200

250

0 0

50

100 150 200 250 Temperature (K )

300

Figure 17 (Left) The MR value of three different LSMO/OSEC/Co spin-valve devices vs. temperature T normalized at T ¼ 14 K; the MR absolute values of these devices are summarized in Table 1 of Ref.[89]. The OSEC interlayers in these devices are Alq3 (green (gray) squares), CVB (red (dark gray) circles), and NPD (blue (black) stars). (Right) The temperature dependence of the spin-1/2 PLDMR (PL/PL) value at 950 G (i.e., spin-1/2) for an evaporated Alq3 film. The inset shows the PLDMR value vs. the applied field H at 220 K. From Wang FJ, Yang CG, and Valy Vardeny Z (2007) Spin response in organic spin valves based on La2/3Sr1/3MnO3 electrodes. Physical Review B 75: 245324–245330.

Organic Spintronics

the voltage polarity together with a steep MR decrease with T, where it vanishes at T >220 K, similar to other MR responses in inorganic tunneling junction devices based on LSMO and Co FM electrodes. In contrast, the spin-1/2 photoluminescence detected magnetic resonance of the organic interlayer, which directly depends on the spin-lattice relaxation rate of polarons in the OS, was found to be temperature independent. Thus, the authors conclude that the steep MR dependence on T is due to the temperature dependence of the interfacial spin polarization of the LSMO electrode, which also drastically decreases up to T > 220 K and that the spin-lattice relaxation time in OS should not be the limiting factor in fabricating room temperature organic spin valves. In order to achieve roomtemperature spin-valve operation with a substantial MR value, spin-injection electrodes other than LSMO, having a large but low temperature-dependent spin polarization, need to be involved. A study of MR in Co/Alq3/Ni nanowires, performed by Pramanik et al. [32], showed MR effect of about 1% at low temperature. From their estimate of the spin-relaxation length by the Jullie`re model, the spin-relaxation time turned out to be extremely long. Values between a few milliseconds and a second were obtained, depending on the mobility value of the Alq3. Pramanik et al. [32] also measured both the longitudinal (T1) and ensemble-averaged transverse (T2 ) spin-relaxation times in organic nanostructures over a broad temperature range. These studies identified the dominant spin-relaxation mechanism in the -conjugated molecule Alq3 as the EY mechanism. The EY mechanism is normally the dominant spinrelaxation mechanism in low carrier-mobility materials. The two most important revelations of these studies are (1) the longitudinal spin-relaxation time (T1) in the Alq3 molecule is exceptionally long above liquid nitrogen temperature, approaching 1 s at 100 K and relatively temperature independent from 1.9 K to 100 K and (2) the transverse spin-relaxation time (T2) is also quite long. In a recent review, Pramanik et al. [90] thoroughly discussed and calculated spin-relaxation times and lengths in the small molecule Alq3 using different device configurations such as spin valves and nanowires. They also presented some experimental results pointing at a possible phonon bottleneck effect in few molecule samples of Alq3 confined in 1–2 nm spaces. Their results indicate that long spinrelaxation time in OS can be very useful for future organic spintronic applications.

127

1.05.4.2.1 Effect of impurity inclusion on the spin-transport property of OS spacers

Starting from the very first report of vertical organic spin valves, it was discussed that inclusion of the FM top electrode in the OS spacer from the evaporation process is an important challenge. Vinzelberg et al. [91] first showed the irreproducibility problem in LSMO/Alq3/Co devices. Due to the uncontrollable Co (top electrode) evaporation, there is often no systematic dependence of device resistance on the spacer layer thickness and device area. They reported spin-valve effects at 4.2 K in devices with a broad resistance interval from 50 to M range, and in some samples the MR changes sign as a function of the bias voltage (Figure 18). From the observed similarity in the bias voltage dependences of the MR in comparison with conventional MTJs with oxide barriers, the authors suggested that the effects could be due to tunneling. The existence of conducting Co chains within the organics was confirmed by transmission electron microscopic/electron energy loss spectroscopic studies on cross-sectional samples from analogous layer stacks as seen in Figure 18. The proposed model implies the realization of the transport through local Co chains embedded in the Alq3 layer and spin-dependent tunneling over barriers at the interface Co grains/Alq3/LSMO. In 2009, another report discussed vividly the effect of magnetic impurities in the spacer layer of polymeric spin valves with the sandwich configuration of LSMO/conjugated polymer/cobalt (Co), showing GMR response [92]. Based upon different deposition rates of Co at the top electrode, two types of devices were fabricated: one with lower device resistance and linear current–voltage (I–V) characteristics and the other with very low inclusion of Co and exhibiting higher device resistance and nonlinear I–V characteristics. An asymmetric DC bias dependence of MR in devices with more Co inclusion was observed, while for the other type of device, the bias dependence was more symmetric. At higher bias, the MR change in both types of device showed no significant difference (5–10%), but at low dc bias it ranged between 50 and 160% MR. This was attributed to the higher tunneling probability of SP carriers from one FM electrode to the other. MTJlike features were observed in the devices with greater Co inclusions. Devices with more Co penetration (Figure 19, Set 1) caused metal particles or clusters in the whole polymer layer during deposition and the ballistic transport through these embedded metal clusters inside the polymer acted

128 Organic Spintronics

(a)

5

2

(b)

R ~ 23 kΩ

R ~ 62 kΩ 0

0

–10

MR (%)

MR (%)

–2 –5

I = 50 μA

T = 4.2 K

–4

–15

–8

–20 –5

(c)

0 H (kOe)

–10 –5

5

4

(d)

R ~ 200 kΩ

0 H (kOe)

5

2 R ~ 1 mΩ 0

MR (%)

3 MR (%)

I = –10 μA

T = 4.2 K –6

2 I = 1 μA

T = 4.2 K

–2 –4

I = 200 nA

T = 35 K

1 –6 0 –5

0

–8 –5

5

0 H (kOe)

H (kOe)

(b)

Au Colayer

Position axis Alq3

Co

Intensity (a.u.)

(a)

5

Alq3

Au

50 nm 0

20

40 60 80 Position (nm)

100

Figure 18 (Left) MR measured at constant current of different LSMO (100 nm)/Alq3 (150 nm)/Co(10 nm)/Al(10 nm) layer stacks at 4.2 K with device resistances from 60 up to 1 M . (Right) TEM/EELS studies on an Alq3/Co interface: (a) TEM bright-field picture with indication of the integration area within the sample range analyzed by EFTEM, and (b) intensity profile of the Co distribution along the position axis see a perpendicular to the Alq3/Co/Au interfaces obtained from EELS data of a Alq3(150 nm)/Co(10 nm)/Au(50 nm) layer stack. From Vinzelberg H, Schumann J, Elefant D, Gangineni RB, Thomas J, and Bu¨chner B (2008) Low temperature tunneling magnetoresistance on (La,Sr)MnO3/Co junctions with organic spacer layers. Journal of Applied Physics 103: 093720-1–093720-5.

as magnetic nanocontacts between the two FM electrodes (Figure 19). This plays an important role in the spin transport, especially at low temperature. The total device resistance per unit area (5  105 cm2) for the devices with more Co inclusion and that for

devices with lesser inclusion (4  108 cm2) (Figure 19, Set 2) are a few orders of magnitude higher than the in-plane resistance per unit area of the LSMO bottom electrode (4.17  103 cm2). Therefore, in the Set 1 devices, the relative

Organic Spintronics

129

Al Co

30 nm 70 nm

RRaP3HT LSMO Set 1

Set 2

Figure 19 Cross-sectional schematic diagram of the Set 1 and Set 2 devices showing different conduction channels for SP carriers in these two device sets. Curved arrows indicate hopping conduction through the clean polymer layer, while straight ¨ sterbacka R arrows indicate channels shorted by Co inclusions (Set 1). From Majumdar S, Majumdar HS, Laiho R, and O (2009) Organic spin-valves: Effect of magnetic impurities on the spin transport properties of polymer spacers. New Journal of Physics 11: 013022-1–013022-11.

1.05.4.3

Organic MTJs

Santos et al. [14] demonstrated SP tunneling through a thin Alq3 barrier sandwiched between a Co (bottom) and Ni80Fe20 (permalloy, Py) contact (top) at room temperature (Figure 21). I–V characteristics and polarization measurements indicate the good quality of the Alq3 barrier without any Co inclusions. The TMR value was improved by adding an Al2O3 layer in between the Co and the Alq3 tunnel barrier, which reduces the formation of interfacial charge states. The highest TMR observed at room temperature was 6% and a substantial TMR value was even

9

(a)

Set 1

(b)

Set 2

10

I = 1 mA

6 I = 1 mA

5

0

0

150

Set 1

(c)

(d)

Set 2

60 I = 10 nA

I = 150 nA

100

40

50

20

–200 –100

MR (%)

3

MR (%)

contributions from the two conduction channels are considered – drift/diffusion through the polymeric spacer and ballistic SP transport through the pinhole nanocontacts [93] that connect the two FM electrodes. On the contrary, in the Set 2 devices, the transport is mainly governed by drift/diffusion through RRaP3HT due to decreased Co penetration in the bulk. Anomalous MR peaks were also observed in these devices as shown in Figure 20, and their origin was explained in terms of the presence of additional scattering centers around the included metal ions and increased spin relaxation due to high magnetic anisotropy in the system. Both types of SVs showed a monotonic decrease in MR with temperature at high bias currents. Lin et al. [94] showed that even the LSMO film morphology plays a very important role in determining the organic spin-valve properties. The origin of ill-defined layers in organic spin valves was investigated by using AFM and Rutherford backscattering (RBS) analysis. It was found that conductive bulges of LSMO film and self-grown pinholes in Alq3 film other than Co inclusions could lead to the formation of an ill-defined layer.

0

100 –200 –100 0

0 100 200

B (mT) Figure 20 The MR% vs. magnetic field (B) curves for (a) Set 1 and (b) Set 2 devices for higher bias current measurements of the milliampere range and (c) Set 1 and (d) Set 2 devices for lower bias current (I) measurements of the nanoampere range in a typical RRaP3HT PSV, showing the appearance of additional peaks with decreasing current for Set 1 RRaP3HT PSV devices and absence of any additional peak in Set 2 RRaP3HT PSVs. Form Majumdar S, Majumdar ¨ sterbacka R (2009) Organic spin-valves: HS, Laiho R, and O Effect of magnetic impurities on the spin transport properties of polymer spacers. New Journal of Physics 11: 013022-1–013022-11.

present above 100 mV. The positive polarization for Co and Py corresponded to the observed positive TMR, but was in contrast to the negative MR reported by Xiong et al. [9]. The authors argued that this is not because of the negative polarization of the Co d-band, as proposed by Xiong et al., but

(a) 8 300 K 77 K 4.2 K

TMR (%)

6

3 2 1 100 200

0

300

T(K)

4

N O Al O

2

8

100

150

SiO Si 2

300 K 4.2 K

50 H (Oe)

3

0

Co

–50

Alq

–100

(b) 10

TMR (%)

N O N

Py

0

5 nm

6

4

2

dl/dV (a.u.)

might originate from the opposite spin asymmetry coefficients of Co and LSMO. They also pointed out that the role of the Co inclusions is not yet well understood. To directly determine the polarization of the tunnel current from Co, Fe, and Py electrodes through the Alq3 barrier, junctions with an Al counterelectrode were cooled down to 0.4 K in a He3 cryostat and dynamic conductance (dI/dV) versus bias was measured. Shown in Figure 21 is dI/dV of a 3.8 nm Al/Al2O3/1:5 nm Alq3/8 nm Co junction and a 3.7 nm Al/3:7 nm Alq3/ 3 nm Co/6 nm Py junction, displaying the characteristic behavior of conduction by tunneling into a superconductor. The Al electrode was superconducting below 2.9 K. Negligible leakages at V ¼ 0 and the sharp peaks at the superconducting gap voltage, seen in the zero field conductance curves, confirms the high quality of the Alq3 tunnel barrier without any Co inclusions. When a magnetic field (H) is applied in the plane of the film, Zeeman splitting of the conductance peaks is observed with the magnitude 2BH. Asymmetry in the conductance curve classically represents the tunnel current. By fitting the dI/dV curve using Maki’s theory [14], the polarization was obtained taking into account the orbital depairing and spin–orbit scattering. For the Co electrode and Al2O3/Alq3 barrier, a P value of 27% was determined. Similarly, P values of 30% for Fe and 38% for Py were determined. This measurement demonstrated that SP tunnel currents from an FM through an OSC indeed can occur. In 2008, the same group reported MTJ devices made using amorphous rubrene (C42H28) as the spacer that allowed direct measurement of the spindiffusion length of 13.3 nm in rubrene [95]. Shin et al. compared the data with amorphous Si or Ge. While no spin-conserved transport has been reported in amorphous Si or Ge, the spin-relaxation length in amorphous rubrene is quite substantial. This is in agreement with the results obtained by Drew et al. [87] and Chincetti et al. [88]. Shin et al. concluded that the absence of dangling-bond defects can result in the spin-transport behavior in amorphous OS. Furthermore, when rubrene barriers were grown on a seed layer, the elastic tunneling characteristics were greatly enhanced. Based on these results the authors concluded that spin-relaxation length in singlecrystalline rubrene can reach even millimeters, showing the potential for organic spintronics development.

RJ(T)RJ/T = 300K)

130 Organic Spintronics

–50

–100

0 50 Voltage (mV)

100

1

H=0 H = 3.5 T, with Al2O3 H = 3.3 T, no Al2O3

T = 0.4 K 0

–1.5

–1.0

–0.5 0.0 0.5 Voltage (mV)

1.0

1.5

Figure 21 (Upper) TMR for an 8 nm Co/0.6 nm Al2O3/ 1.6 nm Alq3/10 nm Py junction. (a) TMR measured with 10 mV bias. The inset shows the temperature dependence of RJ for this junction and the chemical structure of the Alq3 molecule. (b) Bias dependence of the TMR. The inset is a cross-sectional HRTEM image of an MTJ, showing the continuous Alq3 barrier. (Lower) Conductance of a 3.8 nm Al/Al2O3/1.5 nm Alq3/8 nm Co junction (solid squares) and a 3.7 nm Al/3.7 nm Alq3/ 3 nm Co/6 nm Py junction (open circles), with and without an applied magnetic field. From Santos TS, Lee JS, Migdal P, Lekshmi IC, Satpati B, and Moodera JS (2007) Room-temperature tunnel magnetoresistance and spin-polarized tunneling through an organic semiconductor barrier. Physical Review Letters 98: 016601–016604.

Organic Spintronics

1.05.4.4

OLEDs with SP Electrode

The efficiency of the OLEDs has improved immensely over the last few years. Light emission in OLEDs made from -conjugated molecules arises from the radiative decay of singlet excitons, while triplet excitons normally do not contribute to photon emission unless used in phosphors. Due to spin statistics, singlet excitons cannot reach more than 25% of the total number and hence the electroluminiscence efficiency will be limited to 25%. In recent years, there have been several attempts to increase the OLED efficiency by introducing SP electrodes (such as LSMO, Fe) instead of the traditionally used ITO. An SP electrode in an OLED, made by SP metallic FM thin films or half-metallic oxides, should inject carriers whose spins will be aligned in a certain direction. By varying their mutual magnetization with an external magnetic field, the singlet/triplet rate should be modified. Some groups [96,97] reported on OLEDs using different SP electrodes. Although the electroluminescence efficiency was not improved, a red-shift has been observed in the electroluminiscence spectra (Figure 22), the origin of which is still not clear.

1.05.4.5 Valves

1.05.5 Organic Magnetoresistance

12

Intensity (a.u)

10 ITO/AI 100

6

300

400 700

λ (nm)

900

ITO/Fe ITO/Co

4

LSMO/AI

2 0 400

the first organic spintronic device was realized using a single multiwall CNT sandwiched between polycrystalline Co electrodes [98]. The main advantage of using CNT or graphene is the absence of hyperfine interactions which leads to long spin-relaxation time and lengths. Although sizable MR response was observed in these devices at low temperatures, the main problem lies with the irreproducibility, mainly because of the contact resistances [80]. Reasonably long spin-relaxation times (up to 30 ns) and lengths (1.4–50 mm) have been reported [80]. But the estimation varies from sample to sample due to problems in contact resistance. An earlier review on organic spintronics [80] covered this issue in detail. In a recent experiment, Tombros et al. [99] measured spin-relaxation length of about 2 mm and spinrelaxation time of about 150 ps at room temperature in a graphene-based spin valve. In a subsequent report [100], they showed that there is a small difference for spin relaxation of injected electrons with their spins parallel or perpendicular to the graphene plane. They concluded from this observation that the main spin-relaxation mechanism is the DP mechanism, where the spin precesses between scattering events.

CNT and Graphene-Based Spin

Several experiments have been reported on organic spin valves where the NM spacer between the FM electrodes is formed by a CNT or graphene. In fact,

8

131

500

600 λ (nm)

700

Figure 22 Electroluminescence spectra corrected for substrate transmission and CCD camera response. The inset shows the collected spectra for the LSMO–Al OLED in parallel external magnetic field of 0.35 T (black line) and at H ¼ 0 (gray line). From Bergenti I, et al. (2004) Spin polarised electrodes for organic light emitting diodes. Organic Electronics 5: 309–314.

Starting from the late 1980s, it has been reported that OSs can have a sizable magnetic field effect [101]. The interest in this aspect of OS arose from the understanding of the origin of the internal quantum efficiency of OLEDs discovered in 1987 [102]. As mentioned before, in OLEDs, Fermi statistics limit the efficiency at 25%. However, there have been experimental observations reporting otherwise – most interestingly higher than 25% efficiencies. The quest for the origin of this anomaly led to the investigation of magnetic field effect on OS and on OLEDs. One of the very first works toward that was reported by Frankevich et al. [105] in a series of chapters on different materials giving precious insights on the carrier dynamics in various OS materials. They clarified the nature of the intermediate paramagnetic species involved in the photogeneration of charge carriers in various OSs. The result that almost passed unnoticed from these measurements was the variation of a diode current under magnetic field. Almost 3% variation in magnetoconductance (MC) was observed. It was only in 2004 that the potential for immediate

132 Organic Spintronics

0

100 R (MΩ)

V

–2

i

10

–4

1

Cathode Organic

ΔR/R (%)

0.1

–6

5

9V –8 7.5 V 6V

6 7 8 9 Voltage (V)

–10

5.5 V –12

Anode

–14

5V

R

R H17C8

Substrate B

CH

8 17 –16 –100–80 –60 –40 –20 0 20 40 60 80 100 B (mT)

Figure 23 (Left) A schematic drawing of the device and the magnetoresistance experiment. (Right) Magnetoresistance,
R/R curves, measured at room temperature in an ITO (30 nm)/PEDOT(100 nm)/PFO (100 nm)/Ca (50 nm including capping layer) device at different voltages. The inset shows the device resistance as a function of the applied voltage. From Francis TL, Mermer O¨, Veeraraghavan G and Wohlgenannt M (2004) Large magnetoresistance at room temperature in semiconducting polymer sandwich devices. New Journal of Physics 6: 185-1–185-8.

application of this property of OS was considered seriously [13]. Francis et al. reported a considerable magnetic field effect in OLEDs and coined the term ‘organic magnetoresistance’ or (OMAR) (Figure 23). This also led to magnetic manipulation of the OLED emission. Almost simultaneously, Kalinkowski et al. [12] reported on the 6% increase in electrophosphorescence in a magnetic field of 500 mT. This phenomenon is different from the spin valves discussed in the previous section. Here, there is no presence of any known FM components in the device geometry. The conventional OLED structures are used for the experiments.

1.05.5.1

Experimental Observations

In 2004, a report by Francis et al. [13] showed a considerable MR effect in OS-based diode devices at room temperature with small applied fields. In their experiments with the polymer polyfluorine (PFO), small molecule Alq3, and several other PCPs and small molecules, it was shown that MR is universal in nature and it can be positive or negative depending on the material or the operating condition of the device. Mermer et al. [103] also showed that MR line shape obeys the universal line shape of a Lorentzian B2/(B2 þ B20) or a particular non Lorentzian type B2/(|B| þ B0)2 with B0  5 mT in most materials. The effect is found to depend rather weakly on temperature, often changing sign with decreasing temperature (Figure 24).

MR is also reported to be independent of magnetic field direction and impurities. With increasing carrier density, it reaches a maxima and then decreases. In 2007, Prigodin et al. [78a–b] and Yu and Hu [104] reported similar magnetic-fielddependent electrical resistivity and electroluminiscent properties of OS-based diodes. They also observed that with increasing spin–orbit coupling in the material, that is, in Ir(ppy)3-doped Alq3, OMAR effect is appreciably reduced by a factor of 10 and for doping with a Pt-containing organic complex, the OMAR effect is no longer observed (Figure 25). Desai et al. [105] showed that in Alq3-based OLEDs MR occurs only when there is light emission from the device, suggesting the very important role of excitons or charge pairs. Bloom et al. [106] showed that OMAR sign changes as function of the applied voltage and temperature. The transition voltage, where the sign change takes place, is associated with the onset of minority charge injection in the device. Nguyen et al. [40] experimentally verified that OMAR effect can only be observed in systems having hydrogen atoms in their side chains. By measuring C60 devices with different electrode materials, they showed that OMAR effect is absent in molecules without hydrogen atoms, that is, OMAR effect is closely linked to the hyperfine interaction in the material. In 2005, Reufer et al. [107] investigated the singlet and triplet formation in polymer light-emitting diodes (LEDs) by studying both their fluorescence and phosphorescence under magnetic fields. The phosphorescent hydrocarbon polymer is a

Organic Spintronics

133

10

ΔR/R (%)

0.5

10

1.55 V

R(MΩ)

1.0

1.4 V

R(MΩ)

1.5

1 0.1

1 0.1 3

1.4 1.6 1.8 2.0 Voltage (V)

1.7 V

4 5 Voltage (V)

3V

2V

3.5 V

0.0 6V –0.5 CH2(CH2)4CH3 10

–60

R(MΩ)

1.5

1.0 ΔR/R (%)

200 K

300 K

–40

–20

0 B (mT)

20

40

60

1

S

6.5 7.0 7.5 8.0 Voltage (V) 0.5

0.0 8V –0.5

100 K –60

7V

6.5 V –40

–20

0 B (mT)

20

40

60

MR (%)

Figure 24 Magnetoresistance,
R/R curves, in an ITO/PEDOT/RRP3HT(100 nm)/Ca device measured at different temperatures (100, 200, and 300 K). The insets show the device resistance as a function of the applied voltage. From Mermer O¨, Veeraraghavan G, Francis TL, et al. (2005) Large magnetoresistance in nonmagnetic -conjugated semiconductor thin film devices. Physical Review B 72: 205202-1–205202-12.

1 Alq3:PtOEP 0 –1 Alq3:Ir(ppy)3 –2 –3 –4 Alq3 –5 –6 –500 –1000

0 H (Oe)

500

1000

Figure 25 The magnetoresistance as a function of magnetic field for the undoped and doped Alq3 devices at 300 K. From Prigodin VN, Bergeson JD, Lincoln DM, and Epstein AJ (2006) Anomalous room temperature magnetoresistance in organic semiconductors. Synthetic Metals 156: 757–761.

phenyl-substituted derivative of the prototypical conjugated polymer ladder-type poly(p-phenylene). By sensitized phosphorescence measurement in the range 4–300 K, they quantified spin conversion in Coulomb-bound e–h pairs, the precursors to exciton

formation and found that no interconversion between singlet and triplet e–h pair configurations is present. Static magnetic fields are equally unable to induce spin mixing in electroluminescence (Figure 26). In 2009, Majumdar et al. [108a–b] presented the magnetotransport studies of RRP3HT-based diodes and P3HT:1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]-methanofullerene (PCBM) bulk heterojunction solar cells. Bulk heterojunction solar cells are chosen as the suitable model systems because they effectively quench excitons but the probability of forming e–h pairs can be tuned over orders of magnitude by the choice of material and solvent in the blend. The e–h recombination coefficients in this system, directly proportional to the probability for the charge carriers to meet in space, were systematically varied and it was found that a reduced probability of electrons and holes meeting in space, lead to disappearance of OMAR (Figure 27). These results clearly showed that MR is a direct consequence of e–h pair formation.

134 Organic Spintronics

(a)

101

1.10 1.08 1.06 1.04

MR(%)

8T EL intensity

Normalized intensity

1.12

0T

β/βL ~ 1 β/βL ~ 0.5 β/βL ~ 10–1 β/βL ~ 10–3

10–1

1.02 450

1.00 0.98

500 550 Wavelength (nm)

600

2

Energy –0.04

10–2 –300 –200 –100

0

6 4 Magnetic field (T)

8

0 100 B (mT)

200

300

Figure 27 MR% as a function of magnetic field (B) in different devices with varying /L ratio in an RRP3HT diode (), in a RRP3HT:PCBM BHSC (*) made from dicholorobenzene, in an RRP3HT:PCBM BHSC (r) made from chloroform, and in a MDMO-PPV:PCBM BHSC (
). From Majumdar S, Majumdar HS, Aarnio H, Vanderzande D, ¨ sterbacka R (2009) Role of electron-hole pair Laiho R, and O formation in organic magnetoresistance. Physical Review B 79: 201202(R).

(b) IS> IT+> ~ 1 mev IT0> IT–> Magnetic field

–0.02 (r–r)/r

100

0.00 –0.02 –0.04 0.01

1 0.1 Magnetic field (T)

10

Figure 26 Effect of strong magnetic fields on fluorescence and phosphorescence under electrical excitation at 100 K. (a) Relative increase of the fluorescence (circles) and phosphorescence (squares) with magnetic field. (b) Corresponding relative change of the ratio r of singlet to triplet emission to the average ratio r. In both cases, the field was swept up and down and the average value is plotted. The upper inset shows two typical EL spectra on a logarithmic scale recorded at 0 and 8 T. The lower inset depicts the anticipated Zeeman splitting of the PP sublevels. The error bars indicate the standard deviation accounting for changes in the sample during successive sweeps. The error propagation was calculated in (b). From Nguyen TD, Sheng Y, Wohlgenannt M, and Anthopoulos TD (2007) On the role of hydrogen in organic magnetoresistance: A study of C60 devices. Synthetic Metals 157: 930–934.

Wang et al. [109] reported the presence of low and high-field components of MC and magneto-electroluminiscence (MEL) in MEH–PPV and PCBM blend with varying concentration of PCBM. They reported that the positive MC low-field component, which also governs the MEL response, dramatically decreases and broadens in the blends, showing a positive high-field and a negative low-field component. The low-field component is suggested to be due to magnetic-field changes of the spin sublevels mixing through the hyperfine interaction in polaron pair species. The high-field component is suggested

to be due to changes in spin sublevel mixing of charge transfer states, caused by the
g mechanism. In another recent report, Niedermeier et al. [110] showed that OMAR effect in poly(paraphenylene vinylene)-based OLEDs can be increased by device conditioning. Electrically stressing the devices at high current densities as well as illuminating the devices with high-power ultraviolet light, OMAR values can be increased. Depending on type, duration, and intensity of the conditioning process, it was possible to increase the effect to more than 20% at 4 V and 40 mT. However, the conditioning effect is not permanent and after removal of the conditioning procedures, MR values decrease slowly back to its initial value.

1.05.5.2 Theoretical Models – Physical Understanding The general observations have led to a variety of interpretations for the OMAR effect. Prigodin et al. [78a–b] proposed a model called magnetic-fieldinduced singlet–triplet interconversion (MIST) (Figure 28). This model relies on the assumption that charge transport in OS is recombination limited. In the space-charge-limited transport regime both electrons and holes are injected which form either Coulomb-bound e–h pair or excitons, if they are captured in a single molecule. These e–h pairs or excitons can be either in a singlet (S) or a triplet (T) state.

Organic Spintronics

(a) T1

135

T1

T0

S

T0 T–1

T–1

gμ BH

Energy

gμ BH

2J(r) (b)

Separation

S

T0,1,–1

S Figure 28 Schematic dependence of singlet and triplet levels of an electron–hole correlated pair on the spatial separation. Insets: (a) the singlet–triplet interconversion for strong magnetic field; (b) the singlet–triplet interconversion in the absence of field. From Prigodin VN, Bergeson JD, Lincoln DM, and Epstein AJ (2006) Anomalous room temperature magnetoresistance in organic semiconductors. Synthetic Metals 156: 757–761.

In the MIST model, it was proposed that in the presence of a magnetic field degeneracy of the triplet state is lifted and hence excitons in the singlet state can only react with the T0 component, greatly modifying the singlet–triplet interconversion. As the recombination rate and the recombination current depend on the degree of mixing, an external magnetic field changes this current, and hence MR is formed. The second model, namely triplet-polaron quenching model, proposed by Desai et al. [105], suggested that the OMAR effect is intimately linked to the presence of excitons within the device and suggested that it may be caused by the trapping of free carriers by triplets. As the OMAR appears in the OLED devices only when there is light emission, it was predicted that this effect is closely related to the excitons. As the triplet excitons have relatively longer lifetimes than the singlets, the population of triplets is often larger than that the singlets. These triplets act as trapping centers for the free charges, modifying the device current. In the presence of a magnetic field, the paramagnetic centers are quenched and subsequently the trapping of mobile carrier is modified. Hu and Wu [111] reported that the total observed MR effect is the sum of the positive and the negative components. They proposed that e–h pairs or excitons remain in either singlet and triplet states and their interconversion is magnetic-field dependent. The rate of intersystem crossing (KISC) for the polaron pairs has stronger magnetic-field dependence than the rate of intersystem crossing for the exciton pairs as shown schematically in Figure 29.

The singlet polaron pairs dissociate faster than the triplet polaron pairs because the triplets have an extremely long lifetime. The dissociation of singlet polaron pairs into individual secondary charge carriers gives rise to negative MR signal, whereas the reaction between the generated charge carriers and triplet polaron pairs gives rise to positive MR signals. By changing the sign and amount of injected charge carriers, the authors showed that it is possible to tune the MR between positive and negative MR values. Bobbert et al. [112] suggested a mechanism for OMAR called the bipolaron model where the authors described OMAR to be caused by the magnetic-field dependence of bipolaron density. They claimed that excitonic picture is unable to account for the two different observed line shapes and with decreasing minority carrier density, that is, lesser exciton formation probability. Further, MR does not show any linear dependence with minority carrier density, as expected from the excitonic picture. They proposed that the polarons in hydrocarbon molecules, when exposed to a local hyperfine field produced by hydrogen nuclei, can give rise to a randomly oriented classical field Bhf and the total field acting on a particular site I is then Btotal, I ¼ B þ Bhf,i. In out-ofthermal equilibrium situation, a B dependence of the bipolaron density is obtained due to competition between the probability of hopping to an already occupied site to form a bipolaron or to an empty site (Figure 30). Sites with low energies hold at least one polaron and therefore the bipolaron formation probability is maximum there. They identified

136 Organic Spintronics

Charge injection e+h

KISP

(e–h)1

(e–h)3

KISC

S

T

Dissociatioin

–MRS

Charge reaction

Secondary carriers

+MRT

MR = (–MRS) + (+MRT) m=1 ΔEB

m=0 ΔEST m=1

m=1 ΔEB

(e–h)1

(e–h)3

ΔEST

m=0

m = –1

S1 e–h pair states

T1 Excitonic states

Figure 29 (Top) Schematic diagram for positive and negative magnetoresistance components – MRS from the dissociation dominated by singlet excited states and þMRT from the charge reaction dominated by triplet excited states. (eh)1 and (eh)3 are singlet and triplet intermolecular e–h paris. S and T represent singlet and triplet excitons. KISP KISC are ISCs in e–h pair and excitonic states, respectively. (Bottom) The energy levels for e–h pair and excitonic states in an external magnetic field.
EST and
EB are the singlet–triplet energy difference and magnetic splitting energy, respectively. From Hu B and Wu Y (2007) Tuning magnetoresistance between positive and negative values in organic semiconductors. Nature Materials 6: 985–991.

two competing mechanisms contributing to MC – (1) blocking of transport through bipolaron states (negative MC) and (2) an increase in polaron population at the expense of bipolarons with increasing B (positive MC). Despite the above-mentioned experimental results and theoretical models, the phenomenon of spin mixing and spin transport in OS and the actual spin coherence times in OS materials have remained elusive. Recently, McCamay et al. [113] measured spin coherence to provide insight into spin-

relaxation mechanisms, to test the recent models of spin-dependent transport and recombination involving high levels of spin mixing. They demonstrated the technique of coherent manipulation of spins in OLEDs, using nanosecond pulsed electrically detected electron spin resonance to drive singlet– triplet spin Rabi oscillations (Figure 31). By measuring the change in photovoltaic response due to spindependent recombination, they showed spin control of electronic transport and thus directly measured spin coherence over 0.5 ms. They clarified that this

Organic Spintronics

1.0

Environment

(f(B))

B total,α

20 10

B total,β

PAPrα→β rθ→α

PPrα→β α

rβ→θ

β

rα→θ Environment

Environment

50 100 200 0.8 500 1000 0.6

5

0.4

2 1

0.2

0.5

–20

–10

0 B/Bhf

10

20

Figure 30 Hyperfine field average of the function f(B) of equation (2) [Ref.112], determining the bipolaron probability, for various branching ratios b. The lower three thick lines show Lorentzian fits, the upper three fits to the nonLorentzian empirical law. Inset: model as described in the main text, with the black arrow indicating the spin of a polaron present at  (arbitrarily chosen opposite to the local magnetic field) and the gray arrows the spin of a possible additional polaron. From Bobbert PA, Nguyen TD, van Oost FWA, Koopmans B, and Wohlgenannt M (2007) Bipolaron mechanism for organic magnetoresistance. Physical Review Letters 99: 216801-1–216801-4.

surprisingly slow spin dephasing underlines that spin mixing is not responsible for MR in OLEDs. Hence, experimental evidence so far suggests that e–h pair formation is of extreme importance for observing OMAR, but once the excitons are formed from these e–h pairs, their spin states do not change under a static magnetic field.

1.05.6 Conclusions and Open Questions In this chapter, we reviewed the emerging field of organic spintronics, emphasizing both on the fundamental aspects of spin injection, transport, relaxation, and spin dynamics in hybrid inorganic–organic heterostructures and organic diode devices, as well as their potential applications. The fundamental spin physics in purely organic or inorganic–organic hybrid devices is interesting and intriguing. New results are emerging very frequently, which opens new ways to look at the unresolved issues and also questioning our understanding of the basic spin dynamics in these materials and their devices. At the same time, the large MR effect observed at room temperature and with very moderate magnetic

137

fields opens up possibilities for large-scale application of magnetically controlled spin-based organic electronic devices. Organic spin valves, MTJs, and organic LEDs are already quite widely reported in the literature over the last few years, and with better physical understanding they will improve even at a faster pace in future. Several other device options can also be exploited once the interface-related issues are well understood. Therefore, the overall picture is quite encouraging and worth pursuing. However, there are many challenges and open questions which need be addressed. Some major issues are discussed below: 1. In OSs, widely different spin-relaxation lengths have been measured from optical and electrical methods. Spin-valve architectures with electrical injection and detection show quite long spinrelaxation lengths and spin-valve responses for OS spacer thickness up to almost 100 nm. However, recent spectroscopic experiments suggest that they lose their spin polarization within tens of nanometers. This discrepancy is not yet well understood. Due to low spin–orbit and hyperfine coupling in OS, the spin-relaxation length is supposed to be higher in these materials. Therefore, the factors that limit the spin-relaxation length in these materials need to be carefully studied. Several reports of TMR in OS-based devices also need to be taken into consideration. 2. The observation of normal and inverse spin-valve effects in organic spin valves need to be investigated further. Whereas most of the Alq3-based spin valves have shown inverse MR effects, several polymerbased spin valves and Alq3-based MTJs exhibited normal MR effects. Earlier, inverse MR has also been reported for LSMO/SrTiO3/Co and LSMO/Ce0.69La0.31O1.845/Co MTJs and is ascribed to the negative spin polarization of the Co d-band. So far, for the Alq3 spin valves, only a phenomenological model was proposed for explaining the inverse spin-valve effect. Based on the energy diagram of the full LSMO/Alq3/Al2O3/Co device structure, it was shown that the metal/Alq3 interfacial barriers are of about 0.5–1 eV for both interfaces. The presence of these barriers aligns the LUMO level of Alq3 with the spin-down bands of both LSMO and Co considering similar Fermi energy values for Co and LSMO (EF ¼ 4.9–5 eV). Thus, the spin-up electrons injected by either the LSMO (negative voltage) or the Co electrode (positive voltage) propagate by a hopping mechanism

138 Organic Spintronics π-pulse

S

2π-pulse

B1

0

100

200

τ(ns) 300

400

0 –1 –2

ΩR(MHZ)

ΔQ (104 electrons)

500 16 W 32 W 63 W 125 W

–3 –4 –5 80

–6

40

–7 0

0.5

1.0

0

B1 (a.u) Figure 31 Coherent spin control of a polymer OLED: Rabi flopping in the photocurrent for a number of different microwave field parameters. The integrated charge,
Q, as a function of microwave pulse length, , for a number of different microwave p powers P (note that B1 _ P, at constant repetition rate, for clarity, the curves are offset by 1.5  104 e with each decrease in power). The data are accurately described by a transfer function T( ) (solid lines). The Rabi frequency, R, obtained from this fit is shown in the inset as a function of the applied B1 field, the line being a guide to the eye. The spin precesses about the microwave B1 field with the total angle of nutation depending on the microwave pulse length. The time evolution of the orientation of a spin relative to B1 is illustrated with rotating-frame Bloch spheres above the plot. Note that the inhomogeneity of the excited resonance (due to the random, Gaussian distribution of Lande’ factors) implies that either the electron spin, the hole spin, or both the electron and hole spins will precess. It is known that in each of these cases, polaron paris are manipulated between PPS and PPT states. From McCamey DR et al. (2008) Spin Rabi flopping in the photocurrent of a polymer light-emitting diode. Nature Materials 7: 723–728.

along the organic material where they gradually lose part of their spin polarization. Eventually, the electrons tunnel from the LUMO of Alq3 into the spin-down bands of either the Co or LSMO electrode, respectively. It was also shown that depending on Co penetration into the OS layer, the sign of MR can be both normal and inverse. Therefore, the pinhole and impurity concentration plays a major role in this aspect. This issue needs to be clarified further for different molecules and systems in order to achieve more reproducible and stable devices in the future. 3. Another important issue is the spin-relaxation phenomenon in graphene-based spin valves. Due to low atomic mass of carbon, spin–orbit interaction is very small in this carbon allotrope. Further, as the average nuclear spin of carbon atom is small, the carbon hyperfine interaction strength is weak; consequently, the hyperfine interaction in graphene is negligible. Hence, it is expected

that long spin-relaxation length and time should be observed in graphene. However, experimental results show that this is not the case. It is therefore important to understand the reason for this. 4. In terms of the OMAR effect, the biggest point of debate is the origin of this effect. While there is evidence of both the excitonic and bipolaronic nature of this effect, none of the reports could clearly clarify all the experimental observations. The OMAR line shape is also not universal, as suggested before. One recent report suggests that OMAR magnitude and line shape strongly depends on the magnetic history of the samples [114]. OMAR line shape also changes with measuring bias. Further experimental and theoretical results are needed to clarify this issue. 5. Future progress in spintronics is also largely dependent on the material advances. Improved polymeric materials with improved mobilities are frequently emerging, opening up prospects for

Organic Spintronics

better spin transport. Several organic ferromagnets which are magnetic even at room temperature are available, though there is room for improvement in their stability and morphology. With further development of OSs and FM materials, the prospect of an all-organic spintronic devices will not be a distant goal.

13.

14a.

14b.

Acknowledgments

15a.

The authors gratefully acknowledge the financial support from the Academy of Finland through project number 116995, 107684, the Centre of Excellence Program, and the Wihuri Foundation.

15b.

16. 17. 18a.

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