Magnetism and spintronics in carbon nanotubes

4 Magnetism and spintronics in carbon nanotubes 4.1 Introduction Carbon nanotubes (CNT) have a curved, aromatic structure with electronic properties of metals to semiconductors of varying bandgaps as a function of chirality and display excellent electrical, thermal, and mechanical properties (Harris, 1999; Dresselhaus et al., 1996), large specific surface area, which are highly desirable for their enormous potential applications particularly for nano-electronics. Due to the covalently bonded structure, metallic CNTs are ballistic conductors more than 1 μm lengths and can carry current densities of 109 A/cm2 that can be designed to be the ultimate electronic circuits. Thus CNTs have been regarded as the most promising electronic material as silicon devices reach their fundamental scaling limitations. For the last three decades, since the discovery of CNTs by Iijima (1991), synthesizing useful quantities of analytically pure CNTs as well as characterizing and processing into forms suitable for specific applications are largely unsolved problems. To realize the potential advantages of CNTs in electronic circuits, besides controlling the materials characteristics, methods to assemble a large density of functional devices need to be developed. Due to low (small) spin
orbit coupling (SOC) interactions and long spin diffusion lengths ( . 100 nm) due to their ideal π-electron system, CNTs are considered very promising and lots of interests for CNT-based spintronic applications. There are two types of CNTs: single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs). CNTs containing nano-magnetic materials are able to form a perfect spin-transport medium, since electron transport in them is one-dimensional and ballistic with a long spin relaxation time and weak spin
orbital effects. Even pure CNTs, which are nonmagnetic materials, are characterized by a giant magnetoresistance (MR) (GMR) (Cottet et al., 2006; McIntosh et al., 2002). On the other hand, it is obvious that the modification of CNTs would lead to significant differences in their electronic structure and other different properties (Tyagi et al., 2005; Mykhailenko et al., 2007; Borowiak-Palen et al., 2005). Due to very large magnetic shape anisotropies, the encapsulation of magnetic phases in CNTs could provide a feasible approach to achieve magnetic order stabilization against thermal fluctuations in systems having extremely reduced dimensions. The ferromagnetic (FM) nanoclusters are expected to have much better magnetic properties than bulk metals due to their single-domain nature (Grobert et al., 1999) and are very useful for spintronic devices fabrication. Therefore it is desirable to produce CNTs not only with magnetic material inside of the tubes in a specific and controlled way but also further modification with specific species. Beyond the Magnetism and Spintronics in Carbon and Carbon Nanostructured Materials. DOI: https://doi.org/10.1016/B978-0-12-817680-1.00004-4 © 2020 Elsevier Inc. All rights reserved.

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geometrical advantage of a quasi-one-dimensional CNT design, the carbon shells can provide an effective protection against oxidation. The growing of CNTs is, however, a catalyzed-determined process. The most used catalyst materials are Fe, Co, and/or Ni. All these metals show over a wide temperature range of FM properties. Therefore alternative techniques have to be developed in order to either remove completely the catalyst material from the nanotubes or to apply nonmagnetic catalysts. Various purification methods have been employed to remove magnetic impurities, such as chemical treatment, microwave heating, mechanical filtration, and heat treatment in a vacuum or oxidative environment (Rinzler et al., 1998; Strong et al., 2003; Harutyunyan et al., 2002; Zhou et al., 2001). However, a graphitic coating commonly found around FM catalyst particles shields the particles from acid dissolution. Attempts to remove this graphitic coating often result in damage or destruction of SWCNTs (Zhou et al., 2001). Although some groups applied magnetics filtration, the efficiency was low such that ferromagnetism still dominated the magnetic moment of the sample for fields of order a few Tesla (Thien-Nga et al., 2002; Wiltshire et al., 2005; Islam et al., 2005; Kim et al., 2005). To circumvent this problem, other researchers synthesized nanotubes using non-FM catalysts, such as Rh/Pd or Rh/Pt (Goze-Bac et al., 2002). Lipert et al. (2009) show two different ways to obtain CNTs having diamagnetic behaviors (nonmagnetic). In a CNT spin-valve device, spin-polarized electrons are generated at the FM/CNT interface, and the CNT provides a coherent path for the polarized electron, which can be detected at the other FM electrode. A versatile method for assembling CNTs on FM metal contacts from solution was already been developed. The quantum mechanical spin degree of freedom is now widely exploited to control current transport in electronic devices. Based on these regards, CNTs are considered to be a potential material for spin electronics device applications. CNTs exhibit a long electron mean free path (Bachtold et al., 2000; Yao et al., 2000) and weak SOC; thus the spin diffusion length is expected to be extremely large (Dresselhaus et al., 2001). Spin-dependent transport in CNTs was originally demonstrated in MWCNTs with FM contacts (Tsukagoshi et al., 1999). However, there are still several problems to overcome before their potential in this application can be realized. The directional placement of immobilized CNTs in aligned geometries on electrodes represents is one of the critical steps toward the creation of the spintronic devices. The MR effect (Alphenaar et al., 2001; Zhao et al., 2002) in a FM tunnel junction consists of two FM electrodes (source and sensor) separated by a non-FM layer (spacer), which is the key for spintronic devices. Because of the GMR at room temperature and the application for magnetic random access memories (MRAMs) (Ono et al., 1997, 1998), the MR effect is caused due to the spin polarization of the FM electrodes. It has been known since the work of Julliere (1975) that the resistance of single planar FM junctions decreases when the magnetizations of the electrodes rotate from parallel to antiparallel alignment. CNTs can be synthesized by various methods, such as arc discharge (Iijima, 1991), chemical vapor deposition (CVD) (Cassell et al., 1999; Ren et al., 1998; Li et al., 1996), and laser ablation (Maser et al., 1998). In this chapter, magnetization, MR, field-effect transistor (FET)
based transport properties of different catalysts (with/without) on the basis of SWCNT/MWCNTs are reviewed and discussed for spintronic applications.

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4.2 Magnetism of carbon nanotubes Ray et al. (2010) studied the magnetic properties of as-grown and postannealed MWCNTs with embedded iron nanoparticles at room temperature (B300K) and below room temperature (B5K) to investigate the hysteresis of the magnetic behavior. Anisotropy measurements were also performed by automated sample rotation; therefore a magnetic field (H) is applied both parallel and perpendicular to the tube axis. Fig. 4
1A and B and C and D plots the resulting magnetic moment M and magnetic field H measured at B305 and B5K, respectively, of the as-grown and postannealed MWCNTs. Fig. 4
1B shows that after the application of magnetic field parallel to the tube axis, there is an interface between FM and anti-FM materials in the postannealed MWCNTs network, so the coercivity is not symmetric. The FM behavior was apparently eliminated when the magnetic field is applied perpendicular to the tube axis. Ray et al. (2010) observed that MWCNTs exhibit significantly a higher coercivity B750 Oe than its bulk counterpart (Febulk  0.9 Oe), suggesting its potential use as lowdimensional, high-density magnetic recording media. The magnetic moment of the

FIGURE 4–1 Magnetic hysteresis (M
H) loop obtained at 300K when magnetic field is applied perpendicular and parallel direction of (A) as-grown and (B) postannealed MWCNTs. Magnetic hysteresis (M
H) loop obtained at 5K when magnetic field is applied perpendicular and parallel direction of (C) as-grown and (D) postannealed MWCNTs. MWCNTs, Multiwalled carbon nanotubes. Ray, S.C., et al., 2010. High coercivity magnetic multi-wall carbon nanotubes for low-dimensional high-density magnetic recording media. Diamond Relat. Mater. 19, 553
556. Copyright Elsevier Publishing. Reproduced with permission.

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postannealed MWCNTs is nearly 104 times lower than those of as-grown samples, and the magnetization, H, gradually decreases more than 2000 Oe, which is due to the diamagnetic contribution. Ray et al. (2010) strongly believe that the disappearance of FM behavior is attributed to the formation of Fe-carbides and Fe-oxides with a specific composition upon high-temperature annealing or the increase of kinetic bandwidth. The coercivity (Hc) of the as-grown sample significantly exceeds (B750 Oe) that of the bulk Fe counterpart (Febulk  0.9 Oe) (Bozorth, 1951) and Ni/Co nanowire arrays (Whitne et al., 1993; Ozhuharova et al., 2004), and it is comparable with the values obtained elsewhere (Kuo et al., 2003; Geng et al., 2006). These findings are encouraging for various technological applications and suggest a higher magnetic stability. In addition to the structure and alignment of the nanotubes, two other factors are responsible for the significant increase in Hc values: (1) First, Hc is well known to increase as the magnetization becomes higher and the feature size shrinks (Bertotti, 1998; Mills et al., 2006). The embedded Fe nanoparticles found at the bottom of the MWCNTs (Ray et al., 2010) are somewhat smaller than the typical domain of a size that can be magnetized by coherent rotation. It is noted that if the feature or particle size becomes comparable to the domain wall width (for Fe, it is B40 nm), the Hc begins to drop with feature size and magnetic hysteresis decreases as well, which seems quite possible for the postannealed MWCNTs. Moreover, increased surface-to-volume ratio for nanoparticles makes them susceptible to interaction with neighboring magnetic materials. (2) Second, large shape anisotropies of nanotubes can act on encapsulated Fe nanoparticles as shown in Fig. 4
1A. It is interesting to see, although the hysteresis loops look similar for parallel and perpendicular directions for the asgrown samples at the lowest recorded temperatures (B5K), they differ significantly for the postannealed MWCNTs establishing a high magnetic
anisotropic nature of iron particles in the MWCNTs, as shown in Fig. 4
1C and D. At low temperatures, Hc is found to increase for both MWCNTs; however, this dependence particularly for the postannealed samples can be found to be significantly different from previous reports. The coercivity is significantly enhanced at low temperature (B5K) and is nearly 2600 Oe for as-grown MWCNTs and B320 Oe for postannealed MWCNTs. The postannealed MWCNTs show high anisotropic behaviors at this low temperature (B5K) which can be seen in Fig. 4
1D. In fact, the shape of anisotropies helps to stabilize magnetic order against thermal fluctuations in low-dimensional systems. However, if the Fe particles used as the seeds for the nucleation of the nanotube are small enough, say, less than 20 nm (in the present case), the particles will be a single domain and will exhibit very high uniaxial anisotropy due to the stress of the nanotube and the shape. The possible mechanism is expected to be similar to the case studied by Geng et al. (2006) using Fe nanoparticles as catalysts for growing CNTs where the formation of the Fe3C was observed by the x-ray diffraction (XRD) measurements (Ray et al., 2010). These findings are encouraging for various technological applications and suggest higher magnetic stability. Geng et al. (2006) obtained the high coercivity and high loop squareness in Fe nanoparticles
filled CNTs array that could be an interest for magnetic recording as well as magneto-electronic sensors applications. Li et al. (2007) studied the magnetic characterization of Fe nanoparticles
encapsulated SWCNTs and compared with pristine SWCNTs at room temperature (B300K) and below

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room temperature (B5K). Li et al. (2007) found that the Fe nanoparticles
encapsulated SWCNTs have higher magnetization. To gain a better understanding of the magnetic behavior of Fe-filled SWCNTs, Li et al. (2007) performed zero-field-cooled (FC) (ZFC) and FC magnetization measurements. It was noticed that the Fe-filled SWCNTs striking sharp decrease in the FC curve at about 25K, corresponding to the existence of small magnetic particles in the superparamagnetic state. For ZFC curves, as the temperature increases, the magnetization shows an increase because the magnetic moment is thermally activated along the magnetic field direction. For Fe-filled SWCNTs a blocking temperature (Tb 5 transition temperature from FM to superparamagnetic state) peak can be observed in the ZFC curve at about 94K. By contrast, the Tb peak can be identified at near room temperature of 255K for pristine SWCNTs. This was similar to the magnetic behavior of iron-filled MWCNTs suggesting the presence of large magnetic particles in the case of our SWCNTs. Undoubtedly, the above results reveal an obvious difference between pristine and Fe-filled SWCNTs, and superparamagnetic properties of Fe-filled SWCNTs can be observed due to the small size distribution of Fe nanoparticles inside the SWCNTs. Zhang et al. (2001) studied the magnetic properties of Fe-nanoparticles trapped at the tips of the aligned CNTs in the temperature range of 5K
360K and found that the CNTs are highly FM in nature. They observed that the Fe particles behave ferromagnetically with Curie temperature much higher than 350K. Magnetization was measured in the magnetic field along the perpendicular and parallel to the film plane and found that the CNTs are highly anisotropy in nature. They obtained different magnetic parameters, namely, Mr/Ms ratio and coercive field Hc from M
H hysteresis loops and observed that these parameters decrease monotonically with the increase in temperature that can be attributed to the depinning of domain walls in the particles. In this study, HcB2.5 kOe was also obtained, which is very useful for the next-generation high-density recording media. Another advantage of the particles in the tips of the nanotubes is that the walls of the nanotubes act as a nonmagnetic separation, which is essential for the high magnetic recording media to eliminate the dipolar interaction between the neighboring particles. It is very important to note that the tube number density is one of the main factors determining the media recording density. Ray et al. (2017a,b) studied the magnetization of nitrogen-doped multiwall nitrogenated carbon nanotubes (MW-NCNTs) functionalized with chlorine as well as oxygen plasma atmosphere. It was observed that at room temperature, nonfunctionalized MW-NCNTs have diamagnetic behaviors, whereas chlorine- and oxygen-functionalized MW-NCNTs, respectively, hold paramagnetic and FM behaviors. The magnetization M
H hysteresis loop of MW-NCNTs obtained at 300K and 5K is shown in Fig. 4
2A. The behavior of spectral features unambiguously implies a pure diamagnetic behavior, although the nanotubes have contribution of strong magnetic Fe particles as a catalyst in the NCNTs. Lipert et al. (2009) observed the similar diamagnetic behavior of Fe-based MWCNTs, after the postannealing process at a very high temperature of B2500 C. Lipert et al. (2009) claimed that the FM behavior changed to diamagnetic due to complete evaporation of Fe-catalyst particles from the CNTs at this high-temperature annealing. They also observed the synthesis of

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FIGURE 4–2 M
H hysteresis loop of (A) MW-NCNTs, (B) chlorine-functionalized MW-NCNTs, and (C) oxygenfunctionalized MW-NCNTs (black solid ball and red hollow ball are at 300K and 5K, respectively). MFC and MZFC, M
T spectra of (D) MW-NCNTs, (E) chlorine-functionalized MW-NCNTs, and (F) oxygen-functionalized MW-NCNTs. Ray, S.C., et al., 2017a. Change of magnetic behaviour of nitrogenated carbon nanotubes on chlorination/ oxidation. Int. J. Nanotechnol. 14 (1
6), 356
366; Ray, S.C., et al., 2017b. Hall effect studies and magnetic behaviour in Fe-nanoparticle embedded multiwall CNTs. J. Nanosci. Nanotechnol. 17 (12), 9167
9171. By courtesy of Copyright Inder-Science Publication.

diamagnetic behavior for the CNTs using nonmagnetic Re as a catalyst. Ray et al. (2017a,b) expected that the diamagnetic behavior of MW-NCNTs may be due to the presence of nonmagnetic bonding that dominates the Fe catalyst in the MW-NCNTs structure. Furthermore, the oxygen-functionalized NCNTs show strong FM behavioral M
H hysteresis loop (opposite trend of pure NCNTs) unlike nonfunctionalized NCNTs, as shown in Fig. 4
2C. Del Bianco et al. (2009) observed the FM behavioral M
H loop for the core interface of oxygen-passivated Fe nanoparticles. Ray et al. (2017a,b) also expected that the FM behavior may occur due to oxygen passivation with the NCNTs on oxidation (oxygen plasma treatment). In case of chlorine-plasma-treated NCNTs, the M
H loops in Fig. 4
2B are not like either pure NCNTs or NCNTS:O, but interphase of those two (dia- and ferro-) that indicate the possible of paramagnetic behavior. Apart from that, they even expected that these magnetic behavioral changes occur due to the formation of different bonding with

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carbon/nitrogen/Fe catalyst on chlorine-/oxygen-plasma functionalization process. Ray et al. (2017a,b) also studied the thermal evolution of the magnetization of MW-NCNTs (:Cl/O) with temperature (T)-dependent magnetization (M) by the zero-field-cooling (MZFC) and field-cooling (MFC) procedures in an applied magnetic field of 1000 Oe in between 5K and 300K. Fig. 4
2D shows the M
T curve of N-CNTs, whereas N-CNT:Cl and N-CNTs:O are shown in Fig. 4
2E and F, respectively. Ray et al. (2017a,b) found that the MZFC curve gradually deviates from the MFC curve with the decrease of temperature at B255K (for MWNCNTs), B200K (for CNTs:Cl), and B300K (for CNTs:O), when the applied magnetic field is 1000 Oe. Upon further cooling the MZFC plot exhibits a cusp centered at B45K for the MWNCNTs and NCNTs:O, but not in NCNTs:Cl. This variable temperature magnetic data of MW-NCNTs and MW-NCNTs:O clearly indicate that the Fe/N-CNTs exhibit FM behavior below the room temperature, which is attributed to the uncompensated surface spin states or FM Fe clusters, although the M
H curve of nonfunctionalized MW-NCNTs shown in Fig. 4
2A is completely diamagnetic in behavior. It is believed that this FM performance of the Fe/N-CNTs:O comes from the FM Fe clusters with the formation of different bonding with carbon/nitrogen and the uncompensated surface spin states. In case of NCNTs:Cl the MZFC and MFC curves coincide up to B200K, as shown in Fig. 4
2D, when measured at an applied magnetic field of 1000 Oe, which get split below that temperature. A similar behavior has been observed by Del Bianco et al. (2009) to occur in oxygen-passivated Fe nanoparticles. In that case the anti-FM character of Fe2O3 was in the origin of the low-temperature irreversibility. This low-temperature FM phase magnetization is correlated to the fact that at lowest temperature and after MZFC process, the moments of magnetic particle Fe are not fully aligned with the applied field. Furthermore, no cusp is observed in the MZFC plot in N-CNT:Cl indicating non-FM nature. In case of N-CNT:O, it is found that the MZFC curve gradually deviated from the MFC curves with the decrease of temperature at about 300K as shown in Fig. 4
2F, when measured at an applied magnetic field of 1000 Oe. A similar behavior has been observed by Zhang et al. (2008) for CoO/CNTs core
shell nanostructures, when they measured at an applied magnetic field of 100 Oe between 2K and 300K. In our present case, upon further cooling, it is also observed that the MZFC plot exhibits a cusp centered at about  45K and the MFC data sequentially increases, indicating FM behavior at this temperature compared to NCNTs and NCNTs:Cl. It is believed that the FM behavior in NCNTs:O comes from the FM Fe clusters and uncompensated surface spin states owing to the formation of different bonding with carbon/nitrogen/Fe catalyst. Kapoor et al. (2018) studied the magnetization of 3d transition metals and oxides within CNTs by copyrolysis of metallocene and camphor and found that the Fe, Co, and Ni composites with CNTs have excellent magnetic behaviors and could be used for spintronic applications. They emphasized that the use of camphor enabled to obtain Ni- and Co-filled CNT with significantly improved Ms values (up to 12 emu/g), as compared to what one obtains for aerosol-assisted CVD (0.1
0.4 emu/g) reported earlier (Sun et al., 2013; Yang et al., 2016; Terrones et al., 2006). In the case of Ni@CNT, pyrolysis of only nickelocene in powder form is not known to yield well-formed CNT, though there exist reports of copyrolysis of nickelocene with ferrocene in aerosol-assisted CVD for the formation of Ni@CNT. Fig. 4
3A shows

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normalized Ms, limited to low-field scans, for a clear depiction of the coercivity, Hc, for each type of CNTs. The coercivities, Hc, for Fe@CNT and Co@CNT are approximately few 100 Oe and can be further tuned by variations in Tpyro, depending on the application-specific requirement. Here, the Ni@CNT exhibits very low Hc, almost mimicking a superparamagnetic type of behavior. However, the magnetization data is likely to contain contributions from both Ni and Fe in such cases (Yang et al., 2016). Copyrolysis of camphor with nickelocene, as shown in this figure, enables the formation of Ni@CNT with no such ambiguity in the magnetic data. Fig. 4
3B shows the magnetization as a function of temperature for Fe2O3@CNT, clearly exhibiting the Morin transition, which signifies the onset of weak ferromagnetism and piezomagnetism in this CNT
metal oxide hybrid. These CNTs could be synthesized by systematic variation of synthesis parameters and possible to obtain self-organized structures of filled CNT, with narrow diameter and length distribution and reduced residue particle density. The metal@CNT can be used as a template to form oxide@CNT that could be useful for numerous applications in spintronics, magneto-optics, and in the energy sector. Narayanan et al. (2009) studied the magnetic properties of MWCNT
SPION (superparamagnetic iron oxide nanoparticle) composites at low temperature, ZFC and FC experiments. The ZFC shows a blocking at B110K. A peculiar FM ordering is exhibited by the MWCNT
SPION composite above the room temperature due to the FM

FIGURE 4–3 (A) M
H loops at 300K limited to 6 1 kOe, depicting the coercivity (Hc) for Fe@CNTs, Co@CNTs, and Ni@CNTs. (B) M
T at 1 kOe, depicting a magnetic transition at B250K. Kapoor, A., et al., 2018. 3d Transition metals and oxides within carbon nanotubes by co-pyrolysis of metallocene & camphor: high filling efficiency and self-organized structures. Carbon, NY 132, 733
745. Copyright Elsevier Publishing. Reproduced with permission.

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interaction emanating from the clustering of superparamagnetic particles in the constrained volume of an MWCNT. This kind of MWCNT
SPION composite can be envisaged as a good agent for various biomedical applications. Titus et al. (2011) studied the magnetization behavior of nickel nanoparticle
deposited vertically aligned CNTs for the magnetic tunnel junction (MTJ) spintronic application. To probe the magnetic properties the field dependence of the magnetization was measured using a superconducting quantum interface device (SQUID) (Kenane et al., 2006). They observed that the vertically aligned CNTs exhibit FM behavior with a hysteresis loop at 2K and 300K, respectively. The saturation magnetization (Ms), coercivity (Hc), and remnant magnetization (Mr) for both temperatures were obtained from the M
H hysteresis loops. The saturation magnetization values and coercivity for nickel nanoparticle
deposited vertical carbon nanotubes (VCNTs) shows a decrease from 5.3 to 4.4 emu/g and 395 to 115 Oe, respectively, with the increase of temperature, indicating a characteristic FM behavior. To gain a better understanding of the magnetic behavior of nickel nanoparticle
deposited VCNTs, Titus et al. (2011) performed ZFC and FC magnetization measurements. For the ZFC measurement the nickel nanoparticle
deposited VCNTs sample is first cooled from 300K to 2K in zero magnetic field. On the other hand, for FC measurements the sample is cooled in the magnetic field (25 G) from 300K to 2K, and later the magnetization is measured in the warming cycle keeping the field on. The temperature dependence of ZFC and FC measurements under the applied magnetic field of 25 G for nickel nanoparticle
deposited VCNTs which exhibits the main features of FM behavior (Li et al., 2007). The blocking temperature TB (transition temperature from FM to superparamagnetic state) peak can be observed in ZFC curve at B44K. The low value of TB is directly in agreement with smaller size of nickel nanoparticles randomly deposited on the VCNTs (Fonseca et al., 2002; Linderoth et al., 1993).

4.3 Spintronic devices The quantum mechanical spin degree of freedom is exploited to control current transport in electronic devices. The readout of magnetic hard disks is based on the spin-valve effect, that is, the tunability of a conductance through the relative orientation of some FM polarizations (Prinz, 1998). Realizing of spin injection in nanostructures, for example, mesoscopic conductors or molecules, would allow to implement further functionalities. The realization of a “spin transistor” would allow electric field control of the spin-valve effect through an electrostatic gate (Datta et al., 1990; Schäpers et al., 2001). CNTs are particularly interesting, because they should exhibit a long spin lifetime and can be contacted with FM materials. This section presents the state of the art regarding the realization of spin transistor
like devices with CNTs. The most standard method to inject or detect spins in an insulating or conducting element M is to use the spin-valve geometry (Baibich, 1988; Binasch et al., 1989) in which M (CNTs) is connected to two FM leads L and R (Fig. 4
4, left). One has to measure the conductances GP and GAP of the spin valve for lead magnetizations in the parallel (P) and

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antiparallel (AP) configurations. This requires the use of two FMs with different coercive fields (HcL and HcR, respectively) for switching one magnetization with respect to the other with the help of an external magnetic field H (Fig. 4
4, right). The spin signal or MR is then defined as the relative difference MR 5 (GP 2 GAP)/GAP. Let us consider the situation in which the element placed between the two FM contacts is a tunneling barrier with a transmission probability independent of energy (Julliere, 1975). This case, usually referred to as Julliere’s model, describes the principle of magnetic memories and magnetic read From Fermi’s golden rule the transmission probability of the  heads. barrier for spins σE m; k is proportional to the electronic densities of states at the Fermi
energy (FE) Nl;σ 5 Nl 1 1 σɳl pl for spin s at the both contacts, with lAL; R and ɳ A 1 1; 2 1 the direction of the magnetization at contact l. Here, Nl is the spin-averaged density of states (DOS), and pl is the spin polarization at contact l. The conductance GP of the barrier in the parallel configuration is proportional to NLNR[(1 1 pL)(1 1 pR) 1 (1 2 pL)(1 2 pR)], whereas the conductance GAP in the antiparallel configuration is proportional to NLNR[(1 1 pL)(1 1 pR) 1 (1 2 pL)(1 2 pR)]. This leads to MR 5 2pL PR =ð1 2 2 pL PR Þ: If the spin polarizations pL and pR have the same sign the MR of the device is positive because the current flowing in the antiparallel configuration is lower due to the imbalance between NL,σ and NR,σ. The working principle of a spintronic device follows these steps: (1) information is stored into spins as an orientation (i.e., up or down), (2) spin information is carried by mobile electrons along a path or wire, and (3) the information is then read at a final point. The spin orientation of conduction electrons will exist for several nanoseconds making them useful in electronic circuit and chip design. The most basic method of creating a spin-polarized

FIGURE 4–4 Left: Spin-valve geometry. The CNT is connected to two ferromagnetic leads L and R with magnetic ~ is the applied magnetic field and Vsd is source
drain voltage. Right: Resistance curve r polarizations ~ P L and ~ P R. H (H) measured in the spin valve. Increasing [blue line] and then decreasing [red line] H. L and R have different coercive fields HcL and HcR, it is possible to selectively reverse the directions of ~ P L and ~ P R during this cycle. MR 5 (GP 2 GAP)/GAP and MR . 0. MR, Magnetoresistance. Cottet, A., et al., 2006. Nanospintronics with carbon nanotubes. Semicond. Sci. Technol. 21, S78
S95. Copyright IOP Publishing. Reproduced with permission.

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current is to transport current through a FM material and to transmit the electron spin carrying the information to the receiver point. Spin current is, therefore, an important tool to detect spin in spintronic devices. The important avenues for the development of spintronic devices are (1) fabrication of nanoscale nanostructures, including novel magnetic materials, thin films, hybrid structures, and functional materials; (2) research on spin effect (spin injection, and spin transport and detection); (3) demonstration of spintronic devices including GMR and tunnel MR (TMR) devices in MTJs; and (4) study of single electron tunneling (SET) in MTJs. Mohamed et al. (2008) describe an alternative method for realizing a CNT spin FET device by the direct synthesis of SWCNTs on substrates by alcohol catalytic CVD. They observed that the hysteretic MR at low temperatures due to spin-dependent transport and the maximum ratio in resistance variation of MR was found to be 1.8%.

4.3.1 Magnetic tunnel junctions A MTJ can be considered a spintronic device as it is composed of two FM materials, such as nickel, cobalt, or iron, separated by an ultrathin layer of insulator with a thickness of the order of nanometer (1029 m). It exhibits two resistances, low (RP) and high (RAP), depending on the relative direction of FM materials, parallel (P) or antiparallel (AP), respectively. The insulating layer is so thin that electrons can tunnel through the barrier if a bias voltage is applied between the two metal electrodes. In MTJs the tunneling current depends on the relative orientation of magnetizations of the two FM layers, which can be changed by an applied magnetic field. This phenomenon is called TMR. An important factor in TMR is the interaction between the electron spin (S) and angular momentum (L), that is, SOC. An example of SOC is splitting of hydrogen spectrum (Bratkovsky, 1998; Zhang et al., 1998; Dholabhai et al., 2008). The SOC deforms the electron shell as the direction of the magnetization rotates. This deformation also changes the amount of scattering undergone by the conduction electrons when traversing the lattice. There will be minimum resistance if the magnetizations are in parallel orientation, and it will go to maximum with opposite orientations (Fig. 4
4). Therefore such kind of junction can be easily switched between two states of electrical resistance—one with low and the other with very high resistance.

4.3.2 Fabrication of magnetic tunnel junctions The fabrication of MTJs with high TMR ratios is crucial in developing spintronic devices. With the advance of nanotechnology, there are various methods to deposit MTJs, such as molecular-beam epitaxy, magnetron sputtering, electron-beam evaporation and CVD, and so on. In detail, the MTJ’s main components are FM and insulator layers. The FM layers can be fabricated by sputter deposition (magnetron sputtering and ion-beam deposition). The fabrication issue is the magnetic alignment and thickness (deposition rates should be in the angstrom-per-second range). The best way of fabricating insulating layer is still under research. Some of the proven materials are Al2O3 tunnel barriers made by depositing a metallic aluminum layer in the range of 5
15 Å thickness. In addition, ion-beam oxidation,

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glow discharge, plasma, atomic-oxygen exposure, and ultraviolet-stimulated oxygen exposure are also alternate ways of insulator deposition. Since the first report on TMR by Julliere (1975), many studies have been performed to explore this property, especially on Al2O3 insulating layers. The necessity of controlling the magnetic properties of the magnetic layers introduces special requirements on the deposition process. The maintaining of inherent magnetic anisotropy is crucial in the deposition process. This can be set by applying magnetic field during deposition. The thickness and uniformity of the material, the coercivity, magneto-restriction, all are important in controlling the magnetic anisotropy.

4.3.3 Tunnel magnetoresistance in magnetic tunnel junctions Since its discovery, a large number of nano-devices such as single-electron transistor, light emitting diode, FET have been demonstrated. However, these devices are based on charge of the electron. TMR is a magneto-resistive effect that occurs in component consisting of two ferromagnets separated by a thin insulator (MTJ). The interest toward TMR is driven by the fact that MTJs with spin-dependent tunneling (SDT) are expected to provide technical promises that will allow the realization of nanoscale devices in more advanced spintronic applications. Moodera et al. (1995) fabricated the first reproducible TMR up to 24% at room temperature on CoFe/Al2O3/Co or NiFe junction. Today, reproducible TMR value up to 50% can be obtained with three-dimensional ferromagnets making them useful for industrial application (Parkin et al., 1999). TMR characteristics have already been measured in CNTs both experimentally and theoretically (Jensen et al., 2005). Tsukagoshi et al. (1999) demonstrated the MR in a single CNT contacted by FM metal electrodes. The spintronic devices exhibiting TMR using ferromagnet-contacted SWCNTs have been demonstrated by Jensen et al. (2005). Most of the reports on CNT-TMR system are mainly based on single CNT contacted to bulk FM material by an ex situ method (Xiong et al., 2004; Shimada et al., 1998; McEuen et al., 2002). The TMR effect is also known to be sensitive to the tunnel barrier/electrode interface. The barrier sensitivity may be more evident in a system with single CNT. De Teresa et al. (1999) studied FM MTJ with various barrier materials and found that even the sign of the TMR depends on the barrier materials. Yuasa et al. (2004) also investigated the effect of crystal anisotropy of the spin polarization on MTJ using single crystal iron electrodes of various crystal orientations. They found a clear crystal orientation dependence of the TMR; which might reflect the crystal anisotropy of the electronic states in the electrodes. TMR/GMR is known to originate from spin interaction between the magnetic and nonmagnetic particle at the interface and are related to the coercivity value (Bergenti et al., 2004).

4.3.4 Application of magnetic tunnel junctions With wider knowledge on how to manipulate spins (Chappert et al., 2007), we can build more state-of-the-art spintronic devices with extraordinary properties. Extended research into application possibilities of any spintronic effects is, therefore, crucial to realize more advanced spintronic devices. These devices made huge impact on computer technology by enabling higher storage of information in hard drives and faster reading of data in random access memories.

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The first successful application of MTJ was demonstrated in computer read head technology with Al2O3 barrier and MgO barrier MTJ. The magnetic recording density in hard disk drive increased (300
600 Gbit/in.2) considerably in these devices (Sanvito, 2007; Appelbaum et al., 2007; Tsymbal et al., 2007; Chantis et al., 2007). Another application of MTJ is to develop MRAM devices. It has been claimed that MRAM can exceed the speed of SRAM (static RAM), density of DRAM (dynamic RAM), and nonvolatility of flash memory. In addition, the nanodimension device has low power consumption and less heating. MRAM is an upgraded version of SRAM and DRAM where data is stored using spin instead of electrical charges. It overcomes one of the disadvantages of the conventional RAM, the loss of information by power failure. Leading companies, such as IBM, Motorola, and Honeywell, started the MRAM research in 1995, and they were supported by US Defense Advanced Research Projects Agency. The influence of spin transfer in MTJs can be observed by measuring resistive loops as a measure of external applied field and applied voltage. By sweeping the magnetic and electrical field, one can observe sharp drop in resistance which is attributed to the switching from parallel to antiparallel and vice versa. The drop of resistance is associated with the TMR. One of the factors that affect drop of resistance and TMR is DOS at the interface (Tsymbal et al., 2007; Chantis et al., 2007; Burton et al., 2007; Suzuura et al., 2000; Zhu et al., 2006). Spintronics aim to develop electronic devices whose resistance is controlled by the spin of the charge carriers that flow through them (Wolf et al., 2001; Zorpette, 2001; Zutic et al., 2004). This approach is illustrated by the operation of the most basic spintronic device, the spin valve (Julliere, 1975; Slonczewski, 1989; Moodera et al., 1995), which can be formed if two FM electrodes are separated by a thin tunneling barrier. In most cases, its resistance is greater when the two electrodes are magnetized in opposite directions than when they are magnetized in the same direction (Baibich, 1988; Binasch et al., 1989). The relative difference in resistance, the socalled magnetoresistance, is then positive. However, if the transport of carriers inside the device is spin- or energy dependent (Zutic et al., 2004), the opposite can occur and the MR is negative (George et al., 1994). The next step is to construct an analogous device to a FET by using this effect to control spin transport and MR with a voltage applied to a gate (Datta et al., 1990; Schapers et al., 2001). However, several spin relaxation mechanisms have been proposed theoretically for CNT (Semenov et al., 2007, 2010; Borysenko et al., 2008). Semenov et al. (2007, 2010) considered the hyperfine interaction with disordered nuclei spins I 5 1/2 of C13 isotopes (with the natural abundance of 1.10%) in semiconducting CNTs. The anticipated spin relaxation time is about 1 s at 4K, which is still much longer than the experimental observations for spin relaxation time (tens of nanoseconds). Borysenko et al. (2008) considered the anisotropy of the g tensor and flexural phonon modes in semiconducting CNTs. They found that the spin relaxation time can be tens of microseconds at room temperature.

4.4 Spin currents in magnetic tunnel junctions In the view of rapid progress in the fabrication of nanoscale MTJs, spin is a subject of great interest. Spin is a purely quantum mechanical quantity which provides an extra degree of

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freedom for the electron to interact with a magnetic field. In 1922 Stern and Gerlach demonstrated the most direct experimental evidence of the existence and of the quantized nature of the electron spin. The first experimental evidence of SDT was reported by Julliere (1975). Later, Berger (1978) proposed the idea that spin-polarized current act on local magnetization of ferromagnets and leads to GMR. The important property of spin is its weak interaction with the environment and with other spins, resulting in a long coherence or relaxation time, which is a very important parameter in the field of spin-transport and quantum computing. For the successful incorporation of spins into the currently existing electronics, one has to resolve issues such as efficient spin injection, spin transport, control and manipulation of spins, and finally detection of spin-polarized current. Spintronics without magnetism are an attractive pathway for designing semiconductor spintronic devices since SOC enables that the spin is generated and manipulated merely by electric field. By the application of electric field, the electrons move in the lattice generating a magnetic field which acts up on the spin. The spin
orbit interaction on mobile electrons was proved theoretically many decades ago. However, the practical harnessing of this concept is still at an early stage

4.4.1 Spin and charge transport Sahoo et al. (2005a,b) were the first to study the MR of MWCNT/SWCNT by contacting the two FM Pd0.3Ni0.7 strips with either MWCNT or SWCNT that allows to obtain devices with resistances as low as 5.6 kV at 300K. The yield of device resistances below 100 kV, at 300K, was around 50%. Fig. 4
5 shows typical MR curves for the SWCNT (Sahoo et al., 2005a,b). The MR observed is positive (MR 5 5.89%), for a gate voltage Vg 5 4.302 V as shown in Fig. 4
5A, whereas for the same device it is negative (MR 5 22.81%), for a gate voltage Vg 5 4.328 V as shown in Fig. 4
5B. The sensitivity S is of the order of 1%/T or less and can change sign for different Vg. From this figure, one can calculate the local field change ΔHloc required to obtain the observed hysteretic MR. For, Vg 5 23.1 V, one finds ΔHloc 5 22.9/0.2 5 214.5 T, which is negative and way beyond what can be obtained with microstrips. Furthermore, for Vg 5 23.3 V, one would need a positive ΔHloc, since both MR and S have the same negative sign. Such a sign change of the local magnetic field produced by two metallic ferromagnets for different gate voltages. Therefore stray field effects are not dominant in the MR signal for this type of F-MWCNT-F device. In addition, as one can see in Fig. 4
5, S is in general smaller for SWCNTs (Sahoo et al., 2005a,b; Nagabhirava et al., 2006; Man et al., 2006). Man et al. (2006) observed spin-induced MR in the SWCNTs contacted with high-transparency FM electrodes. The MR of SWCNT was measured for different values of the gate voltage, for an applied magnetic field increasing from 2700 to 700 mT (upsweep) and subsequently decreasing from 700 to 2700 mT (down sweep). The widths of the two PdNi contacts in this sample are 150 and 500 nm, resulting in magnetization reversal at magnetic field values of 250 and 125 mT, respectively. A hysteretic feature in the resistance is seen in the expected magnetic field range, corresponding to an antiparallel orientation of the magnetization in the PdNi electrodes. The

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FIGURE 4–5 MR curves for the SWCNT: (A) The MR observed is positive (MR 5 5.89%), for a gate voltage Vg 5 4.302 V. (B) The MR observed for the same device is negative (MR 5 22.81%), for a gate voltage Vg 5 4.328 V. (C) MR observed for an MWCNT connected to two PdNi leads, with different values of Vg. Depending on Vg, both signs of MR and sensitivity S are observed. The amplitude and the sign of S are not correlated with MR. Therefore the stray fields from the ferromagnetic electrodes cannot account for MR observed for this device. (D) Resistance of a Pd
SWCNT
PdNi device as a function of an external magnetic field for two values of Vg. Almost no hysteresis is observed. The maximum amplitude can be estimated (almost within the noise) as G/GB1%, more than an order of magnitude smaller than the observed signal with two PdNi electrodes. MR, Magnetoresistance; MWCNT, multiwalled carbon nanotubes; SWCNT, single-walled carbon nanotubes. Cottet, A., et al., 2006. Nanospintronics with carbon nanotubes. Semicond. Sci. Technol. 21, S78
S95. Copyright IOP Publishing. Reproduced with permission.

feature is superimposed on a smoother background, as expected. The absolute change in resistance has a magnitude of a few tenths of a kiloohm, which is much larger than the total resistance of the PdNi strips. This implies that the effect cannot be accounted for in terms of a change in the resistance of part of the PdNi contacts, which is much lower. The change in MR induced by the magnetization reversal is positive for most values of gate voltage, that is, the antiparallel orientation of the magnetization in the contacts results in an increase of the device resistance. One can conclude that stray field effects do not contribute substantially to MR observed in nanotubes, at least for the PdNi devices realized so far. Fig. 4
5C shows the MR of MWCNTs, whereas Fig. 4
5D shows the MR of SWCNTs at different applied gate voltages Vg. In the following, the results of the MWCNT device are discussed first. Fig. 4
5C shows single traces of the linear response resistance R as a function of the magnetic field H at 1.85K for two sweep directions and four different gate voltages Vg. For all cases the characteristic hysteretic behavior of a spin valve appears. On sweeping the magnetic field from

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2500 to 500 mT, the configuration becomes antiparallel between 0 and 100 mT, whereas it is always parallel for tHt . 100 mT. At Vg 5 23.1 V, for example, R increases from 49.7 to 51.5 kΩ when the sample switches from the parallel to the antiparallel configuration. This yields a normal positive TMR of 12.9%. In contrast, at Vg 5 23.3 V, R switches from 30.5 kΩ in the parallel configuration to a smaller resistance of 29.5 kΩ in the antiparallel configuration, yielding an anomalous negative TMR of 23.5%. Therefore the sign of the TMR changes with the gate voltage, demonstrating a gate-field-tunable MR. Fig. 4
5D shows the G/G measurement performed using a Pd
SWCNT
PdNi device, for two different values of gate voltages, one in the Coulomb valley and the other close to a resonance. The upper bound for G/G is 1.4% in amplitude, which is one order of magnitude lower than the maximum G/G.

4.4.2 Spin polarization As discussed above, spin polarization is an important factor in governing TMR along with the spin transport and spin injection. The spin polarization is a result of a subtle cancellation between two spin channels and is greatly influenced by the atomic, electronic and magnetic structures of the system. To build up on experimental findings, it is also essential to develop an accurate model of the spin polarization and transport of spin current through the FM/non-FM interface and finally into vacuum which is highly sensitive to the chemical and material details of the device. In this context, density functional theories (DFTs) (Arras et al., 2010) of MTJ system that can produce spin polarization effects in the FE are important. DFT is a widely used method for modeling charge/spin carrier transport semiconductors. There is plenty of literature on DFT-based calculations in studying SDT in MTJs (Caffrey et al., 2011; Stilling et al., 2007; Ke et al., 2008). The key components in the modeling are Schottky barrier (τb) and the applied voltage VA against current density. Ab initio Waldron et al. (2007) have demonstrated simulation of MTJs. Chung et al. (2009) report the effect of Schottky barrier profile on SDT in a ferromagnet
insulator
semiconductor system. Ray et al. (2010) studied the high coercivity magnetic MWCNTs for low-dimensional, high-density magnetic recording media. In this study, they used Fe-embedded MWCNTs with B80 mm in length and outer (inner) diameter of 20
50 nm (10
20 nm) and found the coercivity of 2600 and 732 Oe at 5K and 305K, respectively. These values are much higher than that of bulk iron (B0.9 Oe) and Fe/Co/Ni nanoparticles or nanowire arrays (B200
500 Oe) at the room temperature. This high coercivity and the structure of single-domain Fe nanoparticles isolated by anti-FM MWCNTs make it a promising candidate for low-dimensional, high-density magnetic recording media. Tsukagoshi et al. (1999) reported the injection of spin-polarized electrons from FM contacts into MWCNTs and observed the direct evidence for coherent transport of electron spins. A hysteretic MR in several nanotubes with a maximum resistance change of 9%, from which Tsukagoshi et al. (1999) estimated the spin-flip scattering length to be at least 130 nm, an encouraging result for the development of practical nanotube spin-electronic devices. In the MR measurements, Tsukagoshi et al. (1999) performed in 4.2K bath cryostat with the magnetic field (B) from a superconducting magnet directed in the plane of the substrate (BO). Two-terminal resistance is measured using an a.c. lock-in technique with an excitation

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voltage of 100 μV. Tsukagoshi et al. (1999) found that the lead resistance was negligible (B10 Ω) compared with the MWCNT resistance and the Co/MWCNT contact resistance. The field is swept slowly (,10 mT/min); at this sweep rate the MR of MWCNTs contacted by Au/ Pt shows no hysteresis in the applied field. Fig. 4
6 shows the two-terminal differential resistance of three different Co-contacted nanotubes as a function of magnetic field. Each device shows a large hysteretic MR peak. The field was swept first from 2100 to 100 mT (solid line) and then back to 2100 mT (dashed line). In each trace, a resistance peak appears as the magnetic field moves through 0 T. The width of the resistance peak is B50 mT, which is commensurate with the coercive field strength for a thin Co film (Rüdiger et al., 1999). Tsukagoshi et al. (1999) attribute the MR peak to spin-polarized injection (Prinz, 1998; Julliere, 1975; Moodera et al., 1995, 1998; Miyazaki et al., 1995; Meservey et al., 1994; Shang et al., 1998) between the FM contacts and the MWCNT. The magnetization direction of the left and right contacts is represented by the direction of the arrows at the top of the figure. When the magnetizations of the two contacts are parallel, the resistance is lower than when the magnetization of the two contacts is antiparallel. This explanation requires that the spinscattering length in the nanotube is of the order of the contact separation. In addition,

FIGURE 4–6 The solid (dashed) trace corresponds to the positive (negative) sweep direction. The differential resistance shows a large variation among devices—the device shown in (C) has a resistance an order of magnitude lower than the devices shown in (A) and (B). (The magnetic field BO is directed parallel to the substrate, and the temperature is 4.2K.) The percent difference ΔR/RAP between the tunnel resistance in the parallel and the antiparallel states is approximately 6% in (A), 9% in (B), and 2% in (C). Tsukagoshi, K., et al., 1999. Coherent transport of electron spin in a ferromagnetically contacted carbon nanotube. Nature 401, 572
574. Copyright Nature Publishing Group. Reproduced with permission.

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scattering at the contact interfaces must not completely randomize the spin. There is also a large hysteresis in the peak position (B 6 50 mT) between positive and negative sweep directions, indicating the probable influence of the contact magnetization. Similar hysteretic MR is observed in MTJs, where it has been attributed to spin-polarized electron tunneling (Prinz, 1998; Julliere, 1975; Moodera et al., 1995, 1998; Miyazaki et al., 1995; Meservey et al., 1994; Shang et al., 1998). MTJs consist of two FM contacts separated by a thin oxide layer. The conduction electrons within the FM contacts have a preferred spin direction, which is determined by the local magnetization. This causes the formation of majority and minority spin conduction bands with different densities of states at the FE. In the absence of spin scattering the resistance across the tunnel barrier is dependent on the relative alignment of the magnetization of the two contacts. In the antiparallel state the majority spin states are out of alignment, and the junction resistance is higher than in the parallel state in which the majority spin states are aligned. For the nanotube devices in Fig. 4
6 the contact magnetizations align parallel with the magnetic field at B 5 6 100 mT. As we sweep B through 0 T the magnetization polarity switches. The observed peak in the nanotube resistance suggests that the contact magnetizations switch separately and become misaligned as the field is swept. For a MTJ, misalignment occurs because different FM contact materials are used, with different coercivities—the magnetizations are misaligned when B lies between the coercive fields of the two contacts. But this does not explain the misalignment in the nanotube device, because the average coercivity of the two Co contacts should be very similar. In this case the misalignment may be caused by magnetization fluctuations that occur locally, on the scale of the nanotube diameter (30 nm). The average Co domain size, 50 nm (Rüdiger et al., 1999), is considered in the order of the width of the nanotube, so that the nanotube contacts only a small number of magnetic domains. The coercivity of each domain varies and depends on its geometry and the local energy conditions. Edge and surface effects are also important. It is reasonable, then, that there will be a range of B over which the magnetization at the two nanotube contacts will be misaligned. A resistance peak due to the misaligned state occurs, even though the average properties of the two contacts are similar. The small switches in the resistance, seen most clearly in Fig. 4
6, provide additional evidence for local magnetization fluctuations of individual domains. Tsukagoshi et al. (1999) include data in the figure taken from three different samples to give a clear indication of large sample-to-sample variation, typically observed in the magnetoresistance. In all CNTs, Co-contacted CNTs displayed hysteretic MR, with a peak height varying from 2% to 10%. Two CNTs showed a step-like resistance peak as seen in Fig. 4
6A, while the remaining samples showed a smoother peak as seen in Fig. 4
6B and C. It is likely that the sample-to-sample variations are due to inherent random variations in the surface potential over the small nanotube contact area. Previous experiments on nonmagnetically contacted nanotubes have observed large variations in the contact resistance (Langer et al., 1996; Bachtold et al., 1998). In addition, in the ferromagnetically contacted CNTs, it is impossible to control the particular domain structure in contact with the nanotube. Reproducibility might be improved by increasing the thickness and quality of the Co layers, which will increase the area of the magnetic domains. Unfortunately, our maximum film thickness (and hence the domain width) is limited to B100 nm by the

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electron-beam lithographic procedure. The spin-injection model for the nanotube MR requires a sufficiently small amount of spin scattering to occur both within the nanotube, and at the interfaces between the nanotube and the contacts. We can estimate the minimum spin-scattering length in the MWCNT using Julliere’s model for the MTJ. The difference between the tunnel resistance in the parallel (RP) and antiparallel (RAP) states (Julliere, 1975) is given by the following equation: ΔR RAP 2 RP 2P1 P2 5 5 RAP RAP 1 1 P1 P2

where P1 and P2 are the percentage of conduction electrons polarized in the majority spin band in the FM contacts 1 and 2. For Co the polarization has been determined (Meservey et al., 1994) to be 34% giving a maximum resistance change of 21%. In the present best case, ΔR/RAP reaches a maximum value of 9% (Fig. 4
6B), so that B14% of the spin-polarized electrons travel 250 nm through the nanotube without spin-flipping. The spin-scattering length, ls, can then be estimated by assuming that the spin polarization reduces as exp(2l/ls) within the nanotube. This gives ls 5 130 nm. Although fairly long, this is probably an underestimation. The spin-polarization near the ferromagnet/nanotube interface will depend on the interface quality and could be appreciably lower than 34%. Also, we do not take into account spin scattering at the ferromagnet/nanotube interface. Tsukagoshi et al. (1999) also studied the MR of a nanotube contacted with a double Co layer as a function of BO and observed that the technique improves the continuity of the Co film and reduces the contact resistance in comparison with the single Co-layer devices. However, the MR ratio, ΔR/RAP, and the coercive field are less than the single Co-layer devices, as shown in Fig. 4
6, which implies that the magnetization is averaged over many small magnetic domains in the double Co-layer devices and each has relatively weak coercivities. The MR is measured as a function of B perpendicular to the substrate (B\) for the CNTs. In this case the peaks are broader and shifted to higher fields when compared with the BO dependence. This is expected for a FM film with an in-plane easy axis of magnetization (Moodera et al., 1995). The temperaturedependent MR shows that the percentage difference between the resistance in the parallel and antiparallel configurations goes to zero as the temperature increases from 4.2K to 20K. ΔR/RAP decreases almost exponentially with the increase in the temperature. The exact mechanism for the temperature dependence is not yet known. Low atomic number of carbon, the spin
orbit scattering in the CNT (and hence its temperature dependence) should be negligible. However, the spin polarization at the interface will decrease with the increase in temperature if the nanotube/ferromagnet interface is of relatively poor quality.

4.5 Tunnel magnetoresistance in carbon nanotube
based spintronic devices CNTs are molecular tubes of carbon with outstanding properties (Min et al., 2006). They are among the stiffest and strongest materials known and have remarkable electronic behavior

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and many other unique properties. They are attractive for spintronic devices due to their nanoscale size, extremely large spin-flip scattering lengths, and because they can behave as one-dimensional ballistic quantum conductors (Glazman et al., 1988; Monsma et al., 2000; Kamper et al., 1987; Hofer et al., 2003). Experimental investigations on coherent spin transport through Co-contacted CNTs showed that spin could be coherently transported over 130 nm through the CNT (Tsukagoshi et al., 1999). Sahoo et al. (2005a,b) studied the gate dependence TMR of MWCNTs with PdNi electrodes, at T 5 1.85K). Fig. 4
7A shows the TMR observed to oscillate relatively regularly between 25% and 16% on a gate-voltage scale ΔVgTMR such that 0:4 V , ΔVgTMR , 0:75 V. The conductance of the MWCNTs has been studied at lower temperatures (T 5 300 mK), in order to resolve the single-electron states which could not be resolved at the temperature at which the MR was measured. A measurement of the differential conductance dV/dI as a function of source
drain Vsd and gate voltage Vg at T 5 300 mK is also studied for a relatively narrow Vg range. It displays the diamond-like pattern characteristic of single-electron tunneling in a quantum dot. The diamonds vary in size with single-electron addition energies ranging between 0.5 and 0.75 meV, in agreement with previous reports on MWCNT quantum dots with nonFM leads (Buitelaar et al., 2002), where coulomb blockade and energy level quantization were observed. The electron levels are nearly fourfold degenerate (including spin) and their evolution in magnetic field (Zeeman splitting) agrees with a g factor of 2. However, the TMR gate-voltage scale ΔVgTMR measured at T 5 1.85K is much larger than the scale Vge B25 mV for addition of single electrons: it corresponds to the addition of at least 16 electrons rather than 1. A gate-voltage scale that agrees with the TMR signal becomes visible if the linear conductance G at low temperatures is monitored over a wider gate-voltage range. The single-electron conductance peaks are strongly modulated in amplitude, leading to a regular beating pattern with the proper gate-voltage scale of ΔVg  0.4 V. Fig. 4
7B shows the conductance G and the TMR of a SWCNT device measured by Sahoo et al. (2005a,b). The quantum dot behavior is already observed at 1.85K, whereas this was only evident at 0.3K in the MWCNT device. This is consistent with the higher energy scales (both single-electron charging energy and level spacing) for SWCNTs as compared with MWCNTs. The typical single-electron addition energy amounts to B5 meV, whereas it was an order of magnitude smaller in the MWCNT device. The variation of the conductance G and the TMR are simultaneously shown in Fig. 4
6B. They attributed those behaviors to the quantum interference in CNT (Schäpers et al., 2001). First, the TMR changes sign on each conductance resonance. Furthermore, the line shape of the conductance resonances is symmetric, whereas that of the TMR dips is asymmetric. The jump in the G(Vg) data at Vg 5 4.325 V. The amplitude of the TMR ranges from 27% to 117%, which is a higher amplitude than for the MWCNTs. This might be due to the higher charging energy in SWCNTs (Barnas et al., 2000). Normal
SWCNTs
FM (N
SWCNTs
F) devices yield an order of magnitude lower signal proving that the current in the F
tube
F devices is indeed spin polarized. Gunnarsson et al. (2008) investigated the gate-voltage dependence of nonlocal spin signal. Nonlocal spin-valve devices of SWCNT were fabricated, and these devices showed clear quantum dot behavior at low temperatures. They found the nonlocal voltage oscillates around zero with a large amplitude about 1 μV. Similar phenomenon was also reported by Makarovski et al. (2007).

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FIGURE 4–7 (A) The TMR data of MWCNTs connected with two PdNi contacts were measured at T 5 1.85K. The MR oscillates with a period ΔVgTMR B0.4
0.75 V. (B) SWCNT device with a contact separation of L 5 500 nm measured at T 5 1.85K. Measurement (’) of the linear conductance G and the TMR around two resonances. The bars in reflect the error in deducing the TMR signal from R(B) curves. MWCNTs, Multiwalled carbon nanotubes; MR, magnetoresistance; SWCNT, single-walled carbon nanotubes; TMR, tunnel magnetoresistance. Sahoo, S., et al., 2005a. Electric field control of spin transport. Nat. Phys. 1, 99
102; Sahoo, S., et al., 2005b. Electrical spin injection in multiwall carbon nanotubes with transparent ferromagnetic contacts. Appl. Phys. Lett. 86, 112109. Copyright Nature Publishing Group. Reproduced with permission.

Mohamed et al. (2008) studied the MR of SWCNTs from FM electrodes. In Mohamed et al. (2008), the coercive force obtained from the hysteresis curve for both temperatures is B120 Oe. The current I versus voltage V characteristics for the devices was measured and found that resistance of the SWCNT devices is about 10
200 Ω at room temperature. Highresistance CNTs show good FET characteristics (Inami, 2008). These facts also suggest that the channels consist of metallic or semiconducting SWCNTs, but not other conductive carbon materials. In Mohamed et al. (2008), the resistance B186 Ω is almost constant against the magnetic field for a width channel of B500 μm at 300K. However, the resistance peak is B0 Oe at 4.5K, which varies slightly in the direction of the sweep. When the field was swept upward, a peak appeared at about 2110 Oe, whereas in the downward direction, a peak appeared at about 110 Oe. In the case of a device with a channel width of B250 μm, similar behavior to that in the device with the channel width of B500 μm was observed. To ensure

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that this MR effect is not governed by the MR of Co nanoparticles on the electrodes, a control experiment was conducted. Mohamed et al. (2008) measured the MR effect for one of the electrodes at the same temperatures, and no significant change in resistance was observed. This indicates that the hysteretic MR does not originate from the Co nanoparticles themselves. Although the Co FM component is independent of temperature, as mentioned above, MR effects were observed at a low temperature. For spin transport without spin scattering through a material connected to two FM electrodes, the resistance is high when the FM moments in the two electrodes are antiparallel and vice versa (Julliere, 1975). The spin-valve effects observed in MWCNTs and SWCNTs are well explained by this model (Sahoo et al., 2005a,b; Nagabhirava et al., 2006). SWCNTs seem to grow from Co nanoparticles, which form from Co thin film on Mo film while using alcohol catalytic chemical vapor deposition (ACCVD) (Inami et al., 2007). Therefore spin-dependent transport is expected to be governed by the magnetic properties of Co nanoparticles to which the SWCNTs are attached and to be strongly dependent on the size of the Co nanoparticles. Each SWCNT is attached to a Co nanoparticle with a different size. Thus it is expected that each SWCNT should show MR peaks at different field values where parallel and antiparallel magnetization configurations of the electrodes change. However, the magnetic property of an individual Co nanoparticle in contact with an SWCNT cannot be determined. In this study the average magnetic properties and MR effects of Co nanoparticles were observed. The sharp peaks of averaged MR at approximately 6 110 Oe correspond to the average coercive force measured by the SQUID. The origin of this unusual MR hysteresis still remains unclear, but the results of the MR effects are reproducible. A detailed analysis of the spin transport mechanism of these devices is to be carried out in the near future. The ratio of MR is defined by ΔR/R0, where R0 is the resistance in the saturation region. At 4.5K, ΔR/R0 is found to be about 0.7%
1.8%. This value agrees with the previous report on the MR of SWCNTs (Sagnes et al., 2003). To increase ΔR/R0, effective spin injection from the FM electrode to the SWCNTs, and spin-coherent transport in SWCNTs should be realized. For spin injection, improvement in the Co/SWCNT interface quality is necessary. For spin-coherent transport the growth of high-quality SWCNTs and the reduction of L between the electrodes are the most important factors. It is also interesting that the dependence of ΔR/ R0 on L can be used to clarify the spin diffusion length of SWCNTs.

4.5.1 Spin-valve devices of carbon nanotubes Tsukagoshi et al. (1999) fabricated the first two-terminal CNT spin valve device, and spindependent transport was demonstrated through MWCNTs and 9% MR at 4.2K was observed. The MWCNTs were synthesized from graphite rods by the arc discharge. Co electrodes 65nm thick deposited on the nanotube were used to inject spin current into MWCNTs. Later, spin-dependent transport properties in SWCNT or SWCNT network were also been studied in local or nonlocal geometry (Kim et al., 2002; Tombros et al., 2006; Yang et al., 2012; Jensen et al., 2005). Jensen et al. (2005) measured several kinds of two-terminal devices with different electrodes at low temperatures. They observed a large MR up to almost 100% and down to 2150% in the SWCNT devices with two Fe electrodes. However, a 10% MR was also

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observed in the device that only had one magnetic electrode. This phenomenon is quite confusing and need further exploring. Tombros et al. (2006) carried out the nonlocal spin transport measurements in SWCNTs. By separating the charge current from the spin current, they provide an ambitious prove of the spin accumulation induced MR in SWCNT.

4.6 Conclusion and perspectives of carbon nanotubes in spintronics Spin currents have been successfully injected to both SWCNTs and MWCNTs by using different FM electrodes, such as Co, Fe, PdNi, and lanthanum strontium manganite (LSMO). CNT-based spin valve devices show large MR up to 61% and a long spin diffusion length about 50 m at low temperatures, which make CNT a promising material for the spintronic devices. Several spin relaxation mechanisms were discussed, but systematic spin relaxation mechanisms need to be further investigated. The gate tunable properties were studied in local and nonlocal spin valves, indicating that the electric field can be used to control the spin transport in CNTs. By adding spin freedom to the traditional electronics, spintronic devices are expected to combine logic operations, data storage, and communications, making the devices faster and consume less electrical power than the conventional electronic devices. The carbon-based materials are outstanding candidates for this target and have already shown promising future from the recent experimental progress. Spin injection has been realized in graphene, CNT, fullerene, and organic semiconductors. Spin can transport a macroscopic distance in graphene and CNT. The spin valves fabricated from fullerene and organic semiconductors exhibit large MR at room temperature. It has been found in graphene that if a pinhole free and flatness SDT barrier could be obtained, the contact-induced spin relaxation can be reduced effectively. Complicated mechanisms were found in the spin precession in graphene and CNTs, and it needs to be systematically identified. Among these carbon materials, the two-dimensional graphene has drawn lots of attention and been developed quickly. Unlike CNTs, graphene can be tailored into particular shapes and the electronic and magnetic properties are sensitive to the edge structure of graphene. Theory has predicted that the zigzag-edged GNRs are very useful in the future spintronics and many novel spintronic devices based on them have been proposed. However, till now, there are a less number of experimental studies for these proposed devices, which should be the next focus in this field.

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Further reading Liang, W., et al., 2002. Shell filling and exchange coupling in metallic single-walled carbon nanotubes. Phys. Rev. Lett. 88, 126801. Ray, S.C., et al., 2009. High-temperature annealing effects on multiwalled carbon nanotubes: electronic structure, field emission and magnetic behaviors. J. Nanosci. Nanotechnol. 9, 6799
6805. Symbal, E.Y., et al., 2007. Interface effects in spin-dependent tunnelling. Prog. Mater. Sci. 52, 401
420.