Investigation of interface magnetism of complex oxide heterostructures using polarized neutron reflectivity

Current Applied Physics 17 (2017) 615e625

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Investigation of interface magnetism of complex oxide heterostructures using polarized neutron reflectivity Surendra Singh*, S. Basu Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 August 2016 Received in revised form 17 October 2016 Accepted 21 February 2017

Multilayered structures with artificial oxide hetero-interfaces have recently been dominating the field of new states of matter. The unexpected properties and related functionalities at the interface of complex oxide heterostructures as a consequence of the symmetry breaking, electronic reconstruction etc., make these complex oxide interfaces particularly challenging for understanding the fundamental mechanism and interaction across the interfaces. Magnetic modulation, novel magnetic coupling and emerging interface induced magnetization at the interfaces of complex oxide heterostructures have made the search for their technological applications as an intense field of research in recent time. However to study the structure and magnetism in such interfaces require tools with interface specificity. Polarized neutron reflectivity is a non-destructive technique which has played a decisive role in investigation of interfacial magnetic structures and in understanding the underlying physics in this rapidly developing field. This article presents a review of some recent experimental results on emerging magnetization at the interfaces of complex oxide heterostructures specifically investigated using polarized neutron reflectivity. © 2017 Elsevier B.V. All rights reserved.

Keywords: Complex oxide heterostructure Interface magnetism Polarized neutron reflectivity

1. Introduction Nanostructured architecture with reduced dimensionality and/ or enlarged interfacial areas has been used as model systems to investigate the effect of interface on physical properties of complex oxides heterostructures [1e4]. In complex oxide heterostructures, interfaces formed between two different transition metal oxides have attracted intense research interest in a broad range of interesting materials, targeting different functionalities, such as high temperature superconductivity, metal to insulator transition, colossal magnetoresistance, ferromagnetism (FM), ferroelectric (FE), piezoelectric and multiferroic properties. These functionalities are characterized by a strong interplay between the fundamental degrees of freedom in these systems, namely the electronic spin, charge, orbital and the lattice. Coupling between distinct orders parameters, as a consequence of competition/cooperation between these degrees of freedom, across interfaces yields many interesting physical phenomena, like charge transfer [5e8], strain induced coupling [9e13], chemical reconstruction [14e16], spin and orbital reconstructions [17e19], proximity effects [20e27] and tunneling

* Corresponding author. E-mail address: [email protected] (S. Singh). http://dx.doi.org/10.1016/j.cap.2017.02.017 1567-1739/© 2017 Elsevier B.V. All rights reserved.

order parameters across insulator [28]. Oxide interfaces have attracted considerable attention due to their emerging novel behavior which does not exist in the corresponding bulk parent compounds. These studies have opened the possibility of electronic applications based on the novel physical properties of oxide interfaces [29,30]. In many oxide interfaces, they form a cystallographically layered structure, which allows strong directional growth of the films as well as excellent crystalline epitaxy at the interfaces. Development of state-of-the-art thin film growth techniques, e.g. pulsed laser deposition, molecular beam epitaxial (MBE) etc., have driven the fabrication of atomically sharp and epitaxial interfaces in artificial heterostructures of dissimilar materials that have exhibit novel physical phenomena, especially at the interfaces. On the other hand advanced characterization tools have made it possible to study the effects of small perturbations, such as chemical doping [31], strain [9e13,32e34], photons [35] and magnetic/electric fields [31,36e40], on physical properties of interfaces of complex oxide heterostructures [1e4]. Experimental results [2,41e49] have also shown that either structural or electrostatic boundary conditions can be dominant factors in controlling the atomic, electronic, and magnetic structures of complex oxide interfaces. Reduced dimensionality, broken symmetry, spin-orbit coupling,

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quantum confinement, strain, and atomic or electron reconstruction have been studied to a variety of physical phenomena at interfaces. In addition rotation of oxygen octahedral to accommodate lattice constant difference at interfaces, as well as charge and orbital orders at interfaces have presented an unparalleled wealth of physical phenomena in complex oxide interfaces. These parameters resulted in emergence and modifications of magnetism at these interfaces and have been a topic of great interest during the last decade. Induced ferromagnetism at the interfaces of complex nonmagnetic oxide heterostructures has also been motivating intense research for understanding the related phenomena at interfaces both theoretically and experimentally [8,19,22,28,50e60]. Complex oxide interfaces not only modify bulk magnetic properties, but are also capable of creating magnetism in non-magnetic layers [8,18,37,52], altering the nature of a magnetic state [56e59]. Thus, to characterize the structure and magnetism of interfaces we require tools which can probe the interfaces in heterostructures with good spatial resolution. Polarized neutron reflectometry (PNR) is intrinsically sensitive to interfacial magnetism and it has been successively used to resolve the origins of magnetism that arise at interfaces of complex oxides [8,19e25,28,50e61]. In contrast to conventional magnetic measurements, e. g., vibrating sample magnetometer (VSM) and superconducting quantum interference device (SQUID) magnetometer, which are only sensitive to macroscopic magnetization of the whole sample (including substrate: the contribution from the substrates is a serious concern in interfaces and heterostructures), PNR is an ideal tool to probe the interface magnetism with a depth resolution in sub nanometer length scale [62]. Hence the advantage of PNR is that the substrate does not contribute to the magnetic signal unlike in conventional magnetometry measurements where the diamagnetic signal of the substrate dominates over the tiny magnetic signal originating from the sample. Also there are concerns, often raised in literature on using these macroscopic (bulk) magnetometry techniques [63] for understanding nanoscale structures which are not germane to PNR. In addition to interface sensitivity of PNR, it can also address the effect of many other parameters (roughness, interdiffusion, oxygen vacancy, lattice expansion, etc.) on interface magnetism, since PNR simultaneously provides depth dependent chemical and magnetic structural information at nanoscales [62,64,65]. This article reviews the recent discoveries of emergent magnetic properties at complex oxide interfaces in heterostructures. We specifically focus on PNR studies with thin film complex oxide heterostructures. We illustrate these interface induced magnetic properties with a few selected examples, showing how PNR has played crucial role for investigating the induced ferromagnetism and modulation of magnetism at interfaces between two antiferromagnetic complex oxides [8], antiferromagnetic and paramagnetic complex oxides [52], two non-magnetic insulating oxides [61], ferromagnetic and antiferromagnetic (multiferroic) oxides [56e58] and ferromagnetic/insulator/superconducting complex oxides [28]. First we will give a brief introduction to PNR technique. We then discuss above mentioned examples of the complex oxide interfaces showing interesting magnetic phenomena. We conclude by articulating the opportunity in the field, in particular, how to use PNR technique to study these interfaces under external perturbation, i.e. applied stress, temperature electric and magnetic fields, by using suitable sample environment, which is easy to implement in PNR as compared to other macroscopic magnetization techniques. 2. Polarized neutron reflectivity Due to rapid growth in availability of high brilliance synchrotron

and neutron sources and in development of new techniques viz. Xray reflectivity (XRR), neutron reflectivity (NR) and PNR based on these sources it is possible to obtain physical and magnetic structure at sub nanometer length scale. The applicability of these techniques has permeated to almost every discipline of science. XRR and NR are nondestructive tools to determine the chemical structures in multilayers with sub-nanometer depth resolution. However the neutron also carries a spin of magnitude ½ and has a magnetic moment of
1.91mN (nuclear magneton). It interacts with unpaired electrons responsible for magnetism. Thus PNR is ideally suited to measure the nuclear scattering length density and magnetization depth profiles across planar interfaces. Because reflection occurs only when either or both of the nuclear scattering length density or magnetic density (or contrast) change across an interface, the technique is intrinsically sensitive to interfacial magnetism. The specular reflectivity (angle of incidence ¼ angle of reflection) is measured as a function of wave vector transfer Q ((Ki e Kf) see Fig. 1; i.e., the vector difference between the outgoing and incoming wave vectors of the radiation/neutron) and its magnitude is: Q ¼ 4lp sinq; where q is the angle of incidence and l is the wavelength of x-ray/neutron. Specular reflectivity is related to the square of the Fourier transform of the depth dependent (z) scattering length density (SLD) profile rðzÞ (normal to the film surface or along the z-direction) [66,67,69e72]. In case of PNR a spinpolarized neutron beam reflected from the sample surface provides additional information about the correlation between nuclear and magnetic structures and SLD profile rðzÞ consists of nuclear and magnetic SLDs such that r± ðzÞ ¼ rn ðzÞ±CMðzÞ, where C ¼ 2.9109  10
9 Å
2 cm3/emu, and M(z) is the magnetization (emu/cm3) depth profile [70]. The sign þ(
) is determined by the condition when the neutron beam polarization (Fig. 1) is parallel (opposite) to the applied field and corresponds to reflectivities, R±. Polarized neutrons with spin parallel (spin-up, þ) or antiparallel (spin-down,
) to the direction of the external magnetic field applied on the sample, experience the same nuclear scattering potential, but opposite magnetic scattering potentials (Fig. 1(b)). On measuring polarized neutron beams reflected from the magnetic film the two contributions (þand -) can be separated to obtain the depth profiles of both the chemical structure and the magnetization in terms of their corresponding SLD. Generally thin films are saturated with a small applied magnetic field (~few hundred Gauss) and the potential defined in Fig. 1(b) are well accepted. For large applied magnetic field we needed to include a Zeeman shift correction as defined in Ref. [73]. In addition to collecting spin dependent reflectivity from magnetic film as discussed above, if one performs spin analysis of reflected neutron beam then it is possible to get the depth dependant magnetic structure of the film [66e72]. Thus polarization analysis of the specularly reflected beam provides information about the projection of the net magnetization vector onto the sample plane. In this case one typically collect four reflectivities, both spin-flip (Rþ
and R
þ, i.e., reflected neutrons with polarizations opposite to incident polarization), and nonspin-flip (Rþþ and R

, i.e., reflected neutrons with the same polarization) settings [66,67]. In case of Rþþ, the neutron spin polarization is parallel to the sample polarization and for R

the neutron spin polarization is opposite to the sample polarization. The two nonspin-flip cross-sections, Rþþ and R

, are related to the magnetization components parallel to the applied (in-plane magnetization of sample). The remaining two spin-flip cross-sections, Rþ
and R
þ, are related to the magnetization components perpendicular to the applied field. Thus employing all spin dependent neutron reflectivity with polarization analysis provides the direction of in-plane magnetization along the depth of the heterostructures and interfaces. Negligible

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Fig. 1. (a) Schematic of polarized neutron reflectivity geometry. Neutron with spin parallel (or antiparallel) to applied magnetic field falls on heterostructure at an angle of incidence, ðqÞ q and reflected specularly (angle of incidence (q) ¼ angle of reflection (q)). Specular reflectivity is measured as a function of momentum transfer, Q (¼ 4p Sin ). The intensity of the l spin dependent reflected beam (R±, where þ and e are the polarization of neutron with respect to applied field) are measured relative to the incident beam intensity. (b) Represents the components of potential seen by neutron in non-magnetic (NM) layer and magnetic (M) layer. Polarized neutrons with spin parallel (spin-up, þ) and antiparallel (spin-down,
) to the direction of the external magnetic field experience the effective potential which is sum and difference of nuclear scattering potential and magnetic scattering potentials, 2 rb respectively. Nuclear and magnetic scattering potential are defined as: Vn ¼ 2hpm , Vm ¼ mn B ¼ CMðzÞ, (see text) respectively, where r, b, mn, mn and B are nuclear density, nuclear n scattering length, mass of neutron, nuclear magnetron and magnetic field associated with magnetism of the layer.

spin-flip reflectivities indicate that the magnetizations in the layers were collinear with the magnetic field. PNR data can also be represented as the difference between Rþ(Q) and R
(Q) (i.e. normalized spin asymmetry (NSA) ¼ {Rþ(Q) - R
(Q)}/RF, where RF is Fresnel decay in reflectivity) [28,58,68] or the difference divided by the sum (i.e. spin asymmetry (SA) ¼ {Rþ(Q) - R
(Q)}/{Rþ(Q) þ R
(Q)}) [10,28,56,61]. Detail about PNR techniques can be found elsewhere [66e72]. 3. Studies using PNR for magnetization depth profile of oxide heterostructures Complex transition metal oxides including ABO3 perovskites are based on multiple cations bonded with oxygen ions, and they form in remarkably diverse and adaptable structures [74]. In ABO3 perovskite oxides the A cations are typically larger radii alkali, alkali-metal, or lanthanide elements and the B cations are commonly transition metals. The presence of oxygen ligands around different cations is very important to some of the remarkable behavior found in these materials. The oxygen anion provides intra- and inter-atomic exchange interactions within mixed ioniccovalent materials through Coulomb repulsion, which contributes constructively for different functionality at interfaces. Here we review some of the recent works on interfacial magnetism from perovskite interfaces and heterostructures, primarily studied, using PNR technique. 3.1. Emergent and interface induced magnetism at complex oxide interfaces Perovskites with dissimilar B-site cations materials make a 0 BeOeB bonds at interfaces and exchange interaction across this can deviate from that found within the adjoined materials with BeOeB and B0 eOeB0 bonds (Fig. 2(a)). So depending on this interaction at interface there are cases where BeOeB0 coupling is ferromagnetic even when both BeOeB and B0 eOeB0 interactions

are antiferromagnetic. Ueda et al. [75], tuned BeOeB0 interaction in (ABO3)1/(AB0 O3)1 (interface of 1 unit cell each of ABO3 and AB0 O3) superlattices by growing these superlattices along different crystallographic orientations to study long range FM or antiferromagnetism (AFM) phenomena at interfaces. Fig. 2(b) shows the High angle annular dark field (HAADF) Z-contrast image of the superlattice consisting of two different ABO3 perovskites. It can be seen that the superlattice has well-defined interfaces between two perovskites. Existence of different magnetic orders at the complex oxide interfaces have been found, which are different from their constituent oxides. For example, ferromagnetism has been observed at interfaces between two different antiferromagnetic oxides [76e78] and interfaces between a paramagnetic metal and antiferromagnetic insulator [8,52,79e82]. The two different cases are depicted in Fig. 2(c) and (d). 3.1.1. AFM insulator/paramagnetic metal interface Macroscopic (SQUID) magnetometry [79] and resonant x-ray magnetic scattering [82] studies discovered ferromagnetism at the interface of CaMnO3 (CMO)/CaRuO3 (CRO) superlattices, where CMO and CRO are an AFM insulator [83] and a paramagnetic metal [84], respectively. The interfacial FM in this system was theoretically explained in terms of a ferromagnetic double exchange interaction among Mn ions in one unit cell of CMO due to canted AFM [80]. Finite magnetization and exchange bias effect on CMO/ CRO superlattice was observed using macroscopic magnetization technique [8]. However, using the depth sensitivity of the PNR, He et al. [8], demonstrated that the interfacial FM is in fact confined to within a single unit cell of CMO. PNR results also confirmed existence of ferromagnetism in one unit cell of CMO at both CMO/CRO (CMO on CRO) and CRO/CMO (CRO on CMO) interfaces which leads to emergence of exchange bias in this system as a consequence of exchange coupling of interface induced FM and rest of antiferromagnetic CMO layer. Another recent study using PNR also confirms the existence of FM at the interface of LaNiO3 (LNO)/CMO superlattices [52], where

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Fig. 2. (a) Schematic of a perovskite ABO3/AB0 O3 interface. Interfacial strain can modify the bond length and bond angle resulting in new phenomena at interface of two complex perovskite structures. (b) High angle annular dark field (HAADF) image showing the interfaces between two perovskites ABO3/AB0 O3. Schematic diagram of emergence of ferromagnetic (FM) at the interface of complex oxides (c) two antiferromagnetic (AF) insulators and (d) AF insulator and paramagnetic (PM) metal as a results of strong correlation among different degree of freedom at interfaces. (e) Depth profile of nuclear and magnetic scattering length density in CMO (antiferromagnetic insulator)/LNO (paramagnetic metal) heterostructure extracted from PNR measurements [52]. PNR data helped to investigate the ferromagnetism at the interface layer of CMO (thickness ~1 unit cell) at both CMO/LNO and LNO/CMO interfaces.

LNO is a paramagnetic metal. PNR results clearly indicated the existence of FM in one unit cell of CMO at both CMO/LNO (CMO on LNO) and LNO/CMO (LNO on CMO) interfaces, which is dependent of the metallic nature of LNO [52]. Fig. 2(e) show the depth dependent profiles of nuclear and magnetic SLDs obtained from the PNR measurements at 15 K from LNO/CMO superlattice [52]. Using SQUID magnetometry and X-ray magnetic circular dichroism (XMCD) techniques, Grutter et al. [52], also found that FM at interfaces of the LNO/CMO superlattice was below 70 K only, the temperature which is also the metal to insulator (MIT) transition temperature for thin LNO layer. The strong dependence of the FM on the conducting state of LNO in LNO/CMO superlattice was indicative of an interfacial double exchange interaction mediated by the LNO eg band [52]. 3.1.2. Non-magnetic insulator/non-magnetic insulator interface Recent discovery of remarkable interface-induced conductivity and magnetic effects caused by electronic reconstruction at oxide interfaces of non-magnetic insulating perovskites, SrTiO3 (STO) and LaAlO3 (LAO), has also attracted a lot of interest [42,43,85]. Many attempts have been made to explain the large conductivity at the interface of the LAO/STO system using different phenomena like, polar catastrophe [15,86], oxygen vacancies [87,88] or cation diffusion [89,90], and structural distortion or orbital reconstruction [91]. One of the scenario proposed for describing the physics of the metallic states is the ‘‘charge transfer’’ that originates from the charge discontinuity at the interface between polar LAO and nonpolar STO. The conductivity at the interface strongly depend on terminating layer of interface. Fig. 3(a) shows a bilayer of LAO and STO and corresponding polar and nonpolar termination of LAO and STO at interfaces, which is one explanation for obtaining high conductivity at the interface of the LAO/STO system. The first signature of magnetism at the LAO/STO interface was reported in 2007 by Brinkman et al. [43]. Several other signatures of magnetism at STO/LAO interfaces have been followed using, SQUID magnetometry [92], torque magnetometry [93], SQUID microscopy [94], XMCD [95] and magnetic force microscopy [96,97] measurements. Ariando et al. [92] reported a direct measurement of LAO/

STO magnetization using SQUID magnetometry. As shown in Fig. 3(b), a series of hysteresis loops of the magnetization in external magnetic field was observed [92]. This hysteresis persists up to room temperature. Whereas torque magnetometry measurements report moments ~5  10
10 Am2 for 5 unit cells of LAO on STO [93]. While the magnetism in LAO/STO heterostructures showed mixed report, actual location and magnitude of magnetic moments in this system remains unknown. Fitzsimmons et al. [61], developed an innovative measurement protocol where PNR measurements were carried out on two samples (a control sample and other STO/LAO superlattice) simultaneously at high magnetic field ~ 11 T and low temperature ~ 1.5 K. Two samples were compared for spin dependent reflectivity. A number of STO/LAO superlattices, which have showed high conducting interfaces, grown by different groups were tested using PNR technique. Fig. 3(c) shows the spin asymmetry data as well as simulated spin asymmetry profiles corresponding to different values of interface magnetization in STO/LAO superlattice [61]. The study clearly indicated (Fig. 3(c)) that the upper limit for the magnetization averaged over the lateral dimensions of the sample induced by an 11 T magnetic field at 1.7 K is less than 2 G (¼ 2 emu/cm3). The shaded region in Fig. 3(c) represents the range of spin asymmetry profiles, considering different magnetization values reported in literature for the STO/LAO superlattice. However PNR measurement suggested very small magnetization (~2 emu/cm3) at the interface of LAO/STO heterostructures [61]. 3.1.3. Ferromagnetic/multiferroic interface Several fascinating interfacial phenomena that are driven by spin, charge and orbital reconstructions at the interface have also been observed in heterostructures involving oxide ferromagnets such as La0.7Sr0.3MnO3(LSMO) and oxide antiferromagnet such as BiFeO3(BFO). BFO is a single-phase multiferroic material which exhibits magnetoelectric coupling between FE (TC ¼ 1103 K) and AFM (TN ¼ 643 K) order parameters [98]. Theoretical calculations have suggested a weak FM for BFO (~0.03 mB) as a consequence of canting of the AFM structure due to Dzyaloshinskii-Moriya interaction [99]. Most interestingly, an enhanced magnetic moment in

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Fig. 3. (a) Schematic representation of a bilayer of LAO/STO heterostructure and the types of LAO/STO interfaces, illustrating the polar and non-polar surface termination of LAO and STO, respectively, which is responsible for conducting AlO2eLaOeTiO2eSrO interface. (b) SQUID measurement: hysteresis loops taken at different temperature superimposed on top of the temperature dependence of magnetic moments [92]. (c) The spin asymmetry (defined as: (Rþ
R
)/(Rþ þ R
), where R± are spin dependent reflectivity) data as well as simulated profiles corresponding to different values of interface magnetization in STO/LAO superlattice [61]. The shaded region represents the range of magnetization value reported in literature for STO/LAO superlattice, while PNR results indicated very small magnetization (~2 emu/cm3) for STO/LAO superlattice.

the BFO layer in BFO/LSMO heterostructure was observed right at the interface. It was attributed to FeeMn hybridization and orbital reconstruction. The hybridization is associated with charge transfer possibly occurring in close proximity to the BFO/LSMO interface [18,56] and it is quite different from the bulk. Using XMCD measurements, Yu et al. [18] indicated an interface magnetization of 0.6mB/Fe at the BFO/LSMO interface, which is ferromagnetically coupled to the LSMO layer. They inferred that the new magnetic phase induced at the interface between the LSMO and the BFO as a consequence of the electronic orbital reconstruction. Recent PNR measurements [56] from BFO/LSMO heterostructure have suggested a similar magnetization for the whole BFO layer [~6 unit cells thick]. However PNR study from two different BFO/LSMO heterostructures (with different bilayer thickness) grown on different STO substrates, suggested different magnetic couplings: ferromagnetic [56] and antiferromagnetic [57]. The different coupling at the interfaces might have resulted due to different surface terminations of the surface of LSMO in these heterostructures. Ab-initio calculations also confirmed both AFM and FM exchange coupling across the BFO/LSMO interface, suggesting surface termination dependence of the LSMO film [56].

Theoretical calculations also suggested such magnetization can indeed develop via spin-exchange effects in a BFO/LSMO superlattice, resulting in strong induced magnetization for even thinner films (~3 unit cell) [56]. Recent PNR measurements on ultrathin BFO/LSMO heterostructures (layer thickness ~ 2 unit cells for each layer) clearly suggested an increase in magnetization (~0.8 mB/Fe) for whole BFO (~2 unit cell) layer, which is ferromagnetically coupled with the LSMO [58]. Fig. 4 illustrates growth of high quality ultrathin BFO/LSMO superlattice on STO substrate and interface induced magnetization in the BFO layer which is ferromagnetically coupled with the LSMO layer. Fig. 4(a) and (b) show the XRD and XRR measurements from the BFO/LSMO superlattice, respectively. The thickness oscillation peaks (indicated by arrows in Fig. 4(a)) around the STO (002) and epitaxial peak (blue dash line in Fig. 4(a)) clearly suggest a high degree of perfection of atomic structure along the growth direction (normal to the film surface). Further perfection in layer structure is evident from the Bragg peak in XRR measurement (Fig. 4(b)) from superlattice. The extracted structure with electron SLD (ESLD) for bilayer is depicted in Fig. 4 (c). Fig. 4(def) show the NSA (PNR) data from superlattice at 10 K and Fig. 4(gei) show the magnetization profile across a bilayer of the

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Fig. 4. (a) Typical q
2q (out of plane) XRD pattern (in log scale) of the [(LSMO)2/(BFO)2]32 heterostructure (32 repeat of ultrathin (~2 unit cell) bilayer of LSMO and BFO) on (001) SrTiO3 substrate showing high quality, epitaxial growth. (b) X-ray reflectivity (XRR) from heterostructure. (c) The electron scattering length density (ESLD) depth profile of a bilayer of the heterostructure obtained from fit (solid line in (b)) to XRR data. (def) Normalized spin asymetry (NSA) data (PNR data) defined as: (Rþ
R
)/RF, where R± and RF are spin dependent neutron reflectivity and Fresnel decay of reflectivity, at 10 K and an applied in-plane magnetic field of 4.5 kOe. Solid lines in (def) are fit to NSA data considering different magnetization depth profiles as shown in (gei) across a bilayer of LSMO/BFO. Magnetization depth profiles considering zero magnetization (g) and negative magnetization (antiferromagnetic coupling at interface) (h) for whole BFO layer poorly fitted NSA data, while magnetization profile considering positive magnetization (ferromagnetic coupling at interface) for whole ultrathin BFO layer (i) gave best fit to PNR data.

BFO/LSMO for different magnetization depth profiles, which were considered to get the best fit for the NSA data (solid lines in Fig. 4(def). Different magnetization depth profiles were (i) assuming zero magnetization (Fig. 4(g)), (ii) assuming negative magnetization (antiferromagnetic coupling at interface) for whole BFO layer (Fig. 4(h)) and (iii) assuming positive magnetization (ferromagnetic coupling at interface) for whole ultrathin BFO layer (Fig. 4(i)). It is evident from Fig. 4 that the ultrathin BFO layer sandwiched between two ultrathin layers of the LSMO show a definite induced magnetization of ~0.8 mB/Fe. The magnetization for BFO layer is aligned along the direction of magnetization in LSMO layer (ferromagnetic coupling). These results also suggested that detail chemical and magnetic depth profiling of interfaces of a supperlattice can be successfully investigated with a depth resolution of subnanometer length scale using a combination of nondestructive XRR and PNR techniques. Interface induced magnetization in BFO layer in BFO/LSMO superlattices studied using XMCD [18] and PNR [56e58] reported till dates are only observed at low temperatures. The BFO/LSMO system promises a strong interfacial magnetoelectric effect, thus these results are important for realization of the system into device application in the near future. 3.2. Proximity and tunneling phenomena at complex oxide interfaces 3.2.1. LCMO/YBCO interface In addition to modification in local electronic density at

interfaces, the orbital occupancy of valence electrons at the interfaces also alters, leading to an orbital reconstruction. The orbital reconstruction was first observed at the interface between La2/3Ca1/ 3MnO3 (LCMO) and YBa2Cu3O7 (YBCO) [17]. YBCO is a d-wave superconductor and LCMO is a half metallic ferromagnet. LCMO/YBCO heterostructures has been extensively studied as a model system to investigate the phase competition between FM and superconductivity (SC) [20,22,51,100,101]. Chakhailian et al. [22] used XMCD to probe element-specific magnetization in LCMO/YBCO heterostructures, and found emergent Cu net moments at the interface, which were antiferromagnetically coupled to the Mn magnetization. Later it was found that the antiferromagnetic coupling at the interface of LCMO/YBCO heterostructure and Cu moment induced at the interface of LCMO/YBCO heterostructure were resulted from charge transfer and orbital reconstructions at the interfaces [19,102]. The mechanism of orbital reconstruction at LCMO/YBCO interface also explains the inverse spin-switch behavior observed in LCMO/YBCO/LCMO structures where superconductivity is enhanced when the LCMO layer magnetizations are parallel [26]. The applicability of the orbital reconstruction mechanism depends critically on the existence of a super-exchange path linking Mn atoms and planar Cu atoms in the cuprate at both interfaces. Using PNR, XRR and XMCD techniques, Visani et al. [103], observed symmetric interface reconstruction at both cuprate interfaces in LCMO/YBCO/LCMO structures. Fig. 5(a) shows the depthdependent x-ray and/or neutron scattering length density (SLD) profiles of LCMO/YBCO/LCMO trilayers. PNR data were measured at 9.5 K at different applied magnetic fields and corresponding

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Fig. 5. (a) Neutron (nuclear and magnetic) and x-ray SLD depth profiles obtained from the fits to the neutron and x-ray data, respectively, from YBCO/LCMO/YBCO trilayer [103]. The data at H ¼ 186 and 335 Oe were taken successively, after applying a negative saturating field of H ¼
5.54 kOe. (b) PNR data at 4 K for spin-up (jþ>) and spin-down (j
>) polarization from [YBCO(10 nm)/LCMO(10 nm)]10 multilayer [24]. Also shown is unpolarized curves at 300 K. The curves are vertically shifted for clarity. (c) Depth profiles as obtained from fitting the data in (b) (solid lines) of the nuclear (green line) and magnetic (red line) scattering length densities (SLD) which are proportional to the nuclear and magnetic potentials, respectively. Best fit to PNR data at 4 K clearly indicate the presence of magnetic depleted layer at interface and thickness of depleted layer depends on the sequence of deposition. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

magnetization depth profiles are shown in Fig. 5(a). XMCD experiments show that both cuprate interfaces are magnetic with a magnetic moment induced in Cu atoms as expected from symmetric MneOeCu super-exchange paths [103]. Using PNR measurements a reduction and modulation in magnetization at the interfaces of LCMO/YBCO heterostructure were observed below superconducting transition temperature (TSC) [5,21] of YBCO. The suppression in magnetism at interfaces was attributed to the charge transfer between the two materials and the consequent change in the Mn valence [5]. Whereas other study [21] considered two possible scenarios; one with a suppressed magnetization in the LCMO, similar as explained in Ref. [5], and the other one where close to the interface a net magnetic moment is induced in the YBCO layer, which was antiparallel to the LCMO moments. Hoppler et al. [23] also inferred a giant modulation of the in-plane magnetization in LCMO layers below the TSC of LCMO/ YBCO multilayers. They observed magnetic suppression in the whole LCMO layer of thickness ~8.5 nm. However the reduction in magnetization was dependent on the sequence of deposition [23], (i.e. the LCMO grown on the YBCO or the YBCO grown on the LCMO). Other PNR studies also exist which indicate the depletion of magnetization or a magnetic “depleted” (MD) layer at the LCMO/ YBCO interface [24,25,104]. Satapahty et al. [24] observed the coexistence of suppressed Mn magnetization and induced Cu

magnetization at the cuprate-mangantite interfaces. Fig. 5(b) depicts the PNR data at 300 K (above magnetic transition temperature) and 4.5 K (below magnetic and superconducting transition temperature) from LCMO/YBCO superlattice [24]. Fig. 5(c) shows the nuclear and magnetic SLD profiles which gave best fit to PNR data. The results clearly indicate the formation of MD layer of 1e2 nm at the interface of LCMO/YBCO multilayers. Also the different length scales in growth of the MD layer at interfaces depend on the sequence of deposition of the LCMO/YBCO interfaces. The combined XMCD and X-ray resonant magnetic reflectivity (XRMR) data on LCMO/YBCO multilayer further provided a compelling evidence that the FM Cu ions originate from the YBCO layers and Mn (in LCMO) and Cu (in YBCO) moments near interface are antiparallel. 3.2.2. LCMO/STO/YBCO system Using SQUID magnetometry, Yashwant et al. [105], observed an evidence of a modification of the ferromagnetic domain structure in the magnetic layer induced by the superconducting transition in the neighbouring YBCO layer in LCMO/STO/YBCO heterostructure. Recently Prajapat et al. [28], observed the emergence of a MD layer in LCMO, adjacent to the insulating (I) STO layer below the TSC (~60 K) of YBCO in LCMO/STO/YBCO heterostructure. The results suggested that a coupling between superconducting and ferromagnetic layers is very much possible even in a tunneling geometry

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with an intervening insulator layer (STO). Using PNR they measured detailed depth dependent magnetization profile across the TSC for different LCMO/STO/YBCO heterostructures with varying thickness of the insulator layer. The study further demonstrated an increase in the MD layer thickness at LCMO/STO interface on decreasing the thickness of the insulator layer [28]. Fig. 6 illustrates the magnetization depth profiles and corresponding fits to NSA (PNR) data from LCMO/STO/YBCO heterostructure (with STO layer thickness ~ 5 nm) at two temperatures, 100 K (above TSC) and 50 K (below TSC), across TSC. Fig. 6(aec) show the NSA (PNR) data at 100 K and corresponding fits (solid lines) assuming different magnetization depth profiles as shown in Fig. 6(def). Similarly Fig. 6(gei) show the NSA (PNR) data at 50 K. Fig. 6(jel) show the magnetization depth profile models which were used to fit PNR data at 50 K. Fig. 6 clearly demonstrate that a uniform magnetization for whole LCMO layer best fitted the PNR (NSA) data measured at 100 K (well above TSC). Whereas a uniform magnetization with an MD layer at LCMO/STO interface model best fitted the PNR data measured at 50 K (well below TSC). The thickness of MD layer was ~3 nm. Prajapat et al. [28], also showed that when the thickness of insulator layer was reduced to 2.5 nm the MD layer thickness increases to 7 nm. Thus the PNR results from the LCMO/STO/YBCO trilayer systems clearly indicated the existence of a MD layer, related to the superconducting state of the YBCO layer. These results also demonstrate that the thickness of the insulating oxide layer plays an important

role in the magnetic modulation in LCMO of such hybrid systems, confirming that the origin of the observation lies in tunneling of the Cooper pairs across insulator STO. The key factor behind the commonly observed ‘proximity effect’ is predicted to be the conduction band of the ferromagnet which helps in the leakage of the Cooper pairs into the ferromagnet and thereby shows a suppression in the transition temperature in the superconductor and/or formation of a magnetic dead layer on the ferromagnetic side. In case the superconductoreferromagnet is interleaved by an insulator this leakage should therefore stop and there will be breaking of the long range order of the triplet spinpairing, which was not the case in the study by Prajapat et al. [28]. Their results open a way to explore the nature of tunneling of the superconducting order parameter in oxide FM/I/SC systems. Future experiments using advanced imaging techniques in combination with scattering (PNR) techniques on a numbers of complex oxides FM/I/SC heterostructures may provide further insight into the superconductivity induced phenomena in FM/I/SC heterostructure systems. 4. Summary and outlook Interface induced effect in heterostructures are very crucial in magnetism, both understanding the physical principle evolved at interfaces due to strong correlations between different degree of

Fig. 6. Emergence of magnetic depleted (MD) layer at LCMO/STO interface below super conducting transition temperature (TSC) in LCMO/STO/YBCO heterostructure. (aec) Normalized spin asymetry (NSA) data (PNR data) at 100 K (above TSC) and an applied in-plane magnetic field of 300 Oe. Solid lines in (aec) are fit to NSA data considering different magnetization depth profiles as shown in (def) across LCMO/STO/YBCO heterostructure. The magnetization depth profiles considered were uniform magnetization for whole LCMO layer (d), MD layer at air/LCMO interface (e) and MD layer at LCMO/STO interface. We obtained best fit for uniform magnetization for whole LCMO layer at 100 K. Comparing NSA data (gei) at 50 K (below TSC) using similar magnetization profiles (jel) as mentioned above, we found that MD layer (thickness ~3 nm)at LCMO/STO interface gave best fit to PNR data. The PNR investigation clearly suggest strong dependence of magnetization modulation across insulator on superconductivity of the system.

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freedoms and related technological application where magnetism play important role. Many important effects in magnetism are either intrinsically interfacial nature or strongly enhanced at interfaces. The ability to heteroepitaxially engineer interfaces with almost atomic perfection is the key to opening up new directions in exploring the interplay between the fundamental degrees of freedom, which yields properties that had previously been seen only in very limited classes of bulk materials, but now can be explored and developed by choosing different materials of interest. However characterization and control of chemical and structural order is a crucial issue in complex oxide heterostructure. While most of the research to date has revealed remarkable phenomena which are mostly related to properties of ideal interfaces, effects of disorders, epitaxial strain, misfit dislocations, twin and low angle grain boundaries, which are crucial for interface related phenomena, are not negligible. We reviewed several examples of emerging research at the interfaces of complex oxide heterostructure specifically highlighting the unique capability of PNR of resolving outstanding issues at interfaces of complex oxides heterostructures. Due to very good depth sensitivity of the PNR and its inherent interaction with nuclei of the system PNR also reveals the chemical changes at interfaces due to presence of disorders at interfaces. In addition to magnetic properties at interfaces, the effect of external environments, e.g. temperature, magnetic field, electric field, applied stress, light etc., also influence the interface induced magnetism in complex oxide heterostructures. These perturbations in PNR experiment has easily been employed (while it is difficult to adopt required sample environment for macroscopic magnetization technique) to study the magnetic heterostructures. Thus, with properly designed spectrometers and the neutron sources in combination of proper sample environments [64], PNR will continue to be a unique technique to investigate interface-induced magnetism in layered oxide heterostructures and its correlation with external environments. The oxide materials have become an interesting class of materials also because of several intrinsic properties of them. Oxide stoichiometry has been used to tune their properties for a long time. Often the oxide materials themselves are of layered crystallographic structure. They allow extremely good crystalline epitaxy at the interface of two oxides of dissimilar properties. Such films have properties quite different from their parent oxides. The high electron mobility at the interface of LAO/STO heterostructures is one such example discussed earlier. In case of YBCO/STO/LCMO heterostructure one can observe a strong (00l) growth [28]. These structures has signature of tunneling of order parameters over large length scales. Hwang et al. [2] suggested that because of strong correlation between charge, spin and orbital degrees of freedom, strain can strongly affect magnetism in complex oxide interfaces which results into emergent phenomena at oxide interfaces and thus oxide interfaces are critical to much of current magnetism research. Recently a strong coupling of strain on magnetism of manganite thin film was studied using PNR [10,11]. In this study authors have simultaneously measured transport measurements and magnetization as a function of temperature, applied bending strain using PNR technique. The study further demonstrate a technique to measure interface specific magnetization as a function of applied bending stress which will undoubtedly have important impact on a broad range of interfaces of complex oxides including piezeomagnetic and multiferroic materials, which will helps to quantitatively correlate theory and experiments in these fields. In summary, interface induced magnetism occurring at interfaces between correlated oxides of two antiferromagnetic, antiferromagnetic and paramagnetic, two nonmagnetic insulators,

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