Paramagnetic spin Hall magnetoresistance

Physical Review B, 2013

rate spin current from charge current effects. One of the prototype examples for a FMI compound is yttrium iron garnet (Y 3 Fe 5 O 12 , YIG). The interplay between spin and charge transport in FM/NM devices gives rise to interesting physical phenomena. A prominent example is the spin Hall magnetoresistance (SMR) discovered recently in FMI/NM hybrids. The SMR is related to the absorption/reflection of a spin current density J s flowing along the direction normal to the FMI/NM interface. The spin current is generated by a charge current density J q in the NM layer via the spin Hall effect (SHE): 32,33

Adv. Mater. Interfaces, 2019

spin-polarized currents. In recent years, however, the flow of pure spin currents has received much interest [1,2] in the quest of novel and low-energy consumption devices. [3] A key discovery was reported in 2013, when Nakayama et al. [4] and Hahn et al. [5] found out a new kind of magnetoresistance that appears in a nonmagnetic metal (NM) when placed in contact with a ferromagnetic insu-lator (FMI). It turned out that the resistance of the NM varies with the direction in which the FMI is magnetized. The observed effect relies on two ingredients. [6] The first one, so-called spin Hall effect (SHE), in which a charge current (J C), due to spin-orbit coupling, creates a flow of spins (J S) perpendicular to J C and produces a spin accumulation at sample edges with a polarization (σ) which is normal to both J C and J S (Figure 1a,b,e). The charge-to-spin current conversion is given by the spin Hall angle θ SH of the NM layer. Additionally, the created J S is converted back to J C by the inverse spin Hall effect (ISHE) which is the reciprocal effect to SHE, in which a spin current generates a transverse charge current. This is thus a second-order effect in θ SH that lowers the base resistivity of the NM layer with respect to its Drude resis-tivity. The second ingredient is the transport of spins across the NM/FMI interface, which is quantified by the spin-mixing interfacial conductance (G ↑↓). When a charge current is applied along the NM, the transverse spin current may be absorbed by the FMI depending on the direction of σ with respect to the direction of the magnetization (M) of the FMI. When σ is parallel to M, the spin current cannot be absorbed via spin transfer torque into the FMI and thus the electrical resistance of the NM layer remains unaltered; in contrast, when σ is perpendicular to M, spin torque occurs and J S is partially absorbed into the FMI (spin excitations) producing a loss of spin accumulation in the metal and thus a reduction of J C which is equivalent to an increase of resistance. Therefore, the resistance of the NM depends on the direction of M of the neighboring FMI, which can be controlled by appropriate external magnetic field, thus giving rise to the so-called spin Hall magnetoresistance (SMR) [4,7] (Figure 1a,b and Figure 2 (central panel)). Angular-dependent magnetore-sistance measurements (ADMR) and field-dependent magne-toresistance may allow observation of SMR. The magnitude of the observed SMR is determined by θ SH and G ↑↓. Spin currents have emerged as a new tool in spintronics, with promises of more efficient devices. A pure spin current can be generated in a nonmagnetic metallic (NM) layer by a charge current (spin Hall effect). When the NM layer is placed in contact with a magnetic material, a magnetoresistance (spin Hall magnetoresistance) develops in the former via the inverse spin Hall effect (ISHE). In other novel spin-dependent phenomena, such as spin pumping or spin Seebeck effect, spin currents are generated by magnetic resonance or thermal gradients and detected via ISHE in a neighboring normal metal layer. All cases involve spin transport across interfaces between nonmagnetic metallic layers and magnetic materials; quite commonly, magnetic insulators. The structural, compositional, and electronic differences between these materials and their integration to form an interface, challenge the control and understanding of the spin transport across it, which is known to be sensitive to sub-nanometric interface features. Here, the authors review the tremendous progress in material's science achieved during the last few years and illustrate how the spin Hall magnetoresistance can be used as a probe for surface magnetism. The authors end with some views on concerted actions that may allow further progress.

Nano Letters

We present a theory of the spin Hall magnetoresistance of metals in contact with magnetic insulators. We express the spin mixing conductances, which govern the phenomenology of the effect, in terms of the microscopic parameters of the interface and the spin-spin correlation functions of the local moments on the surface of the magnetic insulator. The magnetic-field and temperature dependence of the spin mixing conductances leads to a rich behaviour of the resistance due to an interplay between the Hanle effect and spin mixing at the interface. Our theory provides a useful tool for understanding the experiments on heavy metals in contact with magnetic insulators of different kinds, and it predicts striking behaviours of the magnetoresistance.

Physical Review B

We observe an unusual behavior of the spin Hall magnetoresistance (SMR) in Pt deposited on a tensile-strained LaCoO3 (LCO) thin film, which is a ferromagnetic insulator with the Curie temperature Tc=85K. The SMR displays a strong magnetic-field dependence below Tc, with the SMR amplitude continuing to increase (linearly) with increasing the field far beyond the saturation value of the ferromagnet. The SMR amplitude decreases gradually with raising the temperature across Tc and remains measurable even above Tc. Moreover, no hysteresis is observed in the field dependence of the SMR. These results indicate that a novel lowdimensional magnetic system forms on the surface of LCO and that the LCO/Pt interface decouples magnetically from the rest of the LCO thin film. To explain the experiment, we revisit the derivation of the SMR corrections and relate the spin-mixing conductances to the microscopic quantities describing the magnetism at the interface. Our results can be used as a technique to probe quantum magnetism on the surface of a magnetic insulator. Introduction.-Magnetoresistance has been key for understanding spin-dependent transport in solids [1]. In the last years, new magnetoresistance phenomena were discovered in thin ferromagnetic/normal metal(FM/NM)-based heterostructures [2-18], which originate from the interplay of the spin currents generated in the heterostructure (via the spin Hall effect [19-22] or the Rashba-Edelstein effect [23,24]) with the magnetic moments of the FM layer. Among 2 many applications, these magnetoresistance effects have been used for quantifying spin transport properties such as the spin diffusion length  and the spin Hall angle SH of different NM layers, or the spin-mixing conductance ↑↓ of FM/NM interfaces. More interestingly, unlike other surface-sensitive techniques that suffer from a bulk contribution due to a finite penetration depth, the spin Hall magnetoresistance (SMR) [4-11] uses the spin accumulation at interfaces for sensing the magnetic properties of the very first atomic layer of magnetic insulators (MIs) [25,26]. For instance, SMR has been employed for probing the surface of complex magnetic systems such as ferrimagnetic spinel oxides [11,27], spin-spiral multiferroics [28,29], canted ferrimagnets [30], Y3Fe5O12/antiferromagnetic (YIG/AFM) bilayers [31,32], and synthetic AFMs [33]. LaCoO3 (LCO) presents an intriguing magnetic behavior, which has been studied for decades and is still under debate [34-49]. Bulk LCO is a diamagnetic insulator at low temperature, owing to the low-spin (LS) configuration of Co 3+. The relatively small crystal-field splitting of the Co 3+ 3d-shell results in an increasing population of high-spin (HS) Co 3+ with temperature, reaching 1:1 (LS:HS) above ~150K. The close proximity between crystal-field splitting and exchange energy makes the magnetic properties of LCO particularly susceptible to small changes in inter-ionic distances and coordination. For this reason, tensile-strained LCO thin films grown on particular substrates [such as SrTiO3 (STO)] exhibit FM order at low temperatures [42-49]. However, the magnetic properties of the surface of these films-where the crystal-field symmetry is lowered because of a different stoichiometry at the surface-have not been addressed yet. In this letter, we take the first steps towards understanding the magnetic behavior of the surface of strained LCO films by performing magnetoresistance measurements in STO/LCO/Pt. We find that SMR depends strongly on the magnetic field at all temperatures, both above and below the Curie temperature (Tc) of the film, and more strikingly, no hysteresis in the magnetoresistance is observed. These observations clearly show that the surface magnetism of the LCO film is radically different from its bulk counterpart. We support our measurements with a theoretical model that extends the known expressions for SMR [7,50] and HMR [51,52] in MI/NM bilayers for an arbitrary magnetic ordering (para-, ferri-, ferro-, antiferro-magnet) of the localized magnetic moments at the MI/NM interface. We provide expressions for ↑↓ Gr+iGi [25,53] and the effective spin conductance Gs [54,55] in terms of surface spincorrelators. The experimental data evidence that the surface of LCO behave as a lowdimensional Heisenberg FM. Experimental details.-Growth of epitaxial LCO thin films via polymer-assisted deposition on (001) STO substrates, as well as their structural, electrical, and magnetic characterization, is described in Ref. [46]. The LCO films exhibit a tetragonal distortion, which induces FM ordering below Tc~85K and with a coercive field below 1T at 10K, in agreement to other reports [43-45,47,56]. The films exhibit low surface roughness (<1nm) and are insulating [46]. Pt Hall bar structures (width W 100m, length L 800m and thickness dN 7nm) were

AIP Advances, 2017

Applied Physics Reviews, 2022

Journal of Magnetism and Magnetic Materials, 2002

We review several topics in the field of spin electronics: (i) giant magnetoresistance observed in magnetic multilayers; (ii) magnetization reversal by spin injection and (iii) spin-polarized tunneling in magnetic tunnel junctions combining electrodes of ferromagnetic transition metal and half-metallic oxide. r

Advanced Electronic Materials, 2016

Effective spin mixing conductance (ESMC) across the nonmagnetic metal (NM)/ferromagnet interface, spin Hall conductivity (SHC), and spin diffusion length (SDL) in the NM layer govern the functionality and performance of pure spin current devices. It is shown that all three parameters can be tuned significantly via the spin orbit coupling (SOC) strength of the NM layer by virtue of the unique Pd1‐xPtx/Y3Fe5O12 system. Surprisingly, the ESMC is observed to increase significantly with x changing from 0 to 1.0, due to the enhanced local density of states for Pt‐rich alloys. The SHC in PdPt alloys turns out to be dominated by the skew scattering term. In particular, the skew scattering parameter has for the first time been rigorously demonstrated to increase with increasing SOC strength. Meanwhile, the SDL is found to decrease when Pd atoms are replaced by heavier Pt atoms, validating the SOC induced spin flip scattering model in polyvalent PdPt alloys. A thorough grasp of the dependence...

Journal of Magnetism and Magnetic Materials, 2018

The spin Hall magnetoresistance effects (SMR) in four heterostructures consist of different boundaries of Pt layer has been investigated. The result in this work shows that the two boundaries of Pt layer in all heterostructures both influence the spin current absorption and reflection. The Pt/air interface would weaken the spin current absorption at the Co 2 FeSi/Pt interface on the other side of the Pt layer. On the contrary, the Pt/MgO(0 0 1) interface could boost the spin current absorption strongly at the Co 2 FeSi/Pt interface on the other side of the Pt layer. And this promotion effect is much stronger than the Co 2 FeSi/Pt interface on the spin current absorption at the other Co 2 FeSi/Pt interface. The MgO capping layer may avoid anisotropic magnetoresistance (AMR) induced SMR ratio decreasing at low temperature. This study provides a new way for modulating spin current absorption at the FM/HM interface.

Journal of Applied Physics, 2020

Antiferromagnetic materials promise improved performance for spintronic applications, as they are robust against external magnetic field perturbations and allow for faster magnetization dynamics compared to ferromagnets. The direct observation of the antiferromagnetic state, however, is challenging due to the absence of a macroscopic magnetization. Here, we show that the spin Hall magnetoresistance (SMR) is a versatile tool to probe the antiferromagnetic spin structure via simple electrical transport experiments by investigating the easy-plane antiferromagnetic insulators α-Fe2O3 (hematite) and NiO in bilayer heterostructures with a Pt heavy metal top electrode. While rotating an external magnetic field in three orthogonal planes, we record the longitudinal and the transverse resistivities of Pt and observe characteristic resistivity modulations consistent with the SMR effect. We analyze both their amplitude and phase and compare the data to the results from a prototypical collinear ferrimagnetic Y3Fe5O12/Pt bilayer. The observed magnetic field dependence is explained in a comprehensive model, based on two magnetic sublattices and taking into account magnetic field-induced modifications of the domain structure. Our results show that the SMR allows us to understand the spin configuration and to investigate magnetoelastic effects in antiferromagnetic multi-domain materials. Furthermore, in α-Fe2O3/Pt bilayers, we find an unexpectedly large SMR amplitude of 2.5 × 10 -3 , twice as high as for prototype Y3Fe5O12/Pt bilayers, making the system particularly interesting for room-temperature antiferromagnetic spintronic applications.