Session M50: Generation of Spins and Spin Currents

Efficient charge to spin conversion in 5d transition metal oxide.

With charge-based electronics reaching their limitations, more attention is being focused on developing systems utilizing spin currents. Recent focus has been on developing materials with a large spin Hall angle (SHA), the ratio of the charge to spin current density. Transition metal oxides such as iridium oxide IrO₂ are attractive candidates that have large conductivity and high spin-orbit coupling (SOC). IrO₂ possesses Dirac nodal lines in its band structure which is gapped by its strong SOC [1] which can generate large SHA. In this work, we have fabricated bilayers of IrO₂/Co₄₀Fe₄₀B₂₀­ samples to investigate the SHA in IrO₂ with different thicknesses and crystal structures. X-ray reflectivity, X-ray diffraction and atomic force microscopy have been used to verify the smoothness and crystallinity of IrO₂. We perform ST-FMR measurements on this system for different thicknesses of IrO₂ yielding SHA to be 40% in IrO₂ which is comparable to Pt. The resistivity of IrO₂ thin film is found to be 142 μΩ-cm, putting it in the metallic regime and close to that of CoFeB (120 μΩ-cm). Large SHA and high conductivity makes IrO₂ a robust candidate for further applications such as spin Hall nano-oscillators and magnetization switching.

[1] Sahoo et al. Adv. Quantum Technol. 4, 2000146 (2021)   

Modeling spin transport through insulator interfaces

Experiments have shown spin transport enhancement by a thin antiferromagnetic insulator between a metal and a ferromagnetic insulator [1]. While previous theoretical work [2] was able to reproduce some of the features of those experiments, the role of the interface is taken into account typically by a single parameter that is fit to experimental data.

We consider a model of a thin layer NiO oriented in (111) direction, sandwiched between a metal and a ferromagnetic insulator. We take into account nearest and next-nearest neighbor exchange coupling at the interfaces as well as different magnetic order. We compute the spin current through this system driven either by a thermal gradient or by a spin accumulation using Fermi's Golden Rule [3].

We find a sensitive dependence on the magnetic order as well as on the interface parameters.

[1] Wang et al., Phys. Rev. Lett. 113, 097202 (2014), Phys. Rev. B 91, 220410(R) (2015); Lin et al. Phys. Rev. Lett. 116, 186601 (2016).

[2] Khymyn et al., Phys. Rev. B 93, 224421 (2016); Rezende et al., Phys. Rev. B 93, 054412 (2016).

[3] Bender et al., Phys. Rev. Lett. 108, 246601 (2012).   

Nontrivial magnetic field dependence of the anomalous Hall conductivity due to spatially correlated inhomogeneities

We consider the problem of finding the effective anomalous Hall conductivity (AHC) of an inhomogeneous two-dimensional (2D) magnetic conductor governed by classical transport equations. Using homogenization theory, we proved that the effective AHC typically cannot exceed the bounds of local AHC. Conversely, relaxing the conditions needed for the proof allows one to potentially overcome the bounds. As an example, we explored alternative mechanisms than could lead to the hump or dip features in the magnetic-field dependence of the AHC, usually ascribed to the topological Hall effect. Through perturbation calculations and numerically solving the transport equation with random coefficients in a finite system, we found that such features could be caused by inhomogeneous saturated anomalous Hall resistivity correlated with magnetization domain profile, or by the anisotropic resistance associated with magnetic domain walls.   

Spin Current Generation Using Amorphous Materials

The efficient generation of spin currents is critical to numerous low-energy electronic devices, including the attojoule logic gate and spin orbit torque magnetoresistive random access memory (SOT-MRAM). Efforts to identify materials that act as good spin current sources have focused primarily on crystalline systems. In this talk, it will be shown that amorphous materials are potential candidates to generate spin currents. First, the talk will examine the anomalous Hall angle (AHA) in a series of ferromagnetic amorphous transition metal thin films M_xY_1-x (M=Fe, Co; Y=Si, Ge; x=0.40-0.71). It will be shown that the AHA (=σ_xy/σ_xx) is as large as 5%, which is substantial even for crystalline systems. In the amorphous materials it was found that the AHA increases with increasing Hall conductivity (σ​​​​​​​​​​​​​​_xy). This trend, which is opposite to that which occurs in crystalline systems, is attributed to low σ​​​​​​​_xx, while σ​​​​​​​_xy and M remain high. The talk will then report observation of a large spin-orbit torque in an amorphous non-magnetic Fe_xSi_1-x/cobalt bilayer via spin-torque ferromagnetic resonance and harmonic Hall measurements. The origins of this SOT will be discussed.    

Observation of unidirectional magnetoresistance in collinear-antiferromagnet/heavy-metal bilayers

The interplay between electronic transport and the antiferromagnetic order has attracted a surge of interest. Among numerous electrical probes to read out magnetic order, unidirectional magnetoresistance (UMR), where the resistance changes under the reversal of the current direction, can provide rich insights into the transport properties of spin-orbit coupled systems. However, UMR has never been observed in antiferromagnets before, given the absence of intrinsic spin-dependent scattering. Here, we report that a sizable UMR can emerge in an antiferromagnetic system, specifically in a collinear-antiferromagnet/heavy-metal bilayer. The observed UMR evolves nonlinearly with increasing magnetic field, persists to large values at high magnetic field and, most notably, exhibits a sign change at a critical field, unlike the UMRs observed in ferromagnetic and non-magnetic systems. We attribute a UMR in the antiferromagnetic phase of a FeRh/Pt bilayer to the strong effective magnetic field due to spin canting in FeRh. Our results can motivate the growing fields of non-centrosymmetric and topological systems, and suggest a route to the development of tunable antiferromagnet-based spintronics devices.   

Theory of unidirectional magnetoresistance in collinear-antiferromagnet/heavy-metal bilayers

Recently, there has been increasing interest in the study of nonlinear magnetotransport phenomena in systems with broken inversion and time-reversal symmetries. A notable example of such effects is the unidirectional magnetoresistance (UMR), in which spin-orbit coupling (SOC) and spin-asymmetry in electron scattering play key roles. A UMR effect was recently detected in an antiferromagnetic FeRh/Pt bilayer, despite the lack of intrinsic spin-dependent scattering, when the Neel vector is parallel to the applied current but perpendicular to the external magnetic field. More interestingly, the UMR evolves nonlinearly with the increasing magnetic field and undergoes a sign change even when the direction of the field remains the same. Based on a tight-binding model that takes into account the interfacial SOC and the exchange coupling between electron spins and the sublattice magnetization, we theoretically examine the physical origin of the unexpected UMR effect, and unveil the connection between the UMR and antiferromagnetic ordering. We also argue that the characteristic dependence of the UMR on the magnetic field strength, with a sign change in particular, is unique to antiferromagnetic systems, hence may be regarded as a magnetotransport signature of antiferromagnetic ordering.   

First principles transport calculations of spin transport at Cu/Cr and Cu/V interfaces

Efficient transmission of spin currents is crucial to the operation of many spintronic devices. While the transmission of spin currents in metals depends on the spin diffusion length, the behavior of spin currents near interfaces has escaped description using a single parameter. A recent experimental study shows that heterostructures containing Cu(001)/Cr(001) and Cu(001)/V(001) layers strongly reflect spin currents regardless of the magnetic ground states of Cr (antiferromagnet) and V (paramagnet). To elucidate the physical origin of the peculiar behavior, we model the reflection and transmission of spin currents incident to both interfaces using first-principles calculations. Specifically, our calculations utilize density functional theory (DFT) as implemented in Quantum Espresso followed by Wannier interpolation using Wannier90 to express each system in maximally-localized Wannier functions (MLWF) basis. We then compute the scattering matrices using Green's function approaches to quantify the reflection and transmission of spin currents given an incoming spin current. Our results expand the functionality of magnetic heterostructures as modulators of spin currents, potentially providing a useful tool in the development of spintronic devices.   

Theoretical Study on Spin Swapping Effect of Band-Structure Origin

We theoretically demonstrate the spin swapping effect of band-structure origin, which is mediated by orbital degree of freedom. The proposed mechanism does not require inversion asymmetry or impurity spin-orbit scattering, so is relevant for centrosymmetric ferromagnets. Analytic and tight-binding models reveal that its main contributions originate from k points where bands with different spins and different orbitals are close and thus it has no counterpart in normal metals. The spin swapping conductivities calculated from first-principles calculations for Fe, Co, and Ni are comparable in magnitude to the intrinsic spin Hall conductivity of Pt. Our theory generalizes transverse spin currents generated by ferromagnets and implies the importance of orbital degree of freedom in spin-orbit-coupled transport. In the presentation, we will show detailed analytical and computational results about spin swapping effect of band-structure origin.