Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session G57: Spin and Orbital CurrentsFocus
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Sponsoring Units: GMAG Chair: Giovanni Vignale, University of Missouri Room: Room 303 |
Tuesday, March 7, 2023 11:30AM - 12:06PM |
G57.00001: Spin Seebeck effect in the uniaxial antiferromagnet and magnetoelectric Cr2O3 Invited Speaker: Changjiang Liu Recent years have seen a rapid development of spintronics in the area of spin caloritronics. In particular, the spin Seebeck effect (SSE) has been observed in a diverse class of magnetic materials. In this talk, I will focus on our recent work on the investigation of SSE in the antiferromagntic Cr2O3. We found that the SSE is sensitive to the orientation of the sublattice magnetization of Cr2O3. This was identified by a unique angular dependence of the SSE signal while the sample is rotated relative an external magnetic field. By taking the measurement in the temperature range where the thermal excitation energy is less than the magnetic anisotropy energy, we observed a suppression of the SSE signal. This phenomenon indicates that the SSE signal is mainly contributed by magnons in the bulk Cr2O3. We found that the spin-flop transition in our thin-film Cr2O3 can be clearly measured as an abrupt jump and non-hysteretic SSE signal in the angular dependence measurement. Because a magnetoelectric coupling is allowed in Cr2O3, we further found that the polarization of the magnon spin current in Cr2O3 can be controlled by varying only an electric field while maintaining a constant magnetic field. Our results demonstrate that the SSE can be applied to a large class of antiferromagnets, many of which have multiferroic properties allowing for more efficient electric control of magnon spin currents. |
Tuesday, March 7, 2023 12:06PM - 12:18PM |
G57.00002: Competing local and nonlocal spin Hall magnetoresistance signals in MgO|Ni0.8Zn0.2F2O4|NiO|Pt heterostructure Aditya A Wagh, Priyanka Garg, Krishna Jha, Saswata Roy, Shwetha G Bhat, Suja Elizabeth, P S Anil Kumar Recently, the insertion of passive antiferromagnetic (AFM) layers was found to play a crucial role in magnon spin valves [1]. Electrically and/or thermally injected magnon spin currents and their transport across insulating AFM and ferrimagnetic (FM) layers have been under scrutiny in the past few years. Interestingly, in heterostructure devices consisting of FM|AFM|heavy metal (HM) stack, local spin Hall magnetoresistance (SMR) and non-local magnon-mediated SMR signals compete [2]. |
Tuesday, March 7, 2023 12:18PM - 12:30PM |
G57.00003: Observation of Rashba-related and switchable anomalous Hall effect in an antiferromagnetic metal Haijing Zhang, Seojin Kim, Jihang Zhu, Mario Piva, Marcus Schmidt, Dorsa Fartab, Andrew Mackenzie, Michael Baenitz, michael nicklas, helge rosner, Ashley Cook The anomalous Hall effect (AHE) lies at the heart of a major research field of contemporary condensed matter physics, in which geometrical features of wavefunctions in momentum space control physical observables. Here, we report the observation of an AHE in AgCrSe2, a layered triangular lattice metal that lacks inversion symmetry, and has a sizeable antiferromagnetic coupling between Cr spin 3/2 moments in adjacent layers. In particular, we find that AgCrSe2 exhibits an anomalous Hall resistivity of up to 3 µΩ cm at 2 K, comparable to the largest magnitude observed in any antiferromagnetic system to date. We further demonstrate that the anomalous Hall response in thin layers can be switched on and off by an applied ionic gate, or by controlling the direction of an applied magnetic field. Preliminary theory model suggests the observed AHE is driven by Berry curvature that correlates closely with the Rashba spin-orbit coupling. |
Tuesday, March 7, 2023 12:30PM - 12:42PM |
G57.00004: Edelstein effect and its reciprocity at chiral metal interface Yuta Suzuki, Yusuke Kato Current-induced magnetization and its inverse effect have recently been found in metals with chiral crystal structures [1-4]. A reciprocal relation between the two effects has also been reported [2-4], but has not yet been clarified in accordance with the microscopic theory. Indeed, the reciprocity for the Rashba–Edelstein effect, similar to that for the chiral metals, has been formulated in the case of isolated surfaces [5]. It remains unclear, however, if we can extend such formulation to the case of interfaces with other metals, along with the experimental situations. |
Tuesday, March 7, 2023 12:42PM - 12:54PM |
G57.00005: Magneto-Optical Detection of the Orbital Hall Effect in Chromium Igor Lyalin, Sanaz Alikhah, Marco Berritta, Peter M Oppeneer, Roland K Kawakami The orbital Hall effect has been theoretically predicted but its direct observation is a challenge. Here, we report magneto-optical detection of current-induced orbital accumulation at the surface of a light 3d transition metal Cr. The orbital polarization is in-plane, transverse to the current direction, and scales linearly with current density, consistent with the orbital Hall effect. Comparing the experimental results with ab initio calculations, we determine the orbital Hall angle in a single layer Cr film. From thickness-dependent measurements, we estimate the characteristic length of the orbital transport. |
Tuesday, March 7, 2023 12:54PM - 1:06PM |
G57.00006: Interface-generated orbital currents Timothy Mabe, In Jun Park, Vivek P Amin Spin and orbital currents can be used in spintronic devices to electrically control the magnetic order of ferromagnets and antiferromagnets, providing an attractive write mechanism for magnetic memories. While electrical spin current generation, such as the spin Hall effect, has been extensively studied, orbital current generation is only beginning to be understood. Though a flurry of research is currently underway investigating the orbital Hall effect and its corresponding spin-orbit torques, little effort has been devoted to understanding orbital current generation at interfaces. Theoretical and experimental evidence already suggests that interfaces generate appreciable out-of-plane spin currents under an applied, in-plane electric field. To help fill in the gap in the study of orbital current generation, we present first principles transport calculations of a corresponding effect called of ‘interface-generated orbital currents.’ We calculate the strength and magnetization dependence of interface-generated orbital currents, which, unlike interface-generated spin currents, do not require spin-orbit coupling. Exploiting new sources of angular momentum transfer within magnetic heterostructures—such as interface-generated orbital currents—will help pave the way for more energy efficient magnetic memories. |
Tuesday, March 7, 2023 1:06PM - 1:18PM |
G57.00007: Orbital Hall currents across interfaces from first-principles scattering calculations Max Rang, Paul Kelly The magnetization of a ferromagnetic (FM) -nonmagnetic metal (NM) bilayer can be controlled electrically using the relativistic spin Hall effect to generate a current of spin angular momentum. To study such a system quantitatively, spin-dependent interface parameters like the spin-mixing conductance and spin memory loss need to be known. The orbital Hall effect introduces a current of angular momentum whose origin is nonrelativistic so elements other than the heavy metals need to be considered. Virtually nothing is known about the parameters describing orbital currents. Two systems are of interest: NM|heavy metal (HM) and NM|FM bilayers. In both cases, a current of orbital angular momentum is generated in NM. This is then injected into the HM or FM where spin-orbit coupling converts it into a spin current which then exerts a torque on the local ferromagnetic moment. Computing the orbital Hall conductivity for bulk systems is relatively straightforward but studying NM|HM and NM|FM interfaces requires large supercells making them computationally very demanding. A first-principles scattering formalism that has been used to determine the parameters governing spin-dependent transport has been extended to include orbital currents allowing us to study them in detail.
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Tuesday, March 7, 2023 1:18PM - 1:30PM |
G57.00008: Efficient orbital current generation in nitrided Cu Tian-Yue Chen, Yu-Chan Hsiao, Wei-Bang Liao, Chi-Feng Pai Spin-orbit torque memory is realized as a promising technique for next generation memory and neuromorphic applications. In principle, the sizable SOT is believed to originate from strong spin-orbit coupling (SOC) in heavy metals, topological insulators, and Weyl semimetals. However, recent reports suggest that the light metals can also generate non-negligible SOTs and these SOC-free torques are attributed to the orbital angular momentum induced orbital currents. In this work, we investigate the orbital currents generation in nitrided Cu. We prepare the Pt/Co/CuNx perpendicular magnetic heterostructures and introduce nitrogen into Cu capping layer via reactive sputtering. The hysteresis loop shift measurement confirms that the charge-to-spin conversion efficiency can be enhanced from 2% to ~ 40% through controlling the doping concentration. This significant enhancement is attributed to the orbital currents generated by the nitrided Cu capping layer. Furthermore, we demonstrate the neuromorphic switching measurement, and the multistate behavior shows that the orbital currents can effectively manipulate the perpendicular magnetization. These results provide valuable information for building energy-efficient spin-orbitronic neuromorphic devices. |
Tuesday, March 7, 2023 1:30PM - 1:42PM |
G57.00009: Comprehensive Demonstration of Spin Hall Hanle Effects in Epitaxial Pt Films Andrew H Comstock, Jing Li, Dali Sun, Xiaoshan Xu We demonstrate a nonlinear Hall effect due to the boundary spin accumulation in Pt films grown |
Tuesday, March 7, 2023 1:42PM - 1:54PM |
G57.00010: Long-range exchange spin transport in ferromagnetic nanowires driven by spin Hall current Emma Frantz, Ilya N Krivorotov, Eric A Montoya, Amanatullah Khan Magnetic materials with easy plane magnetic anisotropy are predicted to support long-range spin currents via an exchange-dominated mode described by large-amplitude order parameter winding around the hard anisotropy axis. This mode is often referred to as spin superfluid due to similarity of the order parameter of this classical state to the order parameter of a quantum superfluid. In contrast to spin waves showing exponential spatial decay, this topologically protected exchange mode decays as a power law and thus is expected to support long-range spin currents. Here we use micromagnetic simulations to demonstrate the excitation of this exchange mode in a ferromagnetic nanowire by local injection of spin Hall current into the nanowire. The required easy plane anisotropy is engineered via tuning the values of perpendicular magnetic anisotropy and magnetic shape anisotropy in the nanowire geometry. We explore the dependence of the magnetic ground and excited states of the nanowire on the device and material parameters, and identify the parameter space supporting the long-range spin current. We conclude that this state is practically achievable in nanowires of common ferromagnetic materials. |
Tuesday, March 7, 2023 1:54PM - 2:06PM |
G57.00011: Transverse spin transport in a disordered Pt film Kirill D Belashchenko, Giovanni G Baez Flores, Alexey A Kovalev Spin current and spin accumulation in a free-standing disordered Pt film carrying in-plane charge current are studied using the first-principles nonequilibrium Green's function approach. Disorder is treated within the Anderson model in a supercell embedded between the leads. The spin-Hall conductivity extracted from the data is insensitive to disorder strength, and its magnitude and energy dependence agree with the known intrinsic contribution. The effective spin-diffusion length extracted from the spin accumulation profile is about 1.3 nm (which is similar to or even shorter than the mean-free path), insensitive to disorder strength, and a few times shorter than the bulk spin-diffusion length for longitudinal transport extracted from separate Landauer-Buttiker calculations. These features suggest that spin relaxation near the Pt surface is dominated by the Dyakonov-Perel mechanism. With the Fermi level shifted 0.2 Ry lower, into the region of negative spin-Hall conductivity, the effective spin-diffusion length scales with the conductivity as expected for the Elliot-Yafet mechanism. The interpretation of the data within the spin-diffusion model at both energies requires the inclusion of a spin-relaxing boundary condition with a large spin-loss conductance that increases with disorder strength. These features show that the spin-diffusion model is pushed to its applicability limits in heavy metals with strong spin-orbit coupling such as Pt. We also find that orbital accumulation only penetrates into the bulk of Pt in the presence of spin-orbit coupling and is otherwise a purely surface effect. |
Tuesday, March 7, 2023 2:06PM - 2:18PM |
G57.00012: Absorption of Transverse Spin Current in Ferromagnetic NiCu: Dominance of Bulk Dephasing over Spin-Flip Scattering Youngmin Lim, Shuang Wu, David A Smith, Christoph Klewe, Padraic Shafer, Satoru Emori In ferromagnetic metals, transverse spin currents are thought to be absorbed via dephasing – i.e., destructive interference of spins precessing about the strong exchange field. Yet, due to the ultrashort coherence length of ≈1 nm in typical ferromagnetic thin films, it is difficult to distinguish dephasing in the bulk from spin-flip scattering at the interface. Here, to assess which mechanism dominates, we examine transverse spin-current absorption in ferromagnetic NiCu alloy films with reduced exchange fields. We observe that the coherence length increases with decreasing Curie temperature, as weaker dephasing in the film bulk slows down spin absorption. Moreover, nonmagnetic Cu impurities do not diminish the efficiency of spin-transfer torque from the absorbed spin current. Our findings affirm that transverse spin current is predominantly absorbed by dephasing inside the nanometer-thick ferromagnetic metals, even with high impurity contents. |
Tuesday, March 7, 2023 2:18PM - 2:30PM |
G57.00013: Non-local voltage measurements with wide F/I/N tunnel contacts are strongly affected by generation of circular electric currents Yaroslaw B Bazaliy, Revaz Ramazashvili Electric spin injection is routinely tested by non-local voltage measurements in lateral spin-valves. The advantage of this method is the absence of electric current entering the part of the device where the detecting F/N contact is located. Ideally this leads to the independence of the measured Johnson-Silsbee voltage from the contact size and shape. However, it was predicted [1] that if the detecting contact is wide enough, then circular electric currents are excited in it, altering the non-local voltage and making it device geometry dependent. Calculations [1] were performed in the regime of highly transparent F/N boundary. At the same time, modern measurements often use low transparency F/I/N tunnel contacts, in which circular currents should be strongly suppressed. Thus it seems that tunnel contact measurements should conform to the classic Johnson-Silsbee formula. Here we show that while circular electric currents are indeed suppressed by the tunnel barrier, the voltage modifications persist, may be significant, and have to be accounted for in the data analysis. |
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