Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J41: Spin, Charge, Thermal Interactions, and Conversion |
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Sponsoring Units: GMAG Room: 707 |
Tuesday, March 3, 2020 2:30PM - 2:42PM |
J41.00001: Spin pumping under magnetic field gradient Hiroki Arisawa, Shunsuke Daimon, Yasuyuki Oikawa, Takashi Kikkawa, Eiji Saitoh The spin pumping effect is the generation of a spin current by spin waves in a ferromagnet. Its properties have enthusiastically been investigated mainly in uniform magnetic fields, but they are yet to be explored well in nonuniform magnetic fields. Meanwhile, a recent study reported an interesting behavior of spin waves that the wave number of spin waves is converted to another value during its propagation under inhomogeneous magnetic fields. This fact indicates that spin current emission via the spin pumping effect can be spatially tuned taking advantage of the spin-wave wave number conversion in nonuniform magnetic fields. In this talk, we report spatially nonuniform spin current generation by the spin pumping effect under a magnetic field gradient in a Pt/Y3Fe5O12 bilayer system. We found that a spin current generated by spin waves is nonlocally enhanced depending on the magnetic field gradient. We explain the result on the basis of the wave number conversion of spin waves, considering its dispersion relation spatially varying with the magnetic field gradient. Our results offer a new insight of spin current generation utilizing nonuniform magnetic fields. |
Tuesday, March 3, 2020 2:42PM - 2:54PM |
J41.00002: Drag induced giant anomalous Nernst effect in Ferromagnetic MnBi Bin He, Cuneyt Sahin, Stephen R Boona, Brian Sales, Joseph P C Heremans, Michael Flatté, Claudia Felser Anomalous Nernst effect (ANE) is the thermal equivalent to the Anomalous Hall effect (AHE). It has been observed in various materials and attributed to the Berry curvature of topological materials. In this talk, we present the observation of giant anomalous Nernst effect in MnBi single crystals. In addition to intrinsic contribution from the band structure, magnon drag also contributes to the anomalous Nernst signal in MnBi crystals, resulting in a giant anomalous Nernst thermopower of 18 μV/K at 0.6 T. At the same time, we also observe a magnon drag thermopower and positive magnetothermal conductivity. This magnon drag induce anomalous Nernst effect can be viewed as a self-spin Seebeck effect, which provides alternative path of understanding the electron magnon interaction. |
Tuesday, March 3, 2020 2:54PM - 3:06PM |
J41.00003: Electrically-induced ferromagnetism in diamagnetic FeS2 Jeff Walter, Bryan Voigt, Ezra Day-Roberts, Kei Heltemes, Turan Birol, Rafael Fernandes, Chris Leighton Recent years have seen increasingly impressive demonstrations of all-electrical control of magnetism, including electrolyte-gating-induced ferromagnetism in non-ferromagnetic materials. These demonstrations, however, involve induction of ferromagnetism from some other finite-spin magnetic state, e.g., antiferromagnetic, paramagnetic, etc. In this work we use ionic liquid gating, which can induce electron densities >1014 cm-2, to achieve voltage-induced ferromagnetism in diamagnetic (i.e., zero-spin) FeS2 single crystals. Temperature-dependent transport measurements establish a remarkably reversible positive-bias-induced insulator-metal transition, accompanied by inversion of the FeS2 surface conduction channel to n-type. Anomalous Hall effect measurements then reveal an accompanying onset of voltage-induced soft 2D ferromagnetism, with Curie temperature up to ~20 K. These results are supported by DFT-based tight-binding modelling that indicates induction of Stoner FM by gate-controlled band filling. |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J41.00004: Spin to charge and charge to spin conversion in a Dirac semimetal Wilson Yanez, Run Xiao, Jacob T Held, Di Xiao, Jeffrey G Rable, Enrique González, Yongxi Ou, Anthony R. Richardella, K. Andre Mkhoyan, Nitin Samarth We report evidence of spin to charge and charge to spin conversion at room temperature in thin films of the archetypal three-dimensional Dirac semimetal Cd3As2 grown by molecular beam epitaxy. This has been detected by means of ferromagnetic driven spin pumping (SP) and spin torque ferromagnetic resonance (ST-FMR) in NiFe/Cd3As2 heterostructures. Analysis of the frequency and power dependence of the SP signal and of the symmetric and antisymmetric contributions to the mixing voltage in ST-FMR show that the behavior of these processes is consistent with previously reported spin to charge conversion mechanisms in heavy metals and topological insulators. Finally, we compare the efficiency of these phenomena with the one due to the inverse spin Hall effect in NiFe/Platinum bilayers. |
Tuesday, March 3, 2020 3:18PM - 3:30PM |
J41.00005: Spin Seebeck effect in Pt/SrMnO3 heterostructures Eswara phanindra Vallabhaneni, Arijit Das, Tamalika Banerjee Spin Seebeck effect (SSE) is one of the effective ways of generating pure spin currents driven by thermal gradients and has been extensively explored in spin caloritronics, an active branch in spintronics. In recent years, the focus has shifted to antiferromagnetic insulators (AFM-I), where spin currents created by a thermal gradient has been studied. Thermal gradients across the AFM-I film produce pure spin currents in an adjacent heavy metal layer such as platinum (Pt) due to non-equilibrium distribution of thermal magnons. These spin currents can be detected as a transverse charge current across the Pt layer via inverse spin Hall effect. Here, we have performed systematic investigations of SSE employing two measurement geometries, local and non-local configurations, on oxygen (non-)stoichiometric-SrMnO3 thin films, which exhibit weak-ferromagnetic, insulating behavior down to low temperatures. Controlled experiments as a function of the applied dc bias, magnitude and orientation of applied magnetic field were used to disentangle the role of possible magnetic proximity effects. We observe a quadratic dependence of the SSE signal in Pt/SrMnO3 with the applied current density. SSE is an important tool to detect and study magnon transport in hitherto unexplored complex oxides. |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J41.00006: Longitudinal Spin Seebeck effect in Pyrochlore Iridates with Bulk and Interfacial Dzyaloshinskii-Moriya interaction Bowen Ma, Benedetta Flebus, Gregory A Fiete The longitudinal spin-Seebeck effect (SSE) in magnetic insulator|non-magnetic metal heterostructures has been theoretically studied primarily with the assumption of an isotropic interfacial exchange coupling. Here, we present a general theory of the SSE in the case of an antisymmetric Dzyaloshinskii-Moriya interaction (DMI) at the interface, in addition to the usual Heisenberg form. We numerically evaluate the dependence of the spin current on the temperature and bulk DMI using a pyrochlore iridate as a model insulator with all-in all-out (AIAO) ground state configuration. We also compare the results of different crystalline surfaces arising from different crystalline orientations and conclude that the relative angles between the interfacial moments and DM vectors play a significant role in the spin transfer. Our work extends the theory of the SSE by including the anisotropic nature of the interfacial DMI in magnetic insulator|non-magnetic metal heterostructures and can suggest possible materials to optimize the interfacial spin transfer in spintronic devices. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J41.00007: Spin Seebeck effect in Vanadium Tetracyanoethylene Yuanhua Zheng, Seth Kurfman, Andrew Franson, Ezekiel Johnston-Halperin, Joseph P C Heremans Vanadium tetracyanoethylene (V(TCNE)x, x~2) is an organic based ferrimagnetic coordination compound, with extremely low Gilbert damping which is comparable to yttrium iron garnet. Due to its non-crystalline nature, it is interesting to investigate how spin waves propagate in the material. One possible way is through thermally driven spin pumping, namely spin Seebeck effect (SSE). Here, we present a systematic study on the temperature and thickness dependence of SSE in V(TCNE)x. The thin films (thickness on the order of 100 nm) are grown via chemical vapor deposition on a platinum layer on sapphire substrate. By applying a temperature gradient perpendicular to the V(TCNE)x/Pt, a large SSE signal is collected. Further analysis on the temperature and thickness dependence helps us understand the length scales of the propagation of spin waves. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J41.00008: Temperature Dependence of the Anomalous Nernst Coefficient for Ni80Fe20 Determined with Metallic Nonlocal Spin Valves Rachel Bennet, Alex Hojem, Barry L Zink The anomalous Nernst effect, which generates an out-of-plane charge voltage in response to a thermal gradient perpendicular to the magnetization of a ferromagnet, can play a significant role in any spintronic device where large thermal gradients exist.[1] Since they typically include features deep within the submicron regime, nonlocal spin valves can be made very sensitive to this effect by lowering the substrate thermal conductance.[2] Thus, material parameters are extremely important to the performance of these devices. Although studies have been conducted into the thickness dependence of the anomalous Hall coefficient for permalloy, little is known about the temperature dependence of the anomalous Nernst coefficient.[3,4] |
Tuesday, March 3, 2020 4:06PM - 4:18PM |
J41.00009: Magneto-thermoelectric effect in lanthanum orthoferrite with Pt overlayer Weiwei Lin, Jiaming He, Jianshi Zhou, Bowen Ma, Gregory A Fiete, Chia-Ling Chien Spin caloritronic phenomena, utilizing heat flow to transport spin signals with minimal or no charge current, are of fundamental interest with important applications. We have observed nontrivial magneto-thermoelectric effects in lanthanum orthoferrite (LaFeO3) perovskite single crystals with Pt overlayer at room temperature. The LaFeO3 is a canted antiferromagnetic insulator with weak magnetization of about 0.04 μB per Fe atom along the c-axis, which is only 1% of that of the common ferromagnet iron. The amplitude of magneto-thermovoltage in the Pt/LaFeO3 with an in-plane temperature gradient is comparable to those of the longitudinal spin Seebeck effect and inverse spin Hall effect in Pt/yttrium iron garnet. It provides a sensitive probe of very weak magnetization in the insulator, which can be manipulated by a magnetic field of the order of 10 mT. However, from the thermoelectric measurement configurations and the angular dependence on the applied magnetic field, our results are distinctly different from those of the anomalous Nernst effect and longitudinal spin Seebeck effect. We attribute our results to a thermal magnon transport at the Pt/LaFeO3 interface. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J41.00010: Effects of CoFeAl Alloy Composition on Magnetization Dynamics and Magnetic Thermal Transport Ramya Mohan, Yiwei Sun, Suveen Mathaudhu, Sinisa Coh, Richard Wilson The lifetime of magnetic excitations in metals is governed by scattering rates between magnons and electrons. Recent investigations [1, 2] document how electronic band structure engineering in magnetic alloys allows for suppression of magnon scattering rates; e.g. Co0.25Fe0.75 thin-films display low magnetic damping (α ~ 10-4). My talk will focus on experimental measurements that demonstrate how ultrafast magnetization dynamics and transport properties of CoFeAl magnetic alloys depend on alloy composition. To explore the magnetization dynamics, I use a time-resolved MOKE set-up to observe ultrafast demagnetization in a few hundred femtoseconds followed by precessional dynamics. Through TDTR measurements, I check if lower electron-magnon scattering rates lead to improved thermal transport. My investigation sheds light on how magnon-electron scattering rates govern dynamics on femtosecond (ultrafast demagnetization), nanosecond (magnetic precession and damping), and microsecond time-scales (thermal transport of energy). |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J41.00011: Investigation of thermal Hall conductivity in hexagonal LuFeO3 type multiferroics Hena Das, Sergey Nikolaev Materials those exhibit electric and magnetic ordering simultaneously in their ground states and show electric field control of magnetism, may also exhibit novel/enhanced thermal Hall transport phenomena[Nat. Mater. 16, 797 (2017)] that have immense utilitarian worth in various device applications. In this study, we have investigated topology of the magnonic bands and the magnon-mediated thermal transport of the LuFeO3 type novel multiferroics, in which the geometric origin of the ferroelectricity induced not only a net magnetization but also non-trivial magnetoelectric couplings [Nature Communications 5, 2998 (2014), Nature Materials 13, 163-167 (2014)]. We have employed the framework of linearized spin wave and linear response theories to study thermal conductivity as a function of the ferroelectric trimer lattice distortion. The calculated thermal Hall conductivity shows a non-trivial dependence on the ferroelectric distortion. We have also discussed the thermal transport in multiferroic LuFeO3 and LuMnO3 as case studies involving real materials |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J41.00012: Phase transitions and magnetocaloric effects in the Heusler compounds Ni2Mn0.70Cu0.30Ga and Ni2Mn0.70Cu0.25 Cr0.05Ga Sunday Agbo, Mahmud Khan The structural, magnetic and magnetocaloric properties of Ni2Mn0.70Cu0.25Cr0.05Ga and Ni2Mn0.70Cu0.30Ga Heusler alloys have been investigated by x-ray diffraction, magnetization, transport, and DSC measurements. For the Ni2Mn0.70Cu0.25Cr0.05Ga alloy, a first order coupled magnetostructural phase transformation near room temperature with a thermal hysteresis of <!--[endif]---->1 K was observed. In the vicinity of the transition, large magnetic entropy changes of ~39 J/kg K (while (warming) and ~17 J/kg K (while cooling) was observed for a field change of 50 kOe. For the Ni2Mn0.70Cu0.30Ga material, the first order phase transition with a thermal hysteresis of ~7 K was observed near 345 K. For a field change of 50 kOe, peak magnetic entropy changes of -17 J/kg K and -33 J/kg K were observed for warming and cooling, respectively. The electrical resistivity data showed interesting behavior near the phase transitions. The experimental results will be presented and discussed considering the coupling and de-coupling of the first order structural and second order magnetic phase transitions in the materials.<!--![endif]----> |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J41.00013: Theory of spin magnetic quadrupole moment and temperature-gradient-induced magnetization Atsuo Shitade, Akito Daido, Youichi Yanase We formulate what we call gravitomagnetoelectric (gravito-ME) effect, in which the magnetization is induced by a temperature gradient. Although the Kubo formula for the gravito-ME effect provides an unphysical divergence at zero temperature, we prove that the correct susceptibility is obtained by subtracting the spin magnetic quadrupole moment from the Kubo formula. It vanishes at zero temperature and is related to the ME susceptibility by the Mott relation. We explicitly calculate the gravito-ME susceptibility in a Rashba ferromagnet and show its experimental feasibility. |
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J41.00014: Oscillatory spin pumping in Nb/NiFe/Nb junctions Yunyan Yao, Ranran Cai, Yasumasa Tsutsumi, Yang Ma, Wenyu Xing, yuan ji, Xincheng Xie, See-Hun Yang, Sadamichi Maekawa, Wei Han The quantum size-induced oscilating physical phenomena has attracted a lot of interest in condensed matter physics and materials science. In this talk, I will discuss the oscillatory spin pumping in the Nb(100nm)/NiFe/Nb(100nm) trilayer structure. In the normal state of Nb, via systematical varying the thickness of NiFe from 5nm to 20nm, an oscillating Gilbert damping is observed. This oscillating behavior can be attributed to the quantum-interference effect of angular momentum transfer between the local precessing magnetic moment and conduction electrons in thin NiFe that was theoretically predicted by Mills [1]. As the NiFe layer is thin and coupled to nonmagnetic Nb layers, quantum-interference effect of the spin-polarized electrons shows up, which gives rise to the oscillating spin-transfer torque to the NiFe. This oscillating feature is observed to be largely enhanced below the superconducting critical temperatures in the Nb/NiFe/Nb Junctions. |
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