Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S22: Spin Nernst and Spin Seebeck EffectsFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Gyung-Min Choi, Sungkyunkwan Univ Room: LACC 402A |
Thursday, March 8, 2018 11:15AM - 11:27AM |
S22.00001: Ultrafast Measurements of the Longitudinal Spin Seebeck Effect in Au/TmIG and Au/YIG Michael Gomez, Yawen Liu, Jing Shi, Richard Wilson Spin transport between a magnetic insulator and a normal metal is an area of active research within the spin physics community to further the realization of spintronic devices. We investigate picosecond spin-currents across Au/TmIG and Au/YIG interfaces in response to ultrafast laser heating of the Au metal. In the picoseconds after heating, large interfacial spin currents occur due to a temperature imbalance between electrons and phonons in the metal, and magnons and phonons in the magnetic insulator. We utilize four different optical probes to develop a complete picture of the heat and spin transport in Au/TmIG and Au/YIG. Magneto-optic Kerr effect measurements of Au at a wavelength of 800 nm detects the spin accumulation in the normal metal that results from interfacial spin-currents. Magneto-optic Kerr effect measurements of the YIG/ TmIG at 400 nm monitor the ultrafast decrease in the magnetic moment of TmIG/YIG due to heating from Au electrons. Finally, thermoreflectance measurements at 450 and 950 nm monitor the temperature evolution of the Au electrons and phonons, respectively. Together, these measurements allow us to estimate the magnitude of the transport coefficients responsible for the longitudinal spin-Seebeck effect in these systems. |
Thursday, March 8, 2018 11:27AM - 11:39AM |
S22.00002: Phonon anomaly in the nonlocal spin Seebeck effect Koichi Oyanagi, Ludo Cornelissen, Takashi Kikkawa, Zhiyong Qiu, Timo Kuschel, Gerrit Bauer, Bart Van Wees, Eiji Saitoh The nonlocal spin Seebeck effect is an effective method to clarify the transport properties of thermally excited magnonic spin-current in Y3Fe5O12 (YIG) [1]. Generation of magnonic spin-currents is affected by phonons due to the magneto-elastic coupling in YIG [2]. However, the effect of the magneto-elastic coupling on the magnon transport is yet to be elucidated. In this study, we clarified the role of phonons in the spin-current transport by measuring the magnetic field H dependence of nlSSE. |
Thursday, March 8, 2018 11:39AM - 11:51AM |
S22.00003: Spin Hall magnetoresistance and spin Nernst magnetothermopower in a Rashba system: role of the inverse spin galvanic effect Sebastian Tölle, Michael Dzierzawa, Ulrich Eckern, Cosimo Gorini In ferromagnet/normal-metal bilayers, the sensitivity of the spin Hall magnetoresistance and the spin Nernst magnetothermopower to the boundary conditions at the interface is of central importance. In general, such boundary conditions can be substantially affected by current-induced spin polarizations. In order to quantify the role of the latter, we consider a Rashba two-dimensional electron gas with a ferromagnet attached to one side of the system. The geometry of such a system maximizes the effect of current-induced spin polarization on the boundary conditions, and the spin Hall magnetoresistance is shown to acquire a non-trivial and asymmetric dependence on the magnetization direction of the ferromagnet. The spin Nernst magnetothermopower turns out to be very small due to a cancellation of electrical and thermal contributions, and it vanishes completely in the limit of infinite spin mixing conductance if Elliott-Yafet spin relaxation is neglected. Our findings deviate substantially from the results of previous theoretical considerations based on phenomenological drift-diffusion equations.1 |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S22.00004: Signature of magnon Nernst effect in an antiferromagnetic insulator Yuki Shiomi, Rina Takashima, Eiji Saitoh A magnon Nernst effect, an antiferromagnetic analog of the magnon Hall effect in ferromagnetic insulators, has been studied experimentally for the layered antiferromagnetic insulator MnPS3 in contact with two Pt strips [Y. Shiomi, R. Takashima, and E. Saitoh, Phys. Rev. B 96, 134425 (2017)]. Thermoelectric voltage in the Pt strips grown on MnPS3 single crystals exhibits nonmonotonic temperature dependence at low temperatures, which is unlikely to be explained by electronic origins in Pt but can be ascribed to the inverse spin Hall voltage induced by a magnon Nernst effect. Control of antiferromagnetic domains in the MnPS3 crystal by magnetoelectric cooling is found to modulate the low-temperature thermoelectric voltage in Pt, which is evidence consistent with the emergence of the magnon Nernst effect in Pt-MnPS3 hybrid structures. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S22.00005: Enhancement of the anomalous Nernst effect in ferromagnetic thin films Tsao-Chi Chuang, P. L. Su, P. H. Wuj6, Ssu Yen Huang The anomalous Nernst effect (ANE), generating spin thermoelectric signal through spin-orbit coupling, is an important mechanism to explore the interaction between charge, heat, and spin. Recently, intense researches show that a large ANE signal can be observed in the noncollinear antiferromagnet, thermopile, and multilayer structure. It is commonly thought that the ANE is proportional to magnetization. However, in this work, we find that the magnitude and sign of the ANE exhibit nontrivial thickness dependent behaviors, even in conventional ferromagnets. [1] We study the thickness dependent ANE in Fe, Co, Ni, and Py with in-plane anisotropy and vertical temperature gradient. The sign of the ANE of Fe is opposite to that of Co, Ni, and Py in thicker films, and it can be reversed via decreasing thickness. Most importantly, the anomalous Nernst angles for these FMs can be significantly enhanced by up to one order of magnitude in ultrathin films. By systematically studying the thickness dependence of the electrical and thermal transport properties, we show that the enhanced ANE of FMs is dominated by spin-orbit coupling through the intrinsic and side-jump mechanisms in thin-film. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S22.00006: Nernst thermopower in bismuth - ferromagnetic nanocomposites Koen Vandaele, Bin He, Stephen Boona, Joseph P Heremans Magnetic materials in which thermal transport involves the generation of spin fluxes provide new opportunities to improve the thermal-to-electric energy conversion efficiency over that of conventional, electron-based thermoelectrics. In bulk ferromagnetic (FM) metals e.g. Co, Fe, and Ni, magnon dynamics result in a magnonic contribution to total thermopower, as shown by Watzman et al.1 In thin-film structures composed of a normal metal (NM) film, e.g. Pt, deposited on an electrically insulating FM, e.g. yttrium iron garnet (YIG), a thermally driven spin flux injected from the FM into the NM layer gives rise to inverse spin-Hall voltage in the NM layer. Although these thin-film structures are not suitable for power generation, it was shown by Boona et al.2 that it is possible to obtain a contribution from the spin-Seebeck effect (SSE) to the Nernst thermopower in bulk NM/FM nanocomposites. In this work, Bi will be used as a NM and Co as the FM. Bulk composites of Bi and Co nanoparticles will be studied to determine the SSE contribution to the Nernst thermopower. |
Thursday, March 8, 2018 12:27PM - 1:03PM |
S22.00007: Transverse spin Nernst magnetoresistance induced by thermal spin current in ferromagnet/non-magnet bilayers Invited Speaker: Byong-Guk Park We report thermal generation of pure spin current in non-magnetic materials, which is known as spin Nernst effect by investigating the thermally-induced transverse magnetoresistance in ferromagnet (FM)/non-magnetic heavy metal (HM) bilayers. We observe that the magnitude of the transverse magnetoresistance in FM/HM bilayers is significantly modified by HM and its thickness. This strong dependence of transverse magnetoresistance on HM evidences the spin Nernst effect in HM; the generation of thermally-induced spin current in HM and its subsequent reflection at the FM/HM interface. Our analysis shows that the spin Nernst angles of W and Pt have the opposite sign to their spin Hall angles. Moreover, our estimate implies that the magnitude of the spin Nernst angle would be comparable to that of the spin Hall angle, suggesting an efficient generation of spin current by the spin Nernst effect. |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S22.00008: Spin Nernst Effect in the Paramagnetic Regime of An Antiferromagnetic Insulator Yinhan Zhang, Satoshi Okamoto, Di Xiao We theoretically investigate a pure spin Hall current driven by a longitudinal temperature gradient, i.e. the spin Nernst effect (SNE), in a paramagnetic state of a collinear antiferromagnetic insulator with the Dzyaloshinskii-Moriya interaction. The SNE in magnetic ordered state in such an insulator was proposed by Cheng et al. [PRL 117, 217202 (2016)]. Here we show that the Dzyaloshinskii-Moriya interaction can generate a pure spin Hall current even without magnetic ordering. By using a Schwinger boson mean-field theory, we calculate the temperature dependent SNE in a disordered phase. We also discuss the implication of our results to experimental realizations. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S22.00009: Data-Intensive Spatial Mapping of the Longitudinal Spin Seebeck Effect in Normal Metal/Magnetic Insulator Devices Maxwell Grossnickle, Mohammed Aldosary, Junxue Li, Mark Lohmann, Jing Shi, Nathaniel Gabor The rapidly emerging field of spincaloritronics demands new approaches that access the subtle relationship between spins and heat. As a key benchmark, we demonstrate a data-intensive method for spatially mapping the longitudinal spin Seebeck effect (LSSE) in a ferrimagnetic insulator film (YIG) with a thin metal layer (platinum). By spatially scanning a femtosecond pulsed laser (with a large field of view of up to 2mm) over the Pt/YIG device, we use the mobile thermal gradient of the laser pulse to spatially resolve the photovoltage generated by the inverse spin Hall effect. Rotating the applied magnetic field direction over 360 degrees in both in- and out-of-plane directions allows us to remove contributions of the Seebeck effect and measure only the LSSE signal. We find LSSE decay lengths on the order of 100 microns as well as deviations from expected sinusoidal behavior, even at magnetic fields far in excess (0.5 T) of saturation levels. The rich visualization afforded by our technique will have important implications for the next-generation of spincaloritronic devices utilizing magnetic insulators. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S22.00010: Temperature Dependence of Spin Diffusivity in Yttrium Iron Garnet from the Time Evolution of the Intrinsic Spin-Seebeck Effect John Jamison, Zihao Yang, Jack Brangham, Fengyuan Yang, Roberto Myers Recent experimental and theoretical works describe the production of spin currents within the bulk of yttrium iron garnet (YIG) concomitant with heat flow, known as the intrinsic spin Seebeck effect (SSE). We present pulsed laser measurements of the time-resolved longitudinal spin Seebeck effect (SSE) in Pt/YIG below room temperature, revealing the time-dependent diffusion of magnons produced within the bulk of YIG. Unlike the interface contribution to SSE, which evolves on sub-nanosecond timescales, the bulk magnon diffusion process occurs on time scales spanning hundreds of microseconds. The data are fit to a time-dependent model for coupled spin and heat diffusion in Pt/YIG, from which we extract the spin diffusivity of YIG. At low temperatures the spin diffusivity is orders of magnitude smaller than the thermal diffusivity. The low diffusivities and long diffusion lengths of magnons in YIG imply long magnon lifetimes, indicative of magnons with low wave vector. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S22.00011: Spin Seebeck effect and thermal spin-orbit torque in Ni80Fe20/p-Si bilayers Ravindra Bhardwaj, Paul Lou, Sandeep Kumar The thermal spin current from spin-Seebeck effect has been reported to be more energy efficient than the electrical spin injection methods. But, spin detection has been the one of the bottlenecks since metals with large spin-orbit coupling is an essential requirement. In this work, we report an efficient thermal generation and interfacial detection of spin current. We measured a spin-Seebeck effect in Ni80Fe20 (25 nm)/p-Si (50 nm) (polycrystalline) bilayers without heavy metal spin detector. The p-Si, having the centosymmetric crystal structure, has insignificant intrinsic spin-orbit coupling leading to negligible spin-charge conversion. We report a giant inverse spin-Hall effect, essential for detection of spin-Seebeck effect, in the Ni80Fe20/p-Si bilayer structure, which originates from Rashba spin orbit coupling due to structure inversion asymmetry at the interface. In addition, the thermal spin pumping in p-Si leads to spin current from p-Si to Ni80Fe20 layer due to tunneling spin galvanic effect and spin-Hall effect causing spin-orbit torques. The thermal spin-orbit torques leads to collapse of magnetic hysteresis of 25 nm thick Ni80Fe20 layer for a temperature gradient of 20.84 mK across the bilayer specimen. |
Thursday, March 8, 2018 1:51PM - 2:03PM |
S22.00012: Probing surface magnetic anisotropy via Longitudinal Spin Seebeck Effect on spinels Elias Ferreiro-Vila, Noa Varela, Francisco Rivadulla Spintronics is an emergent field due to the promising applications of spin-dependent charge currents[1]. However, there are some limitations regarding the spin-mixing conductance at the interface of the non-magnetic metal (NM)/ferromagnetic meal (FM). Therefore, it is paramount the study of the role of the magnetic properties at this NM/FM interface. We have studied the Longitudinal Spin Seebeck Effect (LSSE) in Pt (NM)/spinel (FM) interfaces. We have grown two different spinels: CoFe2O4 (CFO) and MnFe2O4 (MFO) by Polymer Assisted Deposition (PAD). Previously studies have confirmed the existence of an intrinsic surface layer with reduced magnetic anisotropy (soft layer), whose relative thickness can be tuned within the growth[2], on CFO films. In this paper, we probe the existence of the mentioned soft layer contribution in the total magnetic anisotropy by controlling the thermal gradient across the CFO film whereas there is no magnetic anisotropy variation across the MFO film. |
Thursday, March 8, 2018 2:03PM - 2:15PM |
S22.00013: Hybridized magnon-phonon excitation in spin Seebeck effect in Bi3-xYxFe5O12 films Takashi Kikkawa, Koichi Oyanagi, Tomosato Hioki, Masahiko Ishida, Zhiyong Qiu, Rafael Ramos, Yusuke Hashimoto, Eiji Saitoh The spin Seebeck effect (SSE) refers to the generation of a spin current in a magnetic material by a temperature gradient. The spin current is detected as an electric voltage via the inverse spin Hall effect in a metal attached to the magnet. Recently we reported that anomalous asymmetric structures appear in the magnetic field dependence of the SSE in Pt/Y3Fe5O12 (YIG) systems when the magnon and phonon dispersion curves of YIG touch, which maximizes the phase space of hybridized magnon-phonon excitation (or magnon polarons) [1-3]. |
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