Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session A38: Spin and Heat TransportsFocus Live

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Sponsoring Units: GMAG DMP FIAP Chair: Pan He, Fudan University 
Monday, March 15, 2021 8:00AM  8:12AM Live 
A38.00001: Spinpolarised current manipulation of Bloch points Marijan Beg, Martin Lang, Hans Fangohr Recently, it was demonstrated that confined helimagnetic nanostructures, composed of grains of different chirality (and consequently, a different sign of DzyaloshinskiiMoriya energy constant), can host stable Bloch points [1]. In this work, using micromagnetic simulations, we explore if those magnetisation states can be manipulated using spinpolarised currents. We start by showing that a zerofield stable Bloch point emerges not only in thinfilm disk samples [1] but also in an extensive range of planar geometries, including nanostrips. By applying an inplane spinpolarised current, we demonstrate that Bloch points, in a planar nanostructure, can be moved in an arbitrary inplane direction. Finally, by applying a spinpolarised current outofplane, we were able to switch the type of Bloch point between headtohead and tailtotail states. Our demonstration of the ability to manipulate Bloch points in planar samples, apart from being of interest in fundamental physics, suggests the possible use of Bloch points in future spintronic devices. 
Monday, March 15, 2021 8:12AM  8:24AM Live 
A38.00002: Dynamic nuclear spin polarization by the Edelstein effect at Bi(111) surfaces Zijian Jiang, Victoria Soghomonian, Jean J. Heremans The appreciable electron density in the semimetal Bi and the strong spinorbit interaction of Bi(111) surfaces allow for the spin transfer between a current of electrons in Bi and its nuclei, in the absence of an external magnetic field. Application of a high current density generates a carrier spin polarization by mainly the Edelstein effect at the Bi(111) surface, which then induces dynamic nuclear polarization via the hyperfine interaction. Quantum transport antilocalization measurements in the Bi(111) thinfilms grown on mica indicate a suppression of antilocalization by the inplane Overhauser field from the nuclear polarization, and allow for the quantification of the Overhauser field. Various delay times between the polarization and measurement result in an exponential decay of the Overhauser field, driven by relaxation time T1. The addition of an external magnetic field does not detectably change the observations. Thus, dynamic nuclear polarization can be achieved without external fields, relying only on the effective electronic field created by carrier spin polarization. 
Monday, March 15, 2021 8:24AM  8:36AM Live 
A38.00003: Enhancement of nuclear spinspin interactions by collective electronic modes Stephen Carr, Eric Barrett, Charles Snider, Dmitri Feldman, Chandrasekhar Ramanathan, Brad Bradley Marston, Vesna Mitrovic Nuclear magnetic resonance (NMR) has been used to great success as an indirect probe of electronic behavior, as changes in the electronic phase can shift or broaden resonance signals. We propose how electronic phases can also introduce feedback loops to nuclear spin behavior. Under strong spinspin interactions, nuclear ensembles will exhibit nonlinear dynamics and give complicated responses to standard pulse protocols. Enhancement mechanisms of the doubleexchange interaction between electrons and nuclei due to the presence of manybody collective modes will be explored, along with implications for observing these nonclassical electronic states in NMR experiments. 
Monday, March 15, 2021 8:36AM  8:48AM Live 
A38.00004: Angular momentum and pseudo angular momentum in magnetic materials Simon Streib In condensed matter systems it is necessary to distinguish between two kinds of momenta: the momentum of the constituents of the material, such as nuclei and electrons, and the pseudomomentum of the emergent fields (or quasiparticles) that are present in the material, e.g., phonons and magnons. The same distinction is also valid for angular momentum and pseudo angular momentum. The latter is conserved when the system is invariant under rotations of the emergent fields. Based on Noether's theorem, I demonstrate that the recently discussed orbital angular momenta of phonons [1,2] and magnons [3,4] are pseudo angular momenta. The conceptual difference between angular momentum and pseudo angular momentum is important for a proper understanding of the transfer of angular momentum in condensed matter systems, for example in spintronics applications. 
Monday, March 15, 2021 8:48AM  9:00AM Live 
A38.00005: Effects of CoFe Alloy Composition on NonElectronic Thermal Transport Ramya Mohan, Victor H. Ortiz, JiCheng Zhao, Sinisa Coh, Richard Wilson CoFe magnetic alloys feature interesting magnetization dynamics that vary as a function of composition. This is, likely, due to their unique electronic band structures that allow for a suppression in electronmagnon scattering at certain CoFe compositions. Recent investigations have shown that Co_{0.25}Fe_{0.75} has low damping (α ~ 10^{4}), comparable to ferrimagnetic insulators. In this talk, I investigate the effect of composition on thermal transport. Using time–domain thermoreflectance (TDTR) analyses, I present a highresolution thermal conductivity map of a bulk CoFe diffusion multiple. This map allows us to correlate the alloy composition to thermal conductivity. I also report on thermal conductivity and electrical resistivities of sputtered thinfilms. To estimate the nonelectronic thermal conductivity, we use the WiedemannFranz law to subtract the electronic contribution. We observe a correlation between the composition, magnetic damping parameter α, and the nonelectronic thermal conductivity. My investigation provides insights into electronmagnon interactions and thermal transport in magnetic metals. 
Monday, March 15, 2021 9:00AM  9:12AM Live 
A38.00006: Anomalous heat and spin transport induced by inplane DM interaction in magnetic insulators Sungjoon Park, Naoto Nagaosa, BohmJung Yang We discuss anomalous heat and spin transport induced by inplane DzyaloshinskiiMoriya interaction (DMI) in twodimensional magnetic insulators with collinear outofplane magnetic order. In such magnets, the DMI does not appear in the linear spin wave theory, so that we usually do not expect the DMI to cause anomalous heat and spin transport. However, we find two mechanisms of anomalous transport that go against this expectation. First, the inplane DMI can induce interaction between magnon phonon. Consequently, magnon and phonon states can hybridize when their energies coincide, resulting in Berry curvature in the anticrossing regions between the magnon and phonon spectrum. As a result of the Berry curvature, we find nonzero thermal Hall and spin Nernst effect. Second, the inplane DMI can induce anomalous heat and spin transport if we go beyond the linear spin wave theory. In particular, we find that the Schwinger boson mean field Hamiltonian of a twodimensional ferromagnet is analogous to that of the twodimensional electron gas with Rashba spinorbit coupling. Therefore, we obtain a new platform to study bosonic analogues of the physics found in Rashba electron gas, such as anomalous Hall effect, spin Hall effect, and RashbaEdelstein effect. 
Monday, March 15, 2021 9:12AM  9:24AM Live 
A38.00007: Interplay of Magnetism and Phonons in Ultrafast Dynamics of Magnetic Topological Insulator MnBi_{2}Te_{4} Peter Kim, Maxwell Poore, Hari Padmanabhan, Vladimir A Stoica, Seng Huat Lee, Zhiqiang Mao, Venkatraman Gopalan, Richard Averitt We investigate the ultrafast dynamics of the antiferromagnetic topological insulator MnBi_{2}Te_{4} using a combination of highsensitivity transient reflectivity and timeresolved magnetooptical measurements. MnBi_{2}Te_{4} has recently generated substantial interest as a stoichiometric topological insulator with intrinsic magnetism, hosting novel symmetryprotected states that are topologically nontrivial. Furthermore, theory predicts that the magnetic ground state lies in proximity to several competing orders due to frustration from large nextnearest neighbor AFM exchange along with strong coupling between electronic, magnetic, and lattice degrees of freedom. This opens the door for the optical control and engineering of nonequilibrium states of matter. Here, we report clear signatures of spinphonon coupling in the ultrafast magnetization dynamics. We provide a detailed account of these dynamic features as a function of magnetic field applied along various crystalline axes, temperature, and pump conditions. 
Monday, March 15, 2021 9:24AM  9:36AM Live 
A38.00008: Influence of Thermal Noise on the Frequency Resolution of a SweepTuned Spectrum Analyzer Peter Elphick, Steven Louis, Vasyl Tyberkevych, Andrei Slavin Nanosized spintorque nanooscillators (STNO) can be very rapidly sweeptuned by variation of the bias direct current, and, therefore, can be used in ultrafast (tuning speed ~100 MHz/ns) sweeptuned spectrum analyzers [1,2]. At relatively high tuning speeds the resolution in frequency determination in such a device is determined by the bandwidth theorem. In contrast, at a relatively low tuning speed the generation linewidth of the STNO, which is determined by the thermal noise, is the limiting factor in the frequency determination accuracy. Our numerical calculations have shown that the external thermal noise not only leads to the broadening of the STNO generation linewidth, but also to the STNO central frequency “jitter”, which can significantly impact the accuracy of the frequency determination. 
Monday, March 15, 2021 9:36AM  9:48AM Live 
A38.00009: Quantitative temperature dependence of the Spin Seebeck effect in a mixed valent manganite Avirup De, Arup Ghosh, RAJESH MANDAL, Satishchandra Ogale, Sunil Nair Spin Seebeck effect offers a pure spin current under the application of a thermal gradient across a magnetic material and has become an active area of research in the field of spin caloritronics. Here, we report on the temperature dependence of the longitudinal spin Seebeck effect (LSSE) in the mixed valent manganite La_{0.7}Ca_{0.3}MnO_{3}. By disentangling the contribution arising due to the anomalous Nernst effect, we observe that in the lowtemperature regime, the LSSE exhibits a T^{0.5 }dependence, which matches well with that predicted by the magnondriven spin current model. Across the double exchange driven paramagneticferromagnetic transition, the LSSE exponent is significantly higher than the magnetization one and also depends on the thickness of the spintocharge conversion layer. These observations highlight the importance of individually ascertaining the temperature evolution of different mechanisms — especially the spin mixing conductance — which contribute to the measured spin Seebeck signal. 
Monday, March 15, 2021 9:48AM  10:00AM Live 
A38.00010: Coherent spinphonon oscillations and demagnetization dynamics in the van der Waals ferromagnet CrI_{3} Prashant Padmanabhan, Finn Lasse Buessen, Roxanne Tutchton, Kevin WC Kwock, Samuel Gilinsky, MinCheol Lee, Michael A McGuire, Srinivasa Singamaneni, Dmitry Yarotski, Arun Paramekanti, JianXin Zhu, Rohit P Prasankumar CrI_{3} is a rare example of a material supporting intrinsic magnetic order at the twodimensional limit. Here, we report on the nonequilibrium lattice and spin dynamics in CrI_{3} across the magnetic phase transition. Our experiments reveal multistep optically induced demagnetization dynamics that are driven by ElliottYafet spinflip scattering due to spinorbit coupling in the manifold of valence band states. In addition, we observe pronounced coherent oscillations in the timeresolved polarization rotation signal under offresonant excitation, which are commensurate with phonon modes of A_{1g} symmetry. Intriguingly, the amplitude of the caxis A_{1g} mode shows a strong pump helicity dependence below the Curie temperature. This correlation between magnetic order and vibrational amplitude suggests a strong coupling between the spin and lattice degrees of freedom. This, in turn, allows structural distortions to modulate magnetic exchange coupling, ultimately yielding spincoupled phonon modes as confirmed by our density functional theory and dynamical simulations of the coupled spinphonon system. Our work highlights the potential for coherent optical control of lowdimensional magnetism, with profound implications for future nanoscale magnetooptical devices. 
Monday, March 15, 2021 10:00AM  10:12AM Live 
A38.00011: Spindependent Seebeck and Nernst effects in a skyrmion ideal gas Andrei S. Zadorozhnyi, Yuri Dahnovsky We theoretically and numerically study spindependent Seebeck and Nernst effects in 2D ferromagnetic materials with the topological spin texture (skyrmion and vortex) ideal gas. From the numerical solution of the matrix Boltzmann equation for a nonequilibrium distribution function and the LippmannSchwinger equation for a Tmatrix we find the strong nonlinear behaviors in the thermoelectric coefficients depending on skyrmion/vortex diameters and electron concentrations. In particular, the dramatic dependences in the Seebeck and Nernst coefficients take place at larger magnetic texture sizes where the abrupt sign flip in the vortex Seebeck and Nernst coefficients occurs in the narrow region of electron concentrations. In this case the normalized Nernst coefficient changes from+5 to 7. The spindependent thermoelectric coefficients are proportional to T at low temperatures for all skyrmion/vortex sizes. 
Monday, March 15, 2021 10:12AM  10:24AM Live 
A38.00012: Thermal transport in yttrium iron garnet at very high magnetic fields Danilo Ratkovski, Luis Balicas, Alimami Bangura, Fernando Machado, Sergio M Rezende The ferrimagnetic insulator yttrium iron garnet (YIG) is one of the most important materials in the active fields of insulatorbased spintronics and spin caloritronics. Despite this, and the fact that this material has been studied for over six decades, the thermal properties of magnons in YIG have not been sufficiently characterized, mainly because at not very low temperatures, they are overwhelmed by the contribution of phonons. Here, we report measurements of the thermal conductivity in YIG under magnetic fields up to 31.4 T to increase the magnon energy gap, to suppress the magnon contribution and isolate that of the phonons relative to their behavior at zerofield. We observe that at a temperature of 20 K, even with a field as large as 31.4 T, the magnon contribution is not completely suppressed. The magnon thermal conductivity, measured by subtracting the value of the total thermal conductivity at 31.4 T from the value at zero field, has a peak at 16 K, with an amplitude that is over five times larger than the one obtained by measuring under a field of only 7 T, as previously reported. 
Monday, March 15, 2021 10:24AM  10:36AM Live 
A38.00013: Large zerofield Hall & Nernst effects and AMR in aligned MnBi / Bi composites Brandi Wooten, Koen Vandaele, Steve R. Boona, Joseph P C Heremans The Hall and Nernst effects involve the detection of a transverse voltage due to the effect of the Lorentz force on the flow of electrons under an applied electric field or temperature gradient. In ferromagnets, the application of a magnetic field affects the transverse resistivity the anisotropic magnetoresistance (AMR). Typically, these require an external applied magnetic field to produce a detectable transverse signal. However, here we show that bulk composites of magnetically aligned, unidirectional MnBi needlelike inclusions inside a Bi matrix produce a robust and large Hall, Nernst, and AMR voltage at zero field. The strong spinorbit coupling in Bi and the large magnetic moment provided by MnBi combine in this composite to give extraordinary AMR ratios. Their zerofield effects come on top of the large infield Nernst and spinSeebeck effects reported previously [1]. This material opens new device prospects for magnetic sensing, microwave propagation, and information filtering. 
Monday, March 15, 2021 10:36AM  10:48AM Live 
A38.00014: Spin current transport in EuIG thin films grown by pulsed laser deposition Wei Yuan, Haoyu Liu, Victor H. Ortiz, Jing Shi In this work, we investigate the spin transport in europium iron garnet (EuIG) thin films using spin Seebeck effect (SSE) and nonlocal transport. We grow highquality epitaxial EuIG thin films on the substrates of (111)oriented SGGG using the high vacuum pulsed laser deposition system followed by 750 degree exsitu rapid thermal annealing. In nonlocal transport devices, the spin current is thermally injected from an AC current flowing in one platinum strip and detected by the other platinum strip located 3 um away using a lockin amplifier at the double frequency via the inverse spin Hall effect. Our field and temperature dependent nonlocal reuslts suggest that it contains some additional contribution with the opposite sign to that of the Fe^{3+ }ions in the lowfield signals. The overall nonlocal responses are consistent with the SSE signals measured in the standard longitudinal geometry. This anomalous behavior may be attributed to the paramagnetic spin current contribution from the SGGG substrate. Our investigation future deeps the understanding of the spin transport in REIGs. 
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