Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D48: Focus Session: Spin Transport and Magnetization Dynamics in Metal-Based Systems: Phononic and Thermal Phenomena |
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Sponsoring Units: DMP FIAP GMAG Chair: Ion Garate, Universite de Sherbrooke Room: Mile High Ballroom 1A |
Monday, March 3, 2014 2:30PM - 2:42PM |
D48.00001: Possible charge analogues of spin transfer torques in bulk superconductors Ion Garate Spin transfer torques (STT) occur when electric currents travel through inhomogeneously magnetized systems and are important for the motion of magnetic textures such as domain walls. Since superconductors are easy-plane ferromagnets in particle-hole (charge) space, it is natural to ask whether any charge duals of STT phenomena exist therein. We find that the superconducting analogue of the adiabatic STT vanishes in a bulk superconductor with a momentum-independent order parameter, while the superconducting counterpart of the nonadiabatic STT does not vanish. This nonvanishing superconducting torque is induced by heat (rather than charge) currents and acts on the charge (rather than spin) degree of freedom. It can become significant in the vicinity of the superconducting transition temperature, where it generates a net quasiparticle charge and alters the dispersion and linewidth of low-frequency collective modes. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D48.00002: Superfluid Spin Transport through Easy-Plane Ferromagnetic Insulators So Takei, Yaroslav Tserkovnyak Superfluid spin transport | dissipationless transport of spin | is theoretically studied in a ferromagnetic insulator with easy-plane anisotropy. We consider an open geometry where spin current is injected into the ferromagnet from one side by a metallic reservoir with a nonequilibrium spin accumulation, and ejected into another metallic reservoir located downstream. Spin transport through the device is studied using a combination of magnetoelectric circuit theory, Landau-Lifshitz-Gilbert phenomenology, and microscopic linear-response theory. We discuss how spin superfluidity can be probed using a magnetically-mediated electron-drag experiment. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D48.00003: Quasi-one-dimensional phonon anomaly in the narrow-gap semiconductor FeSb$_2$ Igor Zaliznyak, Cedomir Petrovic, Rongwei Hu, Andrei Savici, Ovidiu Garlea, Barry Winn FeSb$_2$ has a variety of unusual properties, ranging from the temperature-induced electronic paramagnetism to one-dimensional (1D) metallic conductivity at temperatures below $\sim 300$ K and down to $\sim 30$ K, where it becomes insulating, and to giant thermoelectric power factor. While it is generally acknowledged that these properties result from the tight balance between strong covalent hybridization, electronic correlation and the tendency to band delocalization, what exactly are the mechanisms leading to these unusual behaviors remain unclear. In particular, it is a matter of current debate, whether the giant thermoelectric figure of merit observed in FeSb$_2$ is explained by purely correlated-electronic mechanism, or it results from peculiar interaction of electrons with the lattice vibrations - such as the phonon drag effect. Here we present the inelastic neutron scattering survey of the phonon spectra in FeSb$_2$. It reveals phonon dispersions of one-dimensional character, which mirror the 1D metallicity along the $b-$axis, and the quasi-one-dimensional magnetism observed in the insulating sister material CrSb$_2$. Phonon dispersions undergo dramatic changes in the temperature range where electronic paramagnetism emerges, indicating strong coupling to electrons. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D48.00004: Spontaneous magnetic fluctuations in ultrathin magnetic films at zero field Andrew Balk, John Unguris We use magneto optical Kerr effect (MOKE) microscopy to observe room temperature, zero field magnetic fluctuations in perpendicularly magnetized cobalt films at thicknesses near the in-plane to out-of-plane spin reorientation transition.~ The magnetic behavior of our films resembles that of collections of superparamagnetic particles, in that globally they exhibit zero net moment, while local areas continually undergo thermal magnetic fluctuations between saturated states of the maze-like domain structure.~ Unlike superparamagnetic particles, the fluctuations are not constrained by particle boundaries and thus are subject to both exchange and magnetostatic interactions.~ Due to this we can observe temporal and spatial correlations in the fluctuations.~ Furthermore, we observe that the fluctuations obey dynamics distinct from field-driven Barkhausen jumps.~ We also determine scaling exponents of the fluctuations, finding their areas follow a power law distribution (t $=$1.5), and their temporal noise power spectrum is close to 1/f (a $=$ 1.04).~ Based on these observations, we discuss these films as possible candidates for exhibiting magnetic self-organized criticality. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D48.00005: Crystal structure, Magnetic, and Anomalous Schottky Specific Heat of Rare Earth Dialuminides Arjun Pathak, Karl A. Gschneidner, Jr., Vitalij Pecharsky Materials with structural transformations or distortions coupled to magnetic transitions show interesting magnetostrictive, magnetoresistive, and magnetocaloric behavior and are, therefore, important subject of study in condensed matter physics. The importance of either coupled or decoupled magnetostructural transformations has been shown for many materials starting from high temperature superconductors and perovskites to multifunctional intermetallics. The anomalies close to 0 K encompass another playground for the fundamental physics, and they range from the Kondo effect and heavy fermion behavior to quantum criticality and nuclear Schottky specific heat. These remarkable behaviors are ultimately related to the interplay between localized and delocalized electrons, for which lanthanides are truly the best model provided by nature. In particular, the rare earth dialuminides, which have simple cubic Laves phase structure at room temperature have long been the system of choice to understand the fundamentals of rare earth magnetism and low temperature anomalies. In this presentation, we will discuss the low temperature crystal structure, magnetic and thermodynamic properties of binary and pseudobinary rare earth dialuminides by means of low temperature x-ray diffraction, magnetization and heat capacity measurements. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D48.00006: Berry curvature in magnetostatic waves and associated semiclassical dynamics of wavepackets Shuichi Murakami, Ryo Matsumoto, Ryuichi Shindou Magnons in ferromagnets form band structure and thus they are associated with Berry curvature in momentum space. This Berry curvature of magnons causes various interesting phenomena such as thermal Hall effect [1,2]. In particular the magnetic dipolar interaction can be regarded as a spin-orbit coupling in a wider sense and thus can give rise to nonzero Berry curvature. In my presentation, we describe the magnetostatic waves (magnons) in terms of the bosonic Bogoliubov-de Gennes Hamiltonian [3] and show how the magnon thermal Hall conductivity behaves as a function of magnetic field and temperature. We also present how this Berry curvature affects the dynamics of the magnon wavepacket within semiclassical theory. For example, the wavepacket rotates by itself and will give rise to a radial charge distribution due to relativistic effect. We also show how the magnon trajectory is affected by the Berry curvature. [1] R. Matsumoto, S. Murakami, Phys. Rev. Lett. 106, 197202 (2011). [2] R. Matsumoto, S. Murakami, Phys. Rev. B 84, 184406 (2011). [3] R. Matsumoto, R. Shindou and S. Murakami, preprint. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D48.00007: Thermally induced transparency for short spin wave pulses in yttrium iron garnet (YIG) films Cesar Leonardo Ordonez Romero, Oleg Kolokoltsev, Ivan Gomez Arista, Naser Qureshi, Guillermo Monsiv\'ais Galindo, Hesiquio Vargas Hern\'andez The compensation of spin wave propagation losses plays a very important role in the development of novel magnonic devices. Up to now, however, most of the known amplification methods present relative narrow frequency bandwidths due to their resonant nature. In this work, we present compensation of the propagation losses or pseudo-amplification of travelling spin waves by tailoring the bias magnetic field profile. The thermally-induced non-uniform profile of the magnetization introduced on an Yttrium Iron Garnet (YIG) thin film by a localized spot of a cw argon-ion laser creates the conditions to observe the complete compensation of the spin wave propagation losses. The spin wave evolution was mapped with a time and spaced resolved inductive magneto-dynamic prove system. The experiment was carried out using a uniform sample of single-crystal YIG film grown on a gallium-gadolinium garnet (GGG) substrate. The 2mm-wide, 20mm-long and 6microns-thick YIG strip was saturated with an external magnetic field enabling the set up for the propagation of magneto-static surface waves. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D48.00008: Thermally induced spin accumulation at Al/Co$_2$TiSi and Al/Co$_2$TiGe contacts Voicu Popescu, Benjamin Geisler, Peter Kratzer Spin injection from a ferromagnet in a semiconductor substrate can be accomplished either by applying an external voltage or a temperature gradient. In the latter case, one exploits the Seebeck effect, with the temperature gradient across the contact directly resulting in a difference in chemical potentials in the two spin channels due to the spin-dependence of the Seebeck coefficient. The magnetic Heusler alloys Co$_2$TiSi or Co$_2$TiGe exhibit half-metallic ferromagnetism in their ideal L2$_1$ crystal structure, with a potentially high degree of spin polarization of the injected current. As such, they recommend themselves for integrated spin injectors in combination with the closely lattice-matched Al contact layer. We investigate the possibility of employing Al/Co$_2$TiX/Al (X=Si,Ge) trilayers as thermally driven spin injectors by means of first-principles calculations of the electronic structure and of the thermoelectric transport properties. Our results show that the spin-dependent Seebeck effect is sensitive to the atomic structure of the Heusler/Al interface. In particular, for a thin Co$_2$TiSi or Co$_2$TiGe layer terminated by a TiSi or TiGe atomic plane, the thermal spin accumulation is found to be of the same order of magnitude as the effective Seebeck coefficient. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D48.00009: Evidence for a Magnetic Seebeck effect Sylvain Brechet, Francesco Vetro, Elisa Papa, Stewart Barnes, Jean-Philippe Ansermet Spin caloritronics is mainly focused on studying the effects of a temperature gradient on the time evolution of the local spin average of a classical system. In many experimental situations, the system can be treated as a classical continuum with magnetisation on the scale of interest where the quantum fluctuations average out and the underlying microscopic structure is smoothed out. Recently, we established a clear classical formalism describing the thermodynamics of a matter continuum with magnetisation interacting with external electromagnetic fields. This formalism accounts for the thermal and electric magnetisation accumulations and magnetisation waves. It also predicts that a temperature gradient in the presence of magnetisation waves induces a magnetic induction field, which is the magnetic analog of the Seebeck effect. This thermal gradient modulates the precession and relaxation. The Magnetic Seebeck effect implies that magnetisation waves propagating in the direction of the temperature gradient and the external magnetic induction field are less attenuated, while magnetisation waves propagating in the opposite direction are more attenuated. This effect has been observed on a YIG slab in our laboratory and it is in very good agreement with the thermodynamic prediction. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D48.00010: Modeling the Tunneling Anisotropic Magneto-Seebeck Effect Vivek Amin, Jan Zemen, Jakub Zelezny, Jan Masek, Tomas Jungwirth, Jairo Sinova Due to increasing energy consumption in high-density electronics, the control and recycling of heat generated in nanostructures is highly desirable. The effect of temperature gradients on magnetic nanostructures has thus prompted a renewed interest in the long-known Seebeck effect, as it applies to spin-polarized systems. One such phenomenon is the Magneto-Seebeck (MS) effect, in which the Seebeck coefficient of a magnetic tunnel junction (MTJ) changes based on its magnetization configuration. As a result, the thermal properties of an MTJ can be tuned via magnetic field. We extend the study of this effect to the Tunneling Anisotropic Magnetoresistance (TAMR), in which an MTJ with a single ferromagnetic contact produces a spin-orbit-coupling-induced magneto-transport anisotropy. We present numerical results of this Tunneling Anisotropic Magneto-Seebeck Effect in a CoPt/MgO/Pt heterostructure. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D48.00011: Imaging of In-Plane Magnetization using the Time Resolved Anomalous Nernst Effect Jason Bartell, Darryl Ngai, Gregory Fuchs We report on measurements of the time-resolved anomalous Nernst effect (TRANE) for diffraction-limited imaging of in-plane magnetization using a high resolution optical microscope. In TRANE microscopy, pulsed laser light is used to create a transient thermal gradient perpendicular to the film plane. In response, a voltage is generated by the anomalous Nernst effect. The voltage has an amplitude proportional to the in-plane projection of the magnetic moment along a direction perpendicular to the voltage contacts. We show that the TRANE voltage persists for less than 100 ps in 30 nm thick magnetic samples. Additionally, we demonstrate spatial resolution limited only by the area of the thermal gradient generated by the focused laser pulse. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D48.00012: Asymmetric and Negative Differential Thermal Spin Effect at Magnetic Interfaces: Towards Spin Seebeck Diodes and Transistors Jie Ren, Jian-Xin Zhu We study the nonequilibrium thermal-spin transport across metal-magnetic insulator interfaces. The transport is assisted by the exchange interaction between conduction electrons in the metal and localized spins in the magnetic insulator. We predict the rectification and negative differential spin Seebeck effect (SSE), that is, reversing the temperature bias is able to give asymmetric spin currents and increasing temperature bias could give an anomalously decreasing spin current. We resolve their microscopic mechanism as a consequence of the energy-dependent electronic DOS in the metal. The rectification of spin Peltier effect is also discussed. We then study the asymmetric and negative differential magnon tunneling driven by temperature bias. We show that the many-body magnon interaction that makes the magnonic spectrum temperature-dependent is the crucial factor for the emergence of rectification and negative differential SSEs in magnon tunneling junctions. We show that these asymmetric and negative differential SSEs are relevant for building magnon and spin Seebeck diodes and transistors, which could play important roles in controlling information and energy in functional devices. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D48.00013: Spin Seebeck Effect vs. Anomalous Nernst Effect in Ta/CoFeB /Ta Structures Bowen Yang, Yadong Xu, Mike Schneider, Jing Shi We have studied the spin Seebeck effect (SSE) and anomalous Nernst effect (ANE) in a vertical trilayer structure under a vertical temperature gradient. The structure consists of a 3nm CoFeB layer sandwiched by $\beta $-phase tantalum (Ta) layers. The samples are deposited by magnetron sputtering. The existence of Ta $\beta $-phase is verified by the resistivity and its negative temperature coefficient of resistance(TCR). Under a fixed vertical temperature gradient, the measured transverse thermoelectric voltage is linearly proportional to the total sample resistance when the Ta thickness exceeds 2 nm, which can be explained by a shunting resistor model. When the Ta thickness is below 2 nm, the voltage deviates from the linear resistance dependence and merges to the ANE voltage of the CoFeB single layer, due to a weakened inverse spin Hall effect (ISHE) in Ta thinner than the spin diffusion length. In the linear regime, the slope contains both a varying SSE and a fixed ANE responses, thus the SSE contribution could be quantitatively separated out from the ANE of CoFeB. Our results indicate a large SSE from the $\beta $-phase Ta due to its large Spin Hall Angle. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D48.00014: Measuring spin diffusion length using spin Seebeck effect Harsha Kannan, Xin Fan, John Xiao Ever since its discovery, spin Seebeck effect (SSE) has attracted plenty of attention. The conversion from thermal gradient to spin current has shown great potential in thermal energy harvesting. SSE can also be utilized as a source to generate pure spin current to unveil new physics. Here we show that it is possible to measure spin diffusion length of a heavy metal Pt by studying the SSE as a function of Pt layer thickness. The SSE signal first increases, peaks, and then decreases with increasing Pt layer thickness. By fitting with a drift-diffusion model, we obtain the spin diffusion length of Pt to be about 2nm, close to that obtained from other techniques. Moreover, we can insert a thin layer of Cu in order to remove the possible proximity effect. Similar spin-diffusion length is obtained from this measurement. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D48.00015: Spin-Orbit Caloritronics Aurelien Manchon, Papa Birame Ndiaye, Jung-Hwan Moon, Hyun-Woo Lee, Kyung-Jin Lee Utilizing spin-orbit coupling to enable the electrical manipulation of ferromagnets has recently attracted a considerable amount of interest. This spin-orbit torque [1] appears in magnetic systems displaying inversion symmetry breaking. Another adjacent emerging topic, spin caloritronics [2], aims at exploiting magnonic spin currents driven by temperature gradients, allowing for the transmission of information and the control of magnetic domain walls. In this work, we demonstrate that a magnon flow generates torques on the local magnetization when subjected to Dzyaloshinskii-Moriya interaction (DMI) just as an electron flow generates torques when submitted to Rashba interaction [3]. A direct consequence is the capability to control the magnetization direction of a homogeneous ferromagnet by applying a temperature gradient or local RF excitations. Merging the spin-orbit torques with spin caloritronics is rendered possible by the emergence of DMI in magnetic materials and opens promising avenues in the development of chargeless information technology. [1] Miron et. al, Nature Materials 9, 230 (2010); [2] Bauer, et al. Nat. Mater. 11, 391 (2012); [3] A. Manchon and S. Zhang, Phys. Rev. B 78, 212405 (2008). [Preview Abstract] |
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