Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y14: Focus Session: Thermal and Magnon Spin Currents |
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Sponsoring Units: DMP FIAP GMAG Chair: Gabriel Chaves-O'Flynn, New Jersey Institute of Technology Room: 316 |
Friday, March 22, 2013 8:00AM - 8:36AM |
Y14.00001: GMAG PhD Dissertation Research Award: The Planar Nernst and Seebeck Effects in Ferromagnetic Metal Films with In-Plane Thermal Gradients Invited Speaker: Azure Avery Recently, the spin Seebeck effect (SSE) has attracted a great deal of attention as one possible source of pure spin currents. In response to a thermal gradient ($\nabla T$), the SSE is thought to produce a pure spin current detectable by measuring a transverse voltage ($V_T$) generated by the inverse spin Hall effect. However, recent work on spin-dependent transport in thin film nanostructures supported by bulk substrates suggests that early SSE experiments may have been strongly affected by unintended $\nabla T$ through the supporting substrates. They may also have been affected by thermoelectric effects generated from planar thermal gradients such as transverse thermopower, also known as the planar Nernst effect (PNE), in which a $V_T$ develops in response to a $\nabla T$ applied in the plane of a film with in-plane magnetization. In this talk, we present the first results from experiments designed to probe the SSE and related effects such as the PNE and longitudinal thermopower in 20 nm thick nickel and permalloy thin films deposited on suspended Si-N platforms. In our experiments, the background thermal conduction of the 500 nm thick platforms is at least 1000x smaller than the bulk substrates used previous experiments, thus confining $\nabla T$ to the plane of the film. The results exhibit the $\sin \theta \cos \theta$ angular dependence predicted by the PNE, where $\theta$ is the angle between film magnetization and thermal gradient, rather than the $\cos \theta$ dependence expected from SSE predictions. We demonstrate that the magnetic field dependence of the PNE, anisotropic magnetoresistance, and longitudinal thermopower ($\alpha$) is generated by spin-dependent scattering and present results confirming the Onsager reciprocity between $\alpha$ and the Peltier coefficient. Finally, we present an upper limit for the SSE coefficient in our experiment that is at least an order of magnitude smaller than previously reported by experiments conducted using bulk substrates. I would like to thank my collaborators Barry L. Zink and Matthew R. Pufall and gratefully acknowledge support from the NSF CAREER Grant No. DMR-0847796. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y14.00002: Spin-Seebeck effect in amorphous ferromagnetic alloys Hyungyu Jin, Zihao Yang, Roberto Myers, Joseph Heremans Since its first discovery in 2008 [1], continuous research on spin-Seebeck effect (SSE) has established a theory for the driving mechanisms of SSE: in the presence of a thermal gradient, the spin waves (magnons) present in ferromagnets are brought out of thermal equilibrium. It is suspected that their return to thermal equilibrium is what launches a spin flux, which then is converted into a voltage in a separate material by strong spin-orbit interactions. While it is proven that substrate phonons affect the spin-Seebeck signals [2], another possible mechanism that can drive magnons out of equilibrium can be magnon thermal conductivity. Here, to isolate the magnon and phonon contributions, we investigate the relation between SSE and magnon thermal conductivity in amorphous ferromagnetic alloys (Metglas). Because Metglas has high Curie temperature, yet mostly localized phonon modes, the magnon contribution to SSE is expected to be larger than in crystalline ferromagnets. Experimental SSE data as well as magneto-thermal conductivity data will be presented. \\[4pt] [1] K. Uchida et al., Nature 455, 778 (2008).\\[0pt] [2] C.M. Jaworski et al., PRL 106, 186601 (2011). [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y14.00003: Intrinsic Spin Seebeck Effect in Gold Danru Qu, Ssu-Yen Huang, Jun Hu, Ruqian Wu, Chia-Ling Chien In Spin Seebeck Effect (SSE), a pure spin current can be generated by a temperature gradient ($\nabla $T) and detected by the inverse spin Hall effect usually by Pt. Due to the propensity of out-of-plane $\nabla _{\mathrm{z}}$T through substrate, the SSE in the transverse configuration with an in-plane $\nabla_{\mathrm{x}}$T has been shown contaminated by the anomalous Nernst effect.\footnote{S. Y. Huang, et al. Phys. Rev. Lett. 107, 216604 (2011)} The SSE in the longitudinal configuration with $\nabla_{\mathrm{z}}$T suffers from the magnetic proximity effect (MPE) of Pt in contact with a ferromagnetic material thus also contaminated.\footnote{S. Y. Huang, et al. Phys. Rev. Lett. 109, 107204 (2012)} In this work, we demonstrate that Au does not exhibit MPE and reveals the intrinsic SSE. In contrast to Pt/YIG, Au/YIG shows no anomalous Hall signals, very weak inverse MR, and non-monotonic thickness dependence of spin thermal voltage, thus very weak if any MPE. Our results place an upper limit to the intrinsic SSE of 0.1$\mu $V/K at the Au thickness of 8nm, two orders of magnitude smaller than that in Pt/YIG. Spin-polarized density-functional calculations also show a sizable Pt but a negligible Au magnetic moment in contact with YIG, in agreement with experiments. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y14.00004: Electrical measurements of nonlinear magnetization dynamics Can-Ming Hu, Yongshen Gui, Lihui Bai, Paul Hyde A new approach to measure precisely nonlinear magnetization dynamics is demonstrated by using spin dynamos in combination with sensitive electrical probing techniques. The directly measured intrinsic foldover effect of ferromagnetic resonance in Py unravels a 50-year-old mystery of ferromagnetic metals. Pivotal importance of nonlinear ferromagnetic damping is uncovered via its distinct dependence on the frequency, amplitude, and initial conditions. The experimental results are in excellent agreement with a phenomenological model, which revises the pioneer theoretical work of Anderson and Suhl for nonlinear magnetization dynamics. New evidence for electrically detected pure spin pumping in the nonlinear dynamic regime will be briefly discussed. For more information and references, please check our group website at: http://www.physics.umanitoba.ca/\textasciitilde hu/. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:48AM |
Y14.00005: Irreversible thermodynamics of transport and relaxation of magnetic moments with applications for spin caloritronics Invited Speaker: Sylvain Brechet 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. Here, we establish a clear classical formalism describing the thermodynamics of a matter continuum with magnetic moments interacting with external electromagnetic fields. Taking into account the chemical nature of the current densities -- such as the current density of magnetic moments -- and stress tensors leads to three types of dissipation terms: scalars, vectors and pseudo-vectors. The scalar terms account for the chemical reactivities, the vectorial terms account for the transport and pseudo-vectorial terms account for the relaxation. The vectorial phenomenological relations establish notably the Spin Seebeck effect first observed by Uchida and Saitoh. The pseudo-vectorial phenomenological relations establish in particular the Landau-Lifschitz relaxation of the magnetisation. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y14.00006: Spin wave mode coexistence: A consequence of the Oersted field induced asymmetric energy landscape Randy Dumas, Ezio Iacocca, Stefano Bonetti, Sohrab Sani, Majid Mohseni, Anders Eklund, Johan Persson, Olle Heinonen, Johan Akerman The emerging field of magnonics relies on the systematic generation, manipulation, and detection of spin waves (SWs). Nanocontact spin torque oscillators (NC-STOs) provide an ideal platform to study spin transfer torque induced SW emission [1,2]. In analogy to two species competing for the same food supply it has been argued that only one SW mode can survive in the steady state [3]. However, as evidenced in many experiments clear signatures of mode-hopping are often observed [1,4]. We present a third possibility, namely that under the correct experimental conditions, mode coexistence can be realized. Micromagnetic simulations reveal that the SW modes are spatially separated under the NC. Mode coexistence is facilitated by the local field asymmetries induced by the spatially inhomogeneous Oersted field in the vicinity of the NC and further promoted by SW localization. Finally, both simulation and experiment reveal a weak low frequency signal exactly at the difference of the mode frequencies, consistent with inter-modulation of two coexistent modes. [1] S. Bonetti, et al., PRL 105, 217204 (2010). [2] M. Madami, et al., Nature Nanotechnol. 6, 635 (2011). [3]$^{\, \, }$ F. M. de Aguiar, et al., PRB 75, 132404 (2007). [4]$^{\, \, }$ P. K. Muduli, et al., PRL 108, 207203 (2012). [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y14.00007: Controlling spin-wave propagation with Oersted fields K. Vogt, B. Hillebrands, H. Schultheiss, J.E. Pearson, F.Y. Fradin, S.D. Bader, A. Hoffmann The goal of magnon spintronics is to utilize the coherent propagation of spin waves for low-power data processing. Spin waves carry angular momentum and can transport spin information over distances much larger than the spin diffusion length of metals. However, in thin magnetic films the highly anisotropic dispersion relation leads to strong changes in the spin-wave energy for different angles between their propagation direction and the magnetization orientation. Consequently, spin waves only travel along a straight path if the magnetization direction is fixed by a global external magnetic field. We demonstrate that locally rotating magnetic fields generated via electric current pulses allow to vary the propagation direction of spin waves. Using spatially resolved Brillouin light scattering microscopy the propagation behavior was directly verified.\footnote{K.~Vogt, H.~Schultheiss, S.~Jain, J.E.~Pearson, A.~Hoffmann, S.D.~Bader, and B.~Hillebrands, Appl. Phys. Lett. {\bf 101}, 042410 (2012)} We have modeled the current generated magnetic fields with a finite element code and calculated the magnetic response using micro magnetic simulations. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y14.00008: Mapping microwave fields using the spin Hall effect Vincent Vlaminck, Helmut Schultheiss, John Pearson, Frank Fradin, Samuel Bader, Axel Hoffmann We present measurements of the spatial variation of the spin pumping - inverse spin Hall effect in a palladium/permalloy bilayer via a coplanar waveguide ferromagnetic resonance (CPW-FMR) broadband technique. We show that the inverse spin Hall signal is both inhomogeneous and asymmetric with respect to both the position along the CPW and the excitation port. These frequency dependent asymmetries in the measured voltage are most likely due to an impedance mismatch at the contact points and the asymmetry between the two ends of the CPW. Based on this observation we show how the inverse spin Hall effect can be used as a sensitive probe for mapping the microwave magnetic field distribution in the FMR frequency range. This work emphasizes the importance of characterizing the microwave field homogeneity in every experiment with extended samples. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y14.00009: Probing the Influence of Thermal Spin Torque on Magnetic Tunnel Junction Switching Timothy Phung, Aakash Pushp, Charles Rettner, Brian Hughes, See-Hun Yang, Stuart Parkin It has been established in the past few years that heat flow within a ferromagnet can induce a spin current and an associated voltage. This so called Spin Seebeck effect, initially reported in ferromagnetic metals, has also been observed in magnetic semiconductors, magnetic insulators as well as in strongly spin orbit coupled systems. An open question has been whether heat induced spin currents can be used in switching a magnetic tunnel junction (MTJ) via thermal spin torque (TST). In order to answer this question, we investigate the MTJ switching with TST induced by sharp temperature gradients on the order of 1-10 K/nm. We will describe our experimental setup and present data that show the various roles that temperature plays on the saturation magnetization of the material and on the induced spin currents that influence MTJ switching. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y14.00010: Temperature gradient assisted spin transport in nonlocal lateral spin valves Saidur Rahman Bakaul, Shaojie Hu, Takashi Kimura The advent of non-local spin transport devices (NSTD) provide further possibility for nano spin-electronic devices as these are capable of generating electronic charge-free and non-dissipative pure spin current. The most imperative and primary issue associated with these generic spintronic devices is finding the ways to enhance the amplitude of pure spin current and the simplest way to do that is increasing the excitation charge current density. The bottleneck for this method is the Joule heating, which reduces the pure spin current. However, recent discoveries of spintronic versions of the thermoelectric effects, such as spin-dependent Seebeck and Peltier effects$^{\, }$convincingly imply that, in a properly designed device, the thermal gradient may provide aiding impact for pure spin current. In this work we have experimentally studied the multi terminal NSTDs and observed room temperature enhancement of the spin signal at high bias current. The magnitude of the spin signal is asymmetric with respect to the DC bias polarity. We discuss about the role of different thermoelectric effects on the observed spin signal enhancement. These results are important as it may open the road to tackle the Joule heating induced degradation of spin signal in NSTDs. [Preview Abstract] |
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