Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A1: Invited Session: Spin Caloritronics |
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Sponsoring Units: DCMP GMAG Chair: Kai Liu, University of California, Davis Room: Ballroom I |
Monday, March 18, 2013 8:00AM - 8:36AM |
A1.00001: Longitudinal Spin Seebeck Effect Invited Speaker: Eiji Saitoh The spin Seebeck effect (SSE) refers to the generation of a spin voltage as a result of a temperature gradient in magnetic materials [1-7]. Here, a spin voltage is a potential for electron spins to drive a nonequilibrium spin current; when a conductor is attached to a magnet with a finite spin voltage, it induces a spin injection into the conductor. The SSE is of crucial importance in spintronics and spin caloritronics, since it enables simple and versatile generation of a spin current from heat. The simplest and most straightforward setup of the SSE is the longitudinal configuration [4], in which a spin current flowing parallel to a temperature gradient is measured via the inverse spin Hall effect (ISHE). The longitudinal SSE device consists of a ferromagnetic or ferrimagnetic insulator (FI, e.g. YIG) covered with a paramagnetic metal (PM, e.g. Pt) film. When a temperature gradient is applied perpendicular to the FI/PM interface, an ISHE-induced voltage is generated in the PM layer. In this talk, we report the observation of the longitudinal SSE in various FI/PM systems and provide evidence that the longitudinal SSE is free from thermoelectric artefact [7], i.e., the anomalous Nernst effect caused by extrinsic magnetic proximity [8]. Then, we discuss the longitudinal SSE from an application point of view [6]. We thank E. Saitoh, S. Maekawa, G. E. W. Bauer, X.-F. Jin, H. Adachi, D. Hou, D. Tian, T. Kikkawa, A. Kirihara, and M. Ishida for their support and valuable discussions. \\[4pt] [1] K. Uchida et al., Nature 455, 778 (2008).\\[0pt] [2] K. Uchida et al., Nature Mater. 9, 894 (2010).\\[0pt] [3] C. M. Jaworski et al., Nature Mater. 9, 898 (2010).\\[0pt] [4] K. Uchida et al., Appl. Phys. Lett. 97, 172505 (2010).\\[0pt] [5] K. Uchida et al., Nature Mater. 10, 737 (2011).\\[0pt] [6] A. Kirihara et al., Nature Mater. 11, 686 (2012).\\[0pt] [7] T. Kikkawa et al., arXiv:1211.0139 (2012). \\[0pt] [8] S. Y. Huang et al., Phys. Rev. Lett. 109, 107204 (2012). [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A1.00002: Transport Magnetic Proximity Effects in Platinum Invited Speaker: Ssu-Yen Huang Platinum (Pt) metal, being non-magnetic and having a strong spin-orbit coupling interaction, has been central in detecting pure spin current and establishing most of the recent spin-based phenomena. Thus, it is important to ascertain the transport and magnetic characteristics of thin Pt films in contact with a ferromagnet. In this work, we use both electric and thermal means to conclusively show the transport magnetic proximity effects (MPE) of thin Pt film in contact with ferromagnetic insulator YIG. At thicknesses comparable to, and less than, the spin diffusion length, the strong ferromagnetic characteristics in Pt films on YIG are indistinguishable from those of ferromagnetic permalloy on YIG. [1] The MPE occurs at the interface and decreases exponentially away from the interface, concentrating in only a few monolayers. As a result, the pure spin current detected by a thin Pt is tainted with a spin polarized current. The pure spin current phenomena, such as the inverse spin Hall effect and the spin Seebeck effect, have been contaminated with the anomalous Hall effect and the anomalous Nernst effect respectively. These results raise serious questions about the suitability, and the validity, of using Pt in establishing pure spin current phenomena; on the other hand, a much stronger spin-based effect can be induced by the MPE at the interface. This research is in collaboration with X. Fin, Y. P. Chen, J. Wu, and J. Q. Xiao (University of Delaware), T. Y. Chen (Arizona State University) and D. Qu, W. G. Wang, and C. L. Chien (The Johns Hopkins University).\\[4pt] [1] S. Y. Huang \textit{et al.,} Phys. Rev. Letts. \textbf{109}, 107204 (2012). [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A1.00003: Observation of the planar Nernst effect in Permalloy and Nickel Thin Films with In-plane Thermal Gradients Invited Speaker: Barry Zink The reliable generation of pure spin currents is an important ingredient in future spintronic circuits that may offer lower power consumption and greater processing capabilities than current technology. Over the past few years some groups have reported that such a spin current can be generated simply by applying a thermal gradient to a ferromagnetic material. This effect, called the spin Seebeck effect (SSE), has generated tremendous interest in the interaction of heat, charge and spin in ferromagnetic systems. In this talk we will present our own recent measurements of thermoelectric and thermomagnetic effects in thin film metallic ferromagnets. These are enabled by a micromachined thermal isolation platform that removes potentially confounding effects introduced in such measurements by the presence of a highly thermally conductive bulk substrate. One of the main results is the observation of a transverse thermopower, called the planar Nernst effect (PNE), that is caused by spin-dependent scattering. This PNE should therefore be present in any attempted measurement of the SSE in a metal system where spin-dependent scattering of electrons occurs. Furthermore our ``zero substrate" experiment shows no signal with the expected symmetry of the SSE, suggesting that the presence of the substrate is required to cause such a signal. Further experiments are required to determine if a pure spin current is actually involved in the generation of the signal associated with the SSE in ferromagnetic metal films. This work was performed in collaboration with A. D. Avery, and M. R. Pufall. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:24AM |
A1.00004: Non-universal shot noise in quasiequilibrium spin valves Invited Speaker: Tero Heikkila Shot noise can be used as a diagnostic tool characterizing mesoscopic wires, especially the inelastic scattering in them. This characterization is based on the fact that in the absence of inelastic scattering that carries the energy away from the system, disordered wires are described by a universal Fano factor defined as the ratio of the noise power and the average current. In particular, the value of this Fano factor is invariant even for wires with non-uniform conductivity. We show that this universality breaks down in spin valves with strong electron-electron scattering. The reason for this breakdown is that the inter-spin energy relaxation due to electron-electron scattering in the absence of inter-spin charge relaxation breaks the Wiedemann-Franz relation between charge and heat conductivity. In particular, we predict that the Fano factor gets strongly suppressed for the antiparallel configuration of magnetizations.\\[4pt] T.T. Heikkil\"a and K.E. Nagaev, arXiv:1302.1372 [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 11:00AM |
A1.00005: Charge Voltages from Magnetization Dynamics Invited Speaker: Axel Hoffmann The main challenge of spin caloritronics is to establish a connection between heat currents and spin currents. Towards this end, spin Hall effects have become very important, since they allow to convert a pure spin current into a transverse charge voltage. I will show how these spin Hall effects can be characterized with great accuracy using spin pumping, where the excitation of ferromagnetic resonance generates a pure spin current in an adjacent non-magnetic conductor.\footnote{O.~Mosendz, V.~Vlaminck, J.~E.~Pearson, F.~Y.~Fradin, G.~E.~W.~Bauer, S.~D.~Bader, and A.~Hoffmann, Phys.\ Rev.\ B {\bf 82}, 214403 (2010); O.~Mosendz, J.~E.~Pearson, F.~Y.~Fradin, G.~E.~W.~Bauer, S.~D.~Bader, and A.~Hoffmann, Phys.\ Rev.\ Lett.\ {\bf 104}, 046601 (2010).} The change in the line-width of the ferromagnetic resonance determines the spin-mixing conductance and thus after proper calibration of the {\em rf} magnetic fields and the concomitant opening angles of the magnetization precession, allows to determine the magnitude of the spin current. The charge current generated from inverse spin Hall effect is measured through the associated electrical voltage and the ration of spin and charge current directly determines the spin Hall angle. Furthermore I will present an alternative approach for converting magnetization dynamics into measurable charge voltages. Namely, the dissipation of magnetization dynamics in thin films generally also results in a temperature gradient perpendicular to the film, since the supporting substrate acts as a heat sink. This in turn can generate a transverse voltage through the anomalous Nernst effect. Interestingly this allows to detect spin waves with very good signal to noise\footnote{H.~Schultheiss, J.~E.~Pearson, S.~D.~Bader, and A.~Hoffmann, Phys.\ Rev.\ Lett.\ (in press).} and unlike optical or inductive detection techniques there is practically no lower limit for the wavelength of the detected spin waves. [Preview Abstract] |
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