Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W28: Focus Session: Spin Caloritronics |
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Sponsoring Units: GMAG DMP FIAP Chair: Geoffrey Beach, Massachusetts Insititute of Technology Room: 205 |
Thursday, March 5, 2015 2:30PM - 3:06PM |
W28.00001: Spin-current phenomena at high magnetic fields and high temperatures Invited Speaker: Ken-ichi Uchida In the field of spintronics, many experimental and theoretical studies have been focused on spin-transport phenomena in paramagnet/ferromagnet junction systems, where a spin current plays a central role. After the first demonstration of spin transport in insulator-based systems [1], a Pt/YIG junction system becomes one of the prototype samples. In this system, itinerant spins in Pt and localized magnetic moments in YIG interact with each other via the interface s-d interaction, i.e., the spin-mixing conductance; this interaction is the basic mechanism underlying various spin-current-related phenomena, such as the spin pumping [1], the spin Seebeck effect [2], and the recently-discovered spin Hall magnetoresistance (SMR) [3]. In this talk, we report the observation of the longitudinal spin Seebeck effect (LSSE) [4] and the SMR in Pt/YIG systems at high magnetic fields and high temperatures. The LSSE measurements in a high magnetic field range confirm that the observed voltage in the Pt/YIG systems is of magnon origin, providing a useful way to distinguish the LSSE from the anomalous Nernst effect induced by proximity ferromagnetism in Pt [5]. The LSSE and SMR at high temperatures highlight the importance of the temperature dependence of the spin-mixing conductance at the Pt/YIG interface [6]. These results will be helpful for obtaining full understanding of the mechanism of the LSSE and SMR.\\[4pt] We thank E. Saitoh, S. Maekawa, G. E. W. Bauer, H. Adachi, Y. Ohnuma, T. Kikkawa, S. Daimon, Y. Shiomi, and J. Shiomi for their support and valuable discussions.\\[4pt] [1] Y. Kajiwara et al., Nature 464, 262-266 (2010). \newline [2] K. Uchida et al., Nature 455, 778-781 (2008), Nature Materials 9, 894-897 (2010). \newline [3] H. Nakayama et al., Phys. Rev. Lett. 110, 206601 (2013). \newline [4] K. Uchida et al., Appl. Phys. Lett. 97, 172505 (2010), J. Phys.: Condens. Matter 26, 343202 (2014). \newline [5] T. Kikkawa et al., Phys. Rev. Lett. 110, 067207 (2013), Phys. Rev. B 88, 214403 (2013). \newline [6] K. Uchida et al., Phys. Rev. X 4, 041023 (2014). [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W28.00002: ABSTRACT WITHDRAWN |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W28.00003: Spin current draining effect on heat-driven spin transport Yadong Xu, Bowen Yang, Chi Tang, Zilong Jiang, Jing Shi, Michael Schneider, Renu Whig As a non-magnetic heavy metal is attached to a ferromagnet, a vertically flowing heat-driven spin current is converted to a transverse electric voltage, which is known as the longitudinal spin Seebeck effect. If the ferromagnet is a metal, this voltage is also accompanied by voltages from two other sources, i.e. the anomalous Nernst effect in both the ferromagnet and the proximity-induced ferromagnetic boundary layer. In this work, we have investigated these phenomena in NiFe/Cu/heavy metal multilayer structure. By identifying and carefully separating those effects, we find that in this pure spin current circuit the additional spin current drawn by the heavy metal generates another voltage in the ferromagnetic metal via the inverse spin Hall effect. [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W28.00004: Mechanism of the two sign changes in the spin Seebeck effect of a compensated ferrimagnet Yuichi Ohnuma, Hiroto Adachi, Eiji Saitoh, Sadamichi Maekawa Spin Seebeck effect is the mechanism of thermal spin injection from a precessing ferromagnet into an attached paramagnetic metal [Uchida et al., Nature 455, 778 (2008)]. We have theoretically investigated the spin Seebeck effect in compensated ferrimagnets [Ohnuma et al., Phys. Rev. B 87, 014423 (2013)] and predicted that the sign of the spin Seebeck signal changes at the compensation temperature, which is recently confirmed by an experiment [Gepr\"{a}gs et al., arXiv:1405.4971 (2014)]. Interestingly, the experiment found another sign change at a lower temperature. Here we explain its origin by taking account of sublattice dependence of the exchange coupling at the ferrimagnet/paramagnet interface. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W28.00005: The effect of magnetic anisotropy on spin-dependent thermoelectric effects in nanoscopic systems Maciej Misiorny, J\'{o}zef Barna\'{s} Harnessing of the interplay between transport of charge, spin and energy is a prospect route towards maximizing the functional potential of nanoscopic electronic and spintronic devices. Here, we investigate theoretically spin-related thermoelectric effects in electronic, linear-response transport through a nanoscopic systems exhibiting magnetic anisotropy. As an example, a magnetic tunnel junction with a large-spin impurity $-$either a magnetic atom or molecule$-$ embedded in the barrier is considered. Conduction electrons traversing the junction can then scatter on the impurity, which effectively can lead to angular momentum and energy exchange between the electrons and the impurity. As we show, such processes have a profound effect on the thermoelectric response of the system. Since the scattering mechanism also involves processes when electrons are inelastically scattered back to the same electrode, one can expect the flow of spin and energy also in the absence of charge transport through the junction. This, in turn, results in a finite spin thermopower, and the magnetic anisotropy plays a key role for this effect to occur. [1] M. Misiorny and Barna\'{s}, Phys. Rev. B 89, 235438 (2014). [2] M. Misiorny and Barna\'{s}, arXiv:1411.2741 (submitted for publication). [Preview Abstract] |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W28.00006: Optical detection of Spin-Seebeck Effect in Ferromagnetic thin films Ryan McLaughlin, Dali Sun, Valy Vardeny The field of Spin Caloritronics has attracted great interest because of the generation of spin currents in the presence of temperature gradients, mainly detected by means of an Inverse Spin Hall Effect (ISHE) voltage in metals with strong spin-orbit coupling. However, this method of electrical detection is difficult due to the subtle voltage generated by the ISHE combined with a large number of artifacts such as proximity effect, anisotropic magnetoresistance, anomalous Nerst effect, etc., which makes a quantitative understanding of the Spin Seebeck Effect elusive. Instead, here we demonstrate an \textit{optical} detection of spin accumulation in Ferromagnetic thin films using a custom-built Kerr Rotation sensitive interferometer, enabling us to investigate the pure spin accumulation from Spin Seebeck in the absence of spurious effects. [Preview Abstract] |
Thursday, March 5, 2015 4:06PM - 4:18PM |
W28.00007: Separation of the inverse spin Hall effect and anomalous Nernst effect in a single ferromagnetic metal using on-chip spin Seebeck devices Stephen Wu, Jason Hoffman, John Pearson, Anand Bhattacharya The longitudinal spin Seebeck effect is measured on the ferromagnetic insulator Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ with the ferromagnetic metal Co$_{\mathrm{0.2}}$Fe$_{\mathrm{0.6}}$B$_{\mathrm{0.2}}$ (CoFeB) as the spin detector in a micro-patterned device structure using an on-chip heater. By using a non-magnetic spacer material between the two materials (Ti), it is possible to decouple the two ferromagnetic materials and directly observe pure spin flow from Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ into CoFeB. It is shown, that in a single ferromagnetic metal the inverse spin Hall effect (ISHE) and anomalous Nernst effect (ANE) can occur simultaneously with opposite polarity. Using this and the large difference in the coercive fields between the two magnets, it is possible to unambiguously separate the contributions of the spin Seebeck effect from the ANE and observe the degree to which each effect contributes to the total response within a single experiment. Additionally, by using the spin detector layer as a thermometer, an accurate value for the thermal gradient across the device can be measured. These results match well with thermal simulations of our device structure. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W28.00008: Thermal Hall Effect of Spins in a Paramagnet Hyunyong Lee, Jung Hoon Han, Patrick Lee Theory of Hall transport of spins in a correlated paramagnetic phase is developed. By identifying the thermal Hall current operator in the spin language, which turns out to equal the spin chirality in the pure Heisenberg model, various response functions can be derived straightforwardly. Subsequent reduction to the Schwinger boson representation of spins allows a convenient calculation of thermal and spin Hall coefficients in the paramagnetic regime using self-consistent mean-field theory. Com- parison is made to results from the Holstein-Primakoff reduction of spin operators appropriate for ordered phases. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W28.00009: Ultrafast Time-correlated Measurements of Spin-Seebeck effect in Yttrium Iron Garnet John Jamison, Brandon Giles, Zihao Yang, Roberto Myers Recently, the time dependence of the spin-Seebeck effect (SSE) has been measured using optical pulses. These measurements suggest a time response faster than 5ns[1]. Here we present time-correlated measurements of the spin-Seebeck effect in Yttrium Iron Garnet (YIG) using an ultrafast laser. The pulsed beam is split into two individually modulated beams with a controllable delay time with sub-picosecond time resolution. The laser pulses are absorbed by a top Pt contact which generates a transient thermal gradient resulting in a spin current crossing the interface. The spin current is detected as a transverse voltage arising from the inverse spin Hall effect in Pt. We will present measurements of the time-correlated SSE signal from the two pulses as a function of delay time out to 1 ns. [1] Roschewsky et al., Appl. Phys. Lett.,~104,~ 202410 (2014). [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W28.00010: Spatiotemporal Imaging of Gigahertz Frequency Magnetization Dynamics Using the Time Resolved Anomalous Nernst Effect Jason Bartell, Darryl Ngai, Zhaoqi Leng, G.D. Fuchs We report on the first demonstration of spatiotemporal magnetic microscopy based on the Time Resolved Anomalous Nernst Effect (TRANE). In TRANE microscopy, pulsed laser light is used to create a transient thermal gradient perpendicular to the film plane. The anomalous Nernst effect generates a corresponding transient electric field that is proportional to the cross product of both the thermal gradient and the in-plane projection of the magnetic moment. We demonstrate TRANE microscopy and use it to study the magnetic configuration and excited magnetization dynamics of patterned ferromagnetic structures. We show that the time resolution exceeds 30 ps, allowing measurement of dynamics above 16 GHz. We observe that the spatial resolution using a thermal gradient generated from focused light matches the optical diffraction limit, indicating that lateral thermal diffusion does not limit resolution. Numerical simulations of the time-dependent thermal gradient indicate that the thermal spot can be confined to nanoscale dimensions using, for instance, a plasmon antenna. This could allow TRANE microscopy to achieve bench-top imaging of magnetization with spatial resolution comparable to the domain wall width and temporal resolution in the GHz range. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W28.00011: Non-local thermal spin injection: Mapping the magnon spin diffusion length in Yttrium Iron Garnet (YIG) Brandon L. Giles, Zihao Yang, John Jamison, Roberto C. Myers The non-local spin detection geometry was developed to sample a pure electron spin current in the absence of an electric field, thereby removing parasitic transport effects [1]. Here we demonstrate the non-local detection of magnon spins that are thermally injected via the spin Seebeck effect in single crystal YIG. A laser is used to thermally generate a spin current under an electrically isolated Pt absorbing pad. The spin current is detected on a remote Pt strip via the inverse spin Hall effect ($V_{ISHE}^{non-local})$. Spatial maps of the spin current are acquired by measuring $V_{ISHE}^{non-local} $ while scanning the laser to different absorbing pads. Temperature modeling shows the laser-induced temperature gradient contained within 50$\mu $m of the Pt absorbing pad [2]. Thus, the spin detector is isolated from thermal effects unrelated to the spin current. Although the thermal magnon diffusion length at 21K is $\sim$ 1 $\mu$m [3], $V_{ISHE}^{non-local} $ is detected at displacements of more than 150um with an exponential decay constant of 40 $\mu $m at 25K.\\[4pt] [1] Jedema, \textit{et al}. Nature 416, 713 (2002).\\[0pt] [2] Z. Yang et al. 2015 APS March meeting abstract.\\[0pt] [3] Boona \textit{et al}. Phys. Rev. B 90, 064421 (2014). [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W28.00012: Finite-element modeling of thermal gradients during non-local thermal spin injection Zihao Yang, Brandon Giles, John Jamison, Roberto Myers A new spin Seebeck experiment has been demonstrated, in which a laser is focused on an electrically isolated Pt absorbing pad on yttrium iron garnet (YIG), thermally generating a spin current in YIG.[1] The spins diffuse laterally and are detected non-locally on a remote Pt detector via the inverse spin Hall effect ($V_{ISHE}^{non-local} )$. This geometry is expected to remove parasitic thermal transport voltages unrelated to the magnonic spin current that could contaminate $V_{ISHE}^{non-local} $. To validate this, 3D steady-state heat conduction equations are solved to determine the stray temperature gradient at the Pt detector as a function of distance from the laser heating source. We find that the temperature gradient beneath the Pt detector vanishes when the laser is laterally displaced (along x) by 50$\mu $m. The gradient along the interface normal follows $\nabla T_{z} (x)\sim e^{-1.76x}$ and the gradient parallel to the interface follows $\nabla T_{x} (x)\sim e^{-0.08x}$. Both gradients decay much faster than the measured $V_{ISHE}^{non-local} (x)\sim e^{-0.025x}$ demonstrating the validity of the non-local geometry in probing laterally diffused spin. [1]B. Giles, et al., 2015 APS March meeting abstract [Preview Abstract] |
Thursday, March 5, 2015 5:18PM - 5:30PM |
W28.00013: Effects of heat current on magnetization dynamics Francesco Antonio Vetro, Sylvain Brechet, Jean-Philippe Ansermet The work is aimed at investigating the interplay between spin dynamics and heat currents in single-crystal Yttrium Iron Garnet (YIG). The irreversible thermodynamics for a continuous medium [1] predicts that a thermal gradient, in the presence of magnetization waves, produces a magnetic induction field, thus a magnetic analog of the well-known Seebeck effect. Time-resolved transmission measurements revealed a change in the attenuation of magnetization waves propagating along the thermal gradient when the gradient is reversed. This magnetic damping change can be accounted for by the Magnetic Seebeck effect [2]. In order to characterize this effect further, we have conducted studies on magnetization dynamic in YIG single crystal samples placed in various geometrical configurations, e.g. with YIG disks in which magnetic vortices might be present. Various magnetic resonance schemes were used, e.g. local probes and cavities.\\[4pt] [1] S. D. Brechet and J.-P. Ansermet, Eur. Phys. J. B, vol. 86, no. 7, pp. 1-19, Jul. 2013.\\[0pt] [2] S. D. Brechet, F. A. Vetro, E. Papa, S. E. Barnes and J.-P. Ansermet, Physical Review Letters 111, 8, 087205, Aug. 2013. [Preview Abstract] |
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