Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A47: Magnetization Dynamics I: Ultrafast and SwitchingFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Andrew Berger, NIST Room: 394 |
Monday, March 13, 2017 8:00AM - 8:36AM |
A47.00001: Direct detection of the spin precession of a pure AC spin current using synchrotron x-rays Invited Speaker: Z. Q. Qiu Despite the great progress in spin-current research, the detection of spin-current has mostly remained indirect by measuring the induced effect of spin-current (e.g., inversed spin Hall effect) that could sometimes generate ambiguous interpretations. By synchronizing a microwave waveform with synchrotron x-ray pulses, we directly probed the spin precession of a pure AC spin-current using pump-probe X-ray Magnetic Circular Dichroism (XMCD). In this experiment, the ferromagnetic resonance of a Py layer in Py/Cu/CuMn/Cu/Co pumps an AC spin-current into the Cu/CuMn/Cu spacer layer and the ferromagnetic Co layer. The x-ray pulses, whose frequency is synchronized with the spin precession frequency, then probe element by element of the spin precessions of the CuMn and Co layers. The AC XMCD signal unambiguously identified the ac spin current in the paramagnetic CuMn layer. In addition, phase-resolved measurement identified a bipolar phase behavior of the Co spin precession which is a finger print of spin-current generated spin precessions. [Preview Abstract] |
Monday, March 13, 2017 8:36AM - 8:48AM |
A47.00002: Exploration of spintronic heterostructures for broadband terahertz generation Evan Jasper, M.T. Warren, T.T. Mai, J. Brangham, F. Yang, R. Vald\'es Aguilar The generation of terahertz (THz) radiation for research purposes has historically been dominated by three techniques: photoconductive antennas, optical rectification in optical nonlinear media (ZnTe, GaP, DAST, etc.), and laser-induced air-based plasma THz emission. Each technique offers tradeoffs between signal-to-noise, power, bandwidth, ease of generation, and cost. Recently a new technique has been developed which utilizes the inverse spin Hall effect in a spintronic heterostructure to transform a laser-induced spin-polarized charge current into a transverse charge current and thereby emit a THz pulse. The THz pulses generated by these heterostructures have a broad bandwidth comparable to that offered by air-based plasma techniques, and a power conversion efficiency, low emitter cost, and ease of use similar to that offered by optical rectification in ZnTe. We have measured THz emission of a heterostructure of W$\vert$CoFe$\vert$Pt grown on MgO with an applied in-plane magnetic field, and will report on further exploration of this class of THz emitter. [Preview Abstract] |
Monday, March 13, 2017 8:48AM - 9:00AM |
A47.00003: Terahertz emission from ultrafast spin-charge current at a Rashba interface Qi Zhang, Matthias Benjamin Jungfleisch, Wei Zhang, John E. Pearson, Haidan Wen, Axel Hoffmann Ultrafast broadband terahertz (THz) radiation is highly desired in various fields from fundamental research in condensed matter physics to bio-chemical detection. Conventional ultrafast THz sources rely on either nonlinear optical effects or ultrafast charge currents in semiconductors. Recently, however, it was realized that ultrabroad-band THz radiation can be produced highly effectively by novel spintronics-based emitters that also make use of the electron's spin degree of freedom \footnote{T. Kampfrath et al., Nat. Nanotechnol. 8, 256 (2013).}. Those THz-emitters convert a spin current flow into a terahertz electromagnetic pulse via the inverse spin-Hall effect. In contrast to this bulk conversion process, we demonstrate here that a femtosecond spin current pulse launched from a CoFeB layer can also generate terahertz transients efficiently at a two-dimensional Rashba interface between two non-magnetic materials, i.e., Ag/Bi. Those interfaces have been proven to be efficient means for spin- and charge current interconversion \footnote{W. Zhang et al., J. Appl. Phys. 117, 17C727 (2015).}$^,$\footnote{M. B. Jungfleisch et al., Phys. Rev. B 93, 224419 (2016).}. [Preview Abstract] |
Monday, March 13, 2017 9:00AM - 9:12AM |
A47.00004: THz-Driven Ultrafast Spin-Lattice Scattering in Amorphous Metallic Ferromagnets Stefano Bonetti, Matthias Hoffmann, Meng-Ju Sher, Zhao Chen, See-hun Yang, Mahesh Samant, Stuart Parkin, Hermann Dürr We use single-cycle THz fields and the femtosecond magneto-optical Kerr effect to, respectively, excite and probe the magnetization dynamics in two thin-film ferromagnets with different lattice structures: crystalline Fe and amorphous CoFeB. We observe Landau-Lifshitz-torque magnetization dynamics of comparable magnitude in both systems, but only the amorphous sample shows ultrafast demagnetization caused by the spin-lattice depolarization of the THz-induced ultrafast spin current. Quantitative modeling shows that such spin-lattice scattering events occur on similar time scales than the conventional spin conserving electronic scattering ($\sim30$ fs). This is significantly faster than optical laser-induced demagnetization. THz conductivity measurements point towards the influence of lattice disorder in amorphous CoFeB as the driving force for enhanced spin-lattice scattering. [Preview Abstract] |
Monday, March 13, 2017 9:12AM - 9:24AM |
A47.00005: Ultrafast spin dynamics and switching via the spin transfer torques in antiferromagnet with weak ferromagnet T. H. Kim, P. Gruenberg, S. H. Han, B. K. Cho The spin-torque driven dynamics of the antiferromagnet with canted moments was investigated analytically based on the Landau-Lifshitz-Gilbert-Slonczewski equation with the antiferromagnetic (\textbf{\textit{l}}) and ferromagnetic (\textbf{\textit{m}}) order parameters. Although Dzyaloshinskii-Moriya (DM) torque splits the degenerate resonant mode into Sigma-mode and Gamma-mode, the equation of motion was found to be described by 2-dimansional pendulum model of \textbf{\textit{l}} as like simple antiferromagnet. Because \textbf{\textit{l}} is coupled to \textbf{\textit{m}}, the close examination of m leads both to reveal \textbf{\textit{l}}'s dynamics and to estimate DM energy. For example, the second harmonic of resonant frequency, together with the resonant frequency softening phenomenon, is the evidence for the non-linear behavior of \textbf{\textit{l}}. The precessional ellipticity of m in Sigma-mode determines the DM energy through the following relation; $m_{\mbox{y}} /m_{\mbox{x}} \sim \hslash \omega_{\mbox{sigma}} /D$ where $\omega _{\mbox{sigma}} $ is resonant frequency in Sigma-mode. Finally, we discuss magnetization reversal efficiency by varying DM energy, anisotropy barrier and damping. [Preview Abstract] |
Monday, March 13, 2017 9:24AM - 9:36AM |
A47.00006: Thermal noise model of antiferromagnetic dynamics: A macroscopic approach Xilai Li, Yuriy Semenov, Ki Wook Kim In the search for post-silicon technologies, antiferromagnetic (AFM) spintronics is receiving widespread attention. Due to faster dynamics when compared with its ferromagnetic counterpart, AFM enables ultra-fast magnetization switching and THz oscillations. A crucial factor that affects the stability of antiferromagnetic dynamics is the thermal fluctuation, rarely considered in AFM research. Here, we derive from theory both stochastic dynamic equations for the macroscopic AFM Neel vector (L-vector) and the corresponding Fokker-Plank equation for the L-vector distribution function. For the dynamic equation approach, thermal noise is modeled by a stochastic fluctuating magnetic field that affects the AFM dynamics. The field is correlated within the correlation time and the amplitude is derived from the energy dissipation theory. For the distribution function approach, the inertial behavior of AFM dynamics forces consideration of the generalized space, including both coordinates and velocities. Finally, applying the proposed thermal noise model, we analyze a particular case of L-vector reversal of AFM nanoparticles by voltage controlled perpendicular magnetic anisotropy (PMA) with a tailored pulse width. [Preview Abstract] |
Monday, March 13, 2017 9:36AM - 9:48AM |
A47.00007: All-optical switching by domain wall motion in Co/Pd superlattices with a high-repetition-rate laser Farzaneh Hoveyda, Erich Hohenstein, Serban Smadici All-optical switching (AOS) of magnetization with femtosecond laser pulses has been demonstrated in ferri- and ferromagnetic materials. Current models mostly correlate ultrafast reversal with the uniform rotation of magnetization. In addition, while single-pulse switching in ferrimagnets has been reported, AOS in ferromagnetic Co/Pt has also been observed with overlapping pulses. This suggests the possibility to initiate a magnetization reversal with a high repetition rate laser. In our work, ferromagnetic Co/Pd superlattices, deposited by e-beam evaporation, were scanned under a high repetition rate Ti:Sapphire laser beam. Imaging with polarizing and magnetic force microscopes confirmed that AOS was attained. Intriguingly, measurements with a chopped beam showed a few ms delay before the reversed domains become visible. These domains gradually expanded by domain wall motion. We explain the delay with heat accumulation due to the high-repetition-rate laser pulses. The heat accumulation introduces in-plane thermal gradients, which apply forces on the magnetic domain walls. This suggests a model of the observed reversal by thermally driven domain wall motion. [Preview Abstract] |
Monday, March 13, 2017 9:48AM - 10:00AM |
A47.00008: A powerful and alternative model for all-optical spin switching Guoping Zhang, Yihua Bai, Thomas F George All-optical spin switching (AOS) has attracted enormous attention. Its underlying mechanism has been under intense debate. A few promising mechanisms include pure heating, inverse Faraday effect, magnetic circular dichroism, sublattice exchange interactions and others. However, the laser only interacts with a magnetic medium within the laser pulse duration, and how the helicity is injected into the system is crucial to understanding AOS. Here we propose a far more powerful but much simpler model that is able to explain varieties of switching in both ferromagnets and ferrimagnets. We show that it is the laser-induced optical spin-orbit torque that leads to the spin reversal. The dynamics can be very long if a weak exchange interaction is used. This is the case in rare-earth transition metal alloys. Our theory opens a new door to understanding the intricate switching and may have some important impact in future magnetic storage technology. (1) G. P. Zhang, Y. H. Bai and T. F. George, Switching ferromagnetic spins by an ultrafast laser pulse: Emergence of giant optical spin-orbit torque, Europhys. Lett. 115, 57003 (2016). (2) G. P. Zhang , T. Latta, Z. Babyak, Y. H. Bai and T. F. George, Mod. Phys. Lett. B 30, 1630005 (2016) 1630005. [Preview Abstract] |
Monday, March 13, 2017 10:00AM - 10:12AM |
A47.00009: A fast time-dependent density functional theory method for ultrafast magnetic dynamics induced by laser Zhanghui Chen, Lin-Wang Wang Ultrafast demagnetization has attracted a thriving interest, but the fundamental mechanism is still intensively debated. We present a fast real-time time-dependent density functional theory method to investigate the ultrafast spin dynamics induced by laser in nickel bulk and clusters. The Hamiltonian considers non-collinear magnetic moment, spin-orbital coupling, electron-phonon and electron-photon interaction. An accelerated method with leapfrog prediction of charge matrix is used to solve the time-evolving equation. We have observed remarkable energy gain from laser and spin demagnetization that consists of one time-lag stage and one fast demagnetization stage followed by one slow demagnetization stage. The evolution and conservation of angular momentum show that laser induces a large change of electron orbital angular momentum, which supplies part of the spin moment loss by spin-orbital coupling. Another reservoir from ions also plays important role in demagnetization through rapid electron-ion exchange interaction. Finally, further demonstrations are shown for the impact of initial ionic random dynamic and laser parameters, which helps understand the gap between theory and experiment as well as the thermal driving. [Preview Abstract] |
Monday, March 13, 2017 10:12AM - 10:24AM |
A47.00010: TDDFT studies of Spin-Flip and Spin-Current Ultrafast Demagnetization Peter Elliott, Kevin Krieger, J. Kay Dewhurst, Sangeeta Sharma, E.K.U. Gross We apply the ab-initio simulation method of time dependent density functional theory (TDDFT) to shed light on the underlying physics of ultrafast demagnetization in ferromagnetic materials due to intense laser pulses. A key finding of our previous work is that spin-flips due to the spin-orbit interaction (SOI) can be responsible for ultrafast loss of moment, under the right circumstances. In this work we compare the loss of moment due to 1) spin transport (i.e. transport of the moment from a ferromagnetic layer into a substrate) and 2) spin-flip SOI demagnetization happening in the magnetic layer itself. For the interfaces we study, we find that both processes contribute equally to the demagnetization of the ferromagnet. Furthermore we predict that if the substrate has strong SOI character (e.g. Pt), there can even be SOI type demagnetization of the moment that was transported into the substrate. [Preview Abstract] |
Monday, March 13, 2017 10:24AM - 10:36AM |
A47.00011: Simulation of Ultrafast Spin-Dependent Hot-Electron Transport in Metallic Multilayers Dennis Nenno, Marius Weber, Hans Christian Schneider Spin currents in metallic heterostructures can play an important role in connection with ultrafast demagnetization in ferromagnetic materials, as optically excited hot electrons contribute to the non-equilibrium magnetization over the whole range of a multilayer structure. Here, we present our results on two approaches to this problem. First, we apply the Boltzmann transport equation in the homogeneous metal part of a ferromagnet/metal-bilayer and determine the microscopic distribution function of the spin-polarized hot carriers excited in an adjacent magnetic layer. To solve the Boltzmann equation, we reduce the computational domain to an effectively two-dimensional phase space. Carrier-carrier scattering and interactions with phonons are included at the level of a relaxation-time approximation. We have also derived semi-classical equations of motion for the carriers using a particle-in-cell approach. With this approach, we calculate the dynamics of the electrons travelling through the whole bilayer structure, using as input velocities and electronic lifetimes from ab-initio calculations. \\ D.~M.~Nenno, S.~Kaltenborn and H.~C.~Schneider, Phys.~Rev.~B 94, 115102 (2016) [Preview Abstract] |
Monday, March 13, 2017 10:36AM - 10:48AM |
A47.00012: Nonreciprocal propagation of surface acoustic waves in Ni/LiNbO3 Ryo Sasaki, Yoichi Nii, Yusuke Iguchi, Yoshinori Onose We investigated surface acoustic wave propagation in a Ni/LiNbO$_3$ hybrid device. We found that the absorption and phase velocity are dependent on the sign of the wave vector, which indicates that the surface acoustic wave propagation has nonreciprocal characteristics induced by simultaneous breaking of time reversal and spatial inversion symmetries. The nonreciprocity was reversed by 180$^\circ$ rotation of the magnetic field. The origin of the nonreciprocity is ascribed to interference of shear-type and longitudinal-type magnetoelastic couplings. [Preview Abstract] |
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