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 xrays Invited Speaker: Z. Q. Qiu Despite the great progress in spincurrent research, the detection of spincurrent has mostly remained indirect by measuring the induced effect of spincurrent (e.g., inversed spin Hall effect) that could sometimes generate ambiguous interpretations. By synchronizing a microwave waveform with synchrotron xray pulses, we directly probed the spin precession of a pure AC spincurrent using pumpprobe Xray Magnetic Circular Dichroism (XMCD). In this experiment, the ferromagnetic resonance of a Py layer in Py/Cu/CuMn/Cu/Co pumps an AC spincurrent into the Cu/CuMn/Cu spacer layer and the ferromagnetic Co layer. The xray 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, phaseresolved measurement identified a bipolar phase behavior of the Co spin precession which is a finger print of spincurrent 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 laserinduced airbased plasma THz emission. Each technique offers tradeoffs between signaltonoise, 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 laserinduced spinpolarized 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 airbased 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 inplane 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 spincharge 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 biochemical detection. Conventional ultrafast THz sources rely on either nonlinear optical effects or ultrafast charge currents in semiconductors. Recently, however, it was realized that ultrabroadband THz radiation can be produced highly effectively by novel spintronicsbased emitters that also make use of the electron's spin degree of freedom \footnote{T. Kampfrath et al., Nat. Nanotechnol. 8, 256 (2013).}. Those THzemitters convert a spin current flow into a terahertz electromagnetic pulse via the inverse spinHall 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 twodimensional Rashba interface between two nonmagnetic 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: THzDriven Ultrafast SpinLattice Scattering in Amorphous Metallic Ferromagnets Stefano Bonetti, Matthias Hoffmann, MengJu Sher, Zhao Chen, Seehun Yang, Mahesh Samant, Stuart Parkin, Hermann Dürr We use singlecycle THz fields and the femtosecond magnetooptical Kerr effect to, respectively, excite and probe the magnetization dynamics in two thinfilm ferromagnets with different lattice structures: crystalline Fe and amorphous CoFeB. We observe LandauLifshitztorque magnetization dynamics of comparable magnitude in both systems, but only the amorphous sample shows ultrafast demagnetization caused by the spinlattice depolarization of the THzinduced ultrafast spin current. Quantitative modeling shows that such spinlattice scattering events occur on similar time scales than the conventional spin conserving electronic scattering ($\sim30$ fs). This is significantly faster than optical laserinduced demagnetization. THz conductivity measurements point towards the influence of lattice disorder in amorphous CoFeB as the driving force for enhanced spinlattice 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 spintorque driven dynamics of the antiferromagnet with canted moments was investigated analytically based on the LandauLifshitzGilbertSlonczewski equation with the antiferromagnetic (\textbf{\textit{l}}) and ferromagnetic (\textbf{\textit{m}}) order parameters. Although DzyaloshinskiiMoriya (DM) torque splits the degenerate resonant mode into Sigmamode and Gammamode, the equation of motion was found to be described by 2dimansional 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 nonlinear behavior of \textbf{\textit{l}}. The precessional ellipticity of m in Sigmamode 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 Sigmamode. 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 postsilicon technologies, antiferromagnetic (AFM) spintronics is receiving widespread attention. Due to faster dynamics when compared with its ferromagnetic counterpart, AFM enables ultrafast 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 (Lvector) and the corresponding FokkerPlank equation for the Lvector 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 Lvector 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: Alloptical switching by domain wall motion in Co/Pd superlattices with a highrepetitionrate laser Farzaneh Hoveyda, Erich Hohenstein, Serban Smadici Alloptical 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 singlepulse 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 ebeam 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 highrepetitionrate laser pulses. The heat accumulation introduces inplane 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 alloptical spin switching Guoping Zhang, Yihua Bai, Thomas F George Alloptical 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 laserinduced optical spinorbit 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 rareearth 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 spinorbit 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 timedependent density functional theory method for ultrafast magnetic dynamics induced by laser Zhanghui Chen, LinWang Wang Ultrafast demagnetization has attracted a thriving interest, but the fundamental mechanism is still intensively debated. We present a fast realtime timedependent density functional theory method to investigate the ultrafast spin dynamics induced by laser in nickel bulk and clusters. The Hamiltonian considers noncollinear magnetic moment, spinorbital coupling, electronphonon and electronphoton interaction. An accelerated method with leapfrog prediction of charge matrix is used to solve the timeevolving equation. We have observed remarkable energy gain from laser and spin demagnetization that consists of one timelag 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 spinorbital coupling. Another reservoir from ions also plays important role in demagnetization through rapid electronion 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 SpinFlip and SpinCurrent Ultrafast Demagnetization Peter Elliott, Kevin Krieger, J. Kay Dewhurst, Sangeeta Sharma, E.K.U. Gross We apply the abinitio 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 spinflips due to the spinorbit 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) spinflip 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 SpinDependent HotElectron 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 nonequilibrium 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/metalbilayer and determine the microscopic distribution function of the spinpolarized hot carriers excited in an adjacent magnetic layer. To solve the Boltzmann equation, we reduce the computational domain to an effectively twodimensional phase space. Carriercarrier scattering and interactions with phonons are included at the level of a relaxationtime approximation. We have also derived semiclassical equations of motion for the carriers using a particleincell 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 abinitio 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 sheartype and longitudinaltype magnetoelastic couplings. [Preview Abstract] 
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