Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z8: Focus Session: Spin-Dynamics: Theory and Experiment |
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Sponsoring Units: GMAG Chair: Lifa Zhang, University of Texas at Austin Room: 104 |
Friday, March 7, 2014 11:15AM - 11:27AM |
Z8.00001: Angular Momentum of Phonons and Einstein-de Haas Effect Lifa Zhang, Qian Niu We study angular momentum of phonons in a magnetic crystal. In the presence of a spin-phonon interaction, we obtain a nonzero angular momentum of phonons, which is an odd function of magnetization. At zero temperature, phonon has a zero-point angular momentum besides a zero-point energy. With increasing temperature, the total phonon angular momentum diminishes and approaches to zero in the classical limit. The nonzero phonon angular momentum can have a significant impact on the Einstein-de Haas effect. To obtain the change of angular momentum of electrons, the change of phonon angular momentum needs to be subtracted from the opposite change of lattice angular momentum. Furthermore, the finding of phonon angular momentum gives a potential method to study the spin-phonon interaction. Possible experiments on phonon angular momentum are also discussed. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z8.00002: Angular and Linear Momentum of Excited Ferromagnets Peng Yan, Akashdeep Kamra, Yunshan Cao, Gerrit Bauer The angular momentum vector of a Heisenberg ferromagnet with isotropic exchange interaction is conserved, while under uniaxial crystalline anisotropy the projection of the total spin along the easy axis is a constant of motion. Using Noether's theorem, we prove that these conservation laws persist in the presence of dipole-dipole interactions. However, spin and orbital angular momentum are not conserved separately anymore. We also define the linear momentum of ferromagnetic textures. We illustrate the general principles with special reference to spin transfer torques and identify the emergence of a non-adiabatic effective field acting on domain walls in ferromagnetic insulators [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z8.00003: Dynamic Magnetoelectric Effect in Ferromagnet|Superconductor Tunnel Junctions Mircea Trif, Yaroslav Tserkovnyak We study the magnetization dynamics in a ferromagnet$\mid$insulator$\mid$superconductor tunnel junction and the associated buildup of the electrical polarization. We show that for an open circuit, the induced voltage varies strongly and nonmonotonically with the precessional frequency, and can be enhanced significantly by the superconducting correlations. For frequencies much smaller or much larger than the superconducting gap, the voltage drops to zero, while when these two energy scales are comparable, the voltage is peaked at a value determined by the driving frequency. We comment on the potential utilization of the effect for the low-temperature spatially-resolved spectroscopy of magnetic dynamics. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z8.00004: Dependence of the demagnetization time on the exchange interaction and spin moment Guoping Zhang, Thomas F. George, Mingsu Si In femtomagnetism, the demagnetization time is at the center of laser-induced ultrafast demagnetization. It depends on various intrinsic and extrinsic parameters, but the experimental results are controversial, and in some cases the opposite effects are reported. In this presentation, we directly address how the exchange interaction and magnetic spin moment affect the demagnetization time. We employ a simple model that includes the exchange interaction and spin-orbit coupling. Then we derive an equation of motion for the spin moment change, from which a master equation is found. This equation explicitly shows how the demagnetization time is related to the exchange interaction and spin moment. This result can be directly compared with the latest experimental results. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z8.00005: Theoretical and experimental investigations of the electronic structure configuration during ultrafast demagnetization of Co Emrah Turgut, Patrik Grychtol, Dmitry Zusin, Henry C. Kapteyn, Margaret M. Murnane, Dominik Legut, Karel Carva, Peter M. Oppeneer, Stefan Mathias, Martin Aeschlimann, Claus M. Schneider, Justin Shaw, Ronny Knut, Hans Nembach, Thomas J. Silva We report on theoretical and experimental studies of the electronic structure configuration during the ultrafast demagnetization in Co thin films. After an ultrafast optical laser excitation of a ferromagnetic material, the magnetization of the material decreases rapidly in less than a picosecond. This ultrafast behavior has attracted a significant amount of attention for more than two decades; however, the underlying driving mechanism is still unclear. In this work, we use an extreme ultraviolet, broad-bandwidth, tabletop, ultrafast, and element-selective magnetization probe that employs the transverse magneto-optical Kerr effect to extract the energy- and time-resolved dynamics of the off-diagonal dielectric tensor element that is proportional to the magnetization. We compare our data with theoretical optical predictions based upon \textit{ab-initio} calculations of the electronic structure, with the ultimate goal of determining how the occupation of majority and minority states vs. energy evolves after ultrafast optical pumping. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z8.00006: Fast reversal of magnetic vortex chirality by electric current Weng-Lee Lim, RongHua Liu, Tolek Tyliszczak, Dmitry Berkov, Sergei Urazhdin We demonstrate reversal of magnetic vortex in a microscopic Pt/Permalloy bilayer disk by a nonuniform electric current in the plane of the disk. The switching is detected electronically by measuring the response to a small ac magnetic field, and confirmed by direct imaging with x-ray magnetic dichroism microscopy (XMCD). The magnetic contrasts obtained from time-resolved x-ray imaging indicate a fast and robust switching of magnetic vortex driven by electric current. The time-resolved XMCD measurements show that the characteristic switching time is less than 3 ns. Analysis from micromagnetic simulation shows that the reversal of the magnetic vortex is driven by a combination of the Oersted field due to the charge current and the spin transfer due to spin current generated by the spin Hall effect in Pt. The simulation reveals that the magnetization switching process of the magnetic vortex involves two distinct stages. The switching first proceeds with a fast dynamics and then evolves at a slower dynamics before reaching the final magnetic vortex state with opposite chirality, in agreement with the experimental result. The simulation also shows that the spin transfer torque (STT) accelerates the reversal of magnetic vortex in comparison to the case without STT. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z8.00007: Photo-induced Spin Angular Momentum Transfer into Antiferromagnetic Insulator Fan Fang, Yichun Fan, Xin Ma, J. Zhu, Q. Li, T.P. Ma, Y.Z. Wu, Z.H. Chen, H.B. Zhao, Gunter Luepke Spin angular momentum transfer into antiferromagnetic(AFM) insulator is observed in single crystalline Fe/CoO/MgO(001) heterostructure by time-resolved magneto-optical Kerr effect (TR-MOKE). The transfer process is mediated by the Heisenberg exchange coupling between Fe and CoO spins. Below the Neel temperature(TN) of CoO, the fact that effective Gilbert damping parameter $\alpha$ is independent of external magnetic field and it is enhanced with respect to the intrinsic damping in Fe/MgO, indicates that the damping process involves both the intrinsic spin relaxation and the transfer of Fe spin angular momentum to CoO spins via FM-AFM exchange coupling and then into the lattice by spin-orbit coupling. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z8.00008: Combined molecular and spin dynamics study of collective excitations in BCC iron Dilina Perera, David P. Landau, Don Nicholson, G. Malcolm Stocks Spin dynamics simulations of classical spin systems have revealed a substantial amount of information regarding the collective excitations in magnetic materials. However, much of the previous work has been restricted to lattice-based spin models that completely disregard the effect of lattice vibrations. Combining an empirical many body potential with a spin Hamiltonian parameterized by first principles calculations, we present a compressible magnetic model for BCC iron, which treats the dynamics of translational degrees of freedom on an equal footing with the magnetic (spin) degrees of freedom. This model provides us with a unified framework for performing combined molecular and spin dynamics simulations and make simultaneous quantitative measurements of the spin wave and vibrational spectrum. Results from our simulations reveal that the presence of lattice vibrations leads to softening and damping of spin waves, as well as evidence for a novel form of longitudinal spin wave excitation coupled with the longitudinal phonon mode of the same frequency. Furthermore, we will also discuss the influence of lattice vibrations at different temperatures and the implications of using different atomistic potentials. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z8.00009: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z8.00010: Laser Demagnetization Dynamics in Gadolinium from Time Resolved Photoemission John Bowlan, Bj\"orn Frietsch, Martin Teichmann, Robert Carley, Martin Weinelt The field of ultrafast magnetization dynamics has seen rapid progress in recent years and has the potential to enable magnetic data storage systems orders of magnitude faster than those based on conventional read/write heads. The dynamics of laser demagnetization in ferromagnetic Gadolinium depend on the transfer of energy and angular momentum between the metallic valence electrons and the core-like $4f$ electrons. Angle-Resolved Photoemission (ARPES) with femtosecond XUV laser pulses produced by high harmonic generation enables the direct measurement of the electronic band structure on a sub-picosecond time scale in a ``tabletop'' setup. Photoemission allows the magnetization dynamics of the valence and $4f$ bands to be tracked independently of one another. Thus, time-resolved photoemission is an alternative to experimental methods such as surface magnetic second harmonic generation (MSHG), the magneto-optical Kerr effect (MOKE), and x-ray magnetic circular dichroism (XMCD). We applied this technique to study Gd(0001) films grown epitaxially on a W crystal. We find that the valence electrons demagnetize on a fs time scale, while the 4f electrons respond more slowly. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z8.00011: Pump-probe measurement of short and long-range exchange interactions in a rare-earth magnet using resonant x-ray diffraction Matthew Langner, Sujoy Roy, Yi-De Chuang, Rolf Versteeg, Yi Zhu, Marcus Hertlein, Thornton Glover, Karine Dumesnil, Robert Schoenlein The combined effects of spin-orbit interactions, magnetostriction, and long-range exchange coupling lead to a wide variety of magnetic phases in the rare earth magnets. In dysprosium, core level spins develop a spiral phase as a result of competition between short and long-range RKKY exchange interactions mediated by the conducting electrons. We use time-resolved resonant x-ray diffraction to directly probe the spiral order parameter of the core level magnetism in response to optical pumping of the conduction electrons that mediate the exchange interaction. The dynamics of the diffraction intensity and spiral turn angle occur on different time scales, and through free-energy analysis, we associate these dynamics with changes in the short and long-range exchange coupling. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z8.00012: Numerical Renormalization-Group computation of nuclear magnetic relaxation rates Krissia Zawadzki, Luiz N. Oliveira, Jos\'{e} Wilson M. Pinto We report an essentially exact numerical renormalization-group (NRG) computation of the temperature-dependent NMR rate $1/T_1$ of a probe at a distance $R$ from a magnetic impurity in a metallic host. We split the metallic states into two subsets, A and B. The former comprises electrons $a_k$ in $s$-wave states about the magnetic-impurity site. The coupling between the $a_k$ band and the impurity is described by the Anderson Hamiltonian, diagonalizable by the NRG procedure. Each state $b_k$ in the B subset is a linear combination of an $s$-wave state about the probe site with the degenerate $a_k$, constructed to be orthogonal to all the $a_k$'s. The $b_k$ band hence decouples from the impurity and is analytically treatable. We show that the relaxation rate has three components: (i) a constant associated with the $b_k$'s; (ii) a $T$-dependent term associated with the $a_k$'s, which decays in proportion to $1/(k_FR)^2$, where $k_F$ is the Fermi momentum; and (iii) another $T$-dependent term due to the interference between the $a_k$'s and the $b_k$'s. The interference term shows Friedel oscillations whose amplitude, proportional to $1/k_FR$, can be mapped onto the universal function of $T/T_K$ describing the Kondo resistivity. We compare our findings with results in the literature. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z8.00013: ABSTRACT MOVED TO A54.00014 |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z8.00014: Dynamic Phase Diagram of the DC-Pumped Magnon Condensates Scott Bender, Rembert Duine, Arne Brataas, Yaroslav Tserkovnyak We investigate the effects of nonlinear dynamics and damping by phonons on a system of electronically pumped Bose-Einstein condensed or normal phase magnons in a ferromagnet. The nonlinear effects are crucial to understanding the phenomenon of ``swasing." Meanwhile damping was heretofore neglected, since the pumped magnon condensates previously considered are quasi-equilibrium and considered only a much shorter timescale. We analyze the magnetic phase behavior in the presence of these two new effects, demonstrating the possibility of stable condensate and hysteresis. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z8.00015: Dynamics of a two-dimensional quantum spin liquid: signatures of emergent Majorana fermions and fluxes Johannes Knolle, Dimitry Kovrizhin, John Chalker, Roderich Moessner Topological states of matter present a wide variety of striking new phenomena. Prominent among these is the fractionalisation of electrons into unusual particles: Majorana fermions, Laughlin quasiparticles or magnetic monopoles. Their detection, however, is fundamentally complicated by the lack of any local order, such as, for example, the magnetisation in a ferromagnet. While there are now several instances of candidate topological spin liquids, their identification remains challenging. Here, we provide a complete and exact theoretical study of the dynamical structure factor of a two-dimensional quantum spin liquid in gapless and gapped (abelian and non-abelian) phases. We show that there are direct signatures--qualitative and quantitative--of the Majorana fermions and gauge fluxes emerging in Kitaev's honeycomb model. These include counterintuitive manifestations of quantum number fractionalisation, such as a neutron scattering response with a gap even in the presence of gapless excitations, and a sharp component despite the fractionalisation of electron spin. Our analysis identifies new varieties of the venerable X-ray edge problem and explores connections to the physics of quantum quenches. [Preview Abstract] |
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