Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X50: Nanoscale Magnetic DynamicsFocus
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Sponsoring Units: GMAG DMP Chair: Javier Pulecio, NIST Room: 397 |
Friday, March 17, 2017 8:00AM - 8:12AM |
X50.00001: Nanoscale magnetometry of dynamic magnetization Joost van Bree, Michael Flatt\'e We propose a novel scheme for sensing magnetic properties of materials using the energy stored in the magnetic field of a nearby (scanning) probe. In conventional scanning probe magnetometry a sample perturbs a probe through a magnetic field external to its volume, limiting it to samples with static moments. In our proposed scheme we overcome this limitation by reversing the perturbation; the probe's magnetic field generates a response of the sample, which acts back on the probe and changes its energy. Taking a nitrogen vacancy (NV) spin center in diamond as the probe, we show theoretically this back action effectively changes the fine structure splitting of the spin ground state. Sensitive measurement techniques using coherent detection schemes would then permit detection of the magnetic response of paramagnetic and diamagnetic materials. Not only would this technique extend the class of materials that can be sensed compared to conventional magnetometry, we also show it could be used to measure the thickness of magnetically dead layers with better than 0.1$\AA$ accuracy. [Preview Abstract] |
Friday, March 17, 2017 8:12AM - 8:24AM |
X50.00002: Geometric Aspects of Spin Dynamics Xiaochuan Wu, Ran Cheng, Di Xiao Spin dynamics in nano-magnets usually come up with effective descriptions. Known examples are the Landau-Lifshitz equation for ferromagnets and the nonlinear sigma model for two-sublattice antiferromagnets. However, there lacks a generic theory applicable to arbitrary nano-magnets with multi-sub-lattices and complicated ground states, such as the gamma phase of transition metal alloys. Here we formulate a unified theory of spin dynamics for general nano-magnets via the SU(2) coherent state path integral. We find that all geometric features of the spin dynamics are encoded in the quantum geometric tensor consisting of the Berry curvature and the quantum metric, regardless of the ground state configuration. Our theory encapsulates the well-known descriptions Landau-Lifshitz equation and the nonlinear sigma model as certain limiting cases. [Preview Abstract] |
Friday, March 17, 2017 8:24AM - 8:36AM |
X50.00003: Ultrafast magnetization reversal by induced orbital moment Anirban Kundu, Shufeng Zhang Recent experiments have shown that magnetization switching of rare-earth/transition-metal magnetic compounds can be achieved by intense circularly polarized laser beams. The role of the laser beams is two folds: to heat the sample to a temperature close to the transition temperature and to transfer the angular momentum of the laser beam to the orbital moment. With an induced orbital moment, it might be possible to achieve the magnetization reversal via spin-orbit coupling. Here we quantitatively examine such scenario by estimating the magnitude of the induced orbital moment. The polarized photons interact with itinerant band electrons in two distinct forms: resonant and non-resonant transitions. By using time-dependent perturbation approach, we have obtained the explicit dependence of the induced orbital moment on the characteristics of the underlying band structure after we have properly treated seemingly divergent terms in the perturbation theory. By placing the parameters from experiments in our formalism, we find that the induced orbital momentum is too small to fully account the observed magnetization switching. Other possible switching mechanism are discussed. This work was supported by NSF-ECCS-1404542. [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 9:12AM |
X50.00004: Femtosecond control and dynamics of magnetism at the nanoscale Invited Speaker: Alexey Kimel The idea to change magnetic properties of media with the help of light has long intrigued people. This research direction became especially appealing after the development of femtosecond laser sources which are able to generate sub-100 fs laser pulses. These pulses are among the shortest stimuli in contemporary experimental physics. Such a development has naturally raised the question about feasibility of ultrafast optical control of magnetism and triggered the field of ultrafast magnetism [1]. The action of electric field of light on electronic dipoles, being the largest perturbation in physics of light-matter interaction, conserves the spin of electron. Nevertheless, an effective optical control and detection of antiferromagnetism become possible due to spin-orbit [2] and exchange interactions [3,4]. Here we review the fundamentals and recent progress in the areas of femtosecond magneto-optics and opto-magnetism. A special attention will be paid to optical control of magnetism in magnetic nanostructures [5], opto-magnetic recording with subwavelength spatial resolution [6] and generation of femtosecond nanomagnons [7]. [1] A. Kirilyuk, A. V. Kimel, Th. Rasing, Ultrafast optical manipulation of magnetic order, Review of Modern Physics 82 2731-2784 (2010). [2] A. V. Kimel, A. Kirilyuk, P. A. Usachev, R. V. Pisarev, A. M. Balbashov, R. V. Pisarev, and Th. Rasing, Ultrafast non-thermal control of magnetization by instantaneous photomagnetic pulses, Nature 435 655 (2005). [3] R. Mikhaylovskiy, E. A. Secchi, J. Mentink, M. Eckstein, A. Wu, R. Pisarev, V. Kruglyak, M. Katsnelson, Th. Rasing, and A. V. Kimel, Ultrafast optical modification of exchange interactions in iron oxides, Nature Communications 6, 8190 (2015). [4] R. R. Subkhangulov, A. B. Henriques, P. H. O. Rappl, E. Abramof, Th. Rasing, A. V. Kimel, All-optical manipulation and probing of the d--f exchange interaction in EuTe, Scientific Reports 4, 4368 (2014). [5] L. le Guyader, M. Savoini, S. El Moussaoui, M. Buzzi, A. Tsukamoto, A. Itoh, A. Kirilyuk, T. Rasing, A. V. Kimel {\&} F. Nolting, ``Nanoscale sub-100 picosecond all-optical magnetization switching in GdFeCo microstructures'', Nature Communications 6, 5839 (2015). [6] R. R. Subkhangulov, A. B. Henriques, P. H. O. Rappl, E. Abramof, Th. Rasing, A. V. Kimel, All-optical manipulation and probing of the d--f exchange interaction in EuTe, Scientific Reports 4, 4368 (2014). [7] D. Bossini, S. Dal Conte, Y. Hashimoto, A. Secchi, R. V. Pisarev, Th. Rasing, G. Cerullo, and A. V. Kimel, Macrospin dynamics in antiferromagnets triggered by sub-20 femtosecond injection of nanomagnons, Nature Communications 7, 10645 (2016). [Preview Abstract] |
Friday, March 17, 2017 9:12AM - 9:24AM |
X50.00005: Nonreciprocal dynamics of domain wall in ferromagnetic film Shu Zhang, Oleg Tchernyshyov We model the dynamics of a domain wall in a thin ferromagnetic film with the easy axis perpendicular to the film plane. A domain wall is modeled as a string whose Lagrangian, in addition to the standard string tension and kinetic energy, possesses a Berry phase term reflecting the precessional dynamics of spins. Waves propagating left and right have different speeds on such a string. We solve the equations of motion for a domain wall driven by an external magnetic field. A sudden application of an in-plane field results in the appearance of kinks (slope discontinuities) on the domain wall, which propagate back and forth along the wall. [Preview Abstract] |
Friday, March 17, 2017 9:24AM - 9:36AM |
X50.00006: Berry curvature of spin waves in ferromagnetic films with dipole-exchange interactions Akihiro Okamoto, Shuichi Murakami Berry curvature of magnetostatic waves in ferromagnetic films gives rise to thermal Hall effect. In previous works, its Berry curvature has been studied for the spin waves with dipole interaction only. Nevertheless, spin-wave dispersion is largely affected by the exchange interaction. Thus, in this presentation, we calculated Berry curvature of spin waves in ferromagnetic film with dipole-exchange interactions. The resulting Berry curvature reproduces that in previous works in the magnetostatic limit. It shows new aspects depending on the thickness of the film. Furthermore, the Berry curvature has a peak at the crossings of the eigenmodes, where hybridizations between the modes are significant. We find that its dependence on the energy gap is different from the Berry curvature of electron systems. We also discuss effects of other hybridizations, such as those with electromagnetic waves. [Preview Abstract] |
Friday, March 17, 2017 9:36AM - 9:48AM |
X50.00007: Field dependence of single phonon electron spin lattice relaxation Johan van Tol, Xiaoling Wang Longitudinal and transverse electron spin relaxation processes are of importance for a variety of applications, for example quantum information processing, quantum memory, spin cooling, and dynamic nuclear polarization. We measured the temperature- and field-dependence of electron spin relaxation in a variety of spin systems directly in the time domain with high frequency pulsed electron spin resonance at various fields in the 0.3-12 T range . Here we will focus on the direct single-phonon spin lattice relaxation rates, which are found to have a strong field dependence (B$^{\mathrm{2}}$-B$^{\mathrm{4}})$ in the high frequency/field and low temperature regime. A comparison is made between transition metal impurities in crystals, donor bound electrons in semiconductors, and other systems of interest to quantum information processing. The spin lattice relaxation time (T$_{\mathrm{1}})$ of organic radicals in frozen solutions also show a marked field dependence. This is of importance to dissolution- and solid state dynamical nuclear polarization (DNP) for NMR and MRI studies. The experimental results are compared with existing theoretical models. [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:24AM |
X50.00008: The magnetization dynamics of nano-contact spin-torque vortex oscillators Invited Speaker: Paul Keatley The operation of nano-contact (NC) spin-torque vortex oscillators (STVOs) is underpinned by vortex gyration in response to spin-torque delivered by high density current passing through the magnetic layers of a spin valve. Gyration directly beneath the NC yields radio frequency (RF) emission through the giant magnetoresistance (GMR) effect, which can be readily detected electronically. The magnetization dynamics that extend beyond the NC perimeter contribute little to the GMR signal, but are crucial for synchronization of multiple NC-STVOs that share the same spin valve film. In this work time-resolved scanning Kerr microscopy (TRSKM) was used to directly image the extended dynamics of STVOs phase-locked to an injected RF current. In this talk the dynamics of single 250-nm diameter NCs, and a pair of 100-nm diameter NCs, will be presented. In general the Kerr images reveal well-defined localized and far-field dynamics, driven by spin-torque and RF current Oersted fields respectively. The RF frequency, RF Oersted field, direction of an in-plane magnetic field, and equilibrium magnetic state, all influenced the spatial character of the dynamics observed in single NCs. In the pair of NCs, two modes were observed in the RF emission. Kerr images revealed that a vortex was formed beneath each NC and that the mode with enhanced spectral amplitude and line quality appeared to be correlated with two localized regions oscillating with similar amplitude and phase, while a second weaker mode exhibited amplitude and phase differences. This suggests that the RF emission was generated by collective modes of vortex gyration dynamically coupled via magnetization dynamics and dipolar interactions of the shared magnetic layers. Within the constraints of injection locking, this work demonstrates that TRSKM can provide valuable insight into the spatial character and time-evolution of magnetization dynamics generated by NC-STVOs and the conditions that may favor their synchronization. [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X50.00009: Nanowire spin Hall oscillators: width dependence and spatial mapping Kemal Sobotkiewich, Andrew Smith, Kyongmo An, Xin Ma, Kevin Olsson, Eric Montoya, Ilya Krivorotov, Xiaoqin Li We present experimental studies of auto-oscillatory modes in nanowire Spin Hall Oscillators(SHOs). The nanowires are composed of Pt(7 nm)/Py(5 nm)/AlOx(2 nm). A direct dc current induces the Spin Hall Effect (SHE) in the platinum providing a pure transverse spin current in the Permalloy. This spin current exert an anti-damping spin torque which enables auto-oscillations. In particular, we investigate how the width of the nanowire affects the critical current density required to induce the auto-oscillations and which modes undergo the auto-oscillation. For the latter, the spatial resolution afforded by the micro Brillouin Light Scattering technique ($\mu$-BLS) is crucial. By scanning the beam spot across the sample we were able to distinguish between edge and bulk modes spatially. We determined that they have different threshold currents and frequency shifts with increasing direct current. [Preview Abstract] |
Friday, March 17, 2017 10:36AM - 10:48AM |
X50.00010: Origin of 1/f noise found in ensembles of random telegraph noise oscillators Barry Costanzi, Dan Dahlberg We experimentally observe 1/f noise appearing as an aggregate of 1/f$^2$ signals from magnetic nanodots undergoing random telegraph noise (RTN) oscillations in their magnetizations. 250nm square permalloy dots (10nm thick) exhibit RTN in magnetization at the appropriate applied fields, and these fluctuations are measured in individual dots through the anisotropic magnetoresistance. The RTN in the resistance results in a Lorentzian power spectral density (PSD). Measuring multiple oscillating dots at once, however, shows an emergent 1/f PSD, in agreement with the van der Ziel theory [1] which predicts an aggregate 1/f spectrum for an appropriate distribution of Lorentzian spectra. Our collections of dots exhibit the necessary energy distributions predicted by the van der Ziel theory, but this emergent behavior can be observed for as few as two RTN oscillators, significantly softening Van der Ziel's requirement of a broad distribution of Lorentzians for 1/f to emerge. The RTN fluctuations are as small as one part in 10$^5$ compared to DC voltages while still leading to easily observable 1/f noise, suggesting the plausibility of very small RTN being responsible for 1/f in other systems even if no obvious RTN has yet been observed. [1] A. van der Ziel, Physica \textbf{16}, 359 (1950) [Preview Abstract] |
Friday, March 17, 2017 10:48AM - 11:00AM |
X50.00011: Time reversal symmetry avoided transitions in quantum nonadiabatic processes Fuxiang Li, Nikolai Sinitsyn Kramers degeneracy theorem is one of the basic results in quantum mechanics. According to it, the time- reversal symmetry makes each energy level of a half-integer spin system at least doubly degenerate, meaning the absence of transitions or scatterings between degenerate states if the Hamiltonian does not depend on time explicitly. We generalize this result to the case of explicitly time-dependent spin Hamiltonians. We prove that for a spin system with the total spin being a half integer, if its Hamiltonian and the evolution time interval are symmetric under a specifically defined time reversal operation, the scattering amplitude between an arbitrary initial state and its time reversed counterpart is exactly zero. We also discuss applications of this result to the multistate Landau–Zener (LZ) theory. [Preview Abstract] |
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