Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session R41: Spin Dynamics IIFocus
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Sponsoring Units: GMAG DMP Chair: Victor Brar, University of Wisconsin-Madison Room: BCEC 209 |
Thursday, March 7, 2019 8:00AM - 8:12AM |
R41.00001: Nanometer-Thick YIG-Based Magnonic Crystals with Large Tunable Bandgaps Huajun Qin, Gert-Jan Both, Sampo J. Hämäläinen, Sebastiaan van Dijken Control of information-carrying spin waves in magnonic crystals is essential for the development of magnon-based computing. Crystals comprised of a periodic array of ferromagnetic metals offer versatility in band structure design, but strong magnetic damping restricts their transmission efficiency. Yttrium iron garnet (YIG) with ultralow damping is the palpable alternative, yet its small magnetization limits dynamic dipolar coupling between discrete units in the technological imperative Damon-Eshbach (DE) geometry. Here, we experimentally demonstrate low-loss spin-wave manipulation in one-dimensional magnonic crystals of physically separated nanometer-thick YIG stripes. We enhance the transmission of DE spin waves in allowed minibands by filling the gaps between the stripes with CoFeB. The thus-formed magnonic crystals exhibit tunable bandgaps of 50 MHz - 200 MHz with nearly complete suppression of the spin-wave signal. We also show that efficient Bragg scattering on just two airgaps or two CoFeB stripes already produces clear frequency gaps in spin-wave transmission spectra. |
Thursday, March 7, 2019 8:12AM - 8:24AM |
R41.00002: Ferromagnetic resonance and proximity effect in WTe2/magnetic insulator heterostructure Peng Li, Edbert Jarvis Sie, Jacob Wisser, Lauren Riddiford, Aaron Altman, Aaron Lindenberg, Yuri Suzuki Integration of FMI films with 2D materials or topological insulators has proven to be promising for the demonstration of spin-orbit torque switching and more efficient spintronic devices. As an alternative to YIG, we have identified spinel ferrite MgAl0.5Fe1.5O4 (MAFO) for the spin source material. The Gilbert damping constant of MAFO is 0.001, similar in magnitude to typical YIG. In this work, we have realized bilayer structures of exfoliated nanometer thick WTe2 flakes on top of low damping MAFO thin films. The samples were post-annealed at 300 oC for 5 h at a vacuum pressure, giving rise to strong exchange coupling between the 2D and the MAFO layers. Raman spectroscopy showed that WTe2 maintained the desired phase after processing. Together with atomic force microscopy, the thickness of WTe2 was estimated to be several atomic layers. Magnetoresistance measurements revealed a hysteresis loop that was correlated with the magnetization curves of MAFO. These results suggest proximity-induced ferromagnetism in the WTe2 atoms at the interface. We characterized our samples with spin-torque ferromagnetic resonance. This new WTe2/magnetic insulator system is promising for future spinel ferrite insulator based spin current devices. |
Thursday, March 7, 2019 8:24AM - 8:36AM |
R41.00003: The Effect of Defects on Precessional Dynamics in a Magnetic Field and Resulting Spin Wave Modes in Elliptical Nanomagnets with In-Plane Anisotropy Md Ahsanul Abeed, Sourav Sahoo, David Winters, Anjan Barman, Supriyo Bandyopadhyay We have simulated the precessional dynamics of magnetization in elliptical nanomagnets biased with an in-plane magnetic field along the minor axis and perturbed by a small out-of-plane magnetic field. The precessional dynamics gives rise to spin wave modes in the nanomagnets. We first calculate the time-dependent out-of-plane component of the magnetization at a fixed plane of the magnet at various coordinate points and also the spatially averaged magnetization as a function of time. We then find the Fourier transform of the spatially averaged component to find the dominant precessional frequencies (which do not necessarily correspond to the Kittel frequencies) and calculate the power and phase profiles of the spin wave modes at those frequencies. We find that the spin wave modes are significantly affected by the presence of defects in the nanomagnets. We have studied six different types of defects arising from material voids and thickness variations which are realistic defects that arise during fabrication processes. Our results show that the spin wave modes are significantly affected by defects. This has a serious implications for devices that rely on spin wave modes such as oscillators and neuromorphic computers based on phase locked oscillators. |
Thursday, March 7, 2019 8:36AM - 8:48AM |
R41.00004: Spin-current-mediated rapid magnon localization and coalescence after ultrafast optical pumping of ferrimagnetic alloys Ezio Iacocca, Alex Reid, Alexey Kimel, Theo Rasing, Roy Chantrell, Mark A Hoefer, Thomas Silva, Hermann Dürr We find evidence of rapid magnetic order recovery via nonlinear magnon processes after ultrafast demagnetization of amorphous GdFeCo thin films with perpendicular magnetic anisotropy. The spatial evolution of the magnetization is measured by time-resolved resonant X-ray scattering and modelled by atomistic and multiscale micromagnetic simulations. We identify both localization and coalescence processes. During localization a paramagnetic state evolves into a collection of localized textures while coalescence describes their growth, break-up, and merging. The characteristic length scale during coalescence is found to grow according to a power law in both experiments and simulations. Our results shed light into the physical mechanisms that are important for the picosecond recovery of magnetic order. |
Thursday, March 7, 2019 8:48AM - 9:00AM |
R41.00005: Magnon-mediated Neel torques and magnonic responses to Neel order dynamics Tianlin Li, Vladimir Zyuzin, Alexey Kovalev Using the Neel ground state, we formulate a microscopic linear response theory of magnon-mediated Neel torques generated by the thermal gradient in collinear antiferromagnets. We also describe the inverse effects in which spin and heat currents are induced by the Neel order dynamics. We obtain the extrinsic and intrinsic contributions where the intrinsic contributions are related to the mixed space Berry curvature. We derive a closed equation of motion of Neel dynamics in antiferromagnets related to thermal Neel torque from linearized LLG equation. We confirm that the Neel torque effect is related to the inverse effect by the Onsager relations. We apply our theory to honeycomb antiferromagnet and confirm that a net magnon-mediated heat current can be induced by the Neel order precession. |
Thursday, March 7, 2019 9:00AM - 9:12AM |
R41.00006: General method for atomistic spin-lattice dynamics with first principles accuracy Olle Eriksson, Johan Hellsvik, Anna Delin, Anders Bergman, Lars Bergqvist, Danny Thonig, Diana Iusan, Klas Modin A computationally efficient general first-principles based method is presented, with applications for spin-lattice simulations for solids and clusters. The method is based on a combination of atomistic spin dynamics and molecular dynamics, expressed through a spin-lattice Hamiltonian where the bilinear magnetic term is expanded to second order in displacement. The effect of first-order spin-lattice coupling on the magnon and phonon dispersion in bcc Fe is reported as an example, and is seen to be in good agreement with previous simulations performed with an empirical potential approach. In addition, we also illustrate the abilities of our method on a more conceptual level, by exploring dissipation free spin and lattice motion in small magnetic clusters. |
(Author Not Attending)
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R41.00007: Magnetic Vortex as a Spin Wave Filter Danilo Froes Batista, Joao Paulo Sinnecker Magnonics aims to use spin-waves for data transmission and processing. However, the use of an external field jeopardizes the energy efficiency. The use of domain walls as waveguides has been proposed1, as they exist in remanent states. Also, the waves are channeled within the wall width, due a magnetostatic potential well. |
Thursday, March 7, 2019 9:24AM - 9:36AM |
R41.00008: ABSTRACT WITHDRAWN
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(Author Not Attending)
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R41.00009: Dynamical amplification of magnetoresistances and Hall currents up to the THz regime Filipe Guimaraes, Manuel dos Santos Dias, Juba Bouaziz, Antônio Costa, Roberto B. Muniz, Samir Lounis The quest for device miniaturization, decreased energy consumption, and improved speeds of processing and storage units naturally led to exploiting the connection between electric currents and magnetization dynamics. In this presentation, I demonstrate that ferromagnetic and antiferromagnetic excitations can be triggered by the dynamical spin accumulations induced by the bulk and surface contributions of the spin Hall effect [1]. I also analyze the general concepts of magnetoresistance and Hall effects together with spin-orbit-related mechanisms to demonstrate how dynamical currents can be dramatically enhanced and precisely controlled by applying ac electric fields and static magnetic fields, in a materials-specific approach [2]. This work may also impact experimental techniques that use currents to probe and quantify magnetization precession and the torques that induce it. |
Thursday, March 7, 2019 9:48AM - 10:00AM |
R41.00010: Bound-state spin-wave spectroscopy exploiting nonlinear three-magnon processes Zhenyu Wang, Beining Zhang, Yunshan Cao, Peng Yan One recent breakthrough in the field of magnonics is the experimental realization of reconfigurable spin-wave nanochannels formed by a magnetic domain wall with a width of 10–100 nm. This remarkable progress enables an energy-efficient spin-wave propagation with a well-defined wave vector along its propagating path inside the wall. In the mentioned experiment, a microfocus Brillouin light scattering spectroscopy was taken in a line-scans manner to measure the frequency of the bounded spin wave. Due to their localization nature, the confined spin waves can hardly be detected from outside the wall channel, which guarantees the information security to some extent. In this work, we theoretically propose a scheme to detect/eavesdrop on the spin waves inside the domain-wall nanochannel via nonlinear three-magnon processes [1]. The approach can be parallelly applied for probing the Dzyaloshinskii-Moriya interaction in narrow magnetic stripes [2]. The idea is analytically formulated with micromagnetic simulations performed to verify the theoretical predictions. |
Thursday, March 7, 2019 10:00AM - 10:12AM |
R41.00011: Transverse spin currents carried by surface spin waves in obliquely magnetized magnetic films Cody Trevillian, Vasyl S Tyberkevych Properties of spin waves (SWs) are well-studied in perpendicular and in-plane magnetized magnetic films, but are understood much worse in the case of oblique magnetization (OM). Recent studies [1, 2] show an unexpected and interesting effect in OM films, namely, an appearance of transverse spin currents (TSCs) carried by SWs propagating in the backward-volume-type geometry. Here, we theoretically study this effect for surface SWs propagating approximately perpendicularly to the bias magnetic field (Damon-Eschbach-type geometry). We show, that the TSC resonantly depends on the direction of SW propagation, reaching maximum when SWs propagate several degrees away from the normal to the bias field. The maximum TSC of surface SWs significantly exceeds that of volume SWs, which makes surface SWs preferable for experimental study of TSCs. |
Thursday, March 7, 2019 10:12AM - 10:24AM |
R41.00012: Off-resonance excitation of spin waves by a four-magnon process Robert McMichael, Paul Haney Recent experiments have shown that driving ferromagnetic resonance in thin films generates noisy stray fields over a broad frequency spectrum [1-3]. The noise has been attributed to spin waves excited through a four-magnon mechanism, although the specifics of the mechanism have not been identified. In this paper we present modeling of one such mechanism that relies on nonuniform fluctuations in Ms owing to thermal excitation. With Mz fluctuating, a uniform driving field at frequency f0 will excite spin waves at frequencies fk. The uniform processional mode adds a resonant amplification of the driving field. The predicted spin wave noise power is proportional to input microwave power, T2, and 1/fk4 (temperature T, spin wave frequency fk). We find the most relevant spin waves for this process are near the bottom of the spin wave band. Numerical evaluation yields spin wave noise power that is roughly a factor of 102 weaker than the noise power due to pure thermal excitation. However, this result can be regarded as a lower bound. The effect may be much larger if the Mz fluctuations are due to spin waves that are excited above equilibrium [3]. |
Thursday, March 7, 2019 10:24AM - 10:36AM |
R41.00013: Berry curvature and topological edge modes for coupled waves between magnons and electromagnetic waves Akihiro Okamoto, Shuichi Murakami, Ryuichi Shindou The Chern number characterizes existence of the chiral topological edge modes inside the gap. The chiral topological edge modes are well studied in electronic systems, and they can be realized in photonic crystals and magnonic crystals, in which the time-reversal symmetry is broken. In addition to systems with a single kind of particles, systems consisting of more than two kinds of particles or quasiparticles attract much attention recently, in the context of topological phases. |
Thursday, March 7, 2019 10:36AM - 10:48AM |
R41.00014: Effects of Carbon Content on Magnetic and Microwave Properties of Iron Gallium Carbon Thin Films Xianfeng Liang, Cunzheng Dong, Nian Xiang Sun A systematic investigation of the soft magnetism, delta E effect, magnetostriction, and microwave properties has been carried out on iron gallium carbon (Fe-Ga-C) thin films over a wide carbon content range. The phase transformation of the Fe-Ga-C films from bcc polycrystalline to amorphous phase leads to excellent magnetic softness with a low coercivity of less than 1 Oe, high 4pMs, narrow ferromagnetic resonance (FMR) linewidth at 10 GHz of 20~30 Oe and ultra-low Gilbert damping constant of 0.0027. A record high piezomagnetic coefficient of 9.71 ppm/Oe, high saturation magnetostriction constant of 81.2 ppm, and large delta E effect of -120 GPa at 500 nm are also achieved. The soft magnetism, large delta E effect and piezomagnetic coefficient combined with ultra-low Gilbert damping constant make Fe-Ga-C thin film an attractive alloy for magnetoelectric and other voltage tunable RF/microwave device applications. |
Thursday, March 7, 2019 10:48AM - 11:00AM |
R41.00015: Edge-Solitons Meditated Dynamic Switching of Vortex Chirality and Polarity Yongming Luo, Yi Zheng Wu, Zhenghong Qian, Tiejun Zhou It has attracted great interests to explore efficient methods to control both the polarity and chirality of magnetic vortex in last decades. Here we report a new mechanism to control both the vortex chirality and polarity by electrical current, which is through the formation, decoupling and fusion of edge solitons. Our micromagnetic simulation show that the dynamics of the edge solitons at the disk edge can lead to selective switching of the vortex chirality and polarity. A way to directly write any of the four vortex states from any random states is proposed based on the switching diagram. The switching phase diagram as a function of disk diameters shows that the vortex chirality and polarity switching process is highly size-dependent. As the disk diameter scales down to below 160 nm, the vortex chirality and polarity are coupled together and always switch at the same time, resulting in an unchanged handedness before and after switch. Furthermore, the critical switch current can be as low as 3*10^6 A/cm^2. Our work offers an efficient way to control vortex chirality and polarity, which may have great application potential in the low-energy and high-density memory application. |
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