Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V19: Magnetic Nanoparticles: Spin Waves and StrainFocus

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Sponsoring Units: GMAG DMP Chair: Dustin Gilbert, NIST Natl Inst of Stds & Tech Room: LACC 308A 
Thursday, March 8, 2018 2:30PM  2:42PM 
V19.00001: Fresnel diffraction of spin waves Nicolás Loayza, Matthias Yougfleisch, Axel Hoffmann, Matthieu Bailleul, Vincent Vlaminck Spin waves, which have long been investigated for their potential in microwave electronics, found a renewed interest in the emerging field of magnonics, with promising functionalities for computation applications. In this context, several concepts of optics are applied to spinwaves, which includes recent works on the generation of spinwave beams in continuous thin films from constricted coplanar waveguides (CPW) [1,2]. In this work, we explore experimentally the nature of the emission for the isotropic magnetostatic forward volume wave mode via spinwave spectroscopy [3]. We found that the spin wave propagation remains focused in a beam of width comparable to that of the constriction, and that the spinwave amplitude displays oscillations in the direction transverse to the propagation. Both of these features are understood in the framework of the Fresnel theory of nearfield diffraction. These findings are of particular importance for future exploration of spinwave interferometry. 
Thursday, March 8, 2018 2:42PM  2:54PM 
V19.00002: Dipole exchange spin wave modes in multilayered ferromagnetic films Rodrigo Arias, Ignacio Armijo Dipole exchange spin wave modes are studied theoretically in multilayered ferromagnetic films under different and general configurations using a method that allows to calculate them with ease. The studied geometries correspond to a couple or several ferromagnetic films separated by non magnetic layers, or to periodically repeated configurations. In general there is a magnetic field applied obliquely with respect to the planar directions, boundary conditions of different origin are considered. 
Thursday, March 8, 2018 2:54PM  3:06PM 
V19.00003: Berry curvature of magnons coupled with elastic and electromagnetic waves in ferromagnets Akihiro Okamoto, Shuichi Murakami The Berry curvature is a differentialgeometric quantity defined in terms of the Bloch wavefunctions, and it manifests itself in Hall effects, in the shift of wavepackets, and even in various topological phases. Recently we calculated the Berry curvature of magnons in ferromagnetic films with dipoleexchange interactions. The Berry curvature of magnons is enhanced at the crossings of the eigenmodes due to the hybridizations between them. 
Thursday, March 8, 2018 3:06PM  3:42PM 
V19.00004: Direct imaging of delayed magneto dynamic modes induced by surface acoustic waves Invited Speaker: Ferran Macia Changes in strain can be used to modify electronic and magnetic properties in crystal structures, to manipulate nanoparticles and cells, or to control chemical reactions. The magnetoelastic effectthe change of magnetic properties caused by the elastic deformation (strain) of a magnetic materialhas been proposed as an alternative approach to magnetic fields for the lowpower control of magnetization states of nanoelements since it avoids charge currents, which entail ohmic losses. Multiferroic heterostructures and nanocomposites have exploited this effect in search of electric control of magnetic states, mostly in static regimes. We have studied the effect of dynamic strain accompanying a surface acoustic wave on magnetic nanostructures in thermal equilibrium. We have developed a new experimental technique based on stroboscopic Xray microscopy that provides a pathway to the quantitative study of strain waves and magnetization at the nanoscale. We have simultaneously imaged the temporal evolution of both strain waves and magnetization dynamics of nanostructures at the picosecond timescale and found that magnetization modes have a delayed response to the strain modes, adjustable by the magnetic domain configuration. The results I will present in this talk provide fundamental insight into magnetoelastic coupling in nanostructures having implications in the design of straincontrolled magnetostrictivenanodevices. 
Thursday, March 8, 2018 3:42PM  3:54PM 
V19.00005: Manipulation of Magnetic NanoParticles with Strain Mediated Multiferroics Cai Chen, Robert Dyro, Greg Carman, Abdon Sepulveda Strain mediated multiferroic systems provide an energyefficient path to the manipulation of magnetic nanoparticles for small scale mechanical, electrical, and biological applications. In this system, there is a magnetoleastic disk, a piezoelectric substrate and soft magnetic nano particles near the disk. The simulation consists of 1) voltage generated piezostrains; 2) disk magnetic spin reorientation; and 3) the dynamic nanoparticle movement. The particle dynamic model includes both translation and rotation considering: magnetic force, fluid drag, frictional force. The particle movement is studied for three dynamic cases with different frictional and drag coefficients: underdamped, critically damped and over damped. When the frictional and drag forces are small, the particle follows the rotation with a lag angle at steady state. The lag angle highly depends on the rotating frequency and the frictional/drag coefficients. When the frictional and drag forces further increase, the particle detaches from the disk when a critical velocity is reached. This work provides guidance on how to implement magnetic particle manipulation for a wide range of applications. 
Thursday, March 8, 2018 3:54PM  4:06PM 
V19.00006: MultiParticle Dynamics in Strain Mediated Multiferroics Robert Dyro, Cai Chen, Greg Carman, Abdon Sepulveda Strain mediated multiferroics has opened the possibility of localized control of the magnetization vector which in turn allows for softmagnetic nanoparticle manipulation with high precision. No dynamic model exists to simulate the full multiparticle dynamics in multiferroic systems. This work focuses on developing a 3D multibody model of magnetic particle motion driven by magnetic forces, particle repulsive forces, frictional forces, fluid drag and adhesion forces considering asperities. Magnetic forces and particle repulsive force were evaluated directly from the finite difference model by postprocessing the output nodal fields. Other forces were calculated using analytical expressions. The system studied in this work is a FeGa disk on a PZT substrate. First, we studied the particle distribution when the magnetization is static. The results show good agreement with previous experimental data. Next, the dynamic distribution of particles was studied with the magnetization rotating at constant angular velocity, comparing the distribution under different angular velocities. Particle distribution highly depends on the magnetic properties and particle number. We believe that this is the first model to predict multiparticle dynamics in multiferroic systems with full 3D dynamics. 
Thursday, March 8, 2018 4:06PM  4:18PM 
V19.00007: Broadband Electrically Small Antennas with Strain Powered Multiferroics JinZhao Hu, John Domann, Scott Keller, Greg Carman, Abdon Sepulveda Antennae built from magnetic materials offer a promising concept for reducing antennae’s size to below free space wavelengths (i.e. < λ/50). However, recent studies on strain mediated multiferroic antenna heterostructures show only narrow band operation. To overcome this issue, we studied a system consisting of a magnetoelastic single domain disk placed atop a piezoelectric substrate. The spinning magnetization radiates electromagnetic energy into free space over broadband frequencies, i.e. from veryhigh frequency to ultrahigh frequency. The magnetization is driven with patterned electrodes with periodic input voltages, which produce strain that couples to the magnetoelastic media. Two magnetoelastodynamic modeling approaches were used; one within a micromagnetic finite element formulation and the other in a macro spin framework. The free space radiation is modeled by considering two orthogonal magnetic dipoles rotating out of phase. 
Thursday, March 8, 2018 4:18PM  4:30PM 
V19.00008: Chaotic magnetization dynamics excited by spin transfer torque in elliptical nanodots Eric Montoya, Salvatore Perna, Massimiliano d'Aquino, Claudio Serpico, Ilya Krivorotov Recent theoretical work has shown that acdriven magnetization dynamics near saddle equilibria in magnetic elliptical nanodots exhibits chaotic behavior. The tools of nonlinear dynamical systems allow the definition of the concept of erosion of the stability region of magnetic equilibria in absence of the ac drive. From an energy perspective, the effect of the erosion is similar to a reduction of the magnetic potential barrier separating two stable equilibrium states of the nanodot, as exploited in energy assisted magnetic reversal schemes. In this framework, we study experimentally this phenomenon by measuring thermal telegraph noise of the free magnetic layer in nanoscale elliptical magnetic tunnel junctions (MTJs) in the presence of AC drive provided by the spin transfer torque arising from radiofrequency (RF) current applied to the MTJ. We follow the switching rate of the free layer, which serves as an indicator for the occurrence of chaotic dynamics as a function of both the frequency and power of the applied microwave excitation. The experimental results are compared to analytical solutions and micromagnetic simulations. 
Thursday, March 8, 2018 4:30PM  4:42PM 
V19.00009: A hybrid magnetic randomaccess memory using spinorbit torque and multiferroics Qianchang Wang, John Domann, Guoqiang Yu, Anthony Barra, Kang Wang, Greg Carman Spinorbit torque (SOT) is an energy efficient method to control magnetization with magnetic memory applications. However, using SOT to switch memory bits with perpendicular magnetic anisotropy (PMA) is still a challenge. Here we present a new strainmediated SOT switching mechanism. The strain induced magnetoelastic anisotropy in the multiferroic heterostructure breaks lateral symmetry of SOT device thus produces fieldfree deterministic perpendicular switching. A finite element model and a macrospin model are used to numerically simulate the switching mechanism. Space inversion transformation is performed to show the symmetry breaking of strainmediated SOT system. A relatively small voltage (±0.5V) combined with a modest current (3.5×10^{7} A/cm^{2}) produces 180° perpendicular magnetization reversal. The switching direction (‘up’ or ‘down’) is dictated by the voltage polarity (positive or negative) applied to piezoelectric layer in the multiferroic heterostructure. The switching is fast (up to 10GHz) and does not require precisely timed voltage or current pulses. Based on the strainmediated SOT control mechanism, a new randomaccess memory system is designed with high storage density and high scalability. 
Thursday, March 8, 2018 4:42PM  4:54PM 
V19.00010: Spin superfluid junction oscillator Yizhou Liu, Igor Barsukov, Ilya Krivorotov, Yafis Barlas, Roger Lake A spin oscillator is proposed based on a spin superfluid junction composed of two exchange coupled easyplane ferromagnets. A spin chemical potential across the junction, with its spin polarization perpendicular to the easyplane, can be established by driving electrical currents through two heavy metal contacts. When the spin chemical potential exceeds the threshold, it drives steadystate spin oscillation with a 2π precession angle, which can be electrically measured by the giant magnetoresistance, the anisotropic magnetoresistance, and the inverse spin Hall effect. The oscillation amplitude and frequency can be tuned via the applied D.C. current. An A.C. current applied to the heavy metal contacts, the spin oscillation also exhibits Shapiro steps, whose timeaveraged component is fixed by the modelocking. These features provide signatures for the experimental verification of spin superfluidity in easyplane ferromagnets. This new type of spin nanooscillator could serve as a device in a spintronic based neuromoprhic computing scheme. 
Thursday, March 8, 2018 4:54PM  5:06PM 
V19.00011: Allmagnonic switching and spinvalve using antiferromagnetic magnons Ran Cheng, Di Xiao, JianGang Zhu In a collinear antiferromagnet (AF) with uniaxial anisotropy, symmetry guarantees that magnon excitations are doubly degenerate. The two degenerate modes carry opposite spin angular momentum and can play a similar role as spin1/2 electrons. At an exchange coupled interface with a ferromagnet (F), thermal populations of the two modes are imbalanced, which can form a pure spin current that propagates under a thermal gradient. Here we present a theoretical framework of magnon transport in an insulating F/AF/F heterostructure. We find that a sufficiently large temperature gradient can switch the downstream F via magnon spin current in the AF if the system is initially prepared in an antiparallel configuration. This thermomagnetic switching opens the possibility of allmagnonic spin valve. 
Thursday, March 8, 2018 5:06PM  5:18PM 
V19.00012: High Frequency Spin Dynamics of Magnetic Tunneling Junctions Consisting of Two Perpendicular Free Layers Rajapaksayalage Rajapakse, Nicholas Penthorn, HongWen Jiang Magnetic Tunnel Junctions (MTJ) with one fixed layer and one free layer have been extensively studied as a memory element, a microwave oscillator and a microwave detector. With the advent of harvesting interfacial perpendicular magnetic anisotropy (IPMA), perpendicular MTJs have drawn much recent attention over the prior sate of the art inplane structures. We report our experimental investigation of MTJ structures with two perpendicular freelayers, with different strengths of IPMA. We find high frequency microwave emission can be produced at zero bias field when a small DC current is applied. We will show our understanding on mutual roles played by each IPMA layer. Micromagnetic simulations will be presented demonstrating the spin dynamics of the two free layers. The role of electron heating in these nanoscale structures, on the microwave emission spectrum, will be discussed. The work was supported by Center for Function Accelerated nanoMaterials (FAME). 
Thursday, March 8, 2018 5:18PM  5:30PM 
V19.00013: Spin Wave Field Effect Transistor YuJin Chen, Han Lee, Alejandro Jara, Amanatullah Khan, En Yang, Patrick Braganca, Ilya Krivorotov Spin waves (SWs) have been proposed as information carriers in novel logic devices. Voltagecontrolled magnetic anisotropy (VCMA) is a promising approach to generation and control of spin waves due to its high energy efficiency. We developed a spin wave field effect transistor (SWFET), which can turn on and turn off SW propagation in a ferromagnetic nanowire with a gate voltage via VCMA effect. We experimentally demonstrate a functioning nanoscale SWFET based on a Ta/CoFeB/MgO multilayer nanowire. The device consists of a ferromagnetic nanowire channel magnetized out of the sample plane for SW propagation and a voltage gate on top of the wire. Propagating SWs are excited on one side of the gate via application of a localized ac spin Hall torque at the SW resonance frequency. SW propagation to the other side of the gate is detected via anomalous Hall effect (AHE). Utilizing VCMA, a dc voltage applied to the gate adjusts perpendicular magnetic anisotropy under the gate and transforms the spatially uniform magnetic energy landscape into a potential well or barrier for SW propagation depending on the gate voltage polarity. Our data demonstrate that VCMAinduced magnetic barrier can efficiently suppress SW propagation under the top gate. 
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