Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session B55: Spin Dynamics and TorquesFocus
|
Hide Abstracts |
Sponsoring Units: GMAG Chair: Nanna Zhou Hagström, University of California, Davis Room: Room 305 |
Monday, March 6, 2023 11:30AM - 12:06PM |
B55.00001: Ultrafast Magnetization Switching in Ferro and Ferri-magnets Invited Speaker: Debanjan Polley Ultrafast helicity-independent all-optical switching (HI-AOS) as well as ps current-pulse driven magnetization reversal of ferrimagnets (FEMs) suggest the pathway of realizing ultrafast magnetization reversal of a magnetic tunnel junction (MTJ) based memory element. However, due to low spin-polarization, FEMs aren’t an ideal choice for the free layer of a MTJ device. Now, the spin-polarization of the free layer could be enhanced by coupling a ferromagnet (FM) with the FEM, given a reliable coupling mechanism. Here, we focus on RKKY exchange coupling mediated HI-AOS of a FM, which is magnetically coupled with a FEM. A Co/Pt, coupled to a GdFeCo, had been switched within ~7 ps after ~100 fs optical excitation. However, GdFeCo is not a desirable choice for building a nano-patterned memory cell because of its unstable perpendicular magnetic anisotropy (PMA) below 1 mm, whereas CoGd or synthetic ferrimagnets seems to be a good choice. We recently demonstrated ultrafast magnetization switching in a Co/Pt layer which is magnetically coupled to a CoGd layer via a Pt spacer. CoGd reverses its magnetization after ~1.5 ps following ~100 fs optical excitation. The Co/Pt shows a unique two-step mechanism related to the RKKY coupling driven switching. It demagnetizes much faster then CoGd, then stays demagnetized for some time before actually switching at ~3.5 ps. Considering inter-sublattice, intra-sublattice and indirect RKKY exchange scatterings, we explain the magnetization reversal dynamics of different magnetic components of ferromagnetically and antiferromagnetically coupled as well as decoupled FM-FEM heterostructures with a modified microscopic three-temperature model. The calculated values of switching times agree with the experimental observations. New material systems are constantly being studied which can work as the free layer of the MTJ. Recently, HI-AOS has been demonstrated in a MTJ structure, using Co/Gd bilayer coupled with CoFeB as the free layer. Co/Tb multilayers also show HI-AOS both with ~fs and ~ps optical pulse excitation and thereby expands the material choice of the free layer of a MTJ device. |
Monday, March 6, 2023 12:06PM - 12:18PM |
B55.00002: Bridging the experimental observables in the pump-probe experiment with the ab-initio methods Mohamed F Elhanoty The interaction of coherent pulsed lasers with magnetically ordered materials induces various elementary excitations and drives the material out of equilibrium at timescales as short as the driving laser itself. The pump-probe experiment on magnetic materials can measure the temporal evolution of the magnetic moment at femtosecond timescales. Bridging the macroscopic experimental observables to the quantum mechanical microscopic picture is of fundamental importance to understand the interplay of different degrees of freedom and correctly interpret the experimental measurements. To do this, we devise the cutting-edge TDDFT to present a mixed scheme between the time evolution of the time-dependent Kohn Sham (TDKS) equation and the linear response to transiently calculate the dielectric tensor. By introducing the electron correlations from the Hubbard model to the quasi-particle TDKS Hamiltonian, we find electron correlations play a fundamental role in interpreting the transient shift of the transient absorption spectra. We apply our scheme to Co and Ni and achieve very good agreement with the state-of-the-art pump-probe experiments at the L edge. We further extend our conclusions to the transient MOKE experiments measuring the magneto-optical functions as a probe for the magnetization dynamics via the discrete energies from the high harmonic generation laser sources and discuss possible contradicting results at energy windows below and above the absorption edge |
Monday, March 6, 2023 12:18PM - 12:30PM |
B55.00003: Coherent terahertz control of antiferromagnetic order in the intercalated transition metal dichalcogenide. Azel Murzabekova, Soyeun Kim, Kannan Lu, Junehu Park, Soho Shim, Nina Bielinski, Lazar L Kish, Lisa M DeBeer-Schmitt, Adam A Aczel, Jaeseok Son, Tae W Noh, S. Lance Cooper, Nadya Mason, Greg MacDougall, Andre Schleife, Fahad Mahmood Antiferromagnetic (AFM) materials have been widely discussed as potential replacements for conventional ferromagnetic memory devices because of their stability in external magnetic fields and fast terahertz (THz) switching timescales. Recent work has shown that the AFM metal Fe1/3NbS2 can be electrically switched using remarkably low current densities. However, optical switching of Fe1/3NbS2 using a THz electrical field has not yet been demonstrated. Here, we perform static polar Kerr and THz-induced dynamic Kerr rotation measurements, combined with neutron scattering experiments and density-functional theory (DFT) calculations to investigate the AFM switching mechanisms in Fe1/3NbS2. We confirm the non-collinear nature of the AFM ordering below the Néel temperature and observe a THz-induced Kerr ellipticity that persists at temperatures three times higher than the Néel temperature. These findings underscore the presence of glassy dynamics in Fe1/3NbS2 and highlight the possibility of manipulating antiferromagnetic ordering in metals using THz electric fields. |
Monday, March 6, 2023 12:30PM - 12:42PM |
B55.00004: Visualization of spin-wave propagation dynamics using laser-free GHz stroboscopic transmission electron microscopy Chuhang Liu, Spencer A Reisbick, Myung-Geun Han, Alexandre Pofelski, Chunguang Jing, Yimei Zhu Spin waves, also known as magnons, have the potential to play a vital role as information carriers in spintronics. Their lower energy damping, short wavelength and spin intrinsic nature, promise high-speed data processing in conventional and quantum information transmission. Although several experimental techniques have already been applied to study magnons, imaging spin wave dynamics with sufficient spatial resolution remains a challenge. |
Monday, March 6, 2023 12:42PM - 12:54PM |
B55.00005: Giant spin-charge conversion in ultrathin MnPtSb half-Heusler alloy Emanuele M Longo, Anastasios Markou, Claudia Felser, Marco Fanciulli, Roberto Mantovan Half-metallic Heusler alloys are gathering enormous interest as a platform for the next generation of spintronic devices, due to their unique band structure characterized by highly spin-polarized electronic transport at the Fermi level.[1,2] Here, the broadband ferromagnetic resonance spectroscopy (FMR) is adopted to investigate on the thickness-dependent magnetization dynamic of the MnPtSb(t)/Co(5nm)/Au(5nm) heterostructure down to t= 1nm, demonstrating the possibility to use the MnPtSb alloy as an efficient spin-current generator. Spin pumping-FMR is also employed to determine the spin-to-charge conversion nature in such a system[3], indicating a predominant contribution from the surface states of the MnPtSb compound. Moreover, when interpreted as arising from the Inverse Edelstein Effect (IEE), the spin-charge conversion efficiency extracted at room temperature for the thinner samples reaches λIEE = 3 nm, to our knowledge being the highest room temperature conversion reported so far. |
Monday, March 6, 2023 12:54PM - 1:06PM |
B55.00006: Vertically Graded Single-Layer Fe-Ni Films with Low Damping and Unconventional Spin-Orbit Torques Rachel Maizel, Shuang Wu, Purnima P Balakrishnan, Alexander Grutter, Christy J Kinane, Andrew J Caruana, Prabandha Nakarmi, Bhuwan Nepal, David A Smith, Youngmin Lim, Julia L Jones, Wyatt Thomas, Jing Zhao, Frederick M Michel, Tim Mewes, Satoru Emori Energy-efficient spintronic devices require a large spin-orbit torque (SOT) and low damping to excite magnetic precession. While ultrathin-ferromagnet bilayers attain a larger SOT at the expense of higher damping, single-layer ferromagnets with bulk inversion asymmetry [1-3] may enable both low damping and sizable SOTs. Here, we examine the impact of intentional asymmetry on damping and SOTs in 10-nm-thick symmetric and vertically graded films consisting of two ferromagnetic elements: Fe with low intrinsic damping [4] and Ni with sizable spin-orbit coupling [5]. Out-of-plane ferromagnetic resonance confirms intrinsic damping parameters of ≈0.003 for both symmetric and graded films, indicating that low damping is maintained even with steep compositional gradients. Further, dc-biased spin-torque ferromagnetic resonance reveals a damping-like SOT in each Fe-Ni system, with an effective Spin-Hall angle exceeding ~0.01. Remarkably, we find that the magnitude of the SOT does not correlate with the vertical gradient, pointing to a nuanced origin of the SOT not captured by conventional mechanisms. Our results are a step toward understanding and engineering low-damping single-layer ferromagnetic films for SOT-driven applications. |
Monday, March 6, 2023 1:06PM - 1:18PM |
B55.00007: Hybrid spin Hall nano-oscillators based on ferromagnetic metal/ferrimagnetic insulator heterostructures Haowen Ren, Xin Yu Zheng, Sanyum Channa, Guanzhong Wu, Daisy O'Mahoney, Yuri Suzuki, Andrew D Kent Spin-Hall nano-oscillators (SHNO) are one of the potential devices to achieve current controlled GHz frequency signals in nanoscale devices for neuromorphic computing and creating Ising systems. Traditional SHNO devices, which are usually made of transition metals, exhibit low emission power and high threshold current and are hard to couple from each other. Here we demonstrate a new type of hybrid SHNO based on a permalloy ferromagnetic-metal nanowire and low-damping ferrimagnetic insulator thin films of lithium aluminum ferrite. We associate the improved characteristics (e.g., a lower threshold current and higher output power) of such hybrid SHNO with the simultaneous excitation of both magnetic layers. We further find that the presence of the ferrimagnetic insulator enhances the auto-oscillation amplitude of spin-wave edge modes, consistent with our micromagnetic modeling. This hybrid SHNO expands spintronic applications, providing new means of coupling multiple SHNOs for neuromorphic computing and advancing magnonics. |
Monday, March 6, 2023 1:18PM - 1:30PM |
B55.00008: High-harmonic generation in spin and charge current pumping at ferromagnetic or antiferromagnetic resonance in the presence of spin-orbit coupling Jalil Varela Manjarres, Branislav K Nikolic One of the cornerstone effects in spintronics is spin pumping by dynamical magnetization that is steadily precessing (around, e.g., the z-axis) with frequency ω0, due to absorption of low-power microwaves of frequency ω0 under the resonance conditions and in the absence of any applied bias voltage. The two-decades-old "standard model" of this effect, based on the scattering theory of quantum transport, predicts that component ISz of spin current vector (ISx,ISy,ISz)α ω0 is time-independent while ISx(t) and ISy(t) oscillate harmonically in time with single frequency ω0; whereas pumped charge current is zero I≡0 in the same adiabatic α ω0 limit. Here we employ more general time-dependent quantum transport formalism to predict unforeseen features of spin pumping -- precessing localized magnetic moments within ferromagnetic metal (FM) or antiferromagnetic metal (AFM), whose conduction electrons are exposed to spin-orbit coupling (SOC) of either intrinsic or proximity origin, will pump both spin ISα(t) and charge I(t) currents. All four of these functions harmonically oscillate in time at both even an odd integer multiples nω0 of the driving frequency ω0. Such high-harmonics are cut off at nmax, with possibility to tune nmax≤4 in the chosen for demonstration one-dimensional FM or AFM models with the Rashba SOC by increasing its strength. Finally, we conjecture that two-dimensional magnetic materials offer the optimal setting for experimentally confirming high harmonics in spin pumping as their magnetic ordering at finite temperature crucially relies on magnetic anisotropy originating from strong intrinsic SOC. To demonstrate this, we compute high harmonics for honeycomb lattice of doubly proximitized graphene with both magnetic ordering (due assumed magnetic overlayer) and SOC (due to assumed transition metal dichalcogenide underlayer). |
Monday, March 6, 2023 1:30PM - 1:42PM |
B55.00009: First principles calculation and experimental study of spin pumping and the spin Hall Effect in La0.67Sr0.33MnO3 In Jun Park, Timothy Mabe, Pushpendra Gupta, Abhisek Mishra, Anupama Swain, Subhankar Bedanta, Vivek P Amin The spin Hall effect is one of the most important mechanisms to generate spin currents in spintronic devices. Recently, experiments have shown that anti-damping spin torques and the inverse spin Hall effect occur simultaneously in the La0.67Sr0.33MnO3 (LSMO)/Pt heterostructures [1]. In this talk, we discuss theoretical calculations and experimental measurements of the inverse spin Hall effect in single layer LSMO films. In order to elucidate the physical origin of the experimental observations, we compute the spin Hall conductivity of LSMO using first principles calculations. First, density functional theory (DFT) is employed to calculate the ground state electronic structure of LSMO, as implemented in the Quantum Espresso package. Next, the calculated electronic structure is interpolated into the maximally-localized Wannier functions basis using Wannier90. Finally, using the Kubo formalism, we compute the full spin Hall conductivity tensor of LSMO under an applied electric field to assess the generation of spin current for a given electric field. We find spin Hall conductivity values for LSMO on the order of 10 S/cm, in reasonable agreeance with the experimental results. |
Monday, March 6, 2023 1:42PM - 1:54PM |
B55.00010: Machine learning nonequilibrium electron forces for spin dynamics of itinerant magnets Puhan Zhang, Sheng Zhang, Gia-Wei Chern We present a generalized potential theory for conservative as well as nonconservative forces for the Landau-Lifshitz magnetization dynamics. Importantly, this formulation makes possible an elegant generalization of the Behler-Parrinello machine learning (ML) approach, which is a cornerstone of ML-based quantum molecular dynamics methods, to the modeling of force fields in adiabatic spin dynamics of out-of-equilibrium itinerant magnetic systems. We demonstrate our approach by developing a deep-learning neural network that successfully learns the electron-mediated exchange forces in a driven s-d model computed from the nonequilibrium Green's function method. We show that dynamical simulations with forces predicted from the neural network accurately reproduce the voltage-driven domain-wall propagation. Our work also lays the foundation for ML modeling of spin transfer torques and opens a new avenue for ML-based multi-scale modeling of nonequilibrium dynamical phenomena in itinerant magnets and spintronics. |
Monday, March 6, 2023 1:54PM - 2:06PM |
B55.00011: Influence of Non-Uniform Magnetization Perturbation on Spin-Orbit Torque Measurements Ryan W Greening Measurements of spin–orbit torques in a ferromagnetic/nonmagnetic multilayer are typically based on an assumption that the entire ferromagnetic layer uniformly responds to the spin–orbit torque. This assumption breaks down when the thickness of the ferromagnetic layer is comparable to the dynamic exchange coupling length, which can be as short as a few nanometers in certain measurement geometries. The nonuniform magnetization perturbation coupled with nonuniform contribution of each magnetic sublayer to the magnetoresistance or the Kerr effect may impact the accuracy in the extrapolation of spin–orbit torque, particularly if a thick ferromagnetic layer is used. In this talk, we will discus the results of numerical models to investigate such an impact in three different techniques: the magneto-optic-Kerr-effect method, the second-harmonic method and the spin torque ferromagnetic resonance method. We show that the second-harmonic and magneto-optic-Kerr-effect methods are prone to be influenced by the nonuniform magnetization reorientation, while the spin torque ferromagnetic resonance method is much less impacted. |
Monday, March 6, 2023 2:06PM - 2:18PM |
B55.00012: Optimizing magnetic damping in Fe-Co based ferromagnetic alloys Tzu-Hsiang Lo, Jianchao Qian, Jinho Lim, Jian-Min Zuo, Axel Hoffmann Magnetic damping is central to the performance of ferromagnetic materials in many applications. In metallic ferromagnets, the damping can be due to electronic excitations, as well as due to coupling to the lattice. Previously, it has been demonstrated that Fe75Co25 has ultralow magnetic damping [1], since the composition has the lowest electronic density of states. It has also been shown that the magnetic damping in Fe50Co50 alloys can be reduced by the addition of carbon [2], which results in a structural transformation to an amorphous phase and thereby reduces the coupling of the magnetization dynamics to the lattice. Here, we combine the two strategies and investigate whether the same structural transformation can further reduce the magnetic damping of (Fe75Co25)1-xBx. Magnetometry measurements show a strongly reduced coercivity at around x ≈ 5% and we also characterize the structural transition via transmission electron microscopy and the magnetic damping via broadband ferromagnetic resonance. |
Monday, March 6, 2023 2:18PM - 2:30PM |
B55.00013: Energy Barriers for Thermally Activated Magnetization Reversal in Perpendicular Magnetic Tunnel Junctions Nanopillars in a Transverse field Gabriel D Chaves, Andrew D Kent, Corrado Capriata, Gunnar B Malm Thermally induced transitions between bistable magnetic states of magnetic tunnel junctions (MTJ) nanopillars are of interest for generating random bitstreams and for applications in stochastic computing. An applied field transverse to the easy axis of a perpendicularly magnetized MTJ (pMTJ) can lower the energy barrier (Eb) to these transitions leading to faster fluctuations. Here we present analytical and numerical calculations of Eb considering both coherent (macrospin) reversal and nonuniform magnetization reversal for a variety of nanopillar diameters and applied fields. We show that a nonuniform reversal processes dominate for larger diameters. Our macrospin reversal results are consistent with previous studies [1] but include a correction that accounts for the size-dependent demagnetization fields. Interestingly, we find that this correction results in a nonmonotonic dependence of Eb on diameter. Numerical calculations of Eb using the String-Method [2] show that the transition state is a nonuniform state with a sigmoidal magnetization profile. The latter can be described with an analytical expression that depends on only one spatial dimension, the dimension parallel to the applied field, which is also the preferred direction of profile motion during reversal. Our results provide a useful approach to determining the fluctuation rates of pMTJs as a function of the transverse field, pMTJ diameter and material characteristics. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700