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 Y57: Magnetization and Spin Dynamics II |
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Sponsoring Units: GMAG Chair: Lina Johnsen, Norwegian University of Science and Technology Room: Room 303 |
Friday, March 10, 2023 8:00AM - 8:12AM |
Y57.00001: Novel Approach to Electron Spin Resonance Spectroscopy Powered by Free Electron Laser Antonin Sojka Pulsed electron spin resonance (ESR) spectroscopy is crucial for obtaining information on spin dynamics for the understanding and development of molecular qubits and single molecular magnets [1], antiferromagnetic spintronics [2], and fractionalized magnets [3]. In the last decade, ESR moved to high fields and frequencies (HF-ESR) to achieve better resolution and sensitivity, and to get access to high-spin systems with large zero field splitting (also called single ion anisotropy). The aim of our work is to set up and develop a general-purpose state-of-the-art broadband pulsed HF-ESR spectrometer. Our design uses UCSB’s free electron laser (FEL) [6] as the source of frequency-agile kW power that operates at frequencies from 120 GHz to 500 GHz, sample temperatures in the range of 1.6 K to 300 K, and at magnetic fields up to 16 T. More importantly, we present a novel approach of rapid scan ESR, which is capable of providing T1 relaxation time via imaging spin saturation recovery by combining a high-power pulse from the FEL together with continuous sample irradiation by low power microwave source. Such a design will allow multi-frequency relaxation studies of a variety of samples ranging from bulk materials, powders, single crystals, and air-sensitive samples in liquid solutions. |
Friday, March 10, 2023 8:12AM - 8:24AM |
Y57.00002: Classical dynamics of S=1/2 spin ladders using SU(4) coherent states Jinu Thomas, David A Dahlbom, Steven S Johnston, Cristian Batista The Landau-Lifshitz equations of motion can be derived by taking the classical limit of a spin system based on SU(2) coherent states. Generalizing this procedure from SU(2) to SU(N) gives a classical dynamics of SU(N) coherent states, where N >2 is the dimension of the local Hilbert space. Among other applications, this generalization can be used to model coupled units with strong intra-unit entanglement. For example, the quantum mechanical states of a dimer of two antiferromagnetically coupled S=1/2 spins are SU(4) coherent states. These coherent states include for instance the singlet, which does not have a classical counterpart in the traditional Landau-Lifshitz dynamics. In particular, we can describe a spin ladder of weakly coupled dimers as a chain of SU(4) spins. By using the corresponding classical limit based on SU(4) coherent states, we compute the zero and finite temperature dynamics of the spin ladder and compare against density matrix renormalization group (DMRG) calculations. In the regime where the inter-dimer exchange is not too large, the SU(4) results agree well with DMRG, while requiring only a small fraction of the computational cost. |
Friday, March 10, 2023 8:24AM - 8:36AM |
Y57.00003: Non-thermal magnon excitation in a two-dimensional honeycomb antiferromagnet BaNi2P2O8 Shingo Toyoda, Tatsuki Sato, Taka-hisa Arima, Yusuke Tokunaga, Joseph W Orenstein Antiferromagnetic materials attract much attention due to their inherently ultrafast spin dynamics and potential application to the magnetic memory devices. Among them, two-dimensional easy-plane type antiferromagnets are of particular interest because they show anomalous magnetic behaviors such as BKT transition and long-distance spin transport via spin superfluidity. Non-thermal excitation of spin waves is crucial to investigate the exotic spin phenomena because one can avoid thermal excitations and detect pure spin contributions. |
Friday, March 10, 2023 8:36AM - 8:48AM |
Y57.00004: Multifrequency Pulsed EPR in the 120-400 GHz Range Johan van Tol At high fields and low temperatures both the electron spin lattice relaxation time (T1) and spin-spin Relaxation time (T2) can change dramatically with respect to those at lower fields. Relaxation in spin systems is of crucial interest with respect to various possible applications like quantum information processing, information storage, spintronics, and dynamic nuclear polarization (DNP). High frequencies and fields in combination with low temperatures polarize the electron spins, and allow for considerably longer spin memory times at high fields and frequencies as compared to X-band[1] for more concentrated spin systems. On the other hand, high frequencies lead to a significantly increased contribution from direct single phonon processes in the spin-lattice relaxation, and at low temperatures T1 can be a few orders of magnitude shorter at fields of the order of 10T with respect to the spin lattice relaxation times at typical X-band frequencies. |
Friday, March 10, 2023 8:48AM - 9:00AM |
Y57.00005: Experimental realization of linearly polarized X-ray detected ferromagnetic resonance Christoph Klewe, Satoru Emori, Qian Li, Mengmeng Yang, Benjamin A Gray, Hyung Min Jeon, Brandon Howe, Yuri Suzuki, Zi Q. Qiu, Padraic Shafer, Elke Arenholz We present the first theoretical and experimental evidence of time-resolved dynamic X-ray magnetic linear dichroism (XMLD) measurements of GHz magnetic precessions driven by ferromagnetic resonance in both metallic and insulating thin films [1]. Our findings show a dynamic XMLD in both ferromagnetic Ni80Fe20 and ferrimagnetic Ni0.65Zn0.35Al0.8Fe1.2O4 (NZAFO) for different measurement geometries and linear polarizations. A detailed analysis of the observed signals reveals the importance of separating different harmonic components in the dynamic signal in order to identify the XMLD response without the influence of competing contributions. In particular, RF magnetic resonance elicits a large dynamic XMLD response at the fundamental frequency under experimental geometries with oblique x-ray polarization. The geometric range and experimental sensitivity can be improved by isolating the 2ω Fourier component of the dynamic response. To account for the effect of crystal-field splitting on the angular dependence of the spin-quantization axis with respect to the crystalline axes, we include crystal-field contributions in the analysis of the dynamic XMLD spectral lineshapes, which yields excellent agreement with the experimental observations. These results illustrate the potential of dynamic XMLD and represent a milestone accomplishment towards the study of GHz spin dynamics in systems beyond ferromagnetic order. |
Friday, March 10, 2023 9:00AM - 9:12AM |
Y57.00006: Temperature dependent spin pumping into the topological insulator Bi2Te3 Vinay Sharma, Harsimrat Kaur, Ramesh C Budhani The bismuth and antimony - based three dimensional (3-D) topological insulators (TIs) such as Bi2Te3, Sb2Te3, BiSb display robust spin-to-charge conversion properties due to their conducting topological surface states (TSS) [1]. Here, we report measurements of ferromagnetic resonance (FMR) driven spin pumping from an amorphous ferromagnet (FM) Fe60Co20B20 (FeCoB) into the 3-D TI-Bi2Te3. The FMR induced spin pumping is reflected in the enhanced Gilbert damping of precessing magnetization and a dc voltage (Vdc) developed across the sample at resonance. The observed Vdc signal is a superposition of spin rectification effects (SRE), inverse spin Hall effect (ISHE) and inverse Rashba Edelstein effect (IREE). We have measured the Vdc in such a manner that the spin injection direction is reversed by flipping the sample to separate SRE and ISHE signals, which are even and odd functions of spin injection directions respectively. The role of TSS has been evaluated by measuring FMR and ISHE signals at cryogenic temperatures. It has been observed that the IREE effect is enhanced when a thin layer of silver is sandwiched between the FeCoB and Bi2Te3. |
Friday, March 10, 2023 9:12AM - 9:24AM |
Y57.00007: Extraction of exchange parameters for K2Ni2(SO4)3 from neutron data using neural networks Tianran Chen, Weliang Yao, Haidong Zhou, Alan Tennant K2Ni2(SO4)3 is a quantum spin liquid in which strongly frustrated magnetic interactions induce a highly entangled quantum paramagnet far below the energy scale of the magnetic interactions. Understanding complex phases in such strongly frustrated systems is challenging because traditional simulation techniques have difficulty incorporating multiple competing interactions as well as linking models and data. We performed neutron scattering measurement and used multiple neural network architectures in combination with high-dimensional modeling and numerical methods to address this. A comprehensive data set of diffraction and inelastic neutron scattering of single-crystal K2Ni2(SO4)3 was collected. Long-range magnetic order and continuous spin-wave like excitation were observed at low temperatures, which melt gradually upon heating or applying an external magnetic field. The annealing process and spin dynamics were simulated and dynamical structure factors were computed using the Sunny package in the high-dimensional space of exchange parameters. Variational autoencoders were used to compress information from simulated data. Radial basis networks were utilized as fast surrogates for diffraction and dynamics simulations to theoretically explore the phase diagrams and to predict the exchange parameters with uncertainty. |
Friday, March 10, 2023 9:24AM - 9:36AM |
Y57.00008: RKKY coupling effect on Spin-orbit torque switching in a L10 FePt-gradient film Eunji Lim, Dongchan Jeong, Wonyeong Choi, Dohee Kwon, Sanghoon Kim, Seyeob Jeong Spin–orbit torque (SOT)-induced magnetic switching has attracted much interest due to its potentially high efficiency in terms of switching time and power consumption, energy efficiency and high endurance. The L10 FePt is representative material as the mainstay of magnetic data storage media owing to its high thermal stability for more than 10-year retention and their amenability to miniaturization. For the SOT switching, a structural inversion asymmetry and strong spin-orbit coupling are generally essential. |
Friday, March 10, 2023 9:36AM - 9:48AM |
Y57.00009: Spin-lattice couplings and their effects in transition-metal ferromagnets with ab-initio accuracy Ivan d Miranda, Maryna Pankratova, Olle Eriksson, Anders Bergman In the emergent field of ultrafast magnetization dynamics, understanding the interplay between lattice, spin, and electrons is extremely relevant to the description of several phenomena. For systems with magnetic order, characterized by the existence of a collective motion of spins, both magnetic moments and lattice degrees of freedom are coupled via the electronic medium, which can influence, for instance, both magnon and phonon spectrum and lifetimes. Although a formalism to describe the magnetization dynamics accounting for the spin-lattice coupling (SLC) is known from many years ago [1], there is still a gap in the literature regarding the parameters for real materials, obtained with first-principles accuracy. From a theoretical point of view, this requires computing the magnetic interactions (e.g., Heisenberg exchange, Dzyaloshinskii-Moriya) in a broken inversion symmetry situation, making the use of real-space-based ab-initio methods a suitable way to calculate such parameters. The combination with state-of-the-art spin-lattice-dynamics simulations [2], then, make it possible to the investigate the influence of such SLC parameters in a wide range of materials. |
Friday, March 10, 2023 9:48AM - 10:00AM |
Y57.00010: Role of spin in the composition-dependent electron-phonon coupling in Co-Fe alloys Kevin Moseni, Sinisa Coh We study from first principles the electron-phonon coupling strength in Co-Fe alloys across the full range of compositions. We decompose the Eliashberg spectral function and subsequently the electron-phonon coupling strength into majority-spin and minority-spin parts. The electron-phonon coupling strength changes significantly with composition. We identify spin-dependent quantities that drive the changes in electron phonon coupling strength. Finally, we show that these results are in qualitative agreement with experimental measurements. |
Friday, March 10, 2023 10:00AM - 10:12AM |
Y57.00011: Lattice Contribution to the Magnetocaloric Entropy Change: A Spin-Lattice Dynamics Study Lokanath Patra, Bolin Liao We present an efficient computational approach for understanding the lattice contribution to the magnetocaloric effect in bcc iron (Fe) and hcp gadolinium (Gd) using spin-lattice dynamics, with exchange coupling parameters determined from ab initio calculations. The simulated magnetic transition temperature using the current approach gives better agreement with the experimental measurements. We find that the presence of long-range indirect (RKKY type) exchange in hcp Gd strongly influences the low-frequency long-wavelength phonons at higher applied external magnetic fields. This results in a higher lattice contribution towards the total magnetocaloric entropy change as compared to bcc Fe with short-range direct exchange couplings. The current developed approach provides a viable framework for understanding the magnetocaloric effect in complicated magnetic materials with strong spin-lattice coupling. Our finding suggests that long-range indirect exchange leads to a larger lattice contribution to the magnetocaloric entropy change and is thus beneficial for magnetocaloric materials. |
Friday, March 10, 2023 10:12AM - 10:24AM |
Y57.00012: Spin-1 magnets — a u(3) formalism Kimberly Remund, Rico Pohle, Yutaka Akagi, Judit Romhanyi, Nic Shannon Systems based on spin-1 moments exhibit many fascinating properties, as demonstrated by spin nematics [1,2], Fe-based superconductors [3], and cold atom systems [4]. However, because spin-1 moments support quadrupoles, they cannot be described using O(3) vectors, even in the classical limit. |
Friday, March 10, 2023 10:24AM - 10:36AM |
Y57.00013: Nonlinear orbital magnetization Benjamin Fregoso We calculate the nonlinear electric polarization induced by a magnetic field in crystals (metals and insulators). Two second order, we find general expressions suitable for ab-initio numerical studies of materials nonlinear magnetoelectric response. In the limit of a homogeneous magnetic field, we recover the known ground state orbital magnetization, and the orbital magnetoelectric response including the Chern-Simons (CS) term. In the nonlinear response, there are higher order analogs of the CS term. In general, these terms are small compared with a large background. Applications to recent experiments is also discussed. |
Friday, March 10, 2023 10:36AM - 10:48AM |
Y57.00014: A Schrödinger Formulation of Generalized Landau-Lifshitz Dynamics: Numerical Techniques and Applications David A Dahlbom, Hao Zhang, Cole M Miles, Xiaojian Bai, Cristian Batista, Kipton Barros The Landau-Lifshitz (LL) equations may be derived using the formalism of SU(2) coherent states. For many spin systems, such as those possessing strong onsite anisotropy and spin greater than ½, it is more appropriate to take a classical limit based on coherent states of SU(N), where N > 2. The resulting generalization of the LL equations are naturally derived in the Heisenberg picture, setting up a correspondence between quantum observables and classical variables. We show that one may instead develop an equivalent Schrödinger dynamics directly on coherent states. This formulation involves fewer variables than the Heisenberg approach and possesses a global Hamiltonian structure. The latter fact enables the application of energy-preserving geometric integration schemes. The dynamics may also be coupled to a thermal bath, resulting in a generalization of the stochastic LL equations. Sunny, a software package for simulating spin systems using SU(N) coherent states, has been developed to make these theoretical advancements accessible. Examples are provided, including a physically realistic spin one Hamiltonian that supports both a novel type of skyrmion crystal at low temperatures as well as a metastable skyrmion gas state, the density of which is controlled by quenching rate. |
Friday, March 10, 2023 10:48AM - 11:00AM |
Y57.00015: First-principles study of spin decoherence in VOPc-graphene nanoribbon complexes Xiao Chen, James N Fry, Hai-Ping Cheng Nanographene qubit arrays facilitate quantum-to-quantum transduction between light, charge, and spin, making them an excellent testbed for fundamental science in quantum coherent systems and for the construction of higher-level qubit circuits. In this work we study decoherence of an electronic spin due to coupling with a surrounding spin bath in a one-dimensional array of vanadyl phthalocyanines (VOPc), a candidate of spin-1/2 molecular qubit, on armchair-edged graphene nanoribbons (GNR). Density functional theory (DFT) is used to obtain low energy isomeric atomic structures at different spacings between VOPcs. Decay of spin coherence in Hahn echo experiments was simulated using the cluster correlation expansion (CCE) method with a spin Hamiltonian involving hyperfine and electric field gradient tensors calculated from DFT. We found that GNR induced a significant decrease of the decoherence time T2 compared with isolated single VOPc. Quadrupole interactions cause a large electron spin echo envelope modulation (ESEEM), which is harmful to spin decoherence, at small magnetic field. However, this ESEEM is suppressed upon increase of the field. |
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