Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R42: Magnons and Spin TransportFocus
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Sponsoring Units: GMAG Chair: Steven Bennett, United States Naval Research Laboratory Room: 709/711 |
Thursday, March 5, 2020 8:00AM - 8:36AM |
R42.00001: Manipulation of magnetic coupling and magnon transport in magnetic insulator-ferromagnetic metal hybrid structures Invited Speaker: Yabin Fan Magnetic insulators and ferromagnetic metals are widely used in spintronic structures for the generation of magnon spin current, control of magnetization states, and for detecting magnon spins. Most of the previous spintronic structures are based primarily on either magnetic insulators or ferromagnetic metals, and the heterostructures that integrate both have not been fully explored while they may exhibit new magnetic coupling and magnon transport properties. Here, we introduce a Pt/yttrium iron garnet (YIG)/permalloy(Py) hybrid structure grown on Si substrate, where YIG is a ferrimagnetic insulator and Py is a ferromagnetic metal. Surprisingly, the YIG and the Py layers show antiferromagnetic coupling when external field is small, and the two layers align to the same direction only when in-pane field is large enough, as evidenced by both vibrating-sample magnetometry and polarized neutron reflectometry measurements. More importantly, the parallel and antiparallel magnetization configurations in this YIG/Py structure could be utilized to control magnon spin current in spin-pumping and spin Seebeck experiments. With easy access to grow by magnetron sputtering, fully controllable magnetization configuration, and efficient control of magnon spin current, the Pt/YIG/Py hybrid structure could potentially find applications in spin logic devices that are fabricated on a Si chip. |
Thursday, March 5, 2020 8:36AM - 8:48AM |
R42.00002: Xe-PFIB microstructuring of Yttrium Iron Garnet films for quantum magnonic applications Dmytro Bozhko, Paul Baity, Sergey Danilin, Rair Macedo, Valentino Seferai, Umberto Nasti, Alessandro Casaburi, William Smith, Stephen McVitie, Robert Hadfield, Martin Weides Recently, the growing interest in quantum magnonics triggered intensive investigation of magnetic materials in the quantum limit. One of the main challenges in this area is to achieve long magnon lifetimes at cryogenic temperatures. Among all currently known magnetic materials, single-crystal Yttrium Iron Garnet (YIG, Y3Fe5O12) possesses the lowest damping due to low spin-orbit coupling. Thin YIG films are, however, grown on GGG (Gd3Ga5O12) substrates, which induce high magnetic damping at low temperatures [1]. |
Thursday, March 5, 2020 8:48AM - 9:00AM |
R42.00003: Py-Cu Graded-Index Magnonic Waveguides Kristen Repa, Celia Mercovich, Casey Miller Spin waves (magnons) have many optical analogies, including the law of reflection, which allows total internal reflection (akin to a fiber optic cable). For this reason, we’re working to create a magnonic waveguide made from Py-Cu multilayered films. |
Thursday, March 5, 2020 9:00AM - 9:12AM |
R42.00004: Non-linear spin torque, pumping and cooling in superconductor/ferromagnet systems Risto Ojajärvi, Juuso Manninen, Pauli Virtanen, Tero T Heikkilä We study the effects of the coupling between magnetization dynamics and the electronic degrees of freedom in a heterostructure of a metallic nanomagnet with dynamic magnetization coupled with a superconductor containing a steady spin-splitting field. We predict how this system exhibits a non-linear spin torque, which can be driven either with a temperature difference or a voltage across the interface. We generalize this notion to arbitrary magnetization precession by deriving a Keldysh action for the interface, describing the coupled charge, heat and spin transport in the presence of a precessing magnetization. We characterize the effect of superconductivity on the precession damping and the anti-damping torques. We also predict the full non-linear characteristic of the Onsager counterparts of the torque, showing up via pumped charge and heat currents. For the latter, we predict a spin-pumping cooling effect, where the magnetization dynamics can cool either the nanomagnet or the superconductor. |
Thursday, March 5, 2020 9:12AM - 9:24AM |
R42.00005: Acoustic Ferromagnetic Resonance Assisted Spin-Torque Switching of Perpendicular MTJs Walid Al Misba, Md Mahadi Rajib, Dhritiman Bhattacharya, Jayasimha Atulasimha Resonant Surface Acoustic Wave (SAW) assisted Spin Transfer Torque (STT) can drive the magnetization of a perpendicular magnetic tunnel junction (p-MTJ) and write bits with an order of magnitude lower energy dissipation compared to the conventional STT current approach [1]. Simulations show that such acoustically driven ferromagnetic resonance (FMR) results in the magnetization precessing in a cone with large deflections relative to the perpendicular direction. The larger the precession angle, the lower the STT current required for switching the magnetization. While our prior simulations [1] of such magnetization dynamics were based on macro-spin assumptions, here we will perform micromagnetic simulations to study incoherent switching in the presence of room temperature thermal noise as well as incorporate realistic effects such as material inhomogeneity, grains, surface roughness and lithographic imperfections. The ramification of such incoherency in magnetization dynamics on the switching error will be discussed. Preliminary experimental work will also be presented. |
Thursday, March 5, 2020 9:24AM - 9:36AM |
R42.00006: Transport and Magnetic Properties of Amorphous Fe:Dy Oxide Films. Sara Bey, Olivia Denton, Tatiana Allen, Krishna Koirala, William Roes, Gerd Duscher, Ramki Kalyanaraman Novel amorphous FeDy-Oxide thin films, deposited by e-beam evaporation, show room-temperature magnetism, as well as high electrical conductivity, carrier mobility, and optical transparency that is very promising for a multitude of applications. We have systematically studied transport, optical and magnetic properties of a large set of films deposited on Quartz and SiO2/Si substrates at different oxygen partial pressures [10-6,10-8,10-9 Torr]. Atomic ratios of Fe to Dy in the films (confirmed by EDS) varied from 0.3 to 6, but all samples had resistivity less than 2.5E-03 Ohm*cm. Optical bandgap of as prepared films was 2.4±0.1 eV. The films showed high ordinary Hall mobility (~10 cm2/V-s) which is among the highest for amorphous oxides, as well as anomalous Hall mobility (~102 cm2/V-s). At low temperatures there was two step magnetization that suggests coexistence of two magnetic phases below H=0.08 T and ferromagnetic phase at higher fields. Select films were taken through cyclic annealing (300-700K) which, after 3 cycles, resulted in still amorphous, thermally stable homogeneous FeDyO material. These results will motivate further investigation into the fundamental mechanisms responsible for these properties and how they might be tuned for integration in devices. |
Thursday, March 5, 2020 9:36AM - 9:48AM |
R42.00007: Nonlinear Optical Imaging of Current Induced Spin Switching in Antiferromagnetic Materials Joongwon Lee, Yongjian Tang, Antonio B Mei, Okan Koksal, Darrell Schlom, Daniel Ralph, Farhan Rana Electrical switching of the Neel order in Antiferromagnetic (AF) materials via spin transfer (STT) has attracted considerable attention [1,2]. Experiments have shown that AF switching is non-uniform, influenced by magneto-elastic coupling, and the efficacy of STT in inducing AF switching has also been questioned [3]. In this paper, we show that scanning optical second harmonic generation (SHG) can be used to image and identify magnetic domains in inversion symmetric AF materials. Using this technique, we image magnetic domains in nanometer thick epitaxial AF layers. We show that the SHG technique can image even when AF materials are covered with metallic spin Hall layers such as Pt. We use scanning SHG to study electrical switching in NiO/Pt devices. Our results show that AF switching is non-uniform across our 100 sq-micron devices. We observe regions that show AF switching in agreement with the STT predictions and regions where magnetoelastic effects seem to dominate, and we also see regions which exhibit no evidence of switching. The contributions of various effects will be discussed. [1] Phys. Rev. Lett. 120, 207204 (2018). [2] Scientific Reports 8, 14167 (2018). [3] arXiv:1907.00314 (2019). |
Thursday, March 5, 2020 9:48AM - 10:24AM |
R42.00008: Modeling Spin-Orbit Phenomena at Magnetic Interfaces Invited Speaker: Aurelien Manchon Magnetic materials lacking inversion symmetry constitute a unique platform for the exploration and control of magnetism [1]. In these systems, typically multilayers of transition metal ferromagnets and heavy metals (W, Pt, Bi2Se3 etc.), interfacial spin-orbit coupling promotes a wealth of physical phenomena, among which the emergence of magnetic skyrmions – topological magnetic textures –, spin-orbit torques– an efficient means to electrically control magnetization dynamics –, as well as chiral magnetic damping – energy dissipation that depends on the texture chirality. While most of the initial theoretical progress has been achieved using minimal models (e.g., the Rashba two-dimensional gas), we have recently developed a multiorbital tight-binding model of such heterostructures that enables us to model these various phenomena on equal footing and in a transparent manner [2,3]. Based on the results of this model, I will address various aspects of the interplay between spin transport and magnetism mediated by spin-orbit coupling. I will first discuss the orbital nature of interfacial spin-orbit coupling in multilayers and examine how it facilitates the onset of chiral magnetic textures. I will then present the physics of spin-orbit torques, and inspect their relation to chiral magnetic properties such as Dzyaloshinskii-Moriya interaction and chiral magnetic damping. Finally, I will discuss the current-driven dynamics of magnetic skyrmions and propose various strategies to enhance their mobility, exploiting topological spin currents flowing through the texture [4]. |
Thursday, March 5, 2020 10:24AM - 10:36AM |
R42.00009: The shift of the Curie temperature in bcc-Fe by phonon softening: first-principles phonon and Monte Carlo calculations. Tomonori Tanaka, Yoshihiro Gohda Various estimation techniques based on spin-lattice models are widely used to estimate Curie temperatures in magnetic materials. In spin-lattice models, only magnetic couplings are explicitly treated, while the phonon effect could be incorporated implicitly into the exchange-coupling parameter. However, the validity of this approach is questionable for the cases where the coupling between the phonon and the magnetism is strong. For example, the phonon frequency typically decreases with a change in the magnetic order from ferromagnetic to paramagnetic state. This phonon softening makes paramagnetic states energetically stable, therefore, equilibrium magnetic states in finite temperatures deviate from the case where only magnetic terms are considered. We develop a theoretical scheme to describe the shift of the Curie temperature and apply it to bcc-Fe. Our scheme employs free energy minimization, therefore, the problem can be formulated as a simple optimization problem. |
Thursday, March 5, 2020 10:36AM - 10:48AM |
R42.00010: Magnetotransport properties of granular oxide-segregated CoPtCr films for applications in future magnetic memory technology Morgan Williamson, Maxim Tsoi, Pin-Wei Huang, Ganping A Ju, Cheng CW Wang Magnetotransport properties of granular oxide-segregated CoPtCr films were studied on both macroscopic and microscopic length scales by performing bulk and point-contact magnetoresistance measurements, respectively. Such a perpendicular magnetic medium is used in state-of-the-art hard disc drives and if combined with magnetoresistive phenomena (for read/write operations) may lead to a novel concept for magnetic recording with high areal density. While the bulk measurements on the films showed only small variations in dc resistance as a function of applied magnetic field (magnetoresistance of less than 0.02 %), the point-contact measurements revealed giant-magnetoresistance-like changes in resistance with up to 50,000 % ratios. The observed magnetorestive effect could be attributed to a tunnel magnetoresistance between CoPtCr grains with different coercivity. The tunneling picture of electronic transport in our granular medium was confirmed by the observation of tunneling-like current-voltage characteristics and bias dependence of magnetoresistance; both the point-contact resistance and magnetoresistance were found to decrease with the applied dc bias. |
Thursday, March 5, 2020 10:48AM - 11:00AM |
R42.00011: The spontaneous electrical and spin Hall effect in a collinear antiferromagnet Gen Yin, Jie-Xiang Yu, Roger Lake, Jiadong Zang, Kang L. Wang In collinear antiferromagnets, since the combined operation of time reversal and a half-unit-cell translation is symmetric for the magnetic lattice, a spontaneous Hall effect is usually absent. Here, we show that in a collinear antiferromagnet, MnTe, the non-magnetic atomic structure breaks such symmetry, therefore allows a spontaneous Hall effect, which is known as planar Hall effect. Such spontaneous Hall effect does not result from the intrinsic Berry curvature. Instead, it comes from the extrinsic scattering centers with either positive or negative polarized spins along the easy axis of the Neel vector. This behavior is intriguing since it is by now the most convenient method to read-out (and potentially to write) the information encoded in antiferromagnets for high-speed device applications. Such zero-field planar Hall effect has already been observed in experiments. Our calculation shows that the spontaneous Hall effect and spin Hall effect can be up to 30%, suggesting a vast space to improve the performance for device applications. |
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