Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A25: Topological/Skyrmion Hall Transport and Related Phenomena in Chiral MagnetsInvited
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Sponsoring Units: GMAG DCMP Chair: Mohit Randeria, Ohio State University Room: LACC 403B |
Monday, March 5, 2018 8:00AM - 8:36AM |
A25.00001: Skyrmion dynamics, nucleation, and stability in ferromagnetic thin film multilayers Invited Speaker: Geoffrey Beach Magnetic skyrmions are particle-like spin textures that are topologically protected from being continuously ‘unwound’. Recent work has shown that in transition metal based multilayers, isolated ordered skyrmion lattices and isolated skyrmions with size <50nm can be achieved at room temperature, with current-driven velocities exceeding 100m/s [1]. This talk focuses on recent developments in stabilizing, manipulating, and designing skyrmions in thin-film heterostructures. We first describe current-driven dynamics and nucleation driven by spin-orbit torques. Using pump-probe dynamic imaging, we demonstrate an analogue to the conventional Hall effect, in which the skyrmion trajectory depends on its topological charge much as a particle in a magnetic field is deflected due to its electric charge [2]. We then demonstrate deterministic current-induced skyrmion writing at sub-nanosecond timescales through the combined action of DMI and spin-orbit torque [3], and show that thermal excitation can drive morphological phase transitions between chiral phases in a controlled way [4]. We next present an analytical framework [5] for computing the energy and structure of any skyrmion in any material, which gives insight into skyrmion stability, allows for mapping the full materials parameter space, and provides a means to solve the inverse problem of designing skyrmions with desired properties through informed materials selection. Finally, we apply this framework to experimentally demonstrate ultrafast and ultrasmall skyrmions at room temperature, with sizes down to 10 nm, in appropriately engineered materials. |
Monday, March 5, 2018 8:36AM - 9:12AM |
A25.00002: Robust Zero-Field Skyrmion Formation in FeGe Epitaxial Thin Films Invited Speaker: Fengyuan Yang Magnetic skyrmions have attracted significant interests in recent years due to their potential for magnetic storage applications. B20 phase materials enable magnetic skyrmions due to the spin-orbit coupling and non-centrosymmetric structure. One major effort in this emerging field is the stabilization of skyrmions at room temperature and zero magnetic field. We grow high quality FeGe epitaxial films of 100, 65, and 36 nm thicknesses on Si by UHV off-axis sputtering, which exhibit pure B20 phase [1]. STEM images clearly reveal the B20 lattice of the FeGe film and its epitaxial relationship with Si. The FeGe films were patterned into a Hall bar structure for longitudinal and Hall resistivity measurements. The Hall resistivity loops show three regions of distinct features: 1) a linear background at large fields (> 2 T) due to the ordinary Hall effect, 2) a magnetic reversal at intermediate fields that follows the magnetization hysteresis loop due to the anomalous Hall effect, and 3) a hysteresis loop within ±3000 Oe due to the topological Hall effect. The topological Hall resistivity was extracted by subtracting the anomalous and ordinary Hall effect, demonstrating the existence of the skyrmion phase in FeGe films between 5 and 275 K. The topological Hall resistivity reaches 918 nOhm cm at 250 K, the highest reported to date. In particular, a large remanent topological Hall resistivity (77%) was observed at zero field and 5 K, indicating a robust skyrmion phase without the need of an external magnetic field. In addition, our recent results on skyrmions in oxides bilayers will be discussed. |
Monday, March 5, 2018 9:12AM - 9:48AM |
A25.00003: Skyrmions and Hall Transport Invited Speaker: Bom Kim Skyrmions in chiral magnetic materials are stable, particle-like spin textures that are protected by a topological quantum number. Typical Skyrmion motions are complex, and their transport properties are involved with electric and thermal Hall conductivities along with linear and angular momenta. The identification of these quantities is often subtle, due to the Skyrmions’ extended nature and their interactions with conduction electrons and other backgrounds. Is there a guiding principle for possible physical quantities relevant for Skyrmions and relations among them? This talk answers the question in two parts. First, we note that Skyrmions are allowed due to broken parity symmetry (mirror symmetry). We provide a review on the parity breaking hydrodynamics, Hall viscosity, angular momentum and central extension of a commutation relation between the momentum operators. The underly theme of all these phenomena is parity symmetry breaking. Second, we provide a simple geometric picture of field theory Ward identities and its application to Skyrmion Hall transport that involves thermal, electric Hall conductivities and Hall viscosity. The topological charge density of Skyrmions has a distinct signature in the electric Hall conductivity that is identified in existing experimental data. In particular, we provide a simple and clear way to measure Hall viscosity that has been widely studied theoretically. |
Monday, March 5, 2018 9:48AM - 10:24AM |
A25.00004: Evolution of chiral magnetic textures and their topological Hall signature in Ir/Fe/Co/Pt multilayer films Invited Speaker: Christos Panagopoulos We establish a direct correspondence between skyrmions and their topological Hall signature in multilayer films by combining transport and magnetic force microscopy measurements over a wide range of temperature and applied magnetic field (H). First, we demonstrate the relationship between the areal density of isolated skyrmions and the magnitude of THE over a 200 K temperature range for fixed fields near saturation. Second, as H is swept towards zero, we find that skyrmions aggregate to form worm-like magnetic textures, which carry a large topological charge, and manifest as peaks in the measured THE. Quantitative modelling of these worm textures allows us to explain the field and temperature evolution of THE. Notably, we identify a much larger topological Hall resistivity than prevailing theory predicts for the observed skyrmion density. We suggest that band structure plays an important role in the skyrmion–current coupling, with implications for the impact of the Berry-phase of skyrmions on charge carriers. |
Monday, March 5, 2018 10:24AM - 11:00AM |
A25.00005: Electric-Field-Driven Switching of Individual Magnetic Skyrmions Invited Speaker: Roland Wiesendanger Based on the discovery of nanoscale magnetic skyrmion lattices [1] as well as individual skyrmions [2] in ultrathin magnetic films and bilayers, stabilized by interfacial Dzyaloshinskii-Moriya interactions, applications of magnetic skyrmions for future spintronic devices, such as memory and logic elements, have become feasible [3]. The properties of magnetic skyrmions can widely be tuned, e.g. by multiple interface engineering, leading to skyrmionic states in metallic multilayer systems being stable up to room temperature [4-6] and in zero magnetic field. Alternatively, chemical treatments, e.g. by oxidation or hydrogenation of ultrathin magnetic thin films can serve as a simple route towards tailored skyrmionic states. |
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