Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session F41: Skyrmion Crystals |
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Sponsoring Units: GMAG DMP Chair: Lingfei Wang, Seoul National University Room: 707 |
Tuesday, March 3, 2020 8:00AM - 8:12AM |
F41.00001: Skyrmion crystal from RKKY interaction mediated by 2DEG Zhentao Wang, Ying Su, Shizeng Lin, Cristian Batista Skyrmion crystals have been found in centro-symmetric itinerant magnets, where the long-range Ruderman-Kittel-Kasuya-Yoshida (RKKY) effective coupling is believed to be the dominant interaction between the local moments. In this work we show that the RKKY interaction mediated by a dilute (kF«1) 2D electron gas (2DEG) on a C6 invariant lattice stabilizes a magnetic skyrmion crystal in the presence of magnetic field and easy-axis anisotropy. A remarkable aspect of this mechanism is that the ordering wave vectors of the resulting skyrmion crystal are dictated by the Fermi wave vector: |Qν|=2kF (ν=1,2,3). Consequently, the topological contribution to the Hall conductivity of the 2DEG becomes very large within the weak-coupling regime: the quantized value e2/h is reached when J becomes comparable to the Fermi energy EF. |
Tuesday, March 3, 2020 8:12AM - 8:24AM |
F41.00002: Magnetic hedgehog lattices in noncentrosymmetric metals Shun Okumura, Satoru Hayami, Yasuyuki Kato, Yukitoshi Motome Recently, three-dimensional topological spin textures called the magnetic hedgehog lattices (HLs) were discovered in the B20-type compounds MnSi1-xGex [1]. The HLs have periodic arrays of magnetic monopoles and anti-monopoles, which induce interesting transport phenomena, such as the topological Hall effect and the topological thermoelectric transport. However, their stabilization mechanism has not been fully understood thus far. Here we investigate the ground state of an effective spin model with long-range interactions arising from the itinerant nature of electrons by variational calculations and simulated annealing. We find that the HLs are stabilized even at zero field by the synergetic effect of symmetric and antisymmetric interactions from the spin-charge and spin-orbit couplings, respectively. We also clarify the full phase diagram in the magnetic field, which includes multiple phase transitions with changes in the number of monopoles and anti-monopoles [2]. |
Tuesday, March 3, 2020 8:24AM - 8:36AM |
F41.00003: Ordering in magnetic skyrmion lattices James Stidham, Michel Pleimling Ordering in magnetic skyrmion lattices is an active area of research for skyrmion systems. In this talk, we present recent results obtained using Langevin molecular dynamic simulations, based on a previously derived particle model of skyrmions. Using a Voronoi cell algorithm, we examine the effect of the Magnus force present in skyrmion systems and how it affects ordering when noise is both present and absent in the system. We observe power-law behavior during late time ordering in these skyrmion systems. We also find power-law behavior when looking at the difference in time of consecutive events as the system orders. |
Tuesday, March 3, 2020 8:36AM - 8:48AM |
F41.00004: Formation of square skyrmion lattice in a centrosymmetric magnet without geometrical frustration Khanh Nguyen, Taro Nakajima, Xiuzhen Yu, Shang Gao, Max Hirschberger, Yuichi Yamasaki, Hajime Sagayama, Hironori Nakao, Licong Peng, Kiyomi Nakajima, Kiyou Shibata, Rina Takagi, Taka-hisa Arima, Yoshinori Tokura, Shinichiro Seki Magnetic skyrmion, a topologically stable swirling spin texture, has attracted attention as a particle-like object potentially suitable for high-density information bit. The previous observation of skyrmions has mostly focused on noncentrosymmetric systems with Dzyaloshinskii-Moriya interaction, while the further search of novel mechanisms to realize smaller skyrmion is highly demanded. Here, we report the observation of square skyrmion lattice for a centrosymmetric non-frustrated tetragonal magnet GdRu2Si2 by performing resonant x-ray scattering and Lorentz TEM experiments. The possible origin of observed skyrmion formation has been discussed in terms of four-spin interaction mediated by itinerant electrons. The present results demonstrate that skyrmions can be stabilized even without geometrical frustration and inversion symmetry breaking, and suggest that rare-earth intermetallics with highly-symmetric crystal lattice may be ubiquitously platforms to explore nanometric skyrmions of exotic origins. |
Tuesday, March 3, 2020 8:48AM - 9:00AM |
F41.00005: Dramatic enhancement of the skyrmion phase-space of Cu2OSeO3 driven by high pressure Liangzi Deng, Hung-Cheng Wu, Alexander Litvinchuk, Noah Yuan, Jey-Jau Lee, Rabin Dahal, Liang Fu, helmuth berger, Hung-Duen Yang, Zheng Wu, Melissa Gooch, Paul C. W. Chu Generally, the skyrmion phase in a bulk helimagnet occurs only over a very restricted magnetic-field – temperature phase space and often at low temperatures. We have expanded and enhanced the skyrmion phase region from the small range of (55-58.5 K) to (5-300 K) in single crystal Cu2OSeO3 by pressures up to 42.1 GPa through a series of phase transitions from the cubic P213, through orthorhombic P212121 and monoclinic P21, and finally to the triclinic P1 phase, using our newly developed ultrasensitive high pressure magnetization technique. The results are in agreement with our Ginzburg - Landau free energy analyses that pressure tends to stabilize the skyrmion states and at higher temperatures. These observations also show that the skyrmion state can be achieved at higher temperatures in various crystal symmetries, suggesting the insensitivity of skyrmions to the underlying crystal lattices and thus the possibility of finding more ubiquitous presence of skyrmions in helimagnets. |
Tuesday, March 3, 2020 9:00AM - 9:12AM |
F41.00006: Detection of magnetic skyrmion tubes in cubic B20 FeGe nanostructures Nitish Mathur, Matthew John Stolt, Fehmi Yasin, Philipp Rybakov, Nikolai S. Kiselev, Xiuzhen Yu, Song Jin In chiral helimagnets with the non-centrosymmetric cubic B20 crystal structure, such as FeGe, host magnetic Bloch-type skyrmions due to the Dzyaloshinskii-Moriya interaction (DMI). Although magnetic skyrmions are depicted as two-dimensional spin textures in bulk crystals, in reality, they possess a three-dimensional structure of skyrmions that looks like elongated tubes extending throughout the thickness of the sample. We have synthesized single-crystal FeGe nanowires (NWs) and Fe1-xCoxGe (x<0.1) nanoplates (NPLs) via chemical vapor deposition (CVD) to explore these magnetic skyrmion tubes (SkT) in geometrically confined nanostructures. Lorentz transmission electron microscopy imaging as well as magnetotransport measurements confirmed that skyrmions are stabilized in a wider magnetic field (Bext) and temperature region in these nanostructures in comparison to the bulk materials. Furthermore, magnetoresistance measurements on FeGe nanostructures have shown some unusual features when Bext is parallel to the current direction. In conjunction with magnetic contrast imaging techniques and micromagnetic simulations, we have further explored the spin modulation of SkT with the variation of in-plane applied magnetic field in FeGe nanostructures. |
Tuesday, March 3, 2020 9:12AM - 9:24AM |
F41.00007: Spectroscopic-imaging STM measurement of magnetic skyrmions in a centrosymmetric crystal Yuuki Yasui, Christopher Butler, Khanh Nguyen, Takuya Nomoto, Satoru Hayami, Tetsuo Hanaguri, Yukitoshi Motome, Ryotaro Arita, Taka-hisa Arima, Yoshinori Tokura, Shinichiro Seki The magnetic skyrmion, vortex-like magnetic swirling structure, has attracted attention as a topological quasi-particle. The Dzyaloshinskii-Moriya interaction originating from broken inversion symmetry in the crystal structure is considered to be the key to stabilize the skyrmion. However, a mechanism to stabilize magnetic skyrmions even in centrosymmetric crystals has been proposed [1, 2]. For such centrosymmetric systems, experiments to show how the itenerant electrons interact with the magnetic structure are highly desired for further understanding. |
Tuesday, March 3, 2020 9:24AM - 9:36AM |
F41.00008: Helical and skyrmion phases in amorphous Fe-Ge thick films Xiaoqian Chen, Robert Streubel, Dinah Simone Bouma, Roland Koch, Stephen Douglas Kevan, Frances Hellman, Peter Fischer, Sujoy Roy Magnetic skyrmions are topologically protected solitons with the potential to revolutionize future spintronics and computing due to its low energy cost of manipulation. Skyrmion phase, now found in various single crystals including ferrimagnet FeGe, has recently predicted to also manifest in vector spin frustrated amorphous films. We studied amorphous Fe-Ge films using coherent resonant soft x-ray scattering. We observed magnetic Bragg peaks associated with the helical and skyrmion periodicity and mapped out their field-temperature phase diagram. We also observed spontaneous dynamics for a large range of temperatures. We calculate the respective one-time correlation functions of this fluctuation, whose characteristic curvatures indicate the distinct dynamical behavior of the helical and skyrmion textures. |
Tuesday, March 3, 2020 9:36AM - 9:48AM |
F41.00009: Observation of Robust Néel Skyrmions in Metallic PtMnGa Abhay Kant Srivastava, Parul Devi, Ankit Sharma, Tianping Ma, Hakan Deniz, Holger Meyerheim, Claudia Felser, Stuart Parkin Bloch[1] and anti-skyrmions[2] have attracted much attention since their first observation. Bulk Néel skyrmions have been observed in two compounds, GaV4S8[3] and VOSe2O5[4], but at low temperatures only. Here we report the first observation of bulk Néel skyrmions in a non-centrosymmetric metallic compound PtMnGa by means of Lorentz Transmission Electron Microscopy (LTEM)[5]. Skyrmions in PtMnGa are found to be stable over a wide temperature range up to its Curie temperature (~220K). By contrast with skyrmions observed to date, we find that the skyrmion size in PtMnGa is strongly influenced by the thickness of the lamella in which the skyrmions are observed. Their size increases by a factor of 7 when the thickness is increased from ~90 nm to ~4 µm. Moreover, skyrmions can be stabilized at zero field after suitable field cooling processes and are highly robust against large in-plane magnetic fields (~1 T). These properties make PtMnGa and related compounds an extremely exciting system for future spintronic applications. |
Tuesday, March 3, 2020 9:48AM - 10:00AM |
F41.00010: New low temperature phases in chiral cubic magnets Catherine Pappas, Fengjiao Qian, Lars J. Bannenberg, Ankit Labh, Heribert Wilhelm, Robert Cubitt, Eddy Lelièvre-Berna, Gregory Chaboussant, Lisa DeBeer-Schmitt, Marcus P. Schmidt, Maxim Mostovoy, Andrey Leonov Chiral and skyrmion-hosting cubic magnets have so far been described in terms of a universal magnetic phase diagram composed of helical spiral, conical spiral, and skyrmion crystal phases. A remarkable deviation from this universal behavior occurs in Cu2OSeO3 revealed by neutron scattering and magnetization measurements. Below the upper critical field at which the conical spiral state disappears, the spiral wave vector rotates away from the magnetic field direction. We discuss the physical origin of this new state and suggest that it provides the in-homogeneous magnetic environment required to stabilise new low temperature skyrmionic phases. We discuss the different stabilization and nucleation mechanisms and the thermodynamic stability of these low temperature skyrmionics phases, as they have been investigated by neutron scattering. |
Tuesday, March 3, 2020 10:00AM - 10:12AM |
F41.00011: Weak crystallization of fluctuating skyrmion textures in MnSi Jonas Kindervater, Ioannis Stasinopoulos, Andreas Bauer, Franz Xaver Haslbeck, Felix Rucker, Alfonso Chacon, Sebastian Mühlbauer, Christian Franz, Markus Garst, Dirk Grundler, Christian Pfleiderer We report an experimental study of the emergence of non-trivial topological winding and long-range order across the paramagnetic to skyrmion lattice (SkL) transition in the transition metal helimagnet MnSi. Combining measurements of the susceptibility with small angle neutron scattering, neutron resonance spin echo spectroscopy and all-electrical microwave spectroscopy, we find evidence of skyrmion textures in the paramagnetic state exceeding 103 Å with lifetimes above several 10-9 s. Our experimental findings establish that the paramagnetic to SkL transition in MnSi is well-described by the Landau soft-mode mechanism of weak crystallization, originally proposed in the context of the liquid to crystal transition. As a key aspect of this theoretical model, the modulation-vectors of periodic small amplitude components of the magnetization form triangles that add to zero. In excellent agreement with our experimental findings, these triangles of the modulation-vectors entail the presence of the non-trivial topological winding of skyrmions already in the paramagnetic state of MnSi when approaching the SkL transition. |
Tuesday, March 3, 2020 10:12AM - 10:24AM |
F41.00012: Surface Effects in the Magnetic Textures of B20 MnGe Thin Films Jacob Repicky, Joseph P Corbett, tao Liu, Adam S Ahmed, Jonathan Guerrero-Sanchez, Roland Kawakami, Jay A Gupta Bulk MnGe hosts a three-dimensional skyrmion state with a period of only 3 nm. Using low-temperature (5K) spin-polarized scanning tunneling microscopy, we show that this magnetic texture is modified at the surface of MnGe thin films. We observe a stripe-like phase with a 6-8 nm period and a propagation vector that is not strongly pinned to any direction, but is influenced by step edges, surface termination, and atomic lattice strain. We also report the observation of target skyrmions with triangular shape that is set by the atomic lattice vectors, and a core size of 15 nm. We observe the target state is much more sensitive to magnetic fields than the stripe phase, and that pulses with the STM tip demonstrate the texture can be ‘switched’ between states with different topological charge. By analyzing the voltage and current applied during switching events we can determine their dependence on thermal energy of tunneling electrons, injection of spin-polarized current, or electric fields. Finally, we explore the magnetic texture in different film thicknesses and compare their larger scale film morphology to distinguish between thin film effects and true surface effects. |
Tuesday, March 3, 2020 10:24AM - 10:36AM |
F41.00013: Equilibrium Skyrmions and Antiskyrmions in Strained Bulk Ferromagnets at Room Temperature Daniil Kitchaev, Anton Van der Ven Control over skyrmion formation and topology at room temperature is foundational to the realization of skyrmion-based spintronic devices. Strained bulk ferromagnets are a rich platform for achieving both goals as strain can precisely adjust the effective magnetic Hamiltonian and thereby control the stability of emergent phases. We use phenomenological and symmetry arguments to identify simple strain fields which may stabilize chiral skyrmions over wide temperature ranges for all crystallographic point groups. We then use first-principles calculations, cluster-expansions and Monte Carlo analysis to computationally demonstrate the strain-induced formation of equilibrium skyrmions and antiskyrmions at room temperature in a proof-of-concept material. Finally, we demonstrate how strain may be used to control the relative stability of skyrmions and antiskyrmions, or engineer a state where these topologies are degenerate. |
Tuesday, March 3, 2020 10:36AM - 10:48AM |
F41.00014: Investigating skyrmions using muon-spin spectroscopy Tom Lancaster, Geetha Balakrishnan, Stephen Blundell, Matjaz Gomilsek, Thomas Hicken, Benjamin Huddart, Ales Stefancic, Murray N Wilson We present the results of muon-spin relaxation measurements on materials hosting a skyrmion lattice (SkL) phase. We argue that the muon is primarily sensitive to the dynamics of the SkL and we have used this property to probe the local magnetism of several distinct skyrmion systems. Here we discuss our recent results on the effect of disorder on the Néel-skyrmion system GaV4S(8-y)Sey [1], where we have used low-level substitution to assess the stability of the SkL. We also discuss Zn-substituted Cu2OSeO3, where we show that the splitting of the SkL phase that was previously reported is only observed in polycrystalline samples and reflects the occurrence of several coexisting phases with different Zn content, each distinguished by different magnetic behavior [2]. We support our results with density functional theory calculations of the muon stopping states, that allow us an additional insight into the local environment of the muon probe. |
Tuesday, March 3, 2020 10:48AM - 11:00AM |
F41.00015: Imaging the internal spin structures of skyrmions by Lorentz scanning transmission electron microscopy Zhen Chen, Teng Xu, Gregory Fuchs, Wanjun Jiang, David Muller Magnetic skyrmions are noncollinear spin textures that have been detected and studied using many different techniques. However, a detailed characterization of the internal spin structures of skyrmions, especially for skyrmions with diameters smaller than 100 nm, is experimentally challenging. These experiments are extremely important for defining the chirality and spin topology. Recent experiments show that Lorentz scanning transmission electron microscopy with a high-dynamic-range pixel array detector - Lorentz 4D-STEM, allows direct imaging of magnetic structures at a spatial resolution down to a few nanometers. Here we first show that Lorentz 4D-STEM technique combining with ptychography can be used to detect the detailed spin structures inside skyrmions and spin singularities in skyrmion lattices. We subsequently demonstrate the direct determination of the chirality of Néel-type skyrmions from the magnetic induction field distribution in an inclined sample setup. Using both fast, wide-field Lorentz TEM and quantitative, high-resolution Lorentz STEM, we are able to perform real-space imaging of magnetic spin textures at length scales that span microns down to a few nanometers. |
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