Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B40: Topological Textures in Itinerant MagnetsFocus Live
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Sponsoring Units: GMAG DMP Chair: Sergey Pershoguba, Yale University |
Monday, March 15, 2021 11:30AM - 12:06PM Live |
B40.00001: Square skyrmion, meron and vortex crystals in centrosymmetric tetragonal magnets Invited Speaker: Zhentao Wang The vast majority of the magnetic skyrmion crystals that have been reported in the literature form hexagonal superstructures (triple-Q ordering) and they are induced by a finite magnetic field. Here, we present a new guiding principle for finding square skyrmion, meron and vortex crystals (double-Q ordering) in centrosymmetric tetragonal magnets even in the absence of magnetic field. The phase diagram of the microscopic theory remains qualitatively the same in the continuum limit, allowing us to extract the universal mechanism for the stabilization of these exotic phases. |
Monday, March 15, 2021 12:06PM - 12:18PM Live |
B40.00002: Imaging the coupling between itinerant electrons and localised moments in the centrosymmetric skyrmion magnet GdRu2Si2 Yuuki Yasui, Christopher J. Butler, Nguyen Duy Khanh, Satoru Hayami, Takuya Nomoto, Tetsuo Hanaguri, Yukitoshi Motome, Ryotaro Arita, Taka-hisa Arima, Yoshinori Tokura, Shinichiro Seki Magnetic skyrmions have been considered to be stabilized by the Dzyloshinskii-Moriya interaction originating from inversion-symmetry breaking structures. Recently, however, magnetic skyrmion lattices have been reported in inversion-symmetric Gd based compounds [1]. These skyrmion lattices are proposed to be stabilized by multiple-spin interactions, which are mediated by itinerant electrons [2-4]. |
Monday, March 15, 2021 12:18PM - 12:30PM Live |
B40.00003: Formation mechanism of helical Q structure in Gd-based skyrmion materials Takuya Nomoto, Takashi Koretsune, Ryotaro Arita Recently, short-period skyrmion phases have been observed in Gd-based compounds[1-3]. Using the ab initio local force method, we investigate the formation mechanism of the helical spin structure in GdRu2Si2 and Gd2PdSi3[4]. We calculate the paramagnetic spin susceptibility and find that the Fermi surface nesting is not the origin of the incommensurate modulation, in contrast to the naive scenario based on the Ruderman-Kittel-Kasuya-Yosida mechanism. We then decompose the exchange interactions between the Gd spins into each orbital component, and show that spin-density-wave type interaction between the Gd-5d orbitals is ferromagnetic, but the interaction between the Gd-4f orbitals is antiferromagnetic. We conclude that the competition between these two interactions, namely, the inter-orbital frustration, stabilizes the finite-Q structure. |
Monday, March 15, 2021 12:30PM - 12:42PM Live |
B40.00004: Chiral electric transport signatures in magnetic skyrmions and chiral helimagnets Fabian Lux, Pascal Prass, Frank Freimuth, Stefan Bluegel, Yuriy Mokrousov
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Monday, March 15, 2021 12:42PM - 12:54PM Live |
B40.00005: Construction of low energy effective Hamiltonians using supervised machine learning. Vikram Sharma, Zhentao Wang, Cristian Batista A crucial problem in modern physics is to derive a low energy effective model from a given high energy model. While perturbation theory is the most commonly used approach, there are many instances when such expansions break down. We propose a simple supervised machine learning (ML) algorithm to find the low energy spin Hamiltonian for a given labeled energy data-set from a “high energy” s-d model. The spin Hamiltonian obtained from the ML assisted approach reproduces the phase diagram of the s-d model and reveals the effective four-spin interactions that stabilize a magnetic field induced skyrmion crystal even in absence of spin anisotropy. |
Monday, March 15, 2021 12:54PM - 1:06PM Live |
B40.00006: Decoding of complex magnetic structures from Hall-effect measurements Juba Bouaziz, Hiroshi Ishida, Samir Lounis, Stefan Bluegel It is generally accepted that the Hall response of complex |
Monday, March 15, 2021 1:06PM - 1:18PM Live |
B40.00007: Nonlinear optical responses in chiral magnetic metals Shun Okumura, Takahiro Morimoto, Yasuyuki Kato, Yukitoshi Motome Chiral magnetic metals breaking both spatial inversion and time reversal symmetries carry a potential for novel nonlinear optical responses. We theoretically study such nonlinear optical responses in a chiral conical magnetic state as a prototypical example. By using the second-order response theory, we show that electrons coupled to a chiral conical magnetic texture exhibit a photovoltaic effect and second harmonic generation. In particular, we find that the coefficient of the photovoltaic effect changes in not only the magnitude but also the sign depending on the external magnetic field and the frequency of light. Our results would pave the way for the next-generation optical electronic devices, such as an unconventional solar cell and optical sensor. |
Monday, March 15, 2021 1:18PM - 1:30PM Live |
B40.00008: Defect-implantation for the all-electrical detection of non-collinear spin-textures Imara Lima Fernandes, Mohammed Bouhassoune, Samir Lounis The viability of past, current, and future devices for information technology hinge on their sensitivity to the presence of impurities. The latter can reshape extrinsic Hall effects or the efficiency of magnetoresistance effects, essential for spintronics, and lead to resistivity anomalies, the so-called Kondo effect. We demonstrate that atomic defects enable highly efficient all-electrical detection of spin-swirling textures [1], in particular magnetic skyrmions, which are promising bit candidates in future spintronics devices. The concomitant impurity-driven alteration of the electronic structure and magnetic non-collinearity gives rise to a new spin-mixing magnetoresistance (XMRdefect). Taking advantage of the impurities-induced amplification of the bare transport signal, which depends on their chemical nature, a defect-enhanced XMR (DXMR) is proposed. Both XMR modes are systematized for 3d and 4d transition metal defects implanted at the vicinity of skyrmions in Pd/Fe/Ir(111). The ineluctability of impurities in devices promotes the implementation of defect-enabled XMR modes in reading architectures with immediate implications in magnetic storage technologies. |
Monday, March 15, 2021 1:30PM - 1:42PM Live |
B40.00009: Incommensurate magnetic ordering in CrB2 András Deák, Jerome Jackson, Bendegúz Nyári, László Szunyogh We investigate magnetic ordering in the non-collinear antiferromagnet |
Monday, March 15, 2021 1:42PM - 1:54PM Live |
B40.00010: Skyrmion size dependence of the topological Hall effect: A real-space calculation Akira Matsui, Takuya Nomoto, Ryotaro Arita Since the first observation of the topological Hall effect (THE)[1], skyrmion systems have attracted broad interest in diverse fields, owing to its fascinating fundamental physics and the potential application in advanced spintronics devices. Recent discoveries of short-period skyrmion latteices[2] and theoretical investigations of the interplay between AHE and THE[3] revealed the importance of the comprehensive understanding of THE in a broad range of skyrmion size. |
Monday, March 15, 2021 1:54PM - 2:06PM Live |
B40.00011: Magnetic Fields in the Topological Hall Effect Ralph Skomski, Ahsan Ullah, Rabindra Pahari, Balamurugan Balasubramanian, David J Sellmyer The nanoscale Berry-phase contribution to the topological Hall effect (THE) in thin-film nanostructures is well-known to involve the magnetic flux density (B-field). However, the role of the B field is nontrivial. First, Maxwell's equations are continuum expressions, but electrons are point-like objects that move in a magnetic Lorentz hole where B and H differ by a vacuum constant only. Thin-film demagnetizing fields (D ≈ 1) affect the THE via the Lorentz cavity field only, not through Maxwell's B-field. Second, from the Aharonov-Bohm effect it is known that the vector-potential A is the key consideration in the interpretation of Berry phase effects, not B = μo(M + H). For example, perpendicularly magnetized continuous thin films in zero external field have D = 1 and B = 0, in contrast to the nonzero THE found in some of these systems. Third, magnetic fields in granular thin films, such as deposited ensembles of Co and CoSi nanoparticles, are strongly inhomogeneous, which means that the local magnetic fields, the Berry curvature, and the current density vary greatly across the sample. We show that this variation leads to a generally very large effect that looks like a THE signal but is unrelated to the nanoscale Berry phase. |
Monday, March 15, 2021 2:06PM - 2:18PM Not Participating |
B40.00012: Towards skyrmionic spin textures combined with SrTiO3 two-dimensional electron gases Luis Moreno, Srijani Mallik, Lourdes Marcano, Karim Bouzehouane, Sergio Valencia, Nicolas Reyren, Agnès Barthélémy, Manuel Bibes SrTiO3 (STO) two dimensional electron gases (2DEGs) can be formed by depositing reactive metals such as Al onto STO single crystals1 : the metal oxidizes, forming oxygen vacancies in the STO and thus doping it in electrons. Such 2DEGs can then be used to interconvert spin and charge currents with great efficiency2. In this presentation we will show that the metal oxide (e.g. AlOx) thus formed can be combined with ferromagnetic and heavy-metal-based ultrathin films to generate various types of magnetization configurations with in-plane or out-of-plane anisotropy, depending on their thicknesses. For specific compositions, these multilayers harbor magnetic bubbles, as imaged by magnetic force microscopy and X-ray photoemission electron microscopy, that likely have a skyrmionic character, due to the expected Dzyaloshinskii-Moriya interaction caused by the strong broken inversion symmetry. |
Monday, March 15, 2021 2:18PM - 2:30PM Live |
B40.00013: Strain Induced Modulation of Dzyaloshinskii-Moriya interaction in Pt/Co Bilayer Devices: A Green's Function Approach. Farzad Mahfouzi, Nicholas Kioussis We use first-principles calculations to study the strain induced control of Dzyaloshinskii-Moriya Interaction (DMI) in ultrathin Pt/Co films. We introduce a Greens function approach to calculate the DMI coefficients that compared to the super-cell and Generalized Bloch approaches is computationally more efficient and accurate. We investigate the dependence of the DMI coefficient for Pt/Co[111] bilayer device on the Pt and Co thicknesses. We compare the calculated results versus Co thickness with the reported experimental data, where we observe a relative agreement. Furthermore, we study the effects of normal and shear strains on the DMI tensor elements. We show that normal(shear) strain results in anisotropic Neel- (Bloch-)type DMI. |
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