Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session D54: Skrymions in Heterostructures and Bulk SystemsRecordings Available
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Sponsoring Units: GMAG Chair: Vincent Cros, CNRS/Thales, France Room: McCormick Place W-476 |
Monday, March 14, 2022 3:00PM - 3:12PM |
D54.00001: Room temperature skyrmions in layered magnet Hongrui Zhang, Yu-Tsun Shao, Rui Chen, Xiang Chen, David Raftrey, Ying-Ting Chan, Sandhya Susarla, Weida Wu, Jie Yao, Peter Ercius, Peter J Fischer, David A Muller, Robert J Birgeneau, Ramamoorthy Ramesh The Néel-type skyrmion is usually observed at room temperature in multilayer metal films. Polar single-phase magnets, with broken symmetries of spatial inversion and time reversal, are expected to host Néel-type skyrmion. However, their skyrmionic state has been observed only at low temperatures. In this talk, we demonstrated the experimental observation of a Néel-type skyrmion lattice at room temperature in a layered 2D magnet, 50% Co-doped Fe5GeTe2 (FCGT) system. We studied the evolution of skyrmion at various magnetic fields, temperatures, thicknesses, and current in FCGT nanoflakes using Lorentz electron microscopy, variable-temperature magnetic force microscopy, and magneto-transport measurements. The skyrmion can be stabilized in 100~300 nm nanoflake at the temperature range from 260~340 K. The current-induced skyrmion motion was realized at room temperature, with a threshold current density ~ 2.1 ×106 A/cm-2. This discovery of skyrmions at room temperature in a non-centrosymmetric material opens the way for layered device applications. |
Monday, March 14, 2022 3:12PM - 3:24PM |
D54.00002: Probing Magnetization Textures including Skyrmions using Tabletop Extreme Ultraviolet Resonant Magnetic Scattering Iona Binnie, Guan Gui, Christian Gentry, Emma M Cating-Subramanian, Sergio Montoya, Eric E Fullerton, Henry C Kapteyn, Margaret M Murnane, Chen-Ting Liao Advances in nanotechnologies require faster and more efficient ways of writing and reading information at room temperature. In spintronics, electron spins are a promising candidate for this role. In addition to supporting next-generation spin-based storage and logic technologies, we also need to understand and learn how to manipulate magnetic order on its fundamental time and length scales. Recently, nanoscale topological defects in spin texture called magnetic skyrmions have attracted great interest due to their topological protection with associated high energy barriers for decay. However, a significant improvement in our understanding of magnetization textures at nanometer length and femtosecond time scales is needed to fully utilize their potential – for example, in all-optical switching. Tabletop extreme ultraviolet (EUV) light sources from laser-driven high harmonic generation combine high temporal and spatial resolution and are thus well suited for time-resolved microscopy of nanoscale spin textures. Resonant magnetic scattering (RMS) experiments measure the scattered light when photons interact with the magnetic moments of materials. Here, we present resonant magnetic scattering patterns from skyrmion lattice and other topologically stabilized samples. The skyrmion samples are 80nm thick Fe/Gd multilayer thin-films that support dipole-stabilized, Dzyaloshinsky–Moriya interaction-free (DMI-free), skyrmion lattices. Using EUV light at the Fe M-edge (~55 eV), we observed hexagonal scatter patterns, representing the lattice phase of the skyrmions at room temperature. By exciting other samples with a femtosecond laser pulse, we use a tabletop EUV source to study the creation, annihilation, and phase transitions of skyrmions. |
Monday, March 14, 2022 3:24PM - 3:36PM |
D54.00003: Frustration of coupled helices in weakly perpendicular magnetic thin films Sophie A Morley, Jianheng Li, Ryan Tumbleson, Sergio Montoya, Eric E Fullerton, David Lederman, Stephen D Kevan, Sujoy Roy Anti-symmetric exchange, or the Dzyaloshinskii–Moriya interaction (DMI), is responsible for non-collinear spin textures such as canting, helices and skyrmions. Analogously, some thin films without DMI can have competing magnetic anisotropies which stabilise non-collinear spin textures. Using resonant soft x-ray scattering, we report on the peculiar phenomenon of collective stripe rotation in an FeGd film under an applied field which also accompanies the transition from the stripe phase to the six-fold dipole skyrmion lattice (SkL). By changing the sample tilt, we control the resultant field vector, and in turn the threshold and abruptness of the observed rotation. We performed micromagnetic simulation to reproduce the experimental result and on inspection of the simulated 3D spin structure we identify two strongly coupled helical structures. As the magnetic field is increased in the simulation, the helical domain wall region unwinds before rotation and then reverts to a more wound structure in the rotated state. Our simulations highlight the importance of setting the polarity of the shared center of the coupled helices as well as the frustration effects induced by a combined in and out-of-plane magnetic field. |
Monday, March 14, 2022 3:36PM - 3:48PM |
D54.00004: Imaging small skyrmions at room temperature in Fe-rich Fe1.2Ge epitaxial films Camelia Selcu, Tao Liu, Binbin Wang, Nuria Bagues Salguero, Po-Kuan Wu, Timothy Q Hartnett, Shuyu Cheng, Denis Pelekhov, Roland A Bennett, Joseph P Corbett, Jacob J Repicky, Brendan A McCullian, P Chris Hammel, Jay Gupta, Mohit Randeria, Prasanna V Balachandran, David W McComb, Roland K Kawakami Chiral magnets exhibit a variety of interesting magnetic phases including topological skyrmion spin textures, a candidate for ultrahigh density memory. A widely studied chiral magnet is FeGe which exhibits skyrmions (∼ 70 nm in diameter) up to its Curie temperature of ∼270 K. To raise the Curie temperature of FeGe and achieve skyrmions at room temperature, we adopted a strategy to develop artificial chiral magnets via atomic-layer molecular beam epitaxy (MBE) far from equilibrium. Specifically, by adding excess Fe atoms to epitaxial FeGe films, we engineered a new Fe-rich Fe1.2Ge material which has its Curie temperature above room temperature. Magnetic imaging by Magnetic Force Microscopy (MFM) and Lorentz Transmission Electron Microscopy (LTEM) reveal the presence of small skyrmions (<20 nm diameter) at room temperature, two important characteristics for new magnetic memory. Topological Hall effect measurements confirm the topological nature of the observed spin textures. |
Monday, March 14, 2022 3:48PM - 4:00PM |
D54.00005: Structure and Magnetism of Chiral Mn5Si2 Ahsan Ullah, Balamurugan Balasubramanian, Wenyong Zhang, Shah Valloppilly, Renat Sabirianov, Ralph Skomski, David J Sellmyer Magnets with chiral crystal structures and Dzyaloshinskii-Moriya (DM) interactions have recently attracted much attention as potential spin-electronics materials [1]. Mn-based intermetallic compounds have gained particular interest, because they often favor noncoplanar spin structures including skyrmions [2, 3], and their magnetic and electron-transport properties are also tunable by alloying and varying Mn content [3]. In this work, Mn5+xSi2-x alloys (x = 0, 0.1, and 0.2) were prepared using arc-melting process followed by annealing at 800 °C for 12 hours. Rietveld refinement analysis on x-ray diffraction data shows that the Mn5Si2 compounds form tetragonal structure with a = 8.914 Å and c = 8.719 Å and a space group P41212, which can support spin spirals caused by DM interactions. The effect of Mn/Si ratio on the magnetic properties will be discussed. |
Monday, March 14, 2022 4:00PM - 4:12PM |
D54.00006: Revealing nanoscale chiral magnetic ordering in polycrystalline FeGe thin films using 4-D Lorentz Scanning Transmission Electron Microscopy Xiyue S Zhang, Kayla X Nguyen, Emrah Turgut, Zhen Chen, Celesta S Chang, Yu-Tsun Shao, Gregory D Fuchs, David A Muller We study nanoscale chiral magnetic ordering in sputtered polycrystalline B20 FeGe thin film on [111] Si substrate with Lorentz Scanning transmission electron microscopy. Chiral magnetism is present in bulk B20 FeGe but far less is known about the impact of grain boundaries on the magnetic order. We use 4D-STEM enabled by a new design of pixel-array detector to disentangle contrast from the local magnetic and electrostatic fields and thus image magnetic structures despite varying grain contrast. Atomic resolution images of the grain boundaries show how the crystal chirality correlates with the local magnetic phase. Across a mirror twin boundary where crystal chirality is preserved, our magnetic induction mapping shows the magnetic helicity is preserved in both the helical and skyrmion states. Across grain boundaries where the crystal chirality is reversed, the magnetic induction for helical and skyrmion states show an inversion of magnetic helicity. Both examples indicate the crystal chirality is coupled to magnetic helicity in the polycrystalline FeGe thin film. |
Monday, March 14, 2022 4:12PM - 4:24PM Withdrawn |
D54.00007: Effects of Disorder on the Skyrmion Phase in FeGe Thin Films Michael B Venuti, Tzu-Ming Lu, Khalid Hattar, Eric J Lang, Serena M Eley Magnetic skyrmions are nanoscale whirlpools of magnetic moments that arise certain magnetic materials and are potentially useful as information carriers in next-generation low-energy spintronic devices. To exploit skyrmions in spintronics, we must be able to controllably manipulate them, which necessitates understanding how to create, annihilate, and move them. It also requires understanding how the skyrmion lattice is affected by disorder, dictating how skyrmions will either maneuver around or be pinned by energy barriers within the material’s disorder landscape. Here, we report on the effects of point disorder on the skyrmion phase in epitaxial FeGe thin films. To tune defect densities, we irradiate the films with 2.8 MeV Au ions, varying the dose to control the densities of induced vacancies. We additionally conduct electrical transport measurements, identifying the skyrmion phase by the appearance of a topological Hall effect component, and compare the subsequently constructed field H – temperature T phase diagrams of the irradiated and pristine samples to systematically observe disorder-induced changes in the skyrmion lattice phase. |
Monday, March 14, 2022 4:24PM - 4:36PM |
D54.00008: Tunable Magnetic Skyrmions in Ferrimagnetic Mn4N Wei Zhou, Chung T Ma, Tim Hartnett, Prasanna V Balachandran, Joseph Poon The magnetic skyrmion has large potential in high-density data storage applications due to its unique properties. To get a high data density, the skyrmions should be small at room temperature (RT). Simulations and experiments have found 10–30nm small skyrmions in GdCo thin films at RT, but CoGd thin films would lose their perpendicular magnetic anisotropy at high temperatures (300 oC). Ferrimagnetic Mn4N thin film is one of the candidate materials to host small magnetic skyrmions with much improved thermal stability. Here, we discuss the results of sputter-grown ~15 nm Mn4N thin films on MgO substrate with CuxPt1-x capping layers. The optimal films showed low saturation magnetization and low magnetic anisotropy energy, which were insensitive to the composition of the capping layer. Magnetic force microscopy images showed the diameter of the magnetic skyrmion decreased from 300nm to 50nm, as the Cu concentration was increased from x= 0 to 0.9. The interfacial Dzyaloshinskii-Moriya Interaction (iDMI), as calculated by DFT, decreases with Cu substitution and is likely the source of the decrease in skyrmion radius. The DFT results will be discussed in greater detail in a separate talk. This work provides guidance to achieve smaller Néel-type skyrmions in Mn4N thin films. |
Monday, March 14, 2022 4:36PM - 4:48PM |
D54.00009: Density Functional Theory Investigation of Ferrimagnetic Mn4N as a Skyrmion Host Tim Hartnett, Prasanna V Balachandran, Chung T Ma, Wei Zhou, Joseph Poon Skyrmion devices have potential to cause significant disruption inthe high-density storage industry. To maximize performance, nucleated skyrmions should be small and stable around room temperature. Thinfilms of ferrimagnetic Mn4N show promise with improved stability be-cause of the high N ́eel temperature, low saturation magnetization, and presence of perpendicular magnetic anisotropy in the thin film geometry. However, conditions that govern the skyrmion size in ultra-thin Mn4N films are not known. Here, density functional theory calculations are used to investigate the salient magnetic properties that govern theskyrmion size in Mn4N thin films. We calculate the impact of changing both the interfacial composition as well as orientation on the interfacial Dzyaloshinskii-Moriya interaction (iDMI). We predict that Cu substitution in the Pt capping layer reduces the iDMI, which reveals the promise of capping layer composition engineering for tuning the skyrmion size. Results from DFT are validated by growth of a < 20 nm Mn4N thin film on MgO (001) substrate with CuxPt1−xcapping layer. Imaging using magnetic force microscopy showed that in CuxPt1−x can serve as a knob to tune the skyrmion size (50–300 nm) at room temperature. |
Monday, March 14, 2022 4:48PM - 5:00PM |
D54.00010: Manipulation of Dyzaloshinskii-Moria interaction in Pt/Co/AlOx heterostructure Babu R Sankhi, Elena Echeverria, Hans T Nembach, Justin shaw, Soumya Mandal, Ritesh Sachan, David N McIlroy, Emrah Turgut, Derek Meyers Dzyaloshinskii-Moria interaction originates from the combination of spin-orbit coupling and broken inversion symmetry.This interaction plays a crucial role in establishing and driving spin textures such as skyrmions and chiral domain walls which are essential ingredients for next-generation data storage devices in spintronics. Quantification of this interaction is crucial for the fabrication of such devices. In this talk, we evaluate the interfacial DMI at the ferromagnet/metal oxide interface of Pt/Co/AlOx trilayers using asymmetric domain wall expansion and hysteresis loop shift methods. We find the Dyzaloshinskii-Moria interaction constants ranging from 0.09 mJ/m2 to 1.13 mJ/m2 by tuning annealing time, substrate type, and AlOx thickness. In addition, we also study the influence of oxygen content on DMI at the Co/AlOx interface of trilayers. For this purpose, we perform X-ray photoelectron spectroscopy, transmission electron microscopy, and low-temperature Hall effect characterizations to investigate the interfacial oxygen content for samples annealed for different time intervals. We found a higher degree of oxygen at the interface for 3-hour annealed samples compared to 1-hr annealed one leading to the enhancement of Dyzaloshinskii-Moria constants. |
Monday, March 14, 2022 5:00PM - 5:12PM Withdrawn |
D54.00011: Resonant optical topological Hall conductivity from skyrmions Arun Paramekanti, Sopheak Sorn, Luyi Yang We study the high-frequency Hall conductivity in a two-dimensional model of conduction electrons coupled to a background magnetic skyrmion texture via an effective Hund's coupling term. For an ordered skyrmion crystal, a Kubo formula calculation using the basis of skyrmion crystal Chern bands reveals a resonant Hall response at a frequency set by the Hund's coupling, ω ~ JH. A complementary real-space Kubo formula calculation for an isolated skyrmion in a box reveals a similar resonant Hall response. A linear relation between the area under the Hall resonant curve and the skyrmion density is discovered numerically and is further elucidated using a gradient expansion which is valid for smooth textures and a local approximation based on a spin-trimer calculation. We point out the issue of distinguishing this skyrmion contribution from a similar feature arising from spin-orbit interactions, as demonstrated in a model for Rashba spin-orbit coupled electrons in a collinear ferromagnet, which is analogous to the difficulty of unambiguously separating the dc topological Hall effect from the anomalous Hall effect. The resonant feature in the high-frequency topological Hall effect is proposed to provide a potentially useful local optical signature of skyrmions via probes such as scanning magneto-optical Kerr microscopy. |
Monday, March 14, 2022 5:12PM - 5:24PM Withdrawn |
D54.00012: Magnetic interactions in multiferroic lacunar spinels GaV4S8 and GaV4Se8 Vladislav Borisov, Patrik Thunström, Anna Delin, Olle Eriksson Since the discovery of magnetic skyrmions in MnSi, systems with topological magnetic textures have been intensively studied due to their possible application in nanoscale memory devices. Even more interesting would be multiferroic skyrmionic systems where the magnetic properties can be, in principle, affected by external electric field through the coupling between the magnetic and ferroelectric order parameters. One of the few known examples are lacunar spinels GaV4S8 and GaV4Se8 which host Neel-type skyrmions and sizeable ferroelectric polarization. These spinels present a challenge for theory due to the fact that the actual magnetic units are V4 metal clusters, as suggested by experimental evidence in the literature. In our work, we study the electronic properties of these lacunar spinels using density functional theory and its extensions for correlated systems and calculate the Heisenberg and Dzyaloshinskii-Moriya magnetic interactions, which are necessary ingredients for the skyrmionic behavior, using the RSPt code. The role of magnetization distribution, electronic correlations and ferroelectric polarization is discussed and large-scale simulations are performed to determine the influence of these factors on the magnetic texture. |
Monday, March 14, 2022 5:24PM - 5:36PM |
D54.00013: Weak crystallization of fluctuating skyrmion textures in MnSi Franz Xaver Haslbeck, Jonas Kindervater, Ioannis Stasinopoulos, Andreas Bauer, 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 (SkX) 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 length scales of 103 Å with lifetimes above several 10−9 s. Our experimental findings establish that the paramagnetic to SkX 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 SkX transition. |
Monday, March 14, 2022 5:36PM - 5:48PM |
D54.00014: Antiskyrmions and Bloch Skyrmions in Magnetic Dresselhaus Metals Deepak S Kathyat, Arnob Mukherjee, Sanjeev Kumar We present a microscopic electronic description of how antiskyrmions can be stabilized in a Dresselhaus spin-orbit coupled magnetic metal. Furthermore, we show that the antiskyrmions can be tuned into Bloch skyrmions via a change in sign of hopping integral. The results are based on the state of the art hybrid Monte Carlo simulations. Origin of such topological textures is understood via an effective spin-only model. Our results uncover an important and useful connection between generic features of the band structure and topological spin textures of immense current interest, and present a microscopic explanation of skyrmion formation reported recently in certain magnetic Weyl semimetals. |
Monday, March 14, 2022 5:48PM - 6:00PM |
D54.00015: Soft x-ray 3D imaging of dipole-stabilized magnetic skyrmions in FeGd thin films Emma M Cating-Subramanian, Chen-Ting Liao, Saman Moniri, Iona Binnie, Arjun Rana, Xingyuan Lu, Christian Gentry, Bin Wang, Vanessa Schoeppler, Young-Sang Yu, David A Shapiro, Eric E Fullerton, Yuka Esashi, Nicholas Jenkins, Michael Tanksalvala, Sergio Montoya, Jianwei Miao, Henry C Kapteyn, Margaret M Murnane Topologically stabilized spin structures such as magnetic skyrmions are promising platforms for efficient next-generation technologies including spintronics and low-energy data transport and storage. However, their small size, low-contrast, required fields, and complex textures make them difficult to image directly in three dimensions (3D). In this work we use specially designed FeGd multilayer thin films in which skyrmions are dipole stabilized and exist within the material for extended periods of time (weeks or even months) at room temperature and with no applied field. This creates an additional imaging challenge, however, as the thin-film nature of the sample means that there is only a low-contrast magnetic scatter signal. We therefore extended a soft x-ray vector ptychotomography technique we developed to a system with only magnetic contrast. By combining resonant x-ray magnetic circular dichroism (XMCD) to provide magnetic sensitivity, ptychography for diffraction-limited imaging with phase contrast which is ideal for low-contrast materials, and tomography, we generate full 3D images of dipole stabilized skyrmions at room temperature and with zero applied magnetic field. |
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