Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Z67: Two-Dimensional Magnets IIRecordings Available
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Sponsoring Units: DMP Chair: Kwabena Bediako, University of California Berkeley Room: Hyatt Regency Hotel -Hyde Park |
Friday, March 18, 2022 11:30AM - 11:42AM |
Z67.00001: Exploring Current Flow Using Cryogenic Scanning Magnetometry based on NV Centers in Diamond Seunghyun Park, Shaowen Chen, Uri Vool, Amir Yacoby Magnetic topological insulators are known to exhibit quantum anomalous Hall effect, which has an insulating bulk and quantized edge channels. However, examination of the spatial structure of the edge current as well as its dissipationless nature is still lacking in detail. Here, we introduce a home-built cryogenic diamond nitrogen-vacancy (NV) scanning system with a broad range of working temperatures as a way to explore the details of the dissipationless edge current and the underlying interactions in the system. Our measurement apparatus allows local measurements of magnetic fields with a sensitivity of a few nT/Hz^1/2 and spatial resolution of about 100 nm which are ideally suitable for the study. |
Friday, March 18, 2022 11:42AM - 11:54AM |
Z67.00002: Depth-resolved imaging of two-dimensional materials via standing-wave photoemission microscopy Jay R Paudel, Ryan Muzzio, Henrique P Martins, Sergio V Molina, Florian Kronast, Slavomir Nemsak, Jyoti Katoch, Alexander X Gray Two-dimensional materials and their heterostructures offer an exciting new avenue for the realization of atomically-thin electronic, photonic, and magnetic devices. This study demonstrates the feasibility of investigating single monolayers of transition-metal dichalcogenides in three dimensions, using depth-resolved standing-wave photoemission microscopy (SW-PEEM) [1,2]. This method is based on excitation with soft x-ray standing waves generated by Bragg reflection from a multilayer (Mo/Si) mirror substrate. Depth-resolved evolution of the WS2 valence-band electronic structure and chemical bonding (via core-level spectromicroscopy) is studied by translating the x-ray standing wave vertically through the substrate and the transferred monolayer. Angstrom-level depth resolution and sensitivity to different depths within the monolayer are demonstrated. |
Friday, March 18, 2022 11:54AM - 12:06PM |
Z67.00003: First-principles insights for high-resolution ion microscopy of graphene Alexandra Olmstead, Alina Kononov, Andrew Baczewski, Andre Schleife The unique electronic properties of graphene allow many potential applications in novel electronic devices. However, these applications require high-resolution imaging of the 2D material to ensure a desirable atomic structure. The most precise available techniques detect emitted electrons as an ion beam scans the sample, resolving nanoscale features but struggling to achieve atomic resolution. First-principles modeling can accelerate experimental efforts to improve microscopy methods and offer detailed insights into underlying physics. Using timedependent density functional theory, we simulate 6-200 keV light ions traversing free-standing graphene along different trajectories. We then generate simulated microscopy images from the calculated entrance- and exit-side emitted electron yields. For incident protons, we find highest contrast for a 50 keV beam, and we predict higher contrast for exit-side (forward) emission than the typically detected entrance-side (backward) emission. We also compare contrast achieved by protons and alpha particles. These findings could enable higher resolution imaging of 2D materials, allowing precise screening for the structural properties required by applications. |
Friday, March 18, 2022 12:06PM - 12:18PM |
Z67.00004: In-situ imaging of room-temperature ferromagnetic domains in monolayer vanadium-doped WS2 Ariana Ray, Mingzu Liu, Boyang Zheng, Da Zhou, Mauricio Terrones, David A Muller Monolayer WS2 has recently been demonstrated to host long-range, room-temperature ferromagnetic ordering when substitutionally doped with vanadium atoms. V-doped WS2 and other 2D dilute magnetic semiconductors formed from doped transition metal dichalcogenides are a promising new class of air-stable intrinsic magnetic materials for spintronics applications, but their local magnetic domain structures have not been explored in detail. Here we image magnetic domains in monolayer V-doped WS2 at different dopant concentrations using in-situ Lorentz Transmission Electron Microscopy (TEM). At low dopant concentration and zero applied field, we observe scattered few-micron-sized ferromagnetic domains with in-plane magnetization. As the out-of-plane applied field strength is increased, the magnetization rotates out-of-plane, aligning with the applied field at ~25 milli Tesla. The domain contrast disappears after repeated exposure to the electron beam, suggesting that radiation damage to the WS2 destroys the long-range magnetic ordering. In particular, the n-type doping of electron beam-induced sulfur vacancies is suspected to disrupt the ferromagnetic ordering induced by the p-type vanadium doping. |
Friday, March 18, 2022 12:18PM - 12:30PM |
Z67.00005: Revealing and controlling magnetism in WSe2 via Se-vacancy states Dinh Loc Duong, Tuan Dung Nguyen, Jinbao Jiang, Bumsub Song, Young Hee Lee In van der Waals-layered transition metal dichalcogenides, intrinsic defects such as transition metals and chalcogen vacancies are often observed. Transition metal vacancies generate magnetic properties; for example, the long-range magnetic order in PtSe2 is induced by the Pt vacancies inherited from the formation of occupied mid-gap states. In contrast, the chalcogen vacancies generate unoccupied levels in the bandgap, which do not contribute to the total magnetic moment. In this talk, a gate-tunable magnetic order resonant with Se vacancy states in WSe2 thin films by partially filling electrons will be presented. The Se-vacancy states were probed via photocurrent measurements with gating to convert unoccupied states to partially occupied states associated with photo-excited carrier recombination. The magneto-photoresistance hysteresis was modulated by gating, which is consistent with the density functional calculations. The two energy levels associated with Se vacancies split with increasing laser power, owing to the robust Coulomb interaction and strong spin-orbit coupling. Our results offer a new approach for controlling the magnetic properties of defects in optoelectronic and spintronic devices using van der Waals-layered semiconductors. |
Friday, March 18, 2022 12:30PM - 12:42PM |
Z67.00006: Anisotropic magnetocaloric effect of CrI$_{3}$: A theoretical study Hung B Tran A promising candidate for van-der-Waals magnetic materials such as CrI$_{3}$ for spintronic devices and data storage has attracted considerable attention. In the present study, we derived the Heisen- berg Hamiltonian for CrI3 from density functional calculations using the Liechtenstein formula. Monte–Carlo simulations with the Sucksmith–Thompson method were performed to analyze the effect of magnetic anisotropy energy on the thermodynamic properties. Our method successfully reproduced the negative sign of isothermal magnetic entropy changes when a magnetic field was applied along the hard plane. We found that the temperature dependence of the magnetocrystalline anisotropy energy is not negligible at temperatures slightly above the Curie temperature. We clarified that the origin of this phenomenon is anisotropic magnetic susceptibility and magnetization anisotropy. The difference between the entropy change curves of the easy axis and the hard plane is caused by the temperature dependence of the magnetic anisotropy energy. The energy barrier reduction as magnetic anisotropy energy at finite temperature mainly comes from the entropic term since it tends to maximize the randomness of the magnetization in both direction and value. |
Friday, March 18, 2022 12:42PM - 12:54PM |
Z67.00007: Anisotropic Heisenberg ferromagnets in 2D and electrically tunable ordering mukul kabir, Chandan Singh Despite colossal efforts, room-temperature ferromagnetism in two-dimension remains elusive due to the fundamental spin fluctuation in reduced dimension. Here, we report a dramatic electrical manipulation of magnetic ordering up to room temperature in the monolayers of CrI3 and CrBr3 within the first-principles Heisenberg XXZ model. We develop an anisotropic Heisenberg model with relativistic exchange interactions that are obtained from the first-principles calculations. We demonstrate the crucial importance of magnetic interactions beyond the first-neighbour. The exchange and anisotropic magnetic interactions are externally modulated by a gate-induced charge carrier doping that triggers a non-trivial phase diagram. High-temperature ferromagnetism is associated with a substantial increase in effective ferromagnetic exchange and overall magnetic anisotropy under experimentally attainable hole doping. These results present new possibilities in electrically controlled spintronic and magnetoelectric devices based on atomically thin crystals. |
Friday, March 18, 2022 12:54PM - 1:06PM |
Z67.00008: Significance of Single-ion Anisotropy in d3 Mott Insulator Xiaoyu Liu, Derek Churchill, Hae-Young Kee Magnetic anisotropy is essential in the development of 2D spintronic devices as the Mermin-Wagner theorem forbids long-range magnetic order in 2D magnetic isotropic systems. For spin moment S bigger than 1/2, the single-ion anisotropy is possible via spin-orbit coupling. However, d3 Mott insulators with octahedra environment map to the half-filled t2g-orbitals and the single-ion anisotropy is absent despite the S=3/2 local moment, as spin-orbit coupling is inactive due to zero total angular momentum according to the Hund’s rule. On the other hand, preferred magnetic moment directions have been reported in d3 materials. Here we derive the single-ion anisotropy interaction using the strong-coupling expansion. The cubic crystal field splitting, trigonal distortions, and Hund’s coupling in addition to spin-orbit coupling from both transition metal and anion sites are taken into account. Intriguing Interplay among them determines the easy-plane vs. easy-axis single ion anisotropy. We apply our theory to CrX3 and present the origin of single-ion anisotropy difference in CrCl3 and CrI3. Our theory can be generalized to 4d3 and 5d3 systems. |
Friday, March 18, 2022 1:06PM - 1:18PM |
Z67.00009: Strain-induced magnetic transition of HfSe2 and PtSe2 nanoribbons: A first-principles study Hsin-Mei Ho, Shih-Chuan Lien, Yu-Hui Tang Recent research has focused on both the fundamental properties of low-dimensional materials and the exploration of new properties based on these materials. Different from intrinsically magnetic metals, such as iron and cobalt, the appearance of magnetic states in graphene nanoribbon is ascribed to the zigzag geometry. Similar to the magnetic states in graphene, research has found solid connections between the zigzag edges and the magnetism in transition metal dichalcogenides (TMDs). In this study, our first-principles calculations reveal that magnetic states in HfSe2 and PtSe2 nanoribbons are related to the broken octahedral coordination at zigzag edges. The magnitude of such magnetization in HfSe2 nanoribbon can be modulated by applying strain; remarkably, a magnetic-nonmagnetic transition in PtSe2 nanoribbon is observed, which is explained by analogy with the Anderson impurity model. These findings expand the knowledge of inducing tunable magnetism in non-magnetic 2D materials. |
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