Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F52: Van der Waals Magnets IIFocus Recordings Available
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Sponsoring Units: GMAG DMP FIAP Chair: Roland Kawakami, Ohio State University Room: McCormick Place W-475A |
Tuesday, March 15, 2022 8:00AM - 8:36AM |
F52.00001: Magnetic Dynamics in van der Waals Magnets Probed by Time-Resolved Magneto-Optics Invited Speaker: Xiao-Xiao Zhang The recently discovered atomically-thin magnetic crystals provide a unique playground for understanding magnetization in 2D confinement and developing new approaches to manipulating magnetism for novel spintronic devices. In this talk, I focus on our recent works of time-resolved spin dynamics in 2D magnetic crystals, including CrI3 and FePS3. Ultrafast optical pulse excitations are used to create non-equilibrium states and launch magnon modes, and we analyze the subsequent magnetization relaxations with sub-picosecond time-resolution. The first half of the talk will focus on the coherent magnons in bilayer CrI3. The observed magnon frequencies correspond to the antiferromagnetic resonance, and can be further tuned with electrostatic gating. In the second half of the talk, I will discuss the diverging spin dynamics in few-layered FePS3, which can be attributed to the critical behaviors near the Neel transition temperature. |
Tuesday, March 15, 2022 8:36AM - 8:48AM |
F52.00002: Probing and controlling 2D and 3D magnetic hybrid systems via spin-orbit torque Yunqiu (Kelly) Luo, Thow Min Jerald Cham, Vishakha Gupta, Kin Fai Mak, Jie Shan, Daniel C Ralph The recently discovered van der Waals (vdW) magnets provide a new library of magnetic material systems which can have either ferromagnetic (FM) or antiferromagnetic (AFM) order, different strengths and directions of magnetocrystalline anisotropy, and insulating, semi-conducting, or metallic electrical properties.The vdW interfaces of 2D magnets allow inter-layer exchange to be much weaker than intra-layer exchange. This provides opportunities to explore experimental behaviors that are distinct from conventional 3D magnetic materials. In this talk, I will discuss our recent progress in probing and controlling vdW magnetic hybrid systems via spin-orbit torque. I will also illustrate emerging device concepts based on vdW and 3D hybrid systems to read and write magnetic states through both optical and electrical techniques. |
Tuesday, March 15, 2022 8:48AM - 9:00AM |
F52.00003: Spin stiffness of chromium-based van der Waals ferromagnets Zhixue Shu, Tai Kong Magnetic van der Waals materials attracted much interest after the discovery of long-range magnetic ordering in two-dimensional limit. Measuring and understanding the underlying magnetic exchange and anisotropy energy scales is the key to future material design. In this work, we systematically measure the low temperature magnetization of CrI3, CrGeTe3, CrSiTe3 bulk single crystals. We extracted the spin stiffness constant and spin excitation gap according to revised Bloch's law. Our results are quantitatively consistent with data from inelastic neutron scattering experiments. Among all three studied compounds, larger spin stiffness value leads to higher ferromagnetic transition temperature. Our work provides a fast and reliable way to characterize magnetic excitation gap and spin stiffness using bulk magnetization measurements. |
Tuesday, March 15, 2022 9:00AM - 9:12AM |
F52.00004: Effect of Spin-lattice coupling in CrI$_3$ Monolayer Banashree Sadhukhan, Anders Bergman, Yaroslav O Kvashnin, Johan Hellsvik, Anna Delin The microscopic understanding of how spin and lattice degree of freedom interact is important to gain control over magnetic ordering in ultrafast experiments. The orbital dynamics of the electrons in CrI$_3$ monolayer is expected to have a significant impact on coupled spin-lattice dynamics (SLD) because of strong hybridization of Cr$-3d$ and I-$p$ orbitals. We present a first-principles assessment of magnetic exchange interactions from a full relativistic approach to understand the microscopic origin of the effect of SLD on magnetism. The magnetic exchange interactions are sensitive both to the in-plane motion of magnetic atoms and out-of-plane motion of ligand atoms in full atomistic vibration. Orbital decomposition of isotropic exchange interactions unfold the dependence of ferromagnetic-antiferromagnetic sign inversion on the competition between the antiferromagnetic $t_{2g}-t_{2g}$ and ferromagnetic $t_{2g}-e_{g}$ orbitals which rely on I-Cr-I bond angle and Cr-Cr bond distance. Our calculated spin-lattice coupling (SLC) constants are $\approx$ 10 times larger than bcc Fe which guarantee a larger SLD effect in CrI$_3$ monolayers. SLD reduces symmetry in CrI$_3$ monolayers and enhances the the Dzyaloshinskii-Moriya interaction over Heisenberg interactions. The estimated SLC constants, along with the microscopic orbital analysis can act as a guiding principle for further studies of the thermodynamic properties and combined magnon-phonon excitations in two-dimensional magnets. |
Tuesday, March 15, 2022 9:12AM - 9:24AM |
F52.00005: Coexistence of structural and magnetic phases in the van der Waals ferromagnet CrI3 Jaume Meseguer-Sánchez, Catalin A Popescu, José L García-Muñoz, Efrén Navarro-Moratalla, Zurab Guguchia, Elton Santos
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Tuesday, March 15, 2022 9:24AM - 9:36AM |
F52.00006: Probing Low Energy Spin-Excitations in Multilayer Chromium Trihalides with Nitrogen-Vacancy Centers in Diamond Ruolan Xue, Liqiao Xia, Takashi Taniguchi, Kenji Watanabe, Pablo Jarillo-Herrero, Amir Yacoby Chromium trihalides are Van der Waals magnets that can be exfoliated down to monolayer. They therefore serve as a model system to study magnetism in low dimensions. However, given the small sample volume of multilayers and their weak signal strength, the dynamical properties are hard to explore experimentally. Here we report the use of nitrogen-vacancy (NV) centers in diamond, which are highly sensitive, local magnetic sensors, to probe the low energy excitations in these systems. |
Tuesday, March 15, 2022 9:36AM - 9:48AM |
F52.00007: Probing ac & dc magnetic properties of van der Waals materials with a programmable quantum sensor Xinyue Zhang, Yuxuan Wang, Thomas Tartaglia, Siyuan Ding, Mason J Gray, Kenneth S Burch, Fazel Tafti, Brian B Zhou The dc magnetism of atomically-thin van der Waals (vdW) magnets has been successfully characterized by multiple, although sophisticated, techniques. However exploration of the dynamic regime of vdW magnets, which is crucial for understanding the magnetic phase transition and interactions, has been experimentally elusive. We have recently demonstrated that the nitrogen-vacancy (NV) center in diamond is a quantitative, programmable probe for both the ac and dc magnetic properties of vdW materials. Through quantum control over the NV center's coherent spin precession, we realize sensitive ac susceptometry of exfoliated vdW magnets over a broad phase space in temperature, external dc field, and ac excitation frequency. Our measurements provide detailed information about the domain wall dynamics and ferromagnetic phase transition of few layer CrBr_3. Our magnetometry platform thus opens a generalizable avenue for probing magnetic excitations in wide-ranging 2D materials. |
Tuesday, March 15, 2022 9:48AM - 10:00AM |
F52.00008: Band tuning with magnon amplification and interaction in honeycomb ferromagnets with Dzyaloshinskii–Moriya interaction Hao Sun, Bo Yang, Pinaki Sengupta In this work, we study the magnon-magnon interaction effect in typical honeycomb ferromagnets consisting of stacked van der Waals honeycomb layers, e.g., Chromium trihalides CrX3 compounds (X= F, Cl, Br and I), that display two magnon modes (Dirac magnon). Using the Green function formulism in the presence of Dzyaloshinskii–Moriya interaction, we obtain a spinor Dyson equation up to second-order approximation by the cluster expansion method, showing prominent renormalizations of the single-particle spectrum. Furthermore, we propose a tunable renormalization effect using a parametric magnon amplification scheme. By altering the magnon population at different k spaces, the resulting renormalization effect not only reshapes the band structure, but also modifies the Berry curvature distribution. Our work demonstrates the interplay between band geometry, interactions and the external light field in the bosonic system and can potentially lead to new applications in magnon-based spintronic devices. |
Tuesday, March 15, 2022 10:00AM - 10:12AM |
F52.00009: Spin-Polarized Scanning Tunneling Microscopy of Epitaxial Fe3GeTe2 Alexander Bishop, Robert Walko, Wenyi Zhou, Jay Gupta, Roland K Kawakami Van der Waals ferromagnet Fe3GeTe2 (FGT) has become an exciting material due to its perpendicular magnetic anisotropy and high Curie temperature that can be tuned to above room temperature. In addition, skyrmion spin textures have been observed in exfoliated FGT flakes and FGT/WTe2 heterostructures. However, the microscopic origin of the skyrmion order remains unclear. Here, we utilize molecular beam epitaxy (MBE) for atomic layer-by-layer synthesis of monolayer and multilayer FGT films and heterostructures. Scanning tunneling microscopy (STM) is performed to characterize the atomic-scale structure and Cr tips are employed for spin-polarized STM (SPSTM) to investigate the magnetic hysteresis and domain structure. Structural analysis of the sample surface by STM attests to the high quality of films grown with MBE. SPSTM spectroscopy at different perpendicular magnetic fields reveals spin contrast originating from surface magnetism. Under optimized tunneling conditions with high spin contrast, we observe magnetic switching events as the field is swept and local magnetic hysteresis loops are obtained. We will report our latest results on FGT layer dependence and the relationship between structure and magnetism at the atomic scale. |
Tuesday, March 15, 2022 10:12AM - 10:24AM |
F52.00010: Spin-Valve Effect in Fe5GeTe2/MoS2/Fe5GeTe2 van der Waals Heterostructures Alex J Moon To date, a few two-dimensional(2D) van der Waals(vdWs) ferromagnets have been studied showing potential for spintronic applications. A practical limiting factor has been the low curie temperature (Tc) of the used materials. Here we focus on bulk Fe5GeTe2 which was reported to have a Tc ~ 310K. In this work, we have studied the electrical transport and magnetic properties of Fe5GeTe2, as well as its potential for spin valve devices. Hall effect measurements indicate that its anomalous component is suppressed as the temperature is reduced with respect to the magneto-structural transition occurring at ~120 K or as the temperature is increased up to 300K. Meanwhile, the longitudinal magnetoresistance(MR) shows a maximum of ~ -12.5 % at 140 K. For the Fe5GeTe2/MoS2/Fe5GeTe2 device, we observe a sizeable tunneling current when a thin MoS2 crystal is chosen as the spacer. A sizeable change in magnetoresistivity as a function of the magnetic field is observed at low Ts, an indication for the spin valve mechanism. However, the effect is not yet observable at room T. |
Tuesday, March 15, 2022 10:24AM - 10:36AM |
F52.00011: First-principles study of the insulator-metal transition in CrGeTe3 under pressure Minsung Kim, Kristjan Haule, David Vanderbilt Recently, CrGeTe3 has been experimentally reported to undergo an isostructural insulator-to-metal transition under pressure with unusually large enhancement of the magnetic transition temperature. Here, we study the insulator-metal transition and the electronic structures of the insulating and metallic phases using first-principles calculations based on a combination of density functional theory and embedded dynamical mean field theory (eDMFT). We find that our eDMFT calculations properly reproduce the charge-transfer insulator state at ambient condition and the transition to the metallic phase under pressure, where the inclusion of spin-orbit coupling is essential. We further investigate the character of the metallic states in the paramagnetic and ferromagnetic regimes and clarify the effect of electron correlation in the metallic ferromagnetic states. Our results shed important light on the insulator-metal transition and the resulting correlated phases in CrGeTe3under pressure. |
Tuesday, March 15, 2022 10:36AM - 10:48AM |
F52.00012: Tunable Easy Axis in Bulk and Monolayer 1T-CrTe2 Boyang Zheng, Vincent H Crespi Recent syntheses of the potential room-temperature 2D ferromagnet CrTe2 as thin flakes or monolayers have shown either in-plane [1–4] and out-of-plane easy axes [5–7]. To understand this discrepancy, we have calculated the magnetic anisotropy energy (MAE) for bulk and monolayer systems in the first-principles density functional theory. The calculated energy difference between out-of-plane and in-plane magnetization is much larger than that within the 2D plane, in good agreement with previous work [8, 9]. However, the magnitude and sign of the MAE are very sensitive to the lattice constant and jellium doping level, which explains the discrepancy and suggests that we can potentially tune the easy axis through strain and gating. |
Tuesday, March 15, 2022 10:48AM - 11:00AM |
F52.00013: Magnetic ion implantation: A defect-assisted mechanism for concurrent control of doping-induced magnetism and excited state dynamics of single-crystalline MoS2 Debjani Karmakar, Anasuya Karmakar, Tuhin K Maji, Dipendranath Mandal, Saurabh Lodha, K V Adarsh In few-layered MoS2, frequent occurrence of defect-assisted scattering, non-radiative carrier recombination and presence of mid-gap traps are detrimental to the lifetime of the excited-states. Additionally, the weak magnetic attributes of MoS2 restricts its application in magneto-optics or spin-transport. In this work, using Fe and Mn ion implantation, we demonstrate a simultaneous control of the magnetism and excited-state optical properties of single-crystalline MoS2. Different concentrations of magnetic doping were accomplished by varying the fluence of the Mn and Fe ion beams. XPS measurements indicate the systematic variation of doping and the corresponding shift of the core-level spectra. These systems exhibit a long-range magnetic ordering below 100 K. Transient absorption measurements have pointed out a significant increase of the life-time of both excitons for an optimal doping percentage. The presence of the stark shifts/blue-shifts are ascribed to the increase in the exciton-exciton repulsion with increasing delay time. In the light of the first-principles calculations on multi-layered MoS2 in presence of the intra-plane, inter-plane and interstitial magnetic impurities and point defects like S or Mo vacancies, we have analyzed our experimental observations. |
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