Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session W47: Atomically-Thin and 2D FerromagnetsFocus
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Sponsoring Units: GMAG Chair: Mateusz Goryca, Los Alamos National Laboratory Room: 710/712 |
Friday, March 6, 2020 8:00AM - 8:12AM |
W47.00001: Coexistence of Magnetic Orders in Two-Dimensional Magnet CrI3 Ben Niu, Tang Su, Brian A Francisco, Subhajit Ghosh, Fariborz Kargar, Xiong Huang, Mark I Lohmann, Junxue Li, Yadong Xu, di wu, Alexander Balandin, Jing Shi, Yongtao Cui The magnetic properties in two-dimensional van der Waals materials depend sensitively on structure. CrI3, as an example, has been recently demonstrated to exhibit distinct magnetic properties depending on the layer thickness and stacking order. Bulk CrI3 is ferromagnetic (FM) with a Curie temperature of 61 K and a rhombohedral layer stacking, while few-layer CrI3 has a layered antiferromagnetic (AFM) phase with a lower ordering temperature of 45 K and a monoclinic stacking. In this work, we use cryogenic magnetic force microscopy to investigate CrI3 flakes in the intermediate thickness range (25 – 200 nm) and find that the two types of magnetic orders can coexist in the same flake, with a layer of ~13 nm at each surface being in the layered AFM phase similar to few-layer CrI3 and the rest in the bulk FM phase. The switching of the bulk moment proceeds through a remnant state with nearly compensated magnetic moment along the c-axis, indicating formation of c-axis domains allowed by a weak interlayer coupling strength in the rhombohedral phase. |
Friday, March 6, 2020 8:12AM - 8:24AM |
W47.00002: The honeycomb quantum Heisenberg ferromagnet model with anisotropic exchange interactions Joren Vanherck, Bart Soree, Wim Magnus Since the discovery of magnetism in single-layered CrI3 about two years ago [1], research into the magnetic properties of this type of monolayer materials has ever been growing. However, a fundamental theory describing the origin of two-dimensional ferromagnetism, which is key in understanding its properties, is still missing. |
Friday, March 6, 2020 8:24AM - 8:36AM |
W47.00003: Two-dimensional Ferromagnetic van der Waals CrX3 (X=Cl, Br, I) Monolayers with Enhanced Anisotropy and Curie Temperature Feng Xue, Ruqian Wu Among the recently widely studied van der Waals layered magnets CrX3 (X=Cl, Br, I), CrCl3 monolayer (ML) is particularly puzzling as it is solely shown by experiments to have an in-plane magnetic easy axis and, furthermore, all of previous first-principles calculation results contradict this. Through systematical first-principles calculations, we unveil that its in-plane shape anisotropy that dominates over its weak perpendicular magnetocrystalline anisotropy is responsible for the in-plane magnetic easy axis of CrCl3 ML. To tune the in-plane ferromagnetism of CrCl3 ML into the desirable perpendicular one, we propose substituting Cr with isovalent tungsten (W). We find that CrWCl6 has a strong perpendicular magnetic anisotropy and a high Curie temperature up to 76 K. Our work not only gives insight into understanding the two-dimensional ferromagnetism of van der Waals MLs but also sheds new light on engineering their performances for nanodevices. |
Friday, March 6, 2020 8:36AM - 9:12AM |
W47.00004: Spin wave excitations in van der Waals honeycomb ferromagnets Invited Speaker: Jae-Ho Chung Recent discoveries of robust two-dimensional magnetism brought about a great research interest in van der Waals ferromagnets [1]. Experimental observations of their spin wave excitations are important because the underlying spin Hamiltonian can provide crucial information regarding thermal stability of their long-range order. In this talk, we focus on honeycomb ferromagnets where linear Dirac crossings in their magnon bands similarly to the electronic band of graphene [2]. We used inelastic neutron scattering to observe spin wave excitations in CrI3 and Cr2Ge2Te6 single crystals, where robust ferromagnetism was observed in monolayers [1]. The spin wave band of CrI3 at T = 5 K consisted of two distinctive bands of ferromagnetic excitations separated by a ~ 2 meV gap at the Dirac points [3]. These results can only be understood by considering a Heisenberg Hamiltonian with Dzyaloshinskii-Moriya interaction, thus providing experimental evidence that spin waves in CrI3 can have robust topological properties. As the temperature was increased to and beyond TC = 61 K, the anisotropy gap at the zone center vanished following the power law behavior whereas the stiffness of the spin waves remained finite. These results strongly indicate that the magnetic anisotropy plays a decisive role in determining the Curie temperature in CrI3, which is in contrast with typical three-dimensional Heisenberg ferromagnets where exchange interactions controlling the Curie temperature. In comparison, Cr2Ge2Te6 showed no evidence of Dirac gap but revealed the significant next nearest neighbor exchanges due to Ge ions. Finally, we will report the quantitative estimates of exchange parameters in their spin Hamiltonian, and compared their values with those of CrBr3 and Cr2Si2Te6. |
Friday, March 6, 2020 9:12AM - 9:24AM |
W47.00005: Observation of 2D magnons in atomically thin CrI3 John Cenker, Bevin Huang, Nishchay Suri, Pearl Thijssen, Aaron Miller, Michael McGuire, Di Xiao, Xiaodong Xu The collective excitations in magnetic materials, i.e. spin waves or magnons, may couple to inelastically scattered light. Despite the recent interest in two-dimensional (2D) van der Waals magnets, a direct observation of magnons in the monolayer limit is lacking. Here, we report the observation of 2D magnons in atomically thin CrI3 by magneto-optical Raman measurements. In monolayer and bilayer CrI3, we observe a sharp one-magnon feature at a zero-field energy of ~.3 meV which blue-shifts with a g factor of 2. However, at the metamagnetic transition from the layered antiferromagnetic to spin-polarized state in bilayer, the magnon exhibits a discontinuous red-shift. This confirms that the magnetic anisotropy is much larger than the interlayer exchange in CrI3. In addition, we observe a magnon feature at ~19 meV (~ 4.6 THz) for bilayer and thicker samples, significantly higher than Γ point magnons in standard ferromagnetic systems, i.e. YIG. Our results establish CrI3 as a potential candidate in miniaturized terahertz magnonic devices. |
Friday, March 6, 2020 9:24AM - 9:36AM |
W47.00006: Tuning Inelastic Light Scattering via Symmetry Control in 2D Magnet CrI3 Bevin Huang, John Cenker, Xiaoou Zhang, Essance Ray, Tiancheng Song, Takashi Taniguchi, Kenji Watanabe, Michael McGuire, Di Xiao, Xiaodong Xu The coupling between spin and charge degrees of freedom in a crystal imparts strong optical signatures on scattered electromagnetic waves. This has led to magneto-optical effects with a host of applications, from the sensitive detection of local magnetic order to data storage technologies. Here, we demonstrate a new magneto-optical effect: the tuning of inelastically scattered light through symmetry control in atomically thin chromium triiodide (CrI3). In monolayers, we found an extraordinarily large magneto-optical Raman effect from an A1g phonon mode due to ferromagnetic order. The linearly polarized, inelastically scattered light rotates by ~40o, more than two orders of magnitude larger than the rotation from MOKE under the same experimental conditions. In CrI3 bilayers, we show that the same A1g phonon mode becomes Davydov-split into two modes of opposite parity, exhibiting selection rules that depend on inversion symmetry and magnetic order. Applied magnetic and electric fields allow for magnetoelectrical control over these selection rules. Our work highlights the unique opportunities provided by 2D magnets for controlling underlying symmetries to manipulate Raman optical selection rules and for exploring emergent magneto-optical effects and spin-phonon coupled physics. |
Friday, March 6, 2020 9:36AM - 9:48AM |
W47.00007: Phonon-mediated magnetic order control of bilayer antiferromagnets Martin Rodriguerz-Vega, Zexun Lin, Gaurav Chaudhary, Aritz Leonardo, Maia Vergniory, Greg Fiete We study theoretically the effects of non-linear phonon processes after the ultra-fast photo-excitation of an infrared-active mode in the magnetic order of bilayer antiferromagnets. As a prototypical example, we consider bilayer CrI3, a van der Waals crystal composed of ferromagnetic (FM) layers with out-of-plane magnetization. Between the layers, the exchange coupling is antiferromagnetic (AFM) in the ground state, as determined experimentally, and such coupling is connected with the relative stacking between the layers. We employ group theory methods to determine the nature of the phonons modes and their allowed non-linear coupling terms in the AFM groundstate. Then, we use first-principles calculations to determine their frequencies and coupling strengths. We then use those results as inputs for the phonon equations of motion after photo-excitation and find that the Raman modes involving relative movement between the layers have the potential to influence the interlayer exchange interaction. Finally, we comment on the feasibility of the experimental detection of the changes in the magnetic order. |
Friday, March 6, 2020 9:48AM - 10:00AM |
W47.00008: Strain effect on magnetism of atomically thin CrI3 Qian Song, Connor A Occhialini, Difei Zhang, Jiarui Li, Abraham L Levitan, Riccardo Comin The structural stacking order plays an important role for the magnetic ground state of atomically thin CrI3. While bulk crystals undergo a transition from Monoclinic structure at room temperature to Rhombohedral structure at low temperature, bilayer CrI3 maintains a monoclinic structure for all temperatures. Despite several studies, the absence of a structural transition in bilayer CrI3 remains to be understood. Here we report on the study of bilayer CrI3 on different substrates, using Raman and magneto-optical Kerr effect measurements to reveal the interplay between epitaxial strain, lattice structure, and spin ordering. We find that the lattice mismatch and resulting strain between the substrate and the CrI3 flakes has a dramatic impact on the structure of the atomically thin CrI3. |
Friday, March 6, 2020 10:00AM - 10:12AM |
W47.00009: Intrinsic Ferromagnetism in 2D CrxSe1-x Crystals Mengying Bian, Keke He, Fan Sun, Peijian Wang, Alireza Jalouli, Jonathan P Bird, Yanglong Hou, Hao Zeng In the last two years, several two-dimensional (2D) materials of magnetic order, including CrI3 and Cr2Ge2Te6, have been observed, which represents a breakthrough in the field of 2D magnetism and opens up opportunities for device applications integrating these magnets with other van der Waals (vdW) crystals. However, most of these materials are vdW structures which rely on the mechanical exfoliation technique to obtain ultra-thin flakes. Furthermore, most 2D magnets have poor stability and low magnetic transition temperature, which limit their practical application. To search for 2D materials with improved magnetic properties, one needs to look beyond vdW crystals. In this work, a 2D non-vdW magnetic material, CrxSe1-x, has been synthesized using an ambient pressure chemical vapor deposition method. The magnetic characterization demonstrated that the thin flakes exhibit ferromagnetic properties with ordering temperature above 250 K. These results may pave the way for practical applications of 2D magnets. |
Friday, March 6, 2020 10:12AM - 10:24AM |
W47.00010: Temperature dependent study of magnetization dynamics in CrCl3 – a layered antiferromagnet SUPRIYA MANDAL, Lucky N. Kapoor, Soham Manni, Meghan P. Patankar, A. Thamizhavel, Mandar M Deshmukh Chromium trichloride (CrCl3) is a layered antiferromagnet. Recently it has found renewed interest due to its antiferromagnetic resonance in GHz frequencies, two-dimensional (2D) layered structure with low cleavage energy and complex magnetization dynamics [1]. We report a detailed study of magnetization dynamics of bulk CrCl3 placed on a coplanar waveguide (CPW) as a function of temperature. We observe prominent ferromagnetic, antiferromagnetic and paramagnetic resonances depending on temperature and in-plane magnetic field. We also observe magnon-magnon coupling and presence of multiple spin-wave modes by introducing an additional out-of-plane magnetic field by tilting the sample with respect to magnetic field direction. We extract useful system parameters and study their dependence on temperature and angle of tilting by fitting to experimental data. Ability to easily exfoliate 2D layers of CrCl3 opens up possibility of coupling it to CPW resonators and application in antiferromagnetic spintronics and hybrid quantum systems. |
Friday, March 6, 2020 10:24AM - 10:36AM |
W47.00011: VI3: a 2D Ising ferromagnet ke yang Two-dimensional (2D) magnetic materials are of great current interest for their promising applications |
Friday, March 6, 2020 10:36AM - 10:48AM |
W47.00012: Two-dimensional magnetism in a van der Waals semiconductor Evan Telford, Avalon Dismukes, Kihong Lee, Minghao Cheng, Kenji Watanabe, Takashi Taniguchi, Abhay Pasupathy, Xiaoyang Zhu, Cory Dean, Xavier Roy Since the recent discovery of ferromagnetism in exfoliated flakes of CrI3 and Cr2Ge2Te6, interest in two-dimensional (2D) ferromagnetic systems has increased. Previously predicted to be prohibited by thermal fluctuations, the existence of 2D magnetically ordered systems offers an ideal platform for examining novel quantum phenomena and opens up opportunities for emerging applications, including magneto-optic and magneto-electronic devices, spintronics, and quantum computing. For use in applications, there is a crucial need to develop 2D materials that simultaneously exhibit ferromagnetism and semiconducting behaviors. Towards this end, we recently synthesized an air stable van der Waals magnetic semiconductor. 2D sheets can be prepared through mechanical exfoliation for measurement and incorporation into more complex heterostructures. Using electrical transport, second harmonic generation spectroscopy, photoluminescence spectroscopy, and scanning tunneling spectroscopy, we investigate the electrical and magnetic properties of this material in the 2D limit. We present clear evidence of layered antiferromagnetic ordering at ~130K, below which the spins in each sheet are aligned ferromagnetically parallel to the plane while the interlayer coupling is antiferromagnetic. |
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W47.00013: Giant and nonreciprocal second harmonic generation from layered antiferromagnetism in bilayer CrI3 Zeyuan Sun, Yangfan Yi, Tiancheng Song, Genevieve Clark, Bevin Huang, Yuwei Shan, Shuang Wu, Di Huang, Chunlei Gao, Zhanghai Chen, Michael McGuire, Ting Cao, Di Xiao, Wei-Tao Liu, Wang Yao, Xiaodong Xu, Shiwei Wu Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. Recently, the van der Waals magnet, chromium triiodide (CrI3), emerged as the first layered antiferromagnetic insulator in its few-layer form. In this talk, we present an emergent nonreciprocal second-order nonlinear optical effect in bilayer CrI3. The observed second-harmonic generation (SHG) is giant: several orders of magnitude larger than known magnetization induced SHG and comparable to SHG in the best 2D nonlinear optical materials studied so far. We show that while the parent lattice of bilayer CrI3 is centrosymmetric and thus does not contribute to the SHG signal, the observed nonreciprocal SHG originates purely from the layered antiferromagnetic order, which breaks both spatial inversion and time reversal symmetries. Furthermore, polarization-resolved measurements reveal the underlying C2h symmetry, and thus monoclinic stacking order in CrI3 bilayers, providing key structural information for the microscopic origin of layered antiferromagnetism. Our results indicate that SHG is a highly sensitive probe of subtle magnetic orders and open up possibilities for the use of two-dimensional magnets in nonlinear and nonreciprocal optical devices. |
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