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 R36: 2D MagnetsFocus Live
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Sponsoring Units: GMAG DMP FIAP DCOMP |
Thursday, March 18, 2021 8:00AM - 8:12AM Live |
R36.00001: Electrostatic Control of Magnetism in a van der Waals Ferromagnetic Semiconductor Ivan Verzhbitskiy, Hidekazu Kurebayashi, Haixia Cheng, Jun Zhou, Safe Khan, Yuan Ping Feng, Goki Eda Control of magnetism via electric fields is a long-standing exciting challenge of fundamental significance for future spintronic devices. Recent discovery of two-dimensional magnetism in van der Waals systems highlights their unique potential as a platform to probe the interplay between charge and magnetic ordering [1]. Here, we report the first observation of carrier-induced ferromagnetic order in heavily doped thin crystals of Cr2Ge2Te6 (CGT) [2]. Upon degenerate electron doping, the CGT transistor exhibits clear hysteresis in the magnetoresistance at 200 K, far above the Curie temperature of undoped CGT (61 K). We find the magnetic easy-axis of doped CGT to lie within the plane of the crystal, which is in stark contrast to the out-of-plane easy-axis of the pristine material. We attribute these changes to emergence of the double-exchange interaction mediated by free carriers. This mechanism dominates over superexchange interaction, which is responsible for the ferromagnetic order in undoped CGT. Our findings reveal a unique role of the electric field in tailoring the magnetic anisotropy and leading exchange interaction in semiconducting 2D ferromagnets. |
Thursday, March 18, 2021 8:12AM - 8:24AM Live |
R36.00002: Angle-resolved photoemission spectroscopy and spin dynamics measurements of MBE-grown ferromagnetic CrTe1-δ thin films Yongxi Ou, Timothy S Pillsbury, Jeffrey Rable, Anthony R. Richardella, Nitin Samarth Thin film synthesis via molecular-beam epitaxy (MBE) provides an efficient way for atomic-scale manipulation of crystal structures for transition metal dichalcogenides (TMDs) and their intriguing physical properties in ultrathin films and heterostructures. The emergence of ferromagnetism in two-dimensional (2D) TMDs provides an important platform to understand ferromagnetism in the 2D limit and offers new opportunities in magnetism and spintronics. Here, we report the MBE synthesis of ferromagnetic TMD thin films of CrTe1-δ. We use in vacuo angle-resolved photoemission spectroscopy to directly measure the band structure of these CrTe1-δ thin films and compare the observations with first principles calculations. We also study the ferromagnetic properties of these films via SQUID magnetometry and transport measurements. We further discuss the possibility of using the CrTe1-δ as a 2D ferromagnet in spin-torque assisted spintronics devices. |
Thursday, March 18, 2021 8:24AM - 8:36AM Live |
R36.00003: Néel-type skyrmion in WTe2/Fe3GeTe2 van der Waals heterostructure Yingying Wu, Senfu Zhang, junwei zhang, Gen Yin, Zhiqiang Mao, Xixiang Zhang, Caihua Wan, Xiufeng Han, Kang-Lung Wang Topologically protected skyrmions are highly promising for spintronic devices and applications. Recently, long-range intrinsic magnetic orders in the van der Waals materials provide a new platform for the discovery of novel physics and phenomena. In this work, we demonstrate the Dzyaloshinskii-Moriya interaction and Néel-type skyrmions are induced at the WTe2/Fe3GeTe2 interface, a van der Waals heterostructure. Topological Hall effect has been observed in this heterostructure for temperatures below 100 K. In addition, we use Lorentz transmission electron microscopy and successfully obtain images of Néel-type skyrmion lattice and the stripe-like magnetic domain structures. These are direct evidence for the skyrmions. Furthermore, we estimate interfacial coupling induced Dzyaloshinskii-Moriya interaction in this system, which has a large energy of 1.0 mJm-2. This work first time points out a new direction of spintronics in van der Waals layered heterostructures. |
Thursday, March 18, 2021 8:36AM - 8:48AM Live |
R36.00004: Spin-orbit engineering in ferromagnetic van der Waals materials Fazel Tafti, Thomas Tartaglia, Jose Lado, Julia Y Chan, Faranak Bahrami, Kenneth Burch, Ying Ran Recently, chromium iodide (CrI3), bromide (CrBr3), and chloride (CrCl3) have received much attention due to exhibiting ferromagnetic ordering in 2D which is ideal for ultrathin magneto-optical devices. We will demonstrate that by alloying the halides, specifically by mixing Cl and Br, the magnetic easy axis can be tuned from in-plane to out-of-plane continuously. The alloying process can be extended to all three halides, giving rise to compositions such as CrClBrI, where the three halides are homogeneously mixed in the material. The crystals of CrClBrI are thin and exfoliate easily. We will present triangular phase diagrams of the mixed halides and demonstrate that (a) the Curie and Weiss temperatures can be tuned continuously, (b) a frustration regime exists in these materials near CrCl3, and (c) a metamagnetic transition is observed in the bulk crystals of CrClBrI, reminiscent of a spin-flip transition in bilayers of CrI3. |
Thursday, March 18, 2021 8:48AM - 9:00AM Live |
R36.00005: Structural monoclinicity and its evolution across magnetic phase transitions in few-layer CrI3 Xiaoyu Guo, Wencan Jin, Zhipeng Ye, Gaihua Ye, Bowen Yang, Hyun Ho Kim, Hongchao Xie, Shangjie Tian, Shaohua Yan, Hechang Lei, Adam W Tsen, Rui He, Liuyan Zhao CrI3 has received renewed interest in the past few years and shows intimate lattice-magnetism coupling. It is reported in few-layer CrI3 that monoclinic and rhombohedral layer stackings favor interlayer antiferromagnetic (AFM) and ferromagnetic (FM) couplings respectively. However, the question remains open how the lattice structure responds to the magnetic phase transitions in few-layer CrI3. Here, using polarization-resolved magneto-Raman spectroscopy, we have investigated layer-number, temperature, and magnetic field dependence of Raman spectra. We find that while the fully symmetry Ag and the antisymmetric magnetism-coupled phonon modes show little indication of monoclinicity in few-layer CrI3, the doubly degenerated Eg mode of monolayer CrI3 splits in N-layer (N > 1) flakes and suggests the monoclinic crystal structure. We further show that the aforementioned split in the frequency decreases at lower temperatures, suggestive of the tendency towards the monoclinc-to-rhombohedral structural phase transition that is present in the bulk. Finally, we present the magnetic-field dependence where a further monoclinic distortion is revealed across the magnetic field-induced layered AFM to FM phase transition. |
Thursday, March 18, 2021 9:00AM - 9:12AM Live |
R36.00006: Large cross-polarized Raman signal in CrI3, a first-principles study Ming Lei, Sinisa Coh We find unusually large cross-polarized Raman signature of Ag phonon mode in CrI3, in agreement with experiments. The signal is present only when the following three effects are considered in concert: ferromagnetism on Cr atoms, spin-orbit interaction, and resonant effects. Somewhat surprisingly, we find that the relevant spin-orbit interaction potential originates from iodine atoms, despite magnetism being mostly on chromium atoms. We analyze the Raman signature as a function of magnetic order, the direction of the magnetic moment, energy and polarization of light used for Raman scattering, as well as carrier lifetime. In addition to a strong cross-polarized Raman signal, we also find unusually strong phonon modulated magneto-optical Kerr effect (MOKE) in CrI3. |
Thursday, March 18, 2021 9:12AM - 9:24AM Live |
R36.00007: Competing antiferromagnetic order in transition metal dichalcogenides FexNbS2 Shan Wu, Zhijun Xu, Shannon Haley, Sophie Weber, Eran Maniv, Yiming Qiu, Adam Aczel, Jeffrey Neaton, James Analytis, Robert J Birgeneau The transition metal dichalcogenides (TMDCs) have attracted a great deal of attention due to their potential for new spintronic technologies. Intercalated variants TxMA2 (T=3d transition metal; M = Ta, Nb; A = S, Se) with x=⅓, which are of particular interest, show a rich array of magnetic phenomena. Chiral magnetism has been reported in Cr and Mn intercalation complexes that give rise to anomalous hall effect. The V complex exhibits a uniaxial chiral magnetism. Unlike these materials, FexNbS2 is antiferromagnetic (AFM) with strong anisotropy along the c axis. Interestingly, devices made from Fe1/3+dNbS2 crystal exhibit novel spintronic properties that are coupled to the antiferromagnetic transition and sensitive to the intercalation ratio. We comprehensively investigated the magnetic structures for two successive transitions at and off- stoichiometry. We found that the sensitivity of the transport properties is reflected in the x-dependence of magnetic ground state, where distinct AFM orders compete over a small range of the intercalation ratio. |
Thursday, March 18, 2021 9:24AM - 9:36AM Live |
R36.00008: Temperature- and magnetic field-dependent Raman spectroscopy of layered, antiferromagnetic FePS3 Jeffrey Simpson, Amber McCreary, Thuc Mai, Kevin Garrity, Cindi L Dennis, Angela Hight Walker, Rolando Valdes Aguilar The recent discovery that van der Waals-bonded magnetic materials retain long range magnetic ordering down to a single layer stimulates a thorough Raman spectroscopic study of one such material, FePS3, a large spin (S = 2) Mott insulator where the Fe atoms form a honeycomb lattice. Bulk FePS3 was shown to be a quasi-2D Ising antiferromagnet, with additional features in the Raman spectra emerging below the Néel temperature (TN ≈120 K). Using temperature- and magnetic field-dependent Raman spectroscopy as an optical probe of magnetic structure, we demonstrate that one of these Raman-active modes below TN is a magnon with a frequency of ≈3.7 THz (≈122 cm-1). Contrary to previous work, which interpreted this feature as a phonon, our Raman data shows the expected frequency shifting and splitting of the magnon as a function of temperature and magnetic field, respectively, with a g-factor ≈ 2. Furthermore, we compare our spectra with predictions from density functional theory to discuss the origin and anomalous temperature dependence of additional low-frequency Raman-active modes. |
Thursday, March 18, 2021 9:36AM - 10:12AM Live |
R36.00009: 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. Experimental observations of their spin wave excitations are important because the underlying spin Hamiltonian can provide crucial information regarding the magnetic interactions governing the statics and dynamics of their long-range order. Given the ferromagnetic orderings on honeycomb lattices, their magnon bands are predicted to host the Dirac magnons which are the bosonic counterpart of Dirac fermions observed in the electronic band of graphene [1]. In this talk, we will review and report the past and recent inelastic neutron scattering measurements of spin wave excitations in van der Waals ferromagnets CrX3 (X = Br, Cr, I) and Cr2Z2Te6 (Z = Si, Ge). Their spin wave bands consist of two ferromagnon modes, which exhibit linear E-p dispersion relations at QK = (1/3,1/3). In CrI3 at T = 5 K, we find that these two bands are separated by a ~ 2 meV gaps suggesting that its Dirac magnons are massive [2]. These results can be explained 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 61 K, the anisotropy gap at the zone center vanished following the power law behavior whereas the stiffness of the spin waves remained finite [3]. 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. |
Thursday, March 18, 2021 10:12AM - 10:24AM Live |
R36.00010: Magnetothermal properties of honeycomb ferromagnet CrGeTe3 Joshua Wakefield, Takashi Kurumaji, Takehito Suzuki, Joseph Checkelsky CrGeTe3 has been investigated as a prototypical semiconducting layered van der Waals ferromagnet with a transition temperature of TC = 61 K. The magnetic sublattice is composed of spin 3/2 Cr atoms on a 2D honeycomb lattice within which the Kitaev interaction is thought to play an important role. We report the physical properties of single crystals of CrGeTe3 grown with a self flux method focusing on magnetothermal effects. We observe an acute sensitivity of both specific heat and thermal transport to applied magnetic fields that evolves rapidly across magnetic ordering. We analyze this response in terms of phonon and spin excitations and find evidence for a significant role of both magnons and magnon-phonon coupling. We compare these results with expectations for the proposed magnetic Hamiltonian for the honeycomb network in this material. |
Thursday, March 18, 2021 10:24AM - 10:36AM Live |
R36.00011: Magnetic-Order-Induced Band-Shift and Carrier Dynamics in CrSiTe3 Nanosheets Giriraj Jnawali, Seyyedesadaf Pournia, Iraj Abbasian Shojaei, Leigh Smith, Jacob Gayles, Brenden Ortiz, Stephen D. Wilson Spin-ordering affects band structure as well as scattering processes in magnetic materials. This is rarely explored in layered ferromagnetic semiconductors. Here we show the band-edge structure and photoresponse of the layered ferromagnet CrSiTe3 (CST). Single nanosheets are studied by ultrafast mid-IR transient reflectance (TR) and photocurrent (PC) spectroscopy at 300 K (paramagnetic phase) and 10 K (ferromagnetic phase). We find a significant decrease of the fundamental direct bandgap as well as relatively smaller changes in the transition to mid-gap defects upon cooling the sample to 10 K. Density functional theory calculations are consistent with experiments and suggest possible spin-lattice coupling-induced structural changes at low temperatures. We find an enhanced rapid cooling rate of hot carriers in the ferromagnetic phase followed by a slower carrier recombination lasting nanoseconds. Both optical transitions are also seen in photocurrent spectra from CST devices over a broad mid-IR range. |
Thursday, March 18, 2021 10:36AM - 10:48AM Live |
R36.00012: Techniques for probing 2D magnetism with quantum coherent spins Xinyue Zhang, Yuxuan Wang, Thomas Tartaglia, Fazel Tafti, Brian Zhou The evolution of magnetic devices towards atomically-thin materials and nanostructured devices has posed challenges for conventional probes of magnetic properties. Recently, magnetometry based on the nitrogen-vacancy (NV) center in diamond has emerged as a potentially non-invasive, quantitative, and spatially-resolved probe for two-dimensional condensed matter systems. In this talk, we discuss the development and optimization of techniques for characterizing 2D magnetic systems with NV center magnetometers. We comment on how the attributes of spin-based quantum sensors can elucidate unique perspectives on magnetic interactions, phase transitions, and spatial variations. |
Thursday, March 18, 2021 10:48AM - 11:00AM Not Participating |
R36.00013: Strain-induced change in two-dimensional antiferromagnets Ting Fung Chung, Peiyao Zhang, Quanwei Li, Sui Yang, Xiang Zhang The control and manipulation of magnetic states by strain are the key challenges to better device technology. The recent experiments of two-dimensional (2D) van der Waals magnets have opened a new door for strain control of magnetism. Of particular interest are the thickness-dependent magnetic states in 2D CrI3, which exhibits interlayer anti-ferromagnetic exchange interaction that is sensitive to strain and other external perturbations. Here, we performed optical measurements to examine the effect of strain on the magnetic states of a few-layer CrI3. |
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