Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session F42: 2D Magnetic Materials IIFocus
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Sponsoring Units: GMAG DCMP FIAP DCOMP Chair: Junxue Li, University of California, Riverside Room: 709/711 |
Tuesday, March 3, 2020 8:00AM - 8:12AM |
F42.00001: Distinct magneto-Raman signatures of spin-flip phase transitions in CrI3 Amber McCreary, Thuc Mai, Franz Utermohlen, Jeffrey Simpson, Kevin Garrity, Xiaozhou Feng, Dmitry Shcherbakov, Yanglin Zhu, Jin Hu, Daniel Weber, Kenji Watanabe, Takashi Taniguchi, Joshua Goldberger, Zhiqiang Mao, Chun Ning Lau, Yuanming Lu, Nandini Trivedi, Rolando Valdes Aguilar, Angela Hight Walker The discovery of 2-dimensional (2D) materials, such as CrI3, that retain magnetic ordering at monolayer thickness has resulted in a surge of research in 2D magnetism from both pure and applied perspectives. Here, we report a magneto-Raman spectroscopy study on multilayered CrI3, focusing on two new features in the spectra which appear at temperatures below the magnetic ordering temperature and were previously assigned to high frequency magnons. We observe a striking evolution of the Raman spectra with increasing magnetic field in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field. Our work highlights the sensitivity of the Raman modes to weak interlayer spin ordering in CrI3. In addition, we theoretically examine potential origins for the new modes, which we deduce are unlikely single magnons. |
Tuesday, March 3, 2020 8:12AM - 8:24AM |
F42.00002: Probing Tunneling Magnetoresistances in Chromium Trihalide Junctions via First Principles Calculations Jonathan Heath, Marcelo Kuroda The demonstration of magnetic tunnel junctions in heterostructures formed with two-dimensional materials has spawned research ventures regarding spin transport in these layered systems. Efforts that initiated with CrI3 have now been expanded to other chromium trihalides such as CrBr3 and CrCl3. Owing to their nanometer thicknesses, the electronic and magnetic properties of chromium trihalide systems are affected by quantum confinement effects along with their environments. Here we characterize physical properties of multilayer chromium trihalide/graphene junctions using density functional theory (DFT) and Landauer’s formalism for ballistic transport. We find that band alignments in few-layer chromium trihalide (CrX3 with X = F, Cl, Br and I) slabs change based on the metamagnetic state of the junctions and differ from those of isolated layers. We also delve into metamagnetic dependent complex band structures as well as discuss changes in the band structures of chromium trihalide junctions beyond DFT. Our results suggest that tunneling magnetoresistance values as high as 4,000% can be attained in bilayer heterojunctions. |
Tuesday, March 3, 2020 8:24AM - 8:36AM |
F42.00003: Optically probing low energy magnetic excitations in bilayer CrI3 Hongchao Xie, Zhipeng Ye, Shangjie Tian, Hechang Lei, Rui He, Liuyan Zhao Study of spin waves, collective magnetic excitations, in two-dimensional (2D) magnets emerges as one key yet new topic in the research of 2D magnetism. Among all the 2D magnets studied so far, bilayer CrI3 stands out because of the realization of the layered antiferromagnetism, in which the spins order ferromagnetically within each layer and antiferromagnetically between the two layers. So far, the magnetic excitations in natural bilayer CrI3 have been rarely studied, with the exception of inelastic magneto-tunneling spectroscopy results. A further question of how such magnetic excitations depends on the angular alignment between the two layers and the resulted long-range Moiré superlattice remains totally unexplored. Here, I will first present our ultra-low frequency Raman spectroscopy studies on the spin waves in natural bilayer CrI3 and discuss its comparison to the inelastic magneto-tunneling spectroscopy. I will further show the magnetic field dependence of the spin waves in natural bilayer CrI3. Furthermore, I will explore how these low energy spin waves depend on the twist angle between bilayer CrI3. |
Tuesday, March 3, 2020 8:36AM - 9:12AM |
F42.00004: Current Control of Magnetism in Two-Dimensional Fe3GeTe2 Invited Speaker: Arne Brataas The recent discovery of magnetism in two-dimensional van der Waals systems opens the door to discovering exciting physics. We investigate how a current can control the ferromagnetic properties of such materials. Using symmetry arguments, we identify a recently realized system in which the current-induced spin torque is particularly simple and powerful. In Fe3GeTe2, a single parameter determines the strength of the spin-orbit torque for a uniform magnetization. The spin-orbit torque acts as a contribution to the out-of-equilibrium free energy and introduces new in-plane magnetic anisotropies to the system. Therefore, we can tune the system from an easy-axis ferromagnet via an easy-plane ferromagnet to another easy-axis ferromagnet with increasing current density. This finding enables unprecedented control and provides the possibility to study the Berezinskii-Kosterlitz-Thouless phase transition in the 2D XY model and its associated critical exponents. |
Tuesday, March 3, 2020 9:12AM - 9:24AM |
F42.00005: Enhanced magnetoresistance in mixed van der Waals magnetic tunnel junctions Dahlia Klein, David MacNeill, Märta Tschudin, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero Since the recent isolation of monolayer CrI3, there has been a surge of interest in van der Waals heterostructures incorporating 2D magnets. Studies have demonstrated large tunneling magnetoresistances in van der Waals magnetic tunnel junctions fabricated from the insulating layered chromium trihalides (CrX3) due to the spin filter effect. Here, we report electron tunneling in graphite/CrCl3/CrBr3/graphite van der Waals heterostructures to study the effect of the CrCl3/CrBr3 interface. We find that the magnetoresistance persists above the Néel temperature of the in-plane antiferromagnet CrCl3 with enhanced values compared to magnetic tunnel junctions fabricated from CrCl3 alone. We interpret these data as resulting from the out-of-plane ferromagnet CrBr3 acting as a spin-polarizing layer. |
Tuesday, March 3, 2020 9:24AM - 9:36AM |
F42.00006: Hall micromagnetometry of two-dimensional ferromagnets Minsoo Kim, Piranavan Kumaravadivel, John Birkbeck, Wenjun Kuang, Shuigang Xu, David Hopkinson, Johannes Knolle, Paul A McClarty, Alexey Berdyugin, Moshe Ben Shalom, Roman Gorbachev, Sarah Haigh, Song Liu, James H. Edgar, Konstantin S Novoselov, Irina Grigorieva, Andre Geim The recent advent of atomically-thin ferromagnetic crystals has allowed experimental studies of two-dimensional (2D) magnetism that not only exhibits novel behavior due to the reduced dimensionality but also often serves as a starting point for understanding of the magnetic properties of bulk materials. However, the experimental techniques that were used to explore two-dimensional ferromagnetism could not probe the magnetic field directly. Here we show that ballistic Hall micromagnetometry provides a reliable and convenient way to measure magnetization of individual two-dimensional ferromagnets. Our devices are made by van der Waals assembly in such a way that the investigated ferromagnetic crystal is placed on top of a multiterminal Hall bar made from encapsulated graphene. We apply the micromagnetometry to study atomically-thin chromium tribromide (CrBr3). The material remains ferromagnetic down to monolayer thickness and exhibits strong out-of-plane anisotropy. Magnetic response of CrBr3 varies little with the number of layers and its temperature dependence cannot be described by the simple Ising model of two-dimensional ferromagnetism. |
Tuesday, March 3, 2020 9:36AM - 9:48AM |
F42.00007: The quantum non-Heisenberg nature of two-dimensional CrI3 magnets Elton Santos, Dina Wahab, Mathias Augustin, Samuel Mañas-Valero, Wenjun Kuang, Sarah Jenkins, Eugenio Coronado, Irina Grigorieva, Ivan Jesus Vera Marun, Efren Navarro-Moratalla, Richard F. L. Evans, Kostya S. Novoselov Higher-order exchange interactions and quantum effects are widely known to play an important role in describing the properties of nano magnets. Here we show that the magnetism of recently discovered 2D CrI3 cannot be captured at the level of a simple Ising model as initially thought. Using a complementary suite of magneto-optical Kerr effect microscopy, magnetic force microscopy, correlated first-principles methods and Monte Carlo techniques including higher-order exchange interactions, we identify CrI3 as a quantum non-Heisenberg material. We find that biquadratic exchange interactions are essential to quantitatively describe the magnetism of CrI3 but requiring quantum scaling corrections to reproduce its thermal properties. The quantization of spin-wave excitations at the low temperature regime is reflected on the fluctuations of the magnetization which follow Bose-Einstein rather than the Boltzmann statistics. These fluctuations induce the formation of metastable magnetic domains stabilizing into a single macroscopic magnetization over large surface areas. Such domains have characteristics of N'eel and Bloch types with a narrow domain wall width (3-5 nm). Similar behavior is expected for the majority of 2D vdW magnets where higher-order exchange interactions are appreciable. |
Tuesday, March 3, 2020 9:48AM - 10:00AM |
F42.00008: Substrate-assisted asymmetric electronic gap in artificial magnetic honeycomb lattice Jiasen Guo, Yiyao Chen, George Yumnam, Quinn Cunningham, Deepak K Singh An artificial magnetic honeycomb lattice was prepared via physical vapor deposition. Asymmetric electronic gaps with increasing broadness were observed in the differential conductivity when applying in-plane current at low temperatures ranging from 27K to 40 K. Electrical hysteresis behavior was also observed and became more profound with increasing temperature. It was seen that an in-plane magnetic field parallel or antiparallel to the applied current tends to narrow the aforementioned gaps and reduce the degree of asymmetry drastically. Resistance measurement with applied magnetic field up to 8.5 T shows a temperature-dependent transition from positive magnetoresistance to negative magnetoresistance, which we suspect, is subject to the effect of the substrate. The observed negative magnetoresistance can be explained as the giant magnetoresistance effect due to the ordering of magnetic moment on the artificial honeycomb lattice. |
Tuesday, March 3, 2020 10:00AM - 10:12AM |
F42.00009: Magneto-Raman study of the Neel-type Antiferromagnet MnPSe3 Thuc Mai, Joshua Argo, Amber McCreary, Rolando Valdes Aguilar, Vicky Doan-Nguyen, Angela Hight Walker The discovery of 2D magnetism in van der Waals materials such as CrI3 and Cr2Ge2Te6 demonstrated that long range magnetic ordering can survive in the monolayer limit, given that the material has sufficiently large anisotropy. MnPSe3 is a layered material with Neel-type antiferromagnetic spins lying parallel to the basal plane. We perform a systematic magneto-Raman study of MnPSe3. Multiple excitation laser wavelengths were used. With resonant Raman condition, we study the temperature and magnetic field dependence of the Raman spectrum of single crystal MnPSe3. A drastic shift in the phonon spectrum across TNeel is observed. We compare the spin-phonon coupling between bulk and thin samples. |
Tuesday, March 3, 2020 10:12AM - 10:24AM |
F42.00010: Magnetic structure determination of transition metal dichalcogenide Fe1/3NbS2 Shan Wu, Zhijun Xu, Eran Maniv, Arani Acharya, Spencer Doyle, Caolan John, James Analytis, Robert J Birgeneau In layered transition metal dichalcogenides (TMD) MA2 (M = Ta, Nb, A = S, Se),the weak van der Waals bonding between adjacent layers allows easy intercalation of transition metal atoms. Intercalated magnetic ions, such as T = Fe, Co, Mn, Cr, occupy the vacant octahedral sites between the layers and form an ordered triangular superlattice.This introduces in the magnetic degree of freedom, giving rise to a class of layered magnetic materials with a rich array of magnetic phenomena. Recently, the current-induced switching and magnetic memory effects were reported in FexNbS2 with x close to 1/3 that exhibits an antiferromagnetic transition at TN ~ 45 K. This provides a new avenue to realize the spintronic device. From neutron diffraction experiment,we determined a stripe-type order that develops into three magnetic domains, which is strongly coupled to three nematic domains. The magnetic order parameter can be explained with the development of canted moment, which is associated with competing exchange interactions in the frustrated lattice and the effect of Dzyaloshinskii-Moriya interaction because of the lack of the inversion symmetry. |
Tuesday, March 3, 2020 10:24AM - 10:36AM |
F42.00011: Magnetic interactions and excitations in magnetic 2D van der Waals materials Liqin Ke Magnetic 2D van der Waals (m2DvdW) materials add new functionality, leading to novel new magnetic 2D devices. Using linear-response ab initio methods, we investigate the magnetic interactions and spin excitations in various m2DvdW systems. Dynamical spin susceptibility is calculated using single-particle Hamiltonians constructed within density-functional theory (DFT) and the quasiparticle self-consistent GW (QSGW) methods. Results are compared with available Inelastic Neutron Scattering (INS) measurements. We found that the more elaborative description of electron interactions better describes the magnetic interactions within 2D van der Waals systems. For example, plain DFT overestimates the exchange couplings in CrI3, while the spin-wave spectrum calculated in QSGW is closer to experiments. The effects of Dzyaloshinskii-Moriya interaction on spin excitations are also discussed. |
Tuesday, March 3, 2020 10:36AM - 10:48AM |
F42.00012: Is 2H-MoTe2 an intrinsically magnetic semiconductor? Jonas A. Krieger, Zurab Guguchia, Toni Shiroka, Thomas Prokscha, Andreas Suter, Vladimir Strokov, Zaher Salman Layered transition metal dichalcogenides are currently intensively investigated due to their opto-electronic, superconducting and topological properties as well as their potential usage as mono-layer building blocks. However, despite their layered nature they often exhibit three dimensional (3D) properties in the bulk. A recently detected long-range magnetic order in semiconducting 2H-MoTe2 was attributed to defects [1]. Here we present results of complementary spectroscopic techniques to elucidate the nature and origin of the reported magnetic order. We find that the magnetism is intrinsic with a layered-antiferromagnetic ground state. Our results also show that this magnetism remains unchanged near the surface, opening new possibilities for applications of 2H-MoTe2 in interface engineering. |
Tuesday, March 3, 2020 10:48AM - 11:00AM |
F42.00013: Patterning-induced ferromagnetism of Fe3GeTe2 van der Waals materials beyond room temperature Mengmeng Yang, Qian Li, Cheng Gong, Rajesh V Chopdekar, Alpha T. N'Diaye, John Turner, Gong Chen, Andreas Scholl, Padraic Shafer, Elke Arenholz, Andreas Schmid, Sheng Wang, Kai Liu, Nan Gao, Alemayehu S Admasu, Sang-Wook Cheong, Chanyong Hwang, Jia Li, Feng Wang, Xiang Zhang, Zi Q. Qiu Magnetic van der Waals (vdW) materials have emerged as promising candidates for spintronics applications especially after the recent discovery of intrinsic ferromagnetism in monolayer vdW materials. There has been a critical need for tunable ferromagnetic vdW materials beyond room temperature. Here we report a real space imaging study of itinerant ferromagnet Fe3GeTe2 and the enhancement of its Curie temperature well above ambient temperature. We find that the magnetic long-range order in Fe3GeTe2 is characterized by an unconventional out-of-plane stripe domain phase. In Fe3GeTe2 microstructures patterned by focused ion beam, the out-of-plane stripe domain phase undergoes a surprising transition at 230 K to an in-plane vortex phase that persists beyond room temperature. The discovery of tunable ferromagnetism in Fe3GeTe2 materials opens up vast opportunities for utilizing vdW magnets in room temperature spintronics devices. |
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