Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session P15: Devices from 2D Materials -- MagnetismFocus
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Sponsoring Units: DMP GMAG Chair: Yuanbo Zhang Room: BCEC 154 |
Wednesday, March 6, 2019 2:30PM - 2:42PM |
P15.00001: Ferromagnetic Proximity Induced Coupling and Exchange Splitting Effects in Graphene Dhavala Suri, Gregory Stephen, Wei Kong, Mirko Rocci, Narendra Kumar, Yiping Wang, Kenneth Burch, Jeehwan Kim, Don Heiman, Jagadeesh Moodera Ferromagnet (FM)/2D material interface with broken symmetry is a potential avenue for new physics and to realize exchange field controllable future spintronics devices. Proximity-induced effects of a FM on TI and graphene (G) have been explored previously with EuS as the ferromagnet, inducing magnetic correlations in the adjacent layer [1]. Magnetic semiconductor GdN would enable higher temperature operation. We report successful sputter growth of ultrathin excellent GdN films on high-quality G synthesized on SiC substrate to form G/FM heterostructures. The properties of G are unperturbed with 15 nm thick GdN film grown over it, having a magnetic moment ≈ 7 μB and Curie temperature ≈ 33 K. Temperature-dependent Raman spectrum of electric field tunable GdN/G system shows a clear signature of graphene G peak near 1580 cm-1 and G’ peak near 2700 cm-1 confirming its quality. The induced magnetic correlations in G creating exchange gap in the Dirac surface states by GdN is investigated. Furthermore, patterning a superconductor over this bilayer for Josephson pair tunnelling studies in G will be presented. |
Wednesday, March 6, 2019 2:42PM - 2:54PM |
P15.00002: Layer-modulated magnetism in a two-dimensional transition metal dichalcogenide Ahmet Avsar, Alberto Ciarrocchi, Michele Pizzochero, Dmitrii Unuchek, Oleg Yazyev, Andras Kis While intrinsic magnets rarely occur in nature, long range magnetism in nonmagnetic materials can be introduced in a variety of forms, e.g., adatom intercalation, proximity coupling and defect engineering. The latter has been predicted for several two-dimensional (2D) materials including transition metal dichalcogenides (TMDCs), but such magnetic ordering of spins in the atomically thin limit has not been experimentally realized yet. Here, we will present defect-induced magnetism for an environmentally stable 2D TMDC crystal. By utilizing magneto-transport and polar refractive magnetic circular dichroism techniques, we demonstrate ferromagnetic or anti-ferromagnetic ground state orderings depending on the number of layers, similarly to recently discovered CrI3. Attempts on further manipulation of magnetism within a layer by precise defect engineering will be also shown. Our findings broaden the horizon of 2D magnets to include normally nonmagnetic stable materials by introducing defects. |
Wednesday, March 6, 2019 2:54PM - 3:06PM |
P15.00003: Corrugate graphene systems for Rashba-like valley-orbital coupling Ching-hao Chang, Sheng-Chin Ho, Tse-Ming Chen, Carmine Ortix Local deformation in graphene are gathering great interest because it introduces huge pseudo-magnetic field to modify the electronic states nearby to host novel transport properties [1]. The graphene nanobubble, for instance, is predicted to strongly polarize the valley of an injecting current [2]. In this respect, the nontrivial valley-dependent band structure can be introduced when a local deformation turns into periodic. In this talk, we will show that the Rashba-like valley bands form in corrugated graphene systems. Moreover, the low-energy bands refer to magnetic snake states moving along the corrugations and thus can provide one-dimensional-like ballistic valley transport. |
Wednesday, March 6, 2019 3:06PM - 3:18PM |
P15.00004: Triangular antiferromagnetism on the honeycomb lattice of twisted bilayer graphene Alexandra Thomson, Subir Sachdev, Shubhayu Chatterjee, Mathias Scheurer We present the electronic band structures of states with the same symmetry as the three-sublattice planar antiferromagnetic order of the triangular lattice. Such states can also be defined on the honeycomb lattice provided the spin density waves lie on the bonds. We identify cases which are consistent with observations on twisted bilayer graphene: a correlated insulator with an energy gap, yielding a single doubly-degenerate Fermi surface upon hole doping. We also discuss extensions to metallic states which preserve spin rotation invariance, with fluctuating spin density waves and bulk Z2 topological order. |
Wednesday, March 6, 2019 3:18PM - 3:30PM |
P15.00005: Interplay Between Ferromagnetism and Superconductivity in van der Waals Devices Robert Polski, Harpreet Singh Arora, Jeannette Kemmer, Youngjoon Choi, Hechen Ren, Yiran Zhang, Kenji Watanabe, Takashi Taniguchi, Stevan Nadj-Perge Two-dimensional van der Waals materials have unlocked the ability to stack electronically dissimilar materials into heterostructures that are highly tunable using proximity effects, gate modulation, and varying layer numbers. Interest in superconducting proximity effects, such as Andreev reflection in superconductor-normal (S-N) junctions and Josephson junctions, have thus provoked numerous studies in these systems, but the combination of spin-singlet superconductors (S) and spin-polarized ferromagnets (F) into S-F interfaces has been largely unexplored in van der Waals materials. Proper understanding and control of these interfaces could lead to advances in spintronics and novel realizations of unconventional superconductivity. We used superconducting and ferromagnetic van der Waals materials to study the electrical properties of S-F interfaces through transport measurements as we vary both the temperature and the magnetic field. |
Wednesday, March 6, 2019 3:30PM - 3:42PM |
P15.00006: Voltage Control of a van der Waals Spin-Filter Magnetic Tunnel Junction Tiancheng Song, Matisse Wei-Yuan Tu, Caitlin Carnahan, Xinghan Cai, Takashi Taniguchi, Kenji Watanabe, Michael A McGuire, David Henry Cobden, Di Xiao, Wang Yao, Xiaodong Xu Atomically thin chromium triiodide (CrI3) has recently been identified as a layered antiferromagnetic insulator, in which adjacent ferromagnetic monolayers are antiferromagnetically coupled. This unusual magnetic structure naturally comprises a series of anti-aligned spin filters which can be utilized to make spin-filter magnetic tunnel junctions with very large tunneling magnetoresistance (TMR). Here we report voltage control of TMR formed by four-layer CrI3 sandwiched by monolayer graphene contacts in a dual-gated structure. By varying the gate voltages at fixed magnetic field, the device can be switched reversibly between bistable magnetic states with the same net magnetization but drastically different resistance (by a factor of ten or more). Our work demonstrates new kinds of magnetically moderated transistor action and opens up possibilities for voltage-controlled van der Waals spintronic devices. |
Wednesday, March 6, 2019 3:42PM - 3:54PM |
P15.00007: Controlling magnetism in layered metal halides by doping Zhong Lin, Qianni Jiang, Zaiyao Fei, Bevin Huang, Yue Shi, Jiun-Haw Chu, Xiaodong Xu van der Waals magnets such as chromium triiodide show thickness dependent magnetic properties which can be further controlled by electrostatic gating. Engineering their intrinsic magnetism via compositional and structural modification remains largely unexplored. I will present our recent efforts on doping layered metal halides, including the characterization of bulk crystals and exfoliated flakes down to their 2D limit. |
Wednesday, March 6, 2019 3:54PM - 4:06PM |
P15.00008: Spin and charge conversion in two dimensional magnets Abhishek Solanki, Avinash Rustagi, Pramey Upadhyaya Interconversion between spin and charge degrees of freedom is one of the central goals of spintronics. In recent years, a host of quantum materials-Rashba interfaces, topological insulators, superconductors and non-collinear antiferromagnets-have emerged as promising candidate material systems, where various novel spin-dependent phenomena allow for efficient spin and charge interconversion [npj Quantum Materials 3, 27 (2018)]. More recently, van der Waal magnets (such as CrI3) [Nature 546, 270(2017)] have emerged as a unique two-dimensional platform, where electron-electron interactions can be significantly altered-from being ferromagnetic to antiferromagnetic-by charge doping [Nature Mat. 17, 406 (2018)]. In this work, we develop a phenomenological theory of spin-charge interconversion enabled by electrically controlled spin-spin interactions in these two-dimensional magnets, predicting new efficient schemes for converting between spin and charge. In combination with the unique toolkit of heterostructure engineering offered by the van der Waal materials [Science 353, aac9439 (2016)], the predicted spin charge interconversion could give rise to a new set of nonvolatile spintronics devices. |
Wednesday, March 6, 2019 4:06PM - 4:18PM |
P15.00009: Current-induced torques in heterostructures of 2D van der Waals magnets Vishakha Gupta, Gregory Stiehl, Arnab Bose, Kaifei Kang, Shengwei Jiang, Kin Fai Mak, Jie Shan, Robert Buhrman, Daniel Ralph The recent discovery of magnetic order in layered 2D van der Waals materials has opened a new platform for spintronic devices, allowing studies of current-induced torques in high-quality heterostructures of single-crystal 2D magnets and large spin-orbit coupling (SOC) materials. Here, we integrate mechanically exfoliated thin flakes of the insulating 2D ferromagnet Cr2Ge2Te6 into a heterostructure with heavy metal Tantalum as the large SOC material. We report initial results of current induced deflections of the out-of-plane magnetic moment of Cr2Ge2Te6 and characterize the spin-orbit torque generated. This work represents a first step towards improved strategies for magnetic memory and logic using heterostructures of van der Waals 2D magnets. |
Wednesday, March 6, 2019 4:18PM - 4:30PM |
P15.00010: Ion Implantation Leading to Magnetism in Many-Layer Graphene Alessandro Mazza, Anna L Miettinen, Timothy Charlton, Thomas Ward, Xiaoqing He, Alex A Daykin, Suchismita Guha, Guang Bian, Edward H. Conrad, Paul F Miceli P-orbital magnetism observed in graphene is of interest both to fundamental physics and for its potential application in new classes of spintronic devices. In this talk, we will demonstrate how low energy ion bombardment of graphene with hydrogen can be used to manipulate defect type and concentration to induce room temperature ferromagnetism in layered graphene. SQUID magnetometry measurements show that it is possible to control the magnetic properties by varying the H energy and dose. In-situ x-ray diffraction shows that ion distribution leads to layer expansion and an increase in RMS height variations of the graphene layers. Neutron reflectivity reveals that implanted H ions remain in the sample after dosing via chemisorption. The relationship between these results and magnetometry measurements, which show that ion implantation leads to a ferromagnetic moment at room temperature, will be discussed. |
Wednesday, March 6, 2019 4:30PM - 4:42PM |
P15.00011: Kondo Effect in Graphene Nanoribbons with Magnetic Impurities Ginetom Diniz, Gisele Iorio Luiz, Andrea Latge, Edson Vernek In this work we have performed a detailed analysis of the Kondo effect of a magnetic impurity in graphene nanoribbons with zigzag edge termination [1]. We have considered an adatom coupled to the graphene nanoribbon via a hybridization amplitude in two different configurations: hollow or top site. In addition, the adatom is also weakly coupled to a metallic STM tip by a hybridization function that provides a Kondo screening of its magnetic moment. We have described the entire system by the well-known Anderson-like Hamiltonian [2] whose low-temperature physics is accessed by employing the numerical renormalization group approach [3], which allows us to obtain the thermodynamic properties used to compute the Kondo temperature of the system. In our numerical calculation, two screening regimes were observed: local singlet and Kondo singlet, which strongly depends on the impurity adatom location and on the coupling strength of the carbon sites of the graphene nanoribbon with the adatom impurity. |
Wednesday, March 6, 2019 4:42PM - 4:54PM |
P15.00012: Analysis of Magnetoresistance in CrI3-graphene Heterostructures via First-principles Calculations Jonathan Heath, Marcelo Kuroda Recently, spin transport was demonstrated in heterostructures based on two-dimensional (2D) materials. Here we characterize electronic transport through multilayer CrI3 systems (bilayer, trilayer, and tetralayer) using the density functional theory (DFT) and the Landauer formalism for ballistic transport. Electronic structure of these tunneling junctions reveal that the interplay of quantum confinement and metamagnetic configurations defines the different tunneling rates. Hence, atomistic calculations capturing coupling between layers are key to these descriptions. Our ballistic transport calculations are in agreement with recent experimental measurements of magnetoresistance in graphene/CrI3/graphene [Klein et al., Science 360, 1218 (2018)]. We apply our transport studies for this type of tunneling junction to other metallic leads with Fermi level density of states larger than that of graphene and compare to other Cr halides junctions. While tunneling resistivity is significantly reduced, magnetoresistance ratios do not necessarily increase due to the intricate complex band structure of these systems. The atomistic details provided by this work may prove valuable towards the use of these 2D material-based spintronic devices. |
Wednesday, March 6, 2019 4:54PM - 5:06PM |
P15.00013: Assembly, X-ray diffraction and electronic transport measurement of atomically thin Bi2Sr2CaCu2O8+y high temperature superconducting van der Waals cuprate heterostructures. Nicola Poccia, Shu Yang Frank Zhao, Yuval Ronen, Hyobin Yoo, Ruidan Zhong, Genda Gu, Xiaojing Huang, Hanfei Yan, Yong S. Chu, Kenji Watanabe, Takashi Taniguchi, Svetlana Postolova, Gaetano Campi, Valerii Vinokur, Philip Kim Unique physical properties of low-dimensional electronic systems arise from the fine balance and interplay of often competing phenomena in the reduced dimensionality. We fabricated van der Waals heterostructure based on atomically thin Bi2Sr2CaCu2O8+y (BSCCO) high temperature superconductor. Unlike conventional superconductors, atomically thin BSCCO film exhibits an intrinsic multi-scale heterogeneity and a complete new set of imaging and nanofabrication tools are required to tame this complexity. In this talk, we will show the structural nanoscale heterogeneity using advanced X-ray nanoprobes of an assembled few-unit cells van der Waals cuprate high temperature superconducting heterostructure. Towards the end, I will demonstrate our recent realization of Josephson junctions made of assembled van der Waals cuprate heterostructures. |
Wednesday, March 6, 2019 5:06PM - 5:18PM |
P15.00014: Ultrafast photoemission momentum microscopy Christopher Corder, Peng Zhao, Jin Bakalis, Michael G White, Thomas Allison Understanding the electronic properties in material systems is key to unlocking new technologies. To probe the carrier dynamics on their natural timescale, ultrafast spectroscopies are employed, typically with femtosecond optical lasers. However, the observed features in these measurements are often ambiguous when attempting to identify the carriers involved and their location in the material band structure. Recent advances in ultrafast extreme-ultraviolet sources [1,2] now allow direct observation of carrier energy and momentum using angle-resolved photoelectron spectroscopy with the high data rates and low space-charge required for observing band structures. We demonstrate the capabilities when one of these XUV sources is paired with a time-of-flight momentum microscope [3] enabling time-resolved multidimensional measurements simultaneously spanning energy and two dimensions of momentum space. |
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