Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session P38: 2D Magnetism IIIFocus
|
Hide Abstracts |
Sponsoring Units: GMAG DMP Chair: Roopali Kukreja, University of California, Davis Room: BCEC 206B |
Wednesday, March 6, 2019 2:30PM - 3:06PM |
P38.00001: Properties of Monolayer Vanadium Dichalcogenides grown by molecular beam epitaxy Invited Speaker: Matthias Batzill Ferromagnetic ordering in monolayer vanadium dichalcogenides was predicted by DFT, despite the lack of ferromagnetic ordering in its bulk form. Ferromagnetic 2D materials with high Curie temperatures are highly sought to enable spintronic devices based on van der Waals heterostructures. In this talk we present a discussion of VSe2 and VTe2 monolayers grown by molecular beam epitaxy and study the difference of their monolayer properties compared to the bulk. In addition, to charge order states and structural variations of the monolayer and the bulk, we critically assess the origin of the measured ferromagnetic properties [1] in (sub)monolayer VSe2. |
Wednesday, March 6, 2019 3:06PM - 3:18PM |
P38.00002: Rashba-type Dzyaloshinskii-Moriya interaction in h-BN/Co and graphene/Co heterostructures Jinghua Liang, Ali Hallal, Fatima Ibrahim, Hongxin Yang, Mairbek Chshiev With the recently observed chiral domain wall structures at the graphene/ferromagnet interfaces [1,2], the Dzyaloshinskii-Moriya interaction (DMI) in light element/ferromagnet heterostructures has become of great interest for spintronics. Here, we present a comparative study of the Rashba-type DMI in the h-BN/Co and graphene/Co heterostructures using first-principles calculations. Unexpectedly, they show opposite trend as a function of Co thickness. With the increase of Co thickness, the DMI coefficient of h-BN/Co first increases gradually and then saturates at around 1.1 meV for thick Co layers, whereas the DMI at graphene/Co interfaces first decreases and then stabilizes at about 0.1 meV for large Co thickness. For thick Co layers, the DMI strength of h-BN/Co is one order of magnitude larger than that of graphene/Co. The origin of this discrepancy is attributed to the different contribution from the hybridization between dz2 and dxz orbitals of the interfacial Co layer to the DMI energy source. These results give deeper insight into microscopic formation of DMI and should help to optimize the material selection for future spin-orbitronic devices. |
Wednesday, March 6, 2019 3:18PM - 3:30PM |
P38.00003: Strong and Tunable Spin Lifetime Anisotropy in Dual-Gated Bilayer Graphene Jinsong Xu, Tiancong Zhu, Yunqiu (Kelly) Luo, Yuan-Ming Lu, Roland Kawakami We report the discovery of a strong and tunable spin lifetime anisotropy with excellent out-of-plane spin lifetimes up to 7.8 ns at 100 K in dual-gated bilayer graphene. Remarkably, this realizes the manipulation of spins in graphene by electrically-controlled spin-orbit fields, which is unexpected due to graphene’s weak intrinsic spin-orbit coupling (~12 μeV). We utilize both the in-plane magnetic field Hanle precession and oblique Hanle precession measurements to directly compare the lifetimes of out-of-plane vs. in-plane spins. We find that near the charge neutrality point, the application of a perpendicular electric field opens a band gap and generates an out-of-plane spin-orbit field that stabilizes out-of-plane spins against spin relaxation, leading to a large spin lifetime anisotropy (defined as the ratio between out-of-plane and in-plane spin lifetime) up to ~12 at 100 K. This intriguing behavior occurs because of the unique spin-valley coupled band structure of bilayer graphene. Our results demonstrate the potential for highly tunable spintronic devices based on dual-gated 2D materials. |
Wednesday, March 6, 2019 3:30PM - 3:42PM |
P38.00004: Ultrafast Spin and Charge Dynamics in Monolayer WSe2-Graphene Heterostructures Michael Newburger, Yunqiu (Kelly) Luo, Kathleen McCreary, Iwan Martin, Elizabeth McCormick, Berend Jonker, Roland Kawakami Monolayer transition metal dichalcogenides (TMDs) have garnered much attention due to their long spin/valley lifetimes and ability to optically excite spin/valley polarization. Additionally, one of the great strengths of TMDs is their ability to compliment other materials, such as graphene, by acting as a means of optical spin injection or proximity coupling. Recently proximity mediated charge transfer and optical spin injection has been demonstrated in TMD/graphene heterostructures. However, the spin transfer dynamics across a TMD/graphene interface remain largely unexplored. |
Wednesday, March 6, 2019 3:42PM - 4:18PM |
P38.00005: Organization and magnetic properties of mass-selected FePt nanoparticles deposited on epitaxially grown graphene on Ir(111) Invited Speaker: Florent Tournus The FePt alloy, when chemically ordered in the L10 phase, is among the magnetic materials displaying the highest magnetic anisotropy constant. Therefore it is a perfect candidate for ultra-high density magnetic storage applications, provided nanoparticles can be prepared in such a high anisotropy phase and organized in a 2D array. One path of bottom-up elaboration following a physical route consists in using template surfaces with specific sites regularly distributed. Such a 2D lattice can be obtained with the moiré (hexagonal lattice of 2.5 nm cell parameter) displayed by a graphene layer epitaxially grown on a Ir(111) surface [2]. For the first time, we have characterized the organization and the magnetic properties of FePt nanoparticles on such a moiré pattern. |
Wednesday, March 6, 2019 4:18PM - 4:30PM |
P38.00006: Oscillated Dzyaloshinskii-Moriya interaction at hydrogenated Graphene/Co interfaces Baishun Yang, Qirui Cui, Jinghua Liang, Hongxin Yang Magnetic topological structures such as chiral domain walls and Skyrmions are becoming a hot topic due to its potential application for future information storage. One of the key ingredient for the formation of these spintextures is the Dzyaloshinskii-Moriya interaction (DMI) in symmetry broken systems. Recently, it was reported that DMI can be induced in Co/Gr heterostructure due to the Rashba effect.[1] Considered that hydrogenated graphene (H-graphene) can enhance the spin orbit coupling of carbon atoms. Therefore, here, we propose to use interfaces between Co and H-graphene to tune DMI. Using first principles calculations, we find that the strength of DMI can be largely modulated by the concentration of H coverage on graphene, e.g., the anti-clockwise chiral of Co/Gr could be tuned to clockwise chiral in a Co(2ML)/1/2H-graphene with amplitude up to 1.0 meV/bond. Moreover, there is an obvious oscillation of DMI as a function of Co thickness ranging from 1ML to 7ML. The strength of DMI and this oscillation may origin from the change of SOC energy and electron filling in Co/Gr with different hydrogen concentration. |
Wednesday, March 6, 2019 4:30PM - 4:42PM |
P38.00007: Examination of the magnetic interactions between divacant Fe dimers in graphene Ronald E Putnam, Alexander Balatsky, Jason Haraldsen In this study, we investigate the isolated magnetic interactions between two identical Fe atoms substituted into graphene. Using density functional theory, we simulated the electronic and magnetic properties for a supercell of graphene with variably-spaced iron atoms in the divacant configuration. Overall, we find that the exchange interaction between the two Fe atoms fluctuates from ferromagnetic to antiferromagnetic as a function of the spatial distance in the armchair direction. Given the induced magnetic moment and increased density of states at the Fermi level by the surrounding carbon atoms, we conclude that the exchange interactions between the Fe atoms can be characterized by an RKKY-like interaction. Furthermore, we examined the same interactions for Fe atoms along the zig-zag direction in graphene and find no evidence for an RKKY interaction as this system shows standard superexchange between the transition-metal impurities. Therefore, we determine that Fe-substituted graphene produces a directional-dependent spin interaction, which may provide stability to spintronic and multifunctional devices and applications for graphene. |
Wednesday, March 6, 2019 4:42PM - 4:54PM |
P38.00008: Spin relaxation and proximity effect in WS2/graphene/fluorographene non-local spin valves Adam Friedman, Kathleen McCreary, Jeremy T Robinson, Olaf M Van T Erve, Berend Jonker Control of the spin relaxation in graphene-based structures is necessary to achieve the envisioned utility of graphene in future spintronic devices beyond Moore’s law. Proximity induced spin relaxation caused by contact to a high spin-orbit material, such as WS2, offers a promising avenue to manipulate the spin lifetime [1]. We demonstrate the operation of WS2/graphene/fluorographene non-local spin valves and extract the spin lifetimes for a range of carrier concentrations by Hanle effect measurements. Four-terminal charge transport measurements allow us to calculate the momentum relaxation time as a function of carrier concentration and compare it to the spin lifetime. These data show that the D’yakonov-Perel’ mechanism is the dominant spin relaxation mechanism for WS2/graphene/fluorographene devices. Without WS2, linear scaling between the spin and momentum lifetimes points to spin-flip scattering during strong elastic scattering events strongly coupled to the electron spin. We attribute the change in spin relaxation type in part with the inclusion of WS2 as a substrate to proximity induced spin-orbit coupling due to the adjacent WS2 layer, and we compare our data to the literature. [1] A.L. Friedman, et al. Carbon 131, 18-25 (2018). |
Wednesday, March 6, 2019 4:54PM - 5:06PM |
P38.00009: Room-temperature long-lived spin polarization in Weyl semimetal thin film Qisheng Wang, Hyunsoo Yang The Weyl semimetals WTe2 and MoTe2 show a great potential in generating large spin currents since they possess topologically-protected spin-polarized states and can carry a very large current density. In addition, the intrinsic noncentrosymmetry of WTe2 and MoTe2 endows with a unique property of crystal symmetry-controlled spin-orbit torques. An important question to be answered for developing spintronic devices is how spins relax in WTe2 and MoTe2. In this work, we show a room-temperature spin relaxation time of 1.2 ns (0.4 ns) in CVD-grown WTe2 (MoTe2) thin films using the time-resolved Kerr rotation (TRKR) . Based on ab initio calculation, we identify a mechanism of long-lived spin polarization resulting from a large spin splitting (~40 meV) around the bottom of the conduction band, low electron-hole recombination rate and suppression of backscattering required by time-reversal and lattice symmetry operation. In addition, we find the spin polarization is firmly pinned along the strong internal out-of-plane magnetic field induced by large spin splitting. Our work provides an insight into the physical origin of long-lived spin polarization in Weyl semimetals which could be useful to manipulate spins for a long time at room temperature. |
Wednesday, March 6, 2019 5:06PM - 5:18PM |
P38.00010: Gate-tunable magnetism of C adatoms on graphene Johannes Nokelainen, Igor Rozhansky, Bernardo Barbiellini, Erkki Lahderanta, Katariina Pussi We have performed density functional theory calculations of graphene decorated with carbon adatoms, which bind at the bridge site of a C–C bond. Earlier studies have shown that the C adatoms have magnetic moments and have suggested the possibility of ferromagnetism with high Curie temperature. Here we propose to use a gate voltage to fine tune the magnetic moments from zero to 1μB while changing the magnetic coupling from antiferromagnetism to ferromagnetism and again to antiferromagnetism. These results are rationalized within the Stoner and RKKY models. When the SCAN meta-GGA correction is used, the magnetic moments for zero gate voltage are reduced and the Stoner band ferromagnetism is slightly weakened in the ferromagnetic region. |
Wednesday, March 6, 2019 5:18PM - 5:30PM |
P38.00011: Quench dynamics of localized zero energy states in graphene Deepak Iyer, Matthew Foster In an external magnetic field, graphene forms Landau levels with an 'anomalous' level spacing due to the low energy linear dispersion. This creates a significant difference between the Landau level splitting and Zeeman splitting, allowing for nontrivial spin dynamics within a Landau level. It is expected that the ground state of the half-filled zeroth Landau level supports Skyrmions as low energy charged excitations, which can in principle exhibit collective dynamics under an external time dependent electromagnetic field via direct coupling to the Skyrmion (topological) charge density. In order to understand the dynamics, we study the related problem of massive Dirac fermions in two dimensions after a quench from an initial state containing localized zero-modes (vortices, in particular) into a homogeneous system with a Haldane mass. The Haldane term gives the leading order effect of a high-frequency drive. The one dimensional version of the problem with mass defects also leads to localized zero modes, and fractional charge. We follow the time evolution of various order parameters in these models and look for dynamical signatures of these zero modes. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700