Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Y67: Two-Dimensional Magnets IRecordings Available
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Sponsoring Units: DMP Chair: Kwabena Bediako, University of California Berkeley Room: Hyatt Regency Hotel -Hyde Park |
Friday, March 18, 2022 8:00AM - 8:12AM |
Y67.00001: Cr5Te8/WSe2 heterostructures by van der Waals epitaxy Mengying Bian, Liang Zhu, Xiao Wang, Junho Choi, Rajesh V Chopdekar, Sichen Wei, Lishu Wu, Chang Huai, Austin Marga, Fei Yao, Ting Yu, Scott A Crooker, Xuemei Cheng, Renat Sabirianov, Junhao Lin, Yanglong Hou, Hao Zeng Recently, two-dimensional (2D) van der Waals (vdW) heterostructures enable the exploration of new physics and applications. Many reported 2D heterostructures are formed by exfoliation and restacking of 2D flakes of small sizes and are restricted to vdW materials. In this work, we report epitaxial growth of covalent 2D magnet Cr5Te8 on vdW semiconducting monolayer WSe2 template, with a tunable thickness down to a single unit cell, via two-step chemical vapor deposition. Moiré superlattices were clearly identified by high-resolution scanning transmission electron microscopy images. Strong coupling between Cr5Te8 and WSe2 was demonstrated by the reduction of the Cr valence state near the interface and strong quenching of the photoluminescence of WSe2, which is expected to be absent in fully vdW heterostructures. Ferromagnetism in 2D Cr5Te8 crystals down to two unit cells thicknesses was measured by both reflective magnetic circular dichroism and X-ray magnetic circular dichroism. Rich magnetic domain patterns were identified by photoemission electron microscopy. Our work expands 2D heterostructures into covalent material systems. |
Friday, March 18, 2022 8:12AM - 8:24AM |
Y67.00002: Magnetoelectronic Properties of Graphene - Antiferromagnetic Insulator Heterostructures Anh Nguyen, Ryan Van Haren, Zhehao Ge, Jairo Velasco Jr., David Lederman Recently, graphene has been theorized to exhibit either ferromagnetism or antiferromagnetism induced by the magnetic proximity effect when placed on a proper magnetic substrate. In order to induce proximity magnetoelectronic effects, the substrate needs to satisfy three criteria. First, it should be insulating, so the graphene provides the only transport channel. Second, the orbitals of the antiferromagnet should hybridize with those of graphene sufficiently to induce sizable exchange and spin-orbit coupling fields. And finally, the substrate’s magnetization should be sufficiently weak so that magnetization measurements of the graphene are not obscured by the underlying magnetic layer. The antiferromagnetic (weak ferromagnet) insulator NiF2 grown via the molecular beam epitaxy on MgF2, is a suitable candidate due to its weak antiferromagnetic property. The Graphene-NiF2 heterostructure was fabricated using a stamping technique in a glovebox to prevent interfacial contamination. The proximity coupling was studied with magnetotransport measurement and the electronic structure by measuring electronic spectroscopy using scanning tunneling microscopy at low temperatures. The results of these measurements will be discussed. |
Friday, March 18, 2022 8:24AM - 8:36AM |
Y67.00003: Magnon-phonon hybridization in 2D antiferromagnet MnPSe3 Thuc T Mai, Kevin F Garrity, Amber D McCreary, J. Argo, Jeffrey R Simpson, Vicky Doan-Nguyen, Rolando Valdes Aguilar, Angela R Hight Walker Magnetic excitations in van der Waals (vdW) materials, especially in the two-dimensional (2D) limit, are an exciting research topic from both the fundamental and applied perspectives. Using temperature-dependent, magneto-Raman spectroscopy, we identify the hybridization of two-magnon excitations with two separate Eg phonons in MnPSe3, a magnetic vdW material that hosts in-plane antiferromagnetism. Results from first principles calculations of the phonon and magnon spectra further support our identification. The Raman spectra’s rich temperature dependence through the magnetic transition displays an avoided-crossing behavior in the phonons’ frequency and a concurrent decrease in their lifetimes. We construct a model based on the interaction between a discrete level and a continuum that reproduces these observations. Our results imply a strong hybridization between two discrete phonons and a two-magnon continuum and demonstrates that the magnon-phonon interactions can be observed directly in Raman scattering to provide deep insight into these interactions in 2D magnetic materials. We also systematically study the overall Raman scattering intensity enhancement in the AFM phase. Our experimental and theoretical results point to the cause of spin-ordering-induced resonant Raman scattering effect in MnPSe3. |
Friday, March 18, 2022 8:36AM - 8:48AM |
Y67.00004: 2D Antiferromagnet MnPS3 Induced Magneto-transport in Pt Film Wei-Cheng Liao, Mohammed A Alghamdi, Haoyu Liu, Josiah Keagy, Jing Shi 2D antiferromagnets such as MnPS3 and FePS3, relatively new members in the van der Waals material family, offer interesting physical properties that are of interest to the nanomagnetism and spintronics community. Probing these materials through electrical transport is difficult owing to their insulating nature at low temperatures. Here we have carried out magneto-transport measurements on exfoliated flakes of MnPS3 (< 10 nm) in heterostructures formed with thin Pt (3 nm). The MnPS3/Pt devices are about 4 µm x 8 µm in lateral dimensions and the Pt Hall pattern is defined by electron beam lithography and inductively coupled plasma etching. We observe both nonlinear Hall voltages and high-field negative magnetoresistance(MR) of Pt at low temperatures, indicative of the acquired nature due to the presence of MnPS3 since standalone Pt shows linear Hall responses and positive MR at the same temperatures. These effects become progressively weaker and persist up to 100 K. Additionally, the low-field Hall voltage shows a sign change at ~ 55 K. The observed magneto-transport features may contain both proximity-induced phenomenon as well as other extrinsic effects at the MnPS3 and Pt interface. Further experiments are underway to separate the extrinsic from the proximity-induced effects. |
Friday, March 18, 2022 8:48AM - 9:00AM |
Y67.00005: Magnetic structure of monoaxial chiral antiferromagnet Fe1/3NbS2 and its implications Kannan Lu, Azel Murzabekova, Lazar L Kish, Junehu Park, Soyeun Kim, Zachary Morgan, Xiaoping Wang, Christina Hoffmann, Lisa M DeBeer-Schmitt, Andre Schleife, Adam A Aczel, Fahad Mahmood, Greg MacDougall The intercalated transition metal dichalcogenide Fe1/3NbS2 has recently attracted significant interest due to its demonstrated peculiar coupling between the magnetic and the electronic degrees of freedom, paving the way for novel antiferromagnetic spintronics applications. The basis for understanding this coupling, is the interpretation of the magnetic structure of this material. With this motivation in mind, we have recently collected single crystal neutron diffraction data on Fe1/3NbS2 over a large volume of reciprocal space using neutron wavelength-resolved Laue diffraction at the SNS TOPAZ instrument. In this talk, I will discuss these measurements and the results of our magnetic structure determination for Fe1/3NbS2 using this data. I will further discuss predictions based on this magnetic structure for a static magnetic Kerr effect, and confirmation of this phenomenon by optical experiments performed at the University of Illinois. Finally, I will discuss the implications of our work for novel transport behaviors in the larger family of intercalated transition metal dichalcogenides. |
Friday, March 18, 2022 9:00AM - 9:12AM |
Y67.00006: Experimental realization of a single-layer multiferroic Qian Song, Connor A Occhialini, Emre Ergecen, Batyr Ilyas, Danila Amoroso, Paolo Barone, Jesse Kapeghian, Kenji Watanabe, Takashi Taniguchi, Antia S Botana, Silvia Picozzi, Nuh Gedik, Riccardo Comin Multiferroic materials have garnered wide interest for their exceptional static and dynamical magnetoelectric properties. In particular, type-II multiferroics exhibit an inversion-symmetry-breaking magnetic order which directly induces a ferroelectric polarization through various mechanisms, such as the inverse Dzyaloshinskii-Moriya effects. This intrinsic coupling between the magnetic and dipolar order parameters results in record-strength magnetoelectric effects. Two-dimensional materials possessing such intrinsic multiferroic properties have been long sought for harnessing magnetoelectriccoupling in nanoelectronic devices. Here, we report the discovery of type-II multiferroic order in a single atomic layer of the transition metal-based van der Waals material NiI2. We use circular dichroic Raman measurements to directly probe the magneto-chiral ground state and its long-lived electromagnon modes which reflect the presence of dynamic magnetoelectric coupling. From birefringence and second-harmonic generation measurements, we detect a highly anisotropic electronic state simultaneously breaking three-fold rotationaland inversion symmetry, and supporting polar order. The evolution of the optical signatures as a function of temperature and layer number surprisingly reveals an ordered magnetic, polar state that persists down to the ultrathin limit of monolayer NiI2. These observations establish NiI2 and transition metal dihalides as a new platform for studying emergent multiferroic phenomena, chiral magnetic textures and ferroelectricity in the two-dimensional limit. |
Friday, March 18, 2022 9:12AM - 9:24AM |
Y67.00007: Layer dependence of anisotropy field and Curie temperature of 2D ferromagnets: Fe5Ge2Te2, Fe3GeTe2 Mohammed A Alghamdi, Weicheng Liao, Diana Luong, Palani Jothi, Boniface Fokwa, Jing Shi Layered ferromagnets, a subset of the van der Waals family, exhibit many fascinating properties that have attracted a great deal of attention in the condensed matter physics community. Fe5Ge2Te2 (FG2T) and Fe3GeTe2 (FGT) stand out among others due to their unusually strong magnetic anisotropy perpendicular to the atomic layers, metallicity, and high Curie temperatures. We have fabricated FG2T and FGT nanodevices with a range of thicknesses down to monolayers by exfoliating flakes from crystals grown by solid-state reaction. We protect the thin flakes by depositing a thin Al2O3 film. Our transport measurements indicate that the Al2O3 protected devices are stable. By performing transport measurements at different temperatures with the external field oriented in-plane and out-of-plane, we obtain the longitudinal resistivity, anomalous Hall resistivity, Curie temperature, coercive field, as well as saturation field for all devices. We find the anomalous Hall conductivity is proportional to the number of layers and it is higher for FG2T than FGT. While the Curie temperature in FG2T is higher than that in FGT, the anisotropy field of the former is smaller. |
Friday, March 18, 2022 9:24AM - 9:36AM |
Y67.00008: Carrier doping enhanced ferromagnetism in Fe5−xGeTe2 Xiang Chen, Yu-Tsun Shao, Rui Chen, Sandhya Susarla, Tom Hogan, Yu He, Hongrui Zhang, Siqi Wang, Jie Yao, Peter J Fischer, David A Muller, Ramamoorthy Ramesh, Robert J Birgeneau Low dimensional magnetic systems beyond the realm of Mermin–Wagner theorem is of particular interests [1]. Recently, Fe5−xGeTe2 as a layered two-dimensional (2D) van der Waals (vdW) magnetic metal with an above-room-temperature magnetic transition temperature Tc is discovered [2-3]. This compound is particularly interesting because of its large Tc, strong magnetization and Fe-content tunable moment direction. In addition, it has been reported that Co doping can efficiently tune the lattice structure and magnetic ground states [4-5]. Here, we report another system of carrier doping in Fe5−xGeTe2, which dramatically alter the magnetic properties. Our work reveals a new arena for studying the 2D vdW magnetic metals with strong room temperature ferromagnetism. |
Friday, March 18, 2022 9:36AM - 9:48AM |
Y67.00009: History-dependent Magnetic and Morphological Phases in the 2D van der Waals Magnets Fe5GeTe2 and Fe3GeTe2 Kai Litzius, Max T Birch, Lukas Powalla, Sebastian Wintz, Fabian Alten, Michael Miller, Markus Weigand, Klaus Kern, Marko Burghard, Gisela Schütz Recently, the combination of 2-dimensional (2D) magnetism with the field of spintronics, i.e. the manipulation of magnetic states with electric currents, has started to gain much traction in modern solid-state physics due to many prospective applications. Observations of magnetic skyrmions in the 2D itinerant ferromagnet Fe3GeTe2 have been reported, opening further possibilities for technological implementation. However, the stability of the different magnetic states and morphological phases in FexGeTe2 remains an unresolved issue. In this work, we utilize real-space imaging to determine thickness-dependent magnetic phase diagrams of exfoliated Fe3GeTe2 and Fe5GeTe2 films. Our findings show besides complex, history-dependent magnetization states also that changes in the crystalline structure significantly alter the magnetic behavior, tuning the available in- and out-of-plane components. Ultimately, the choice of material and a proper nucleation mechanism result in the stabilization of a variety of (meta-) stable magnetic configurations, including skyrmions, at zero field, and at a single temperature. These findings open novel perspectives for designing van der Waal heterostructure-based devices incorporating topological spin textures. |
Friday, March 18, 2022 9:48AM - 10:00AM |
Y67.00010: 2D ferromagnet/topological insulator van der Waals heterostructures grown by molecular beam epitaxy Sotirios Fragkos, Polychronis Tsipas, Nicholas Figueredo Prestes, Panagiotis Pappas, Michael Heuken, Jean Marie George, Athanasios Dimoulas Near-room-temperature gate tunable 2D vdW ferromagnets (FM) such as Fe3GeTe2 (FGT) and CrTe2 [1,2] have ignited interest for the study of the structural and magnetic properties of epitaxial FM/TI heterostructures [3] with the aim to obtain efficient charge-to-spin conversion [4] and spin-orbit torques for spintronics. In this work we present MBE-grown CrxTey and FGT on several substrates. The CrTe2 phase is confirmed by RHEED, STM, Raman and ARPES, supported by DFT calculations of the electronic band structure. Probed by SQUID magnetometry, both CrTe2 and FGT are ferromagnetic and show perpendicular magnetic anisotropy. 5 ML FGT show a large anomalous hall effect (AHE) which persists up to 309K. Bi2Te3/FGT heterostructure shows partial but also consistent magnetization reversal induced by pulsed current through the Bi2Te3. The associated σxyAHE=24 [Ωcm]-1 of CrTe2 is compatible with the contribution from the Berry curvature predicted by our theoretical calculations. Bi2Te3/CrTe2 heterostructure is probed by second harmonic technique revealing large damping and field-like torques. |
Friday, March 18, 2022 10:00AM - 10:12AM |
Y67.00011: Topological Hall effect in graphene/Fe3GeTe2 van der Waals heterostructures Pawan K Srivastava, Budhi Singh, Yasir Hassan, Minwoong Joe, Changgu Lee The recent discovery of long-range magnetic order in atomically thin van der Waals (vdW) materials provides a platform to utilize two-dimensional magnetism for diverse functionalities and applications. Assembling a layered magnet with other vdW magnetic/non-magnetic materials could be used to discover several physical phenomena such as exchange bias effect, spin Hall Effect, magnetic proximity, etc. Probing magnetic proximity in graphene could provide an efficient way to utilize the unique properties of graphene in spintronics applications. Here, we experimentally investigate the magnetic proximity in graphene/Fe3GeTe2 vdW heterostructures probed by anomalous Hall measurements. Transverse magnetoresistance data shows the signatures of the topological Hall Effect, which is driven by non-coplanar spin texture at the graphene/Fe3GeTe2 interface. This can be explained by the interplay between symmetric exchange and antisymmetric Dzyaloshinskii–Moriya interaction due to Rashba type spin-orbit coupling at the interface. Further, the stability of non-coplanar spin texture at the graphene/Fe3GeTe2 interface has been discussed in the framework of temperature-dependent magnetocrystalline anisotropy energy. |
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