Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G42: Spin Textures and Chiral Magnetism in 2D MaterialsFocus

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Sponsoring Units: GMAG DMP Chair: Hongxin Yang, Chinese Academy of Sciences Room: 709/711 
Tuesday, March 3, 2020 11:15AM  11:27AM 
G42.00001: Topological MagneticSpin Structures in TwoDimensional Van Der Waals Cr_{2}Ge_{2}Te_{6} MyungGeun Han, Joseph Garlow, Yu Liu, Huiqin Zhang, Donald DiMarzio, Mark Knight, Cedomir Petrovic, Deep Jariwala, Yimei Zhu Longrange ferromagnetic order down to atomic layers provides an important degree of freedom in engineering twodimensional (2D) materials and their heterostructure devices for spintronics, valleytronics and magnetic tunnel junction switches. Using direct imaging by cryoLorentz transmission electron microscopy we report that topologically nontrivial magneticspin states, skyrmionic bubbles, can be realized in exfoliated insulating 2D van der Waals Cr_{2}Ge_{2}Te_{6}. Due to the competition between dipolar interactions and uniaxial magnetic anisotropy, hexagonallypacked nanoscale bubble lattices emerge in the ab plane by field cooling. Despite a range of topological spin textures arising due to pair formation and annihilation of Bloch lines, bubble lattices with single chirality are prevalent. Our observation of topologicallynontrivial homochiral skyrmionic bubbles in exfoliated vdW materials provides a new avenue for novel quantum states in atomicallythin insulators for magnetoelectronic and quantum devices. 
Tuesday, March 3, 2020 11:27AM  11:39AM 
G42.00002: Multiple ferromagnetic transitions and structural distortion in the van der Waals ferromagnet VI_{3} at ambient and finite pressures Elena Gati, Yuji Inagaki, Tai Kong, Robert J. Cava, Yuji Furukawa, Sergey L. Bud'ko, Paul C Canfield Magnetic vanderWaals (vdW) materials are considered as promising candidate systems to realize lowdimensional ferromagnetism in semiconductors. Here, we present a thermodynamic (specific heat and magnetization) and microscopic NMR study on bulk single crystals of the recently discovered ferromagnetic vdW material VI_{3 }at ambient and finite pressures [1]. Our results show that VI_{3} undergoes a structural transition, which is subsequently followed by two ferromagnetic transitions at ambient pressure, giving rise to two distinct magneticallyordered V sites for low temperatures. Upon increasing pressure, the two magnetic transitions merge at p ~ 0.6 GPa, and for even higher pressures (p ~ 1.25 GPa) merge with the structural transition. From these observations, we infer that the magnetic structure in bulk single crystals of VI_{3} is complex, with magnetoelastic coupling being of significant importance. 
Tuesday, March 3, 2020 11:39AM  11:51AM 
G42.00003: First principles Calculation of Dzyaloshinskii–Moriya interaction in 2D magnetic van der Waals heterostructures Kai Huang, DingFu Shao, Evgeny Y Tsymbal Magnetic skyrmions are nanoscale spin textures promising for nextgeneration spintronic applications. Recent studies have demonstrated that skyrmions can be generated at the interface between a ferromagnetic layer and a nonmagnetic heavymetalbased layer due to a large Dzyaloshinskii–Moriya interaction (DMI) induced by broken inversion symmetry and strong spinorbit coupling. This usually requires multilayer heterostructures with sizable thickness, which limit the nanoscale application. Recent discoveries of twodimensional (2D) magnets and the related van der Waals heterostructures offer the possibility for skyrmions to emerge at the atomic scale layer thickness. Here, we predict the emergence of a large DMI in bilayer magnetic van der Waals heterostructures composed of a 2D ferromagnetic metal Fe_{3}GeTe_{2} monolayer and a nonmagnetic monolayer. Based on firstprinciples density functional theory calculations, we find that the DMI, the exchange coupling, and the magnetic anisotropy of the magnetic van der Waals heterostructures can be modulated by the interfacial proximity effect, leading to the tunable skyrmion behaviors. Our work indicates the 2D magnetic van der Waals heterostructures are promising platforms for the skyrmionbased spintronics. 
Tuesday, March 3, 2020 11:51AM  12:03PM 
G42.00004: Realization of very large DzyaloshinskiiMoriya interaction and skyrmion states in twodimensional Janus manganese dichalcogenides Jinghua Liang, weiwei wang, Haifeng Du, Mairbek Chshiev, Albert Fert, Hongxin Yang The DzyaloshinskiiMoriya interaction (DMI), which only exists in noncentrosymmetric systems, is responsible for the formation of exotic chiral magnetic states. However, it is absent in most theoretically predicted and experimentally confirmed twodimensional (2D) magnetic thin films so far. In this report, we perform firstprinciples calculations to demonstrate that significant DMI can be obtained in a series of Janus monolayers of manganese dichalcogenides MnXY (X/Y = S, Se, Te, X ≠ Y) in which the difference between X and Y on the opposites sides of Mn breaks the inversion symmetry. In particular, the DMI amplitudes of MnSeTe and MnSTe are comparable to those of stateoftheart ferromagnet/heavy metal (FM/HM) heterostructures. Moreover, by performing Monte Carlo simulations, we find that the MnSeTe and MnSTe monolayers can host stable skyrmion states with the application of a moderate external magnetic field. The present results pave the way for new device concepts utilizing chiral magnetic structures in specially designed 2D ferromagnetic materials. 
Tuesday, March 3, 2020 12:03PM  12:15PM 
G42.00005: Topological Spin Textures in Janus Monolayers of Chromium Trihalides Cr(I,X)_{3} Changsong Xu, Junsheng Feng, Yousra Nahas, Sergei Prohorenko, Hongjun Xiang, Laurent Bellaiche Topological magnetic states are promising for ultradense memory and logic devices. Recent progresses in twodimensional magnets encourage the idea to realize topological states, such as skrmions and merons, in freestanding monolayers. However, monolayers such as CrI_{3} lack DzyaloshinskiiMoriya interactions (DMI) and thus do not naturally exhibit skyrmions/merons but rather a ferromagnetic state. Here we propose the fabrication of Cr(I,X)_{3} Janus monolayers, in which the Cr atoms are covalently bonded to the underlying I ions and toplayer Br or Cl atoms. By performing firstprinciples calculations and MonteCarlo simulations, we identify strong enough DMI, which leads to not only helical cycloid phases, but also to topologically nontrivial states, such as the intrinsic domain wall skyrmions in Cr(I,Br)_{3} and the magneticfieldinduced bimerons in Cr(I,Cl)_{3}. Microscopic origins of such spin textures are revealed as well. 
Tuesday, March 3, 2020 12:15PM  12:27PM 
G42.00006: Electrical excitation of superfluid and stringlike domain wall modes in layered van der Waal antiferromagnets Mohammad Mushfiqur Rahman, Avinash Rustagi, Yaroslav Tserkovnyak, Pramey Upadhyaya Efficient excitation of magnons (collective spin excitation in magnetically ordered materials) has been a limiting factor in designing lowdissipation magnonic devices. The recent emergence of lowdimensional van der Waal magnets [such as CrI3] with demonstrated electrical control of magnetic order [Nature Mat. 17, 406 (2018)] opens up new opportunities in the field of magnonics. Motivated by these developments, we theoretically demonstrate the electrical excitation of superfluid and stringlike modes harbored by the antiferromagnetically coupled domain walls of bilayer CrI3. Furthermore, we show that dc magnetic fields provide a handle for selectively exciting the mode of choice. 
Tuesday, March 3, 2020 12:27PM  12:39PM 
G42.00007: Merons in Monolayer CrCl_{3} Xiaobo Lu, Ruixiang Fei, Li Yang Noncollinear spin textures in lowdimensional magnetic systems such as skyrmions, magnetic bobbles and merons have been studied for decades with their extraordinary properties derived from their chirality and topological nature. Here, using first principles and Monto Carlo simulations, we propose that monolayer chromium chloride (CrCl_{3}) can be a promising candidate to observe paired magnetic vortex type and antivortex type topological defects, which are socalled merons. Through this paper, we demonstrate the existence of vortex and antivortex type meron pairings within the low temperature range (below 5K). Moreover, higherorder combinations of those meron pairs which are similar to the “quadrupole” excitations are also identified. Finally, including the inplane and outofplane external magnetic field, we show the robustness of merons’ pairing and a rich phase space to tune the hybridizations between the ferromagnetism and meron excitations. 
Tuesday, March 3, 2020 12:39PM  12:51PM 
G42.00008: Anomalous spinHall effect in 2D Crbased materials Yuri Dahnovsky, Andrei S. Zadorozhnyi We study the THE in various systems on 2D magnetic materials (CrIS_{3}, CrCl_{3}, and CrBr_{3}). From the estimations a skyrmion size is about 3040 nm. The computations of a spin Hall effect and magnetoresistance require the calculations of the nonequilibrium distribution function, which we will determine from the kinetic Boltzmann equation. The distribution function describes by a 4 × 4 matrix because of different spin projections and the spindependent momenta of conducting electrons. The matrix elements contain various parameters such as effective masses, exchange constants, etc., that will be found from the first principle calculations. The reason why such system exhibit topological Hall effect is because the scattering transition probability matrix has an antisymmetric part with respect to the scattering angle. The scattering asymmetry acts as an effective magnetic field, which sign can be either the same for both spin projections of an incident electron, hence leading to a topological charge Hall effect, or opposite for different electron spin projections, leading to the topological spin Hall effect 
Tuesday, March 3, 2020 12:51PM  1:03PM 
G42.00009: Skyrmions in TMDbased antiferromagnetic triangular lattices Aldo Raeliarijaona, Wuzhang Fang, PoHao Chang, Kirill Belashchenko, Alexey Kovalev We study the adsorption of magnetic transition metal atoms (Cr, Mn, Fe, and Co) on top of a TMDs such as MoS2 or WSe2, where the transition metal atoms form a triangular lattice. We conducted Monte Carlo simulations and analytical studies to obtain and characterize the magnetic ground state and determine the phase diagram for systems with AFM triangular lattice. The Heisenberg exchange parameters, singleion anisotropy constants, and DzyaloshinskiiMoriya vectors were extracted from firstprinciples density functional calculations. We found that without external magnetic field spirals are the most stable textures. Furthermore, we demonstrate the possibility of stabilizing antiferromagnetic skyrmion lattices living on 3 sublattices in such system under the influence of an external magnetic field. 
Tuesday, March 3, 2020 1:03PM  1:39PM 
G42.00010: Chirally coupled nanomagnets Invited Speaker: Zhaochu Luo Magnetically coupled nanomagnets have many potential applications including nonvolatile memories, logic gates and sensors. In order to realize functional 2D networks of coupled nanoscale magnetic elements such as those used for nanomagnet logic [1] and artificial spin ice [2], it is desirable to engineer effective lateral magnetic couplings in a controllable way. Up to now, this has been achieved by exploiting the longrange dipolar interaction. However, the dipolar interaction is nonlocal and scales inversely with the magnet volume, so limiting its use in applications involving nanometer sized structures and thin films. 
Tuesday, March 3, 2020 1:39PM  1:51PM 
G42.00011: Tuning Magnetic Order with Iron Intercalation in Transition Metal Dichalcogenides Caolan John, Spencer Doyle, Eran Maniv, James Analytis The transition metal dichalcogenides are a class of twodimensional materials currently under intense research due to their attractive electronic properties. Through the process of intercalation, magnetic atoms can be inserted between the layers of these materials to introduce long range magnetic order, enabling exploration of magnetism in these systems. I will present magnetization and thermodynamic measurements that indicate antiferromagnetic order in iron intercalated NbS_{2}. Crucially, we can use intercalation to control the strength of an emergent spin glass state below intercalation values of x = 1/3 in Fe_{x}NbS_{2}. The cooperation between this glassy phase and the antiferromagnetic order allow for the generation of substantial bias fields in the system. 
Tuesday, March 3, 2020 1:51PM  2:03PM 
G42.00012: Electrical switching in a magnetically intercalated transition metal dichalcogenide Eran Maniv, Nityan Nair, Caolan John, Spencer Doyle, Joseph Orenstein, James Analytis Advances in controlling the correlated behavior of transition metal dichalcogenides have opened a new frontier of manybody physics in two dimensions. A field where these materials have yet to make a deep impact is antiferromagnetic spintronics – a relatively new research direction promising technologies with fast switching times, insensitivity to magnetic perturbations, and reduced crosstalk. Here, we present measurements on the intercalated TMD Fe_{1/3}NbS_{2} which exhibits antiferromagnetic ordering below 42K. We find that remarkably low current densities of order 10^{4} A/cm^{2} can reorient the magnetic order, which can be detected through changes in the sample resistance, demonstrating its use as an electronicallyaccessible antiferromagnetic switch. Fe_{1/3}NbS_{2} is part of a larger family of magnetically intercalated TMDs, some of which may exhibit switching at room temperature, forming a platform from which to build tunable antiferromagnetic spintronic devices. 
Tuesday, March 3, 2020 2:03PM  2:15PM 
G42.00013: Evidence for a pressureinduced gapped spinliquid ground state in a coupled ladder antiferromagnet C_{9}H_{18}N_{2}CuBr_{4} Tao Hong, Tao Ying, Qing Huang, Sachith Dissanayake, Yiming Qiu, Mark M Turnbull, Andrey Podlesnyak, Yan Wu, Huibo Cao, David Tennant, Kai Schmidt, Stefan Wessel Here we present a comprehensive neutron scattering study on a spin1/2 coupled ladder antiferromagnet C_{9}H_{18}N_{2}CuBr_{4} (DLCB for short) under applied hydrostatic pressure. In DLCB, the interladder coupling is sufficiently strong to drive the system to the longrange antiferromagnetic ordering phase below T_{N}=2 K [1]. Analysis of the spin Hamiltonian suggests that DLCB is close to the quantum critical point in two dimensions at ambient pressure and zero field [2]. Singlecrystal neutron diffraction measurements under pressure suggest that the magnetic order breaks down above a ciritical pressure P_{c}~1.0 GPa. By contrasting with quantum Monte Carlo calculations of the dynamic structure factor, the followup inelastic neutron scattering study above P_{c} reveals evidence of a gapped spinliquid phase with the Z_{2 }topological order, characterized by excitation spectra of fully gapped visons and deconfined spinons. 
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