Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session X41: Novel Magnetic Materials |
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Sponsoring Units: GMAG DMP Chair: Christopher Safranski, IBM TJ Watson Research Center Room: 707 |
Friday, March 6, 2020 11:15AM - 11:27AM |
X41.00001: Crystal growth and characterization of the magnetic properties of intercalated transition metal dichalcogenides Kannan Lu, Deepak Sapkota, Adam Aczel, Lisa DeBeer-Schmitt, Yan Wu, David Mandrus, Greg MacDougall The intercalated transition metal dichalcogenide (TMDC) Cr1/3NbS2 has attracted significant interest in recent years due to reports of a magnetic soliton lattice in this material in finite fields. This novel topological state of matter is understood to result from competing Zeeman, Dzyaloshinskii-Moriya and ferromagnetic exchange terms in the spin Hamiltonian. Despite this theoretical understanding, there are few other experimental realizations of soliton lattice materials beyond the singular compound Cr1/3NbS2. With this motivation in mind, we have undertaken a concerted effort at Illinois to grow large single crystals of additional intercalated TMDCs, and characterize them with a series of techniques, including magnetization, heat capacity, x-ray and neutron scattering. In this talk, I will report on these data and their implications for the magnetic ground states for each of the materials studied. In particular, I will report on our use of powder x-ray diffraction and magnetization to infer information about the centrosymmetry of the structure in these materials, and thus the potential for DM interactions. I will supplement with neutron diffraction from single crystals to comment on ordered ground states, and their potential to contain soliton phases in finite fields. |
Friday, March 6, 2020 11:27AM - 11:39AM |
X41.00002: Vestigial Potts nematicity in a triangular lattice magnet Jörn Venderbos, Rafael Fernandes Motivated by recent experimental evidence for spontaneous rotational symmetry breaking in the hexagonal antiferromagnet Fe1/3NbS2, this talk discusses the general theory of nematic order in magnetic system with hexagonal symmetry, focusing in particular on the possibility of a vestigial nematic phase. In stark contrast to tetragonal systems, such as the iron-pnictides, nematic order in hexagonal systems is described by the 3-state Potts model. We develop the general theory of (single-q) magnetic order at degenerate symmetry-related wave vectors, and show that distinct magnetic phases can be elegantly captured by composite order parameters: bilinears of the primary magnetic order parameters which transform irreducibly under the symmetry group. Singling out the composite nematic order parameter, we extend the Landau-type mean field theory with magnetic fluctuations and show that a vestigial nematic phase, i.e., a non-magnetic phase with broken rotation symmetry, can exist. We discuss experimental signatures and interpret our results in light of the reported measurements on Fe1/3NbS2. |
Friday, March 6, 2020 11:39AM - 11:51AM |
X41.00003: Effect of Confinement on Magnetism and Skyrmionic Properties of CoSi Nanoparticles Rabindra Pahari, Balamurugan Balasubramanian, Ahsan Ullah, Rohit Pathak, Arti Kashyap, Ralph Skomski, George C Hadjipanayis, David Sellmyer Quantum-confinement and surface effects often result in unusual magnetic ordering, modified ordering temperatures, and different spin structures in nanoparticles as compared to the corresponding bulk alloys [1, 2]. In sharp contrast to non-magnetic bulk equiatomic CoSi, we show a magnetic ordering at room temperature with a net magnetic moment m = 0.11 μB/Co for CoSi nanoparticles. The nanoparticles have an average size of 11.6 nm, and crystallize in the cubic B20 structure, which supports spin spirals and skyrmions with broken helicity caused by non-zero Dzyaloshinskii–Moriya (DM) interactions [3]. DC susceptibility and anomalous Hall resistivity measurements are used to discuss the effect of confinement within the small diameter of CoSi nanoparticles on the room-temperature skyrmionic properties, which are essential for exploiting these complex spin structures for practical applications. |
Friday, March 6, 2020 11:51AM - 12:03PM |
X41.00004: New magnetic orderings in elemental Neodymium Hasitha Suriya Arachchige, Lisa DeBeer-Schmitt, David Parker, Andrew May, Markus Bleuel, Ganesh Pokharel, Wei Tian, Cristian Batista, David Mandrus, Andrew D Christianson The element neodymium is one of the most magnetically complex elements. Previous reports find a myriad of magnetic phases as a function of temperature and field below the antiferromagnetic transition temperature of 19.9 K [1-3]. Here we present a small angle neutron scattering (SANS) study of neodymium to probe whether the complex magnetic interactions on the double hexagonal closed packed (DHCP) crystal structure of neodymium can host novel spin textures. The SANS measurements reveal several new features. Firstly, the measurements establish regions in the HT phase diagram where scattering patterns characteristic of a skyrmion lattice are present. Additionally, we find new low field phase boundaries and pockets that bear some semblance to existing phase diagrams of skrymion materials. |
Friday, March 6, 2020 12:03PM - 12:15PM |
X41.00005: Magnetic textures in single crystals of Ni1-xFexBr2 Binod Rai, Hasitha Suriya Arachchige, Ganesh Pokharel, Yaohua Liu, Shizeng Lin, Cristian Batista, David Mandrus, Andrew D Christianson, Andrew May Recently, frustrated systems with finite easy axis anisotropy have been proposed as a route to realize topological spin textures. In this context, Ni1-xFexBr2 has been predicted to host a field-induced skyrmion phase. This presentation will discuss the magnetic properties of Ni1-xFexBr2 single crystals and the magnetic field-temperature phase diagram. NiBr2 crystallizes in the centrosymmetric CdCl2 structure type. It exhibits a commensurate antiferromagnetic phase below TN = 48 K and an incommensurate helimagnetic phase below TIC = 22 K, both with easy plane anisotropy. The helimagnetic structure of Ni1-xFexBr2 is tuned as a function of composition and applied magnetic field, as demonstrated using bulk measurements and neutron scattering. A change in magnetic anisotropy from easy plane to easy axis is observed around x = 0.07, which is expected as a key feature to realize a skyrmion phase in this system. |
Friday, March 6, 2020 12:15PM - 12:27PM |
X41.00006: Imaging skyrmions in B20 epitaxial thin films grown on Si substrates using variable temperature Magnetic Force Microscopy (MFM) Camelia Selcu, tao Liu, Denis Pelekhov, Roland Bennett, Binbin Wang, Nuria Bagues Salguero, Jacob Repicky, Joseph P Corbett, Brendan McCullian, Yunqiu (Kelly) Luo, Shuyu Chen, Adam S Ahmed, P Chris Hammel, Mohit Randeria, Jay A Gupta, David W. McComb, Roland Kawakami In the search for the next-generation magnetic storage, tuning the size of the skyrmions with high Curie temperatures (Tc) promotes the possibility to use them for such application. We have successfully grown Fe-rich Fe1.2Ge thin films using Molecular Beam Epitaxy (MBE) with a Tc higher than the room temperature. Furthermore, we have observed skyrmions in the Fe-rich Fe1.2Ge thin films at room temperature. Here, we report the first time imaging of the skyrmions in FeGe and Fe rich Fe1.2Ge epitaxial thin films grown on Si substrates using Magnetic Force Microscopy (MFM). Based on the MFM images, skyrmions were observed in temperatures and magnetic fields consistent with the topological Hall measurements for the FeGe thin films. Additionally, the topological Hall resistance scales down with the density of skyrmions at lower temperatures in these films. Also, we observed skyrmion motion and annihilation which might be driven by the stray field of the MFM tip probably due to the defects in the FeGe films. |
Friday, March 6, 2020 12:27PM - 12:39PM |
X41.00007: Frustration Stabilized Skyrmions studied with X-ray Scattering and μSR Murray Wilson, Matjaz Gomilsek, Thomas Hicken, Samuel Moody, Max T. Birch, Ales Stefancic, Geetha Balakrishnan, Tom Lancaster, Peter D. Hatton Skyrmions are a topologically protected magnetic texture that attracts attention both for its interesting physics, and for potential spintronics applications. Central to the realization of applications is the requirement to have small skyrmions. Recently, skyrmions have been reported to exist in two new materials, Gd2PdSi3 [1] and Gd3Ru4Al12 [2]. The skyrmions in these materials are reportedly extremely small (2.3 – 3 nm compared to 20 – 60 nm in other materials), and, in contrast to most known skyrmion materials, are thought to be stabilized by frustration, rather than by the antisymmetric Dzyaloshinskii-Moriya interaction. |
Friday, March 6, 2020 12:39PM - 12:51PM |
X41.00008: Lattice dynamics of antiperovskite manganese nitrides exhibiting large negative and positive thermal expansion Sheena Patel, Stjepan B Hrkac, Jeffrey Brock, Nelson Hua, Haidan Wen, Oleg Shpyrko, Eric Fullerton Antiperovskite manganese nitrides Mn3AN (where A is a metal or semiconducting element) have strong charge-spin-lattice interactions resulting in magnetovolume effects such as large positive (PTE) and negative thermal expansion (NTE) over transitions between their ferrimagnetic, antiferromagnetic, and paramagnetic phases [1]. We grow films of Mn3Cu1-xGexN with x = 0.1 to 0.5, which exhibit tunable NTE coefficients over a 50 K region, occurring anywhere from around 100 K to around 400 K depending on x. We also grow films of Mn3Cu1-xNixN with x = 0.09 to 0.10, which have a first order hysteretic PTE of 1-1.5% near 100 K and exhibit NTE for a total lattice contraction of roughly 0.5% over a 25 K range around 175 K. These regions are linked to transitions in magnetic phases. To probe the coupling of the structure and magnetism, we have measured the X-ray diffraction following photoexcitation within and through the interesting regions at the Advanced Photon Source. We present the complex lattice dynamics and discuss the spin-lattice coupling driving their response. |
Friday, March 6, 2020 12:51PM - 1:03PM |
X41.00009: Non-periodic magnetic structure in chiral helimagnet Mn1/3NbS2 Sunil Karna, Michalis Charilaou, Andras Kovács, Lisa DeBeer-Schmitt, David P Young, John Ditusa We have investigated the magnetic state of Mn1/3NbS2 through ac-magnetic susceptibility, Small-angle Neutron Scattering (SANS), Lorentz Transmission Electron Microscopy (LTEM) and micromagnetic simulations. The ac susceptibility displays temperature, field, and frequency dependencies which define a complex phase diagram below the critical temperature for magnetic ordering, TC = 45 K. SANS reveals a streak of magnetic scattering along the c-axis near Q = 0 appearing below TC, demonstrating a disordered ferromagnetic (FM) or helical spin ordering in this system. The width of this streak shortens and becomes more intense near TC and is gradually suppressed by the application of H along the beam. Micromagnetic simulations of thin lamella are in agreement with LTEM images of Mn1/3NbS2 where extended FM regions result from a shape anisotropy in thin samples that are separated by the chiral domain walls. |
Friday, March 6, 2020 1:03PM - 1:15PM |
X41.00010: Local structural distortions and magnetism in lacunar spinel compounds Julia Zuo, Emily C Schueller, Ram Seshadri, Stephen Wilson Lacunar spinels AB4Q8 (A=Al, Ga, Ge; B=V, Nb, Mo, Ta; Q=S, Se) display a variety of novel phenomena including skyrmion lattices and pressure-induced insulator-metal transitions and superconductivity. The high temperature structure features unique tetrahedral transition-metal clusters with S=1/2 moments on an FCC lattice. Low temperature distortions of these clusters are crucial to the properties of lacunar spinels. In skyrmion hosts GaV4S8 and GaV4Se8, a low temperature rhombohedral distortion allows for polar and magnetic order. In GaNb4Se8 and GaTa4Se8, antiferromagnetic interactions lead to geometric frustration and a magnetic transition associated with a tetragonal distortion. We present results of local structural characterization on lacunar spinels in connection to their magnetic properties. |
Friday, March 6, 2020 1:15PM - 1:27PM |
X41.00011: Magnetic texture evolution within uniaxial kagome ferromagnets immediately below Curie temperature Akira Sugawara, Tetsuya Akashi, Mohamed A Kassem, Yoshikazu Tabata, Takeshi Waki, Hiroyuki Nakamura Magnetic textures such like skyrmion and skrmionic bubbles attracts attention in connection with topology of electronic structures. Such texture formation usually occurred slightly below Curie temperature (Tc) due to competition of micromagnetic parameters that change rapidly with approaching Tc. We examined magnetic texture within two uniaxial kagome ferromagnets: Co3Sn2S2 (Tc=176 K) studied extensively as a Weyl-semimetal and Fe3Sn2 (Tc= 657 K) interested for its massive Dirac fermions. In-situ Lorentz microscopy was performed during cooling across Tc under zero field. In Co3Sn2S2, nucleation of bubble domains followed by their spontaneous reorganization occurred in narrow temperature range immediately below Tc,, whereas the domain wall motion was mostly frozen below 160 K. The morphology of the domains also depended on cooling rate through domain wall creep associated with thermal depinning. The magnetic phase diagram is influenced by slow spin dynamics also, hence is not determined uniquely as a function of field and temperature. In contrast such creep motion did not occur in Fe3Sn2 immediately below Tc, whereas domain wall motion occurred continuously in wide temperature range down toward room temperature. |
Friday, March 6, 2020 1:27PM - 1:39PM |
X41.00012: Phase Boundary Near a Magnetic Percolation Transition Gaurav Khairnar, Cameron J Lerch, Thomas Vojta Hexagonal ferrites such as PbFe12O19, SrFe12O19, BaFe12O19 show interesting magnetic and ferroelectric properties at low temperatures. The magnetic ordering temperature of these compounds can be tuned by random site dilution replacing Fe ions by Ga ions, for example in PbFe12−xGaxO19. According to recent experiments, the critical temperature of these compounds varies as Tc ∝ (1 − x/xc )2/3, where xc is very close to the percolation threshold of the lattice [1]. Motivated by these results, we re-examine the shape of the magnetic phase boundary over the entire x range from 0 to xc. We perform large scale Monte-Carlo simulation of diluted XY and Heisenberg models for both cubic lattices and lattices representing the crystal structure of the hexaferrites. In particular, we study the critical behavior of the percolation transition and width of the critical region. |
Friday, March 6, 2020 1:39PM - 1:51PM |
X41.00013: Driving the magnetic transition by chemical substitution in Cs1-xRbxFeCl3 Lena Stoppel, Shohei Hayashida, Zewu Yan, Severian Gvasaliya, Andrey Podlesnyak, Andrey Zheludev We report the observation of a chemical-substitution driven phase transition from a gapped quantum paramagnetic phase to one with long range order in Cs1-xRbxFeCl3. The x = 0 compound in this series of triangular-lattice antiferromagnets has a spin-singlet ground state due to strong easy-plane magnetic anisotropy. In contrast, the x = 1 material orders magnetically in a 120° structure [1]. Calorimetric and magnetic experiments performed on a series of samples with 0 ≤ x ≤ 1 reveal that in the low-temperature limit magnetic order appears at x ~ 0.35. Inelastic neutron scattering experiments show that this coincides with the closure of the gap in the spin excitation spectrum. It appears that disorder effects in this material are more pronounced than those in the only other known phase transition of this type, namely in DTNX [2]. |
Friday, March 6, 2020 1:51PM - 2:03PM |
X41.00014: Intrinsic stability of magnetic anti-skyrmions in the tetragonal inverse Heusler compound Mn1.4Pt0.9Pd0.1Sn Rana Saha, Abhay Kant Srivastava, Tianping Ma, Jagannath Jena, Peter Werner, Vivek Kumar, Claudia Felser, Stuart Parkin Magnetic anti-skyrmions [1] are one of several chiral spin textures that are of great current interest both for their topological characteristics and potential spintronic applications. Anti-skyrmions were recently observed in the inverse tetragonal Heusler material Mn1.4Pt0.9Pd0.1Sn. Here we show, using Lorentz transmission electron microscopy, that anti-skyrmions are found over a wide range of temperature and magnetic field in wedged lamellae formed from single crystals of Mn1.4Pt0.9Pd0.1Sn for thicknesses ranging up to ~250 nm [2]. The temperature-field stability window of the anti-skyrmions varies little with thickness. Using micromagnetic simulations we show that this intrinsic stability of anti-skyrmions can be accounted for by the symmetry of the crystal lattice which is imposed on that of the Dzyaloshinskii-Moriya exchange interaction. These distinctive behaviors of anti-skyrmions makes them particularly attractive for spintronic applications. |
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