Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B22: Chiral Magnetism in Single Crystals and Bulk MaterialsFocus
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Sponsoring Units: GMAG DMP Chair: Stefan Bluegel, Forschungszentrum Julich GmbH Room: LACC 402A |
Monday, March 5, 2018 11:15AM - 11:27AM |
B22.00001: Spin excitations of the skyrmion lattice in MnSi David Fobes, Tobias Weber, Johannes Waizner, Max Kugler, Andreas Bauer, Robert Georgii, Peter Link, G. Ehlers, Robert Bewley, Christian Pfleiderer, Peter Boeni, Markus Garst, Marc Janoschek In the less than a decade since skyrmions, topologically-protected three-dimensional spin textures, were originally discovered in MnSi, considerable research efforts have gone towards exploiting their properties for future applications in spintronics. In non-centrosymmetric materials, it is well understood that skyrmions arise due to competition between ferromagnetic exchange and the Dzyaloshinskii-Moriya interaction, which directly relates to their potential for applications, since individual skyrmion size is proportional to the ratio of these interactions. Although the strength of the underlying magnetic interactions can typically be inferred directly from the spin wave dispersion, the mesoscopic size of skyrmions implies a tiny magnetic Brillouin zone, requiring momentum-transfer resolution beyond current state-of-the-art in neutron spectroscopy to resolve the spin waves, an outstanding experimental challenge. Here we overcome this challenge via a new generation of resolution deconvolution, enabling mesoscopic neutron spectroscopy of the spin excitations associated with the skyrmion lattice. The spin wave dispersion and underlying interactions determined in the skyrmion phase of MnSi through this new approach is in excellent agreement with our mean-field Ginzburg-Landau model. |
Monday, March 5, 2018 11:27AM - 11:39AM |
B22.00002: Exploring magnetism in Mn1-xIrxSi Chetan Dhital, Lisa DeBeer-Schmitt, Weiwei Xie, Qiang Zhang, David Young, John Ditusa The combination of the uniform exchange (J) and the Dzyalloshinskii-Moria interaction (D) are fundamental interactions that produce the twisted magnetic structures, helices and skyrmion lattices, in B20 structured compounds such as MnSi. However, it is still not clear which parameter or parameters primarily control these interactions and which model of magnetism is suitable for describing MnSi. To compare the effect of increased spin-orbit interaction and the increased valance electron density on the magnetic behavior of MnSi, we explored the effect of iridium doping in MnSi, Mn1-xIrxSi [1], and compared our results to similar studies on Mn1-x(Fe,Co)xSi. The similarity of the behavior of Mn1-xIrxSi and Mn1-xCoxSi as observed in our magnetization and small angle neutron scattering measurements indicate that the electronic structure surprisingly controls the magnetic behavior of this system. |
Monday, March 5, 2018 11:39AM - 11:51AM |
B22.00003: Magnetic structures and metamagnetic transition of non-centrosymmetric helimagnet ScFeGe Sunil Karna, J. Herbert, W. Phelan, Q. Zhang, D. Young, Rongying Jin, G. Cao, Yan Wu, Huibo Cao, W. Tian, Clarina Dela Cruz, Philip Adams, Chetan Dhital, A. khasanov, A. Roy, Phillip Sprunger, M. Khan, Weiwei Xie, Dana Browne, John Ditusa We have investigated the magnetic, transport and structural properties of non-centrosymmetric (NCS) ScFeGe that crystallizes in the Fe2P-type hexagonal crystal structure with lattice parameters c < a. The temperature dependence of the magnetization indicates a magnetic transition at TN = 40 K. The single-crystal neutron diffraction studies indicates a helical magnetic order with wavevector k = (0 0 0.193) and a magnetic moment ≈ 0.53µB (confined to the ab plane), indicating highly itinerant magnetic system. Itinerant magnetic systems where the Dzyaloshinskii–Moriya interaction is allowed are very interesting because of the effect on the magnetic states and because of fundamental questions about the origins of the interaction itself. We also demonstrate that the newly synthesized NCS helimagnet ScFeGe shows metamagnetic transition simultaneously with the appearance of a long wavelength (≈390 Å) modulation of the magnetic structure. Our observations also strongly suggest the existence of magnetostructural coupling in ScFeGe, which is likely driven by spin-orbit coupling and the DM interaction. In addition, our charge transport measurements display a discontinuous change to the resistivity along with a change in the sign of the magnetoresistance (MR) at TN. |
Monday, March 5, 2018 11:51AM - 12:03PM |
B22.00004: Monte Carlo modeling the phase diagram of magnets with the Dzyaloshinskii - Moriya interaction Alexander Belemuk, Sergei Stishov We use classical Monte Carlo calculations to model the high-pressure behavior of the phase transition in the helical magnets. We vary values of the exchange interaction constant J and the Dzyaloshinskii-Moriya interaction constant D, which is equivalent to changing spin-spin distances, as occurs in real systems under pressure. The system under study is self-similar at D/J=constant, and its properties are defined by the single variable J/T, where T is temperature. The existence of the first order phase transition critically depends on the ratio D/J. A variation of J |
Monday, March 5, 2018 12:03PM - 12:15PM |
B22.00005: Low-temperature spin-Seebeck effect in the helimagnetic insulator Cu2OSeO3 Artem Akopyan, Narayan Prasai, Joshua Cohn, Benjamin Trump, Guy Marcus, Tyrel McQueen We report measurements of the longitudinal spin-Seebeck coefficient (σLSSE) in single crystalsa of the helimagnetic insulator Cu2OSeO3 (with 10-nm thick Pt contacts) for the temperature range 0.5 K ≤ T ≤15 K. The magnitude of the maximum σLSSE ≈ 20 μV/K at T=5-6 K is the largest known for insulating magnets, consistent with the unprecedentedly large magnon thermal conductivity reported for this compound.b The temperature dependence of σLSSE as well as the applicability of various spin-Seebeck theories will be discussed. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B22.00006: Characterization of Zn Doped Insulating Magnetic Skyrmion Material Cu2OSeO3 Paul Neves, Dustin Gilbert, I-Lin Liu, Kathryn Krycka, Julie Borchers, Nicholas Butch Cu2OSeO3 has recently received significant attention as the only known insulating skyrmion material, offering interesting insights into the exchange interactions within magnetic materials. Magnetization studies have suggested that Zn substitutions on the Cu site can split the skyrmion phase into two separate temperature/field regions. However, the skyrmion phase splitting in (Cu1-xZnx)2OSeO3 has not been directly confirmed with structural measurements, and the mechanism for this split is unclear. In this talk, I discuss our work to synthesize and characterize (Cu0.9Zn0.1)2OSeO3, results from our magnetization and small angle neutron scattering (SANS) measurements, and their ramifications. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B22.00007: Néel-type skyrmion lattice in tetragonal polar magnet VOSe2O5 Takashi Kurumaji, Taro Nakajima, Victor Ukleev, Artem Feoktystov, Taka-hisa Arima, Kazuhisa Kakurai, Yoshinori Tokura VOSe2O5 has been known to form a tetragonal polar lattice with the space group P4cc and to exhibit a ferrimagnetic-like transition at TC = 7.5 K [1-3]. We synthesized the single crystals and performed the detailed magnetization study to reveal the versatile magnetic phases in a low-field region (H < 100 Oe) including the “A phase” close to the TC. We perform the SANS investigation and obtain the experimental evidence for the phase transition between cycloidal helimagnetic phase and the triangular skyrmion-lattice phase to demonstrate the Neel-type skyrmion lattice formation in the present system. We identified another magnetic phase of an incommensurate spin texture at lower temperatures under a finite magnetic field, tentatively assigned to a square skyrmion-lattice phase. These findings exemplify the versatile features of Néel-type skyrmions in bulk materials, and provide a further opportunity to explore the physics of topological spin textures in polar magnets. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B22.00008: Single to multiparticle excitations in the S=5/2 chiral magnet - MnSb2O6 Jana Pasztorova, Jose Rodriguez, Peter Gehring, Manila Songvilay, Navid Qureshi, Chris Stock Magnons, in classical systems, are well defined and long-lived transverse excitations of the localized ordered moment [1]. However, the single quasiparticle picture breaks down in low dimensional and low spin magnets [2] where neutron scattering experiments differ from predictions of harmonic spin-wave theory, displaying a momentum and energy broadened continuum of scattering. Noncollinear magnets are systems which may host a similar breakdown of harmonic spin waves. MnSb2O6 is a chiral magnet based upon localized S=5/2 moments with a Neel temperature of 12 K. We apply neutron spherical polarimetry and high resolution neutron inelastic scattering on single crystals to confirm the magnetic and nuclear structures, and to map out the low-energy spin fluctuations associated with the Neel ordered state. At higher energy transfers, a momentum and energy broadened continuum of scattering, which kinematically occurs at twice the energy of the low-energy harmonic spin-waves, is observed. MnSb2O6 therefore shows the breakdown of the single magnon picture in a high spin ordered chiral magnet. |
Monday, March 5, 2018 12:51PM - 1:03PM |
B22.00009: Terahertz spectroscopy of strongly frustrated helical magnet Fe3PO4O3 Nicholas Crescimanno, Thuc Mai, Michael Tarne, Kate Ross, James Neilson, Rolando Valdes Aguilar The hexagonal lattice in Fe3PO4O3 is made up of three triangularly coordinated Fe units that make the magnetic interactions strongly geometrically frustrated. Along the c-axis, layers of these triangular units arrange in a larger, triangular lattice pattern, making the magnetic interactions also frustrated. Even with this level of frustration, magnetic order occurs below TN ~ 165 K in the form of a long-wavelength helical pattern. We use time-domain terahertz (THz) spectroscopy to study the magnetic excitations in powder samples of this material. We find an overall softening of the magnetic spectrum around TN, but with no well-defined sharp resonant modes. However, below ~40 K we find a well defined resonant mode centered around ~ 900 GHz suggesting the existence of an additional transition below this temperature. We will discuss the possible explanations of this behavior within linear spin-wave theory. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B22.00010: Neutron Investigation of the Phase Transitions and Dynamics of Multiferroic GaV4S8 Jeffrey Lynn, William Ratcliff, Markus Bleuel, Lunyong Zhang, Sang-Wook Cheong We have carried out neutron diffraction and small angle neutron scattering measurements on a high quality single crystal of the cubic lacunar spinel GaV4S8 multiferroic as a function of magnetic field and temperature. The system undergoes a structural transition at 44 K to rhombohedral symmetry and becomes ferroelectric, and orders magnetically below ~13 K. The magnetic order parameter has been measured and indicates a ferromagnetic response below 8 K, while the intensities strongly suggest that the single spin is distributed across the V4 molecular unit rather than residing on a single V ion. Field-dependent SANS data exhibit the cycloidal and skyrmion phases, which have been investigated in detail as a function of temperature and applied magnetic field. In addition, the neutron time-tagging technique has been employed with a periodic perturbative magnetic field. Varying the amplitude and frequency of the perturbation has been used to reveal important details of the time dependence of the Néel-type skyrmions as well as other aspects of the magnetic order. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B22.00011: First-Order Magnetic Transitions and Metastability in the Néel Skyrmion Lattice Host GaV4S8 Eleanor Clements, Raja Das, Ganesh Pokharel, David Mandrus, Hariharan Srikanth, Manh-Huong Phan The lacunar spinel GaV4S8 is a unique example of a polar noncentrosymmetric magnetic material which hosts an achiral Néel skyrmion lattice (SkL) below TC ~ 13 K. Until recently, the bulk SkL phase was experimentally observed only in chiral crystals. In contrast to its chiral counterparts, the Néel SkL, which is described as a superposition of spin cycloids, is stable over a relatively large temperature and magnetic field range. While its proximity to TC suggests that thermal fluctuations play a role in stabilization of the SkL, the phase evolution is not well understood. In this study, a detailed analysis of the phase transitions in a GaV4S8 single crystal are investigated. The linear and nonlinear components of the ac susceptibility as a function of temperature and magnetic field are examined for a frequency range, f = 11 – 10 ,000 Hz. Particularly, the metastability of the spin cycloid and SkL states are investigated via field history experiments. The hysteretic behavior of the first-order metamagnetic cycloid to SkL transition is presented via complementary dc and ac susceptibility measurements. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B22.00012: Phase transitions in chiral magnets and spin fluctuations Alexander Belemuk, Sergei Stishov
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Monday, March 5, 2018 1:39PM - 1:51PM |
B22.00013: Magnetoentropic signatures of phase transitions in room temperature skyrmion hosts Joshua Bocarsly, Ryan Need, Ram Seshadri, Stephen Wilson FeGe and CoxZnyMnz have been demonstrated to host skyrmions at temperatures near and above room temperature, opening up the possibility of a number of practical applications. However, the details of the bulk magnetic phase diagrams have been a subject of much debate. In large part, this is due to the experimental difficulties associated with studying subtle materials properties at high temperature, particularly when large single crystals are not available. Here, we adapt tools developed for studying magnetocalorics to gain new insight about the magnetism of these skyrmion hosts using DC magnetization data. The techniques we have developed are easily applied to almost any material at any temperature between 2 K and 1000 K+, and allow for the rapid determination of a magnetic phase diagram (< 24 hrs.) while also giving quantitative latent heats of field-driven phase transformations. We clearly identify the first-order topological phase transitions as well as the Brazovskii transition and tricritical point in FeGe and demonstrate the entropic implications of each of these transitions. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B22.00014: Quantum dynamics of skyrmions in chiral magnets Christina Psaroudaki, Silas Hoffman, Jelena Klinovaja, Daniel Loss Magnetic skyrmions are topologically protected spin structures that have emerged as attractive candidates for magnetic storage applications in a rapidly developing field known as skyrmionics. Skyrmions are typically described as classical magnetic textures which propagate with high mobility due to the absence of a mass, while damping is usually included phenomenologically. |
Monday, March 5, 2018 2:03PM - 2:15PM |
B22.00015: Skyrmion State in Elemental Co Induced by Strong Dzyaloshinskii-Moriya Interaction Hubin Luo, Hongbin Zhang, J Liu Skyrmions are stable spin textures with chiral rotations from one spin-up moment to another across the center. Most of the materials bearing the Skyrmion state are magnetic ultrathin films deposited on heavy metals. Based on theoretical calculations, we show that the Skyrmion state exists also in the metastable phase of Co. Strong Dyzaloshiskii-Moriya (DM) interactions with meanwhile large isotropic exchange have been found in this material. Detailed analysis reveals that the DM interaction is caused by directional anisotropic hopping between atomic orbitals, following the perturbation theory. Although the DM interactions between different atomic pairs partly cancels with each other, the net interaction is sufficient to induce a chiral left-handed screw spiral. The spiral state has a wavelength about 180 nanometers along either [100] or [110] crystalline directions. The critical external magnetic field needed to stabilize the Skyrmion state is estimated to be 2.9 T at zero temperature, which is significantly lower than that required to stabilize the Skyrmion state in magnetic ultrathin films on heavy metals. |
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