Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X28: General Magnetism |
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Sponsoring Units: GMAG Chair: Chris Leighton, University of Minnesota Room: 291 |
Friday, March 17, 2017 8:00AM - 8:12AM |
X28.00001: Origin of the spin reorientation transitions in antiferromagnetic MnPt-based alloys Po-Hao Chang, Kirill Belashchenko Antiferromagnetic MnPt exhibits a spin reorientation transition (SRT) as a function of temperature, and off-stoichiometric Mn-Pt alloys also display SRTs as a function of concentration. Here we describe the origin of these SRTs using first-principles calculations based on the coherent potential approximation, treating chemical and thermally-induced spin disorder on equal footing. We find that the experimentally observed SRTs are related to specific features in the band structure, and we perform a detailed analysis of the effects of temperature and concentration on the magnetocrystalline anisotropy. [Preview Abstract] |
Friday, March 17, 2017 8:12AM - 8:24AM |
X28.00002: Origin and tuning of the magnetic anisotropy in Fe$_2$P-based alloys Ivan Zhuravlev, V.P. Antropov, K. D. Belashchenko Fe$_2$P-based alloys have been studied for years due to their potential magnetocaloric applications. In addition, Fe$_2$P demonstrates record-high magnetocrystalline anisotropy (MCA) for systems with no heavy elements. While the Curie temperature $T_C$ in pure Fe$_2$P is too low for applications, this system appears to be highly tunable, and its $T_C$ can be greatly increased by alloying with many other $d$ and $p$ elements. Here we present the electronic structure analysis of magnetic properties of these alloys, searching for systems with higher $T_C$ while preserving high MCA. The microscopic origin of the dominant contribution to MCA and its concentration dependence is revealed. We further find that co-alloying with Co or Ni and Si is a promising strategy for achieving high Curie temperature and MCA, which is more favorable compared to individual alloying by Co/Ni or Si due to the compensation of their effects on the band occupation. [Preview Abstract] |
Friday, March 17, 2017 8:24AM - 8:36AM |
X28.00003: Physical properties and \textbf{spin excitations in the lacunar spinels AV}$_{\mathrm{\mathbf{4}}}$\textbf{S}$_{\mathrm{\mathbf{8}}}$\textbf{ (A }$=$\textbf{ Ga, Ge)} Ganesh Pokharel, Andrew Christianson, David Mandrus In the lacunar spinels AV$_{\mathrm{4}}$S$_{\mathrm{8}}$ (A $=$ Ga, Ge), the interplay of spin, charge, and orbital degrees of freedom results in a complex phase diagram which includes: ferroelectric, orbitally ordered, and N\'{e}el type skyrmion phases. Below 12.7 K GaV$_{\mathrm{4}}$S$_{\mathrm{8}}$ exhibits cycloidal and ferromagnetic order and the application of a magnetic field results in a N\'{e}el type skyrmion spin structure. On the other hand, GeV$_{\mathrm{4}}$S$_{\mathrm{8}}$ orders antiferromagentically below 18 K. To illuminate the underlying physics driving the formation of these novel phases, we have measured the magnetization, resistivity, thermal conductivity, and inelastic neutron scattering spectrum of these spinels. The inelastic neutron scattering data shows broadened spin excitations which extend to 6 meV within the magnetically order phases for both GaV$_{\mathrm{4}}$S$_{\mathrm{8}}$ and GeV$_{\mathrm{4}}$S$_{\mathrm{8}}$. The similarity of the spectra for ferromagnetic GaV$_{\mathrm{4}}$S$_{\mathrm{8}}$ and antiferromagnetic GeV$_{\mathrm{4}}$S$_{\mathrm{8}}$ reflects the close balance of ferromagnetic and antiferromagnetic interactions in these materials. [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 8:48AM |
X28.00004: Magnetotransport under pressure in helical magnet Au2Mn I-Lin Liu, MARIA PASCALE, NICHOLAS BUTCH The binary compound $Au_{2}Mn$ is well studied because of its magnetic spiral structure. This material is also known to have temperature-dependent giant magnetoresistance. We measured the magnetoresistance and Hall effect as a function of both temperature and pressure. Results will be discussed in relation to neutron diffraction under pressure and the stability of the helical phase. [Preview Abstract] |
Friday, March 17, 2017 8:48AM - 9:00AM |
X28.00005: Magnetism and magneto-transport in the chiral helimagnet Cr$_{\mathrm{1/3}}$NbS$_{\mathrm{2}}$: Microscopic insights from angle-resolved photoemission and time-resolved optical spectroscopy Nicholas Sirica, Paolo Vilmercati, Michael Koehler, Deepak Sopkota, David Mandrus, Norman Mannella, Federica Bondino, Igor Pis, Silvia Nappini, Pranab Das, Ivana Vobornik, Jun Fujii, Hamoon Hedayat, Davide Bugini, Claudia Dellera, Ettore Carpene, Sung-Kwan Mo, David Parker The recent discovery of the soliton lattice, and the intriguing interplay between magnetic and transport degrees of freedom, make the chiral helimagnet Cr1/3NbS2 a very promising material both for technological applications, and for elucidating the connection between non-trivial spin textures and the microscopic interactions allowed in a crystalline lattice lacking in inversion symmetry. In this talk, we present recent results of photoemission and time-resolved optical spectroscopy on Cr1/3NbS2. Most notably, the data reveal that the Fermi surface is partially composed of Cr states, and that such states may give rise to a possible half metallicity as suggested by the characteristically long demagnetization dynamic measured from time-resolved magneto-optical Kerr effect (MOKE). Finally, it will be discussed how these findings are inconsistent with a description of magnetism and magnetotransport in this material based solely on spin ordering arguments. [Preview Abstract] |
Friday, March 17, 2017 9:00AM - 9:12AM |
X28.00006: Origin of the higher order harmonics in the long-period helimagnet Cr$_{\mathrm{1/3}}$NbS$_{\mathrm{2}}$ Lisa DeBeer-Schmitt, Adam Aczel, Travis Williams, Nirmal Ghimire, Ling Li, Michael McGuire, Stephen Kuhn, Morten Eskildsen, David Mandrus Transition metal dichalcogenides have been shown to have a chiral, hexagonal space group. Previous SANS work by this group have found in Cr$_{\mathrm{1/3}}$NbS$_{\mathrm{2}}$ a non-centrosymmetric long-period helimagnetic structure with higher order harmonics both at zero field and in an applied field. A perfect helical state (ie sinusoidal modulation), would display no higher order peaks at zero field. Odd higher order peaks can be explained by a "squaring up" of the sin wave but not even harmonics that have been seen which implies the magnetic ground state of Cr$_{\mathrm{1/3}}$NbS$_{\mathrm{2}}$ is not completely understood. [Preview Abstract] |
Friday, March 17, 2017 9:12AM - 9:24AM |
X28.00007: Inelastic neutron scattering study of the itinerant skyrmion host MnSi Xiang Chen, Matthew Stone, Alexander Kolesnikov, Dmitry Reznik, Stephen Wilson The helimagnet MnSi has been the subject of considerable focus since the discovery of a magnetic skyrmion lattice within its phase diagram at finite temperature and magnetic field. Prior to this discovery, MnSi was also established as a prototypical weak itinerant magnet and stood as a rare, clear example of conventional spin wave excitations damping into the Stoner continuum predicted for magnetic metals. Previous inelastic neutron scattering experiments however were unable to fully explore magnetic excitations in this continuum to high energies, where novel phenomena such as a magnetic Higgs-analog amplitude mode are predicted. Here we report our recent inelastic neutron scattering measurements targeted at exploring the high energy spin dynamics in this compound. Particular focus will be given to the dispersion of excitations within the previously established continuum as well as the evolution of the local spin susceptibility at high frequencies. [Preview Abstract] |
Friday, March 17, 2017 9:24AM - 9:36AM |
X28.00008: Magnon-induced long-range correlations and their neutron-scattering signature in quantum magnets S Bharadwaj, D Belitz, T. R. Kirkpatrick In the ordered phases of classical ferromagnets (FMs) and antiferromagnets (AFMs), the long-range correlations induced by the magnons lead to a singular wavenumber dependence of the longitudinal order-parameter susceptibility: $\chi_L\sim 1/k^{4-d}$ for $2 |
Friday, March 17, 2017 9:36AM - 9:48AM |
X28.00009: Excitation Modes and Gap Scaling in Quantum Critical Dimerized Antiferromagnets Stefan Wessel, Maximilian Lohoefer Results are presented from large-scale quantum Monte Carlo simulations on the excitation modes in two- and three-dimensional quantum antiferromagnets of coupled spin dimers accross their quantum critical points. The identification of the amplitude (Higgs) mode from dynamical spin and singlet spectral functions are contrasted in dependence of the systems's dimensionality and compared to universal scaling predictions. For the three-dimensional case, we furthermore identify characteristic multiplicative logarithmic corrections in the exciation gap scaling atop the leading mean-field behavior. These are in accord with field-theoretical predictions based on an effective description of the quantum critical spin system in terms of an asymptotically-free field theory. The width of the Higgs mode resonance is observed to scale linearly with the Higgs mass near criticality, indicative of this critically well-defined excitation mode of the symmetry broken phase. [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:00AM |
X28.00010: Magnetism and charge density wave in GdNiC2 and NdNiC2 Tom Klimczuk, Kamil Kolincio, Michal Wianiarski, Judyta Strychalska-Nowak, Karolina Górnicka The RNiC$_{2}$ compounds form in an orthorhombic Amm2 crystal structure with Ni and the rare-earth (R) metal chains along the crystallographic a-axis. This system is of particular interest because both a CDW and a long range magnetic ordering phases have been observed together. We report the specific heat, magnetic, magnetotransport and galvanomagnetic properties of GdNiC$_{2}$ and NdNiC$_{2}$ antiferromagnets. Complex B-T phase diagrams were built based on the specific heat data. Large negative magnetoresistance due to Zeeman splitting of the electronic bands and partial destruction of a charge density wave ground state is observed above T$_{N}$. The magnetoresistance and Hall measurements show that at low temperatures a magnetic field induced transformation from antiferromagnetic order to a metamagnetic phase results in the partial suppression of the CDW. This project is financially supported by National Science Centre (Poland), grant number: UMO-2015/19/B/ST3/03127. [Preview Abstract] |
Friday, March 17, 2017 10:00AM - 10:12AM |
X28.00011: Magnetic properties of Fe-based ferromagnets: a first-principles study Olga Vekilova, Olle Eriksson, Heike Herper The increasing importance of permanent magnets to the modern society has resulted in renewed interest in the design of new magnetic materials that are cheap and abundant. A good permanent magnet must have a high Curie temperature, high saturation magnetization and uniaxial anisotropy energy. Fe-rich systems like the hcp Fe$_{\mathrm{3}}$Sn compound are promising candidates. However, as it turned out to be an easy plane magnet. One of the possibilities for changing the easy axis direction is through alloying. Here we use theoretical modelling to discover new advanced permanent magnets. We study from first principles the electronic structure of Fe-based alloys, that exhibit ferromagnetic properties naturally, in combination with phase stabilizers, such as Sn, Sb, Ga, Ge and lighter elements such as Si and N in order to understand the origin of magnetic properties and find new ferromagnetic phases with uniaxial anisotropy that can be used for development of advanced permanent magnets. The stability of the phases is elucidated theoretically by the calculation of formation enthalpies. The trends in the calculated magnetocrystalline anisotropies and Curie temperatures of these compounds are analyzed and suggestions for the better permanent magnets are formulated. [Preview Abstract] |
Friday, March 17, 2017 10:12AM - 10:24AM |
X28.00012: Non-integral-spin bosonic excitations in untextured magnets Akashdeep Kamra, Utkarsh Agrawal, Wolfgang Belzig Interactions are responsible for intriguing physics, e.g. emergence of exotic ground states and excitations, in a wide range of systems. Here we theoretically demonstrate that dipole-dipole interactions lead to bosonic eigen-excitations with spin ranging from zero to above $\hbar$ in magnets with uniformly magnetized ground states. These exotic excitations can be interpreted as quantum coherent conglomerates of magnons, the eigen-excitations when the dipolar interactions are disregarded. We further find that the eigenmodes in an easy-axis antiferromagnet are spin-zero quasiparticles instead of the widely believed spin $\pm \hbar$ magnons. The latter re-emerge when the symmetry is broken by a sufficiently large applied magnetic field. The spin greater than $\hbar$ is accompanied by vacuum fluctuations and may be considered a weak form of frustration. [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X28.00013: Phase Diagram of the 3-D Coulomb Glass Amin Barzegar, Wenlong Wang, Juan Carlos Andresen, Helmut G. Katzgraber The existence of an equilibrium glass phase for charges in a disordered potential with long-range Coulomb interactions still remains controversial. Here, we conduct an extensive study of the disorder-temperature phase diagram of the Coulomb glass model in three space dimensions. We use population annealing Monte Carlo to equilibrate the system down to very low temperatures. Our results suggest an antiferromagnetic phase for the field values $h \la 0.15$. We also measure the corresponding critical exponents that show a good agreement with previous numerical studies. Outside of the antiferromagnetic region, we observe no sign of a Coulomb-glass phase at temperatures as low as $T=0.001$ (dimensionless units). This indicates that glassy properties in the Coulomb glass are either restricted to extremely low or zero temperature, they do not exist at all, or they are heavily suppressed by finite-size effects. [Preview Abstract] |
Friday, March 17, 2017 10:36AM - 10:48AM |
X28.00014: Dependence of grain size and defect density on the magnetic properties of mechanically alloyed Fe90W10 powder Dhananjay Kumar, Nana Kwame Yamoah, M.A. Koten, J.E. Shield, J Narayan Mechanical alloying was used to synthesize nanocrystalline Fe$_{\mathrm{90}}$W$_{\mathrm{10}}$ powders from high purity Fe and W powders. The Scherer-Debye and Williamson--Hall equations revealed that grain size reduction and defect creation were achieved during the milling process. There is a decrease of grain size from about 53 nm to about 6 nm after 80 h of milling. Williamson and Smallman's equation was also used to calculate the dislocation density. The result shows an increase in the dislocation density with increasing milling time. The grain size and defect characteristics were correlated with magnetic measurements. As W is continually dissolved in the Fe lattice, the change in coercivity seems to be minimal until the completion of solid solution when there is a sudden increase in coercivity. The increase in coercivity is explained by an increase in anisotropy due to an additional source of anisotropy arising from strain during the milling process. There was also a decrease in saturation magnetization as a result of the grain size reduction. [Preview Abstract] |
Friday, March 17, 2017 10:48AM - 11:00AM |
X28.00015: On the local magnetic moment formation and on the magnetic hyperfine field at isoelectronic noble impurities (Cu, Ag, Au) diluted in GdZn: period effect . C. M. Chaves, A. L. de Oliveira, N.A. de Oliveira, A. Troper In this work we study the period effect on the local magnetic moment and the related magnetic hyperfine field at non-magnetic s-p impurities (Cu, Ag, Au) diluted in GdZn, the impurity occupying a Zn site. The period effect refers to the difference of the local moment and the hyperfine fields in these noble isoelectronic (nd10ms1, n $=$ 3, 4, 5, m $=$ 4, 5, 6) impurities that apparently would have similar moments and hyperfine fields, which is not the case. We show that the di\textunderscore erence on the local moments is due to the differences of volumes of the impurities with respect to the host Zn ion while the difference of the hyperfine fields is due to different contact parameter A(Zimp), which depend mainly on the principal quantum numbers of the impurity shell m. We further extend our model to calculate the temperature variation of the local magnetic moment and of the hyperfine field for each impurity; for this calculation we adopt a functional integral approach in the static approximation . . [Preview Abstract] |
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