Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z29: Cooperative Phenomena In Magnetism II |
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Sponsoring Units: GMAG Chair: Daniel Parshall, National Institute of Standards and Technology Room: 206A |
Friday, March 6, 2015 11:15AM - 11:27AM |
Z29.00001: High precision measurements of quantum critical properties for 3D quantum antiferromagnets Zi Yang Meng, Yan Qi Qing, Bruce Normand, Anders Sandvik Using large-scale quantum Monte Carlo (QMC) simulations, we study the quantum phase transitions in three-dimensional S=1/2 dimerized Heisenberg antiferromagnets. We obtain high precision results on the quantum critical properties of the transition from antiferromagnetically ordered phase to the magnetically disordered dimerized phase. With careful finite size scaling analysis and improved estimator of physical observables in the QMC simulations, we are able to extract the precise logarithmic corrections to quantum phase transition in our system governed by the 3+1 O(3) universality class. Finite temperature quantum critical properties in excitation spectra are obtained as well. [Preview Abstract] |
Friday, March 6, 2015 11:27AM - 11:39AM |
Z29.00002: Calculation of the Curie temperature of Ni using first principles based Wang-Landau Monte-Carlo Markus Eisenbach, Junqi Yin, Ying Wai Li, Don Nicholson We combine constrained first principles density functional with a Wang-Landau Monte Carlo algorithm to calculate the Curie temperature of Ni. Mapping the magnetic interactions in Ni onto a Heisenberg like model to underestimates the Curie temperature. Using a model we show that the addition of the magnitude of the local magnetic moments can account for the difference in the calculated Curie temperature. For ab initio calculations, we have extended our Locally Selfconsistent Multiple Scattering (LSMS) code to constrain the magnitude of the local moments in addition to their direction and apply the Replica Exchange Wang-Landau method to sample the larger phase space efficiently to investigate Ni where the fluctuation in the magnitude of the local magnetic moments is of importance equal to their directional fluctuations. We will present our results for Ni where we compare calculations that consider only the moment directions and those including fluctuations of the magnetic moment magnitude on the Curie temperature. This research was sponsored by the Department of Energy, Offices of Basic Energy Science and Advanced Computing. We used Oak Ridge Leadership Computing Facility resources at Oak Ridge National Laboratory, supported by US DOE under contract DE-AC05-00OR22725. [Preview Abstract] |
Friday, March 6, 2015 11:39AM - 11:51AM |
Z29.00003: Thermodynamic properties of a 2D itinerant ferromagnet - a sign-problem free quantum Monte Carlo study Sheng-Long Xu, Yi Li, Congjun Wu We investigate thermodynamic properties of itinerant ferromagnetism by using the non-perturbative method of quantum Monte-Carlo simulation, which is shown free of the sign problem in a multi-orbital Hubbard model in the square lattice in a large region of fermion density. The spin magnetic susceptibility is local-moment-like exhibiting the Curie-Weiss law in the off-critical temperature region, while the compressibility typically exhibits the itinerant nature, which is finite and weakly temperature-dependent. The spin magnetic susceptibility further grows exponentially as approaching zero temperature for the SU(2) invariant models. The long-range ferromagnetic ordering appears when the symmetry is reduced to the Ising class, and the Curie temperature can be accurately determined. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z29.00004: A benchmark study of the two-dimensional Hubbard model with auxiliary-field quantum Monte Carlo Mingpu Qin, Hao Shi, Shiwei Zhang The ground state properties of the two-dimensional Hubbard model are calculated with the auxiliary-field quantum Monte Carlo (AFQMC) method. With general twist boundary conditions, the shell effect is eliminated. We use large lattice sizes ($L\times L$ lattices with $L$ up to $24$) and average over many twist angles to extrapolate to the thermodynamic limit and ensure convergence of the calculated physical quantities. At half filling, we obtain accurate results for the ground-state energy, sublattice magnetization and double occupancy, and other correlation functions for interactions ranging from $U/t = 2$ to $8$. We then study the doped cases with a constraint to control the ``minus sign'' problem. We apply the latest development \footnote{Hao~Shi, Shiwei~Zhang, Phys. Rev. B. {\bf 88}, 125132 (2013); Hao~Shi, Carlos~A.~Jim\'enez-Hoyos, R.~Rodr\'iguez-Guzm\'an, Gustavo~E.~Scuseria, and Shiwei~Zhang, Phys. Rev. B. {\bf 89}, 125129 (2014) } in the constrained path AFQMC technique to benchmark the effect of different trial wave functions, and study ground-state properties in the thermodynamic limit. The different competing phases close to half filling are investigated. [Preview Abstract] |
Friday, March 6, 2015 12:03PM - 12:15PM |
Z29.00005: Entropic and magnetic properties of Ni-Mn-In magnetocaloric materials Jing-Han Chen, Nickolaus Bruno, Ibrahim Karaman, Yujin Huang, Jianguo Li, Joseph H. Ross, Jr. We report magnetization and field-dependent calorimetry studies of phase transitions in Ni-Mn-In. Off-stoichiometric alloys based on NiMnIn have drawn attention due to the coupled first order magnetic and structural transformation, and the large magnetocaloric entropy associated with this martensitic transformation. We have analyzed materials with compositions NiMnIn, NiMnInand NiMnIn, which differ in that the former exhibits a paramagnetic to antiferromagnetic transition, while the others exhibit an additional ferromagnetic transition. Our results show that in the Nimaterials, the total entropy change at the phase transition can be modeled solely according to a magnetic contribution due to local moments on the Mn sites. On the other hand, NiMnInincludes a larger contribution which can be described in terms of a magneto-elastic coupling. This we will discuss in terms of the Bean-Rodbell model and a renomalization of the Debye temperature coupled with magnetism. We will also discuss the low-temperature properties, which show divergent behavior including antiferromagnetic, ferrimagnetic and superparamagnetic behavior. [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z29.00006: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z29.00007: Magnetoelectric effect in non-centrosymmetric Kondo lattices Ilya Vekhter, Leonid Isaev We study magnetoelectric (ME) response in Kondo lattices without the center of inversion. In such materials the conduction electrons move under the influence of an odd in momentum spin-orbit interaction (SOI). The interplay between this SOI and Kondo screening enables manipulation of the net magnetization of the system by an applied electric field. As a simple model for this phenomenon, we consider a Kondo bilayer (a pair of two-dimensional Kondo lattices) with the Rashba-type SOI, and treat it within a generalized hybridization mean field theory. We demonstrate that the ME response, strongest inside the heavy-fermion phase, has a very pronounced dependence on the magnitude of the spin-orbit coupling. These results provide a new pathway to the ME effect in strongly correlated materials. [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z29.00008: High Field Pulsed Magnets for Neutron Scattering at the Spallation Neutron Source G. E. Granroth, J. Lee, E. Fogh, N. B. Christensen, R. Toft-Petersen, H. Nojiri A High Field Pulsed Magnet (HFPM) setup, is in use at the Spallation Nuetron Source(SNS), Oak Ridge National Laboratory. With this device, we recently measured the high field magnetic spin structure of LiNiPO$_4$. The results of this study will be highlighted as an example of possible measurements that can be performed with this device. To further extend the HFPM capabilities at SNS, we have learned to design and wind these coils in house. This contribution will summarize the magnet coil design optimization procedure. Specifically by varying the geometry of the multi-layer coil, we arrive at a design that balances the maximum field strength, neutron scattering angle, and the field homogeneity for a specific set of parameters. We will show that a 6.3kJ capacitor bank, can provide a magnetic field as high as 30T for a maximum scattering angle around 40$^\circ$ with homogeneity of $\pm4\%$ in a 2mm diameter spherical volume. We will also compare the calculations to measurements from a recently wound test coil. [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z29.00009: Mechanism of the magnetically-induced ferroelectricity in (ND$_{4})_{2}$[FeCl$_{5}$$\cdot$D$_{2}$O] multiferroic molecular compound J. Alberto Rodriguez-Velamazan, Oscar Fabelo, Angel Millan, Javier Campo, Roger Johnson, Laurent Chapon In magnetoelectric multiferroic materials, a subtle competition between different magnetic interactions is at the origin of multiferroicity, as this competition is the responsible of the formation of a magnetic structure able to induce ferroelectric polarization. The difficulty in controlling this interplay of interactions explains the scarce number of these materials reported. (ND4)2[FeCl5$\cdot$D2O] is a rare case where improper ferroelectricity has been observed in a molecular material. We have used single crystal and powder neutron diffraction to study its crystal and magnetic structures and hence determine the mechanism of multiferroicity in this compound. From the crystal structure determinations above and below 79 K, we have observed an order-disorder phase transition, which is related with the ordering of the ammonium counterion. Below TN, at zero magnetic field, we have determined the magnetic structure, which corresponds with a cycloidal spin arrangement where the magnetic moments are contained in the ac-plane and propagate along the c-direction. All the cycloids in (ND4)2[FeCl5$\cdot$D2O] compound have the same chirality and therefore the ferroelectricity can be explained via the inverse Dzyaloshinskii-Moriya mechanism. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z29.00010: Spin dynamics in the bilinear-biquadratic spin-1 Heisenberg model on the triangular lattice Stefan Wessel, Annika Voell We study thermodynamic properties as well as the dynamical spin and quadrupolar structure factors of the SU(2)-symmetric spin-1 Heisenberg model with bilinear-biquadratic exchange interactions on the triangular lattice. Based on a sign-problem-free quantum Monte Carlo approach, we access both the ferromagnetic and the ferroquadrupolar ordered, spin nematic phase as well as the SU(3)-symmetric point that separates these phases. Signatures of Goldstone soft-modes in the dynamical spin and the quadrupolar structure factors are identified, and the properties of the low-energy excitations are compared to the thermodynamic behavior observed at finite temperatures as well as to Schwinger-boson flavor-wave theory. [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z29.00011: The Higgs amplitude mode in the ferromagnetic phase of ZrZn$_{2}$ Linden Hayes, Kevin Bedell Bedell and Blagoev derived the collective spin modes of a ferromagnet in the low temperature limit [1]. This derivation yielded both the Nambu-Goldstone mode, already known for ferromagnetic systems, as well as another massive, gapped mode. This mode was found to be associated with fluctuations in the order parameter by Zhang, Farinas, and Bedell and was identified as the Higgs amplitude mode, which had never been identified in a weak ferromagnetic system [2]. They applied their model to the weak ferromagnet MnSi using existing experimental results for the material, though an experiment run to detect the Higgs mode in MnSi has thus far been inconclusive. In this talk we fit the model to ZrZn$_{2}$, a weak itinerant ferromagnet that has a wealth of experimental data. These data are applied to the model in order to describe the dynamical structure function defined by Ferromagnetic Fermi Liquid Theory, which yields two distinct peaks from the Nambu-Goldstone mode and the Higgs amplitude mode. We then show that this result can be used in a neutron scattering experiment to detect the Higgs amplitude mode in ZrZn$_{2}$. [1] Bedell and Blagoev, Phil. Mag. Lett. (2001) [2] Zhang, Farinas and Bedell, arXiv: 1305.4674 (2014) [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z29.00012: Anisotropic Non-Spin Wave Magnetic Excitations in FeGe$_2$ Carl Adams, Matthew Stone, Stuart Calder, Douglas Abernathy Inelastic neutron scattering measurements on a single crystal sample of FeGe$_2$ were performed. FeGe$_2$ is a metallic antiferromagnet with a tetragonal crystal structure. It has been known for sometime that the excitations along the tetragonal c-axis extend over a broad range of energies with an effective exchange constant that is 15 times greater in magnitude that the exchange constant in the basal plane. Despite this unusual feature the observed excitations could be described using a classical Heisenberg Hamiltonian. Our recent measurements using both time-of-flight and reactor based inelastic scattering reveal magnetic excitations that are again highly anisotropic, but in this case the anisotropy is evident in the basal plane. The excitations are quite intense, broadly dispersive, and along the (110) directions. Neither the anisotropy nor the excitations themselves can be described in the context of a spin-wave model. We suspect that these excitations are related to a Fermi surface effect. [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z29.00013: Evolution of spin wave excitations with Co-doping in the spinel MnV$_2$O$_4$ Steven Hahn, Jie Ma, Jun Hee Lee, Tao Hong, Huibo Cao, Adam Aczel, Zhiling Dun, Matthew Stone, Wei Tian, Yiming Qiu, John Copley, Haidong Zhou, Randy Fishman, Masaaki Matsuda Spin waves were measured at several levels of Co-doping in the spinel system MnV$_2$O$_4$ by inelastic neutron scattering and analyzed with first-principles-guided spin models. Co-doping creates a rich phase diagram encompassing the transition from localized- to itinerant-electron regimes. Increasing Co concentration weakens the single-ion anisotropy and increases both the magnitude and isotropy of the nearest-neighbor exchange interactions. First principles calculations emphasize the the distinctly different microscopic origins of the two-in-two-out magnetic structure at the Mn-rich and Co-rich limits. [Preview Abstract] |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z29.00014: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 2:03PM - 2:15PM |
Z29.00015: Magnetic fluctuations and specific heat in Na$_x$CoO$_2$ near a Lifshitz Fermi surface topological transition Sergey Slizovskiy, Joseph Betouras, Andrey Chubukov We analyze the temperature and doping dependence of the specific heat $C(T)$ in Na$_x$CoO$_2$. This material was conjectured to undergo a Lifshitz -type topological transition at $x =x_c =0.62$, in which a new electron Fermi pocket emerges at the $\Gamma$ point, in addition to the existing hole pocket with large $k_F$. The data show that near $x =x_c$, the temperature dependence of $C(T)/T$ at low $T$ gets stronger as $x$ approaches $x_c$ from below and then reverses the trend and changes sign at $x \geq x_c$. We argue that this behavior can be quantitatively explained within the spin-fluctuation theory. We show that magnetic fluctuations are enhanced near $x_c$ at momenta around $k_F$ and their dynamics changes between $x \leq x_c$ and $x >x_c$, when the new pocket forms. We demonstrate that this explains the temperature dependence of $C(T)/T$. We show that at larger $x$ ($x > 0.65$) the system enters a magnetic quantum critical regime where $C(T)/T$ roughly scales as $\log T$. This behavior extends to progressively lower $T$ as $x$ increases towards a magnetic instability at $x \approx 0.75$. [Preview Abstract] |
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