Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H41: Cooperative Phenomena: Phase Transition in Magnetic Systems |
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Sponsoring Units: GMAG Chair: Tom Kaplan, Michigan State University Room: LACC 150A |
Tuesday, March 22, 2005 8:00AM - 8:12AM |
H41.00001: Spin-wave spectrum and magnetic phase diagram of hexagonal HoMnO$_{3}$ O.P. Vajk, M. Kenzelmann, J.W. Lynn, S.B. Kim, S.-W. Cheong We present neutron scattering measurements of the magnetic order and spin-wave dynamics in single crystals of hexagonal HoMnO$_{3}$, an antiferromagnetic ferroelectric material. Ferroelectric ordering of Ho$^{3+}$ ions along the c axis occurs around 875K, and the triangular lattice of S=2 moments on Mn$^{3+}$ ions order in a 120$^{\circ}$ antiferromagnetic structure below 72K. Below 40K, these moments undergo a spin reorientation transition to another 120$^{\circ}$ structure, with significant coupling between the two order parameters observable at the transition. We have mapped out the spin-wave spectrum, and our results are well-described by a Heisenberg model on a triangular lattice with a nearest-neighbor exchange of 2.44 meV and a temperature-dependent anisotropy. Measurements of the spin structure in a magnetic field applied along the ferroelectric ordering direction reveal multiple spin-reorientation transitions along with significant hysteresis. Applied electric field results will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 8:12AM - 8:24AM |
H41.00002: Magnetic phase diagram of RFe$_4$Al$_8$ (R$=$Dy,Gd,Lu) single crystals M. Angst, A. Kreyssig, A.I. Goldman, P.C. Canfield Single crystals of RFe$_4$Al$_8$ (R$=$Dy,Gd,Lu) were grown using self-flux at high temperatures $T$ and investigated by magnetization, electrical transport, and x-ray measurements. Crystals grow typically in form of long needles ($\| [001]$) with facets $\bot [110]$ and good crystallinity. Iron moments order below $T_N \approx 175\,{\rm K}$ (R$=$Dy), $150\,{\rm K}$ (R$=$Gd) and $195\,{\rm K}$ (R$=$Lu). Measurements on LuFe$_4$Al$_8$ indicate that fields $H$ up to $7\,{\rm T}$ do not noticeably shift $T_N$. There is no clear signature of Dy moment ordering at lower $T$, in zero field. However, there is evidence of a metamagnetic transition (likely first order) to a low $T$ high $H$ ($\bot[001]$) ferrimagnetic phase, attributed to Dy ordering. Gd moments order via a first order transition at about $30\,{\rm K} $ ($H=0$). The transition is shifted to lower $T$ by applying $H\|[001]$ (to zero temperature in about $1\,{\rm T}$). The resistivity along $[001]$ jumps to a higher value below the transition. The $H-T$ phase diagrams as well as the need for the microscopic determination of the magnetic structure will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 8:24AM - 8:36AM |
H41.00003: The magnetic phase diagram of single-crystal antiferromagnetic Ce$_2$Fe$_{17}$ Y. Janssen$^1$, Y. A. Mozharivskyj$^{1,2}$, S. Jia$^{1,3}$, P. C. Canfield$^{1,3}$ The binary intermetallic compound Ce$_{2}$Fe$_{17}$ crystallizes in the rhombohedral Th$_{2}$Zn$_{17}$-type structure. Unlike other R$_{2}$Fe$_{17}$ (R=rare earth), which are ferro- or ferrimagnetic, Ce$_{2}$Fe$_{17}$ displays antiferromagnetic behavior below its ordering temperature of $\sim $ 215 K. At lower temperatures, a second transition takes place, to an antiferromagnetic state for pure, and a ferromagnetic state for impure $_{ }$(e.g. Ta doped) Ce$_{2}$Fe$_{17}$. For antiferromagnetic samples, the lower temperature transition is accompanied by a feature in the resistivity that is consistent with the formation of a superzone gap. This study focuses on single crystals of pure Ce$_{2}$Fe$_{17}$. As a first step, the magnetic phase diagram for antiferromagnetic Ce$_{2}$Fe$_{17}$ has been determined by means of magnetization and resistance measurements. Ames Laboratory is operated for the US Department of Energy by Iowa State University under contract number W-7405-ENG-82. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H41.00004: Critical Behavior of a Compressible Ising Antiferromagnet at Constant Pressure Xiaoliang Zhu, F. Tavazza, D.P. Landau The effect of elastic degrees of freedom in Ising models has long been a subject of interest. %Theoretical studies on compressible Ising models were done by Bergman and Halperin\footnote{D. J. Bergman and B. I. Halperin, Phys. Rev. B {\bf 13}, 2145 (1976)}, A review of the theoretical studies was given by D\"unweg\footnote{B. D\"unweg, Habilitationsschrift, MPIP, Mainz, Germany (2000)} who predicts different behaviors for ferromagnetic and antiferromagnetic systems. %and recently by D\"unweg\footnote{B. D\"unweg, Habilitationsschrift, Max-Planck-Institut f{\"u}r Polymerforschung, Mainz, Germany (2000)}. %Using Monte Carlo simulations, Laradji and Landau\footnote{Phys. Rev. B {\bf 51}, 4894 (1995)} %M. Laradji and D. P. Landau, Phys. Rev. B {\bf 51}, 4894 (1995)} %studied the phase behavior of ferromagnet at constant pressure %and their results agree well with the theoretical predictions, %but Tavazza {\em et el.} \footnote{Phys. Rev. B {\bf xx}, xx (2004)} F. Tavazza and D. P. Landau and J. Adler, Phys. Rev. B {\bf xx}, xx (2004)} %found discrepancy with the theory on ferromagnet at constant volume. Numerical results for ferromagnetic Ising models don't always agree with the theoretical predications \footnote{M. Laradji and D. P. Landau, Phys. Rev. B {\bf 51}, 4894 (1995)} \footnote{B. D{\"u}nweg and D. P. Landau, Phys.\ Rev.\ B {\bf 48}, 14182 (1993)} \footnote{F. Tavazza, D. P. Landau and J. Adler, Phys. Rev. B (in press)}, and to further test the theory we present our Monte Carlo simulations of an elastic antiferromagnetic Ising model. Spins interact via a Stillinger-Weber-like potential and data were obtained over a wide range of system sizes (from 512 to 110,592 spins). We find a phase diagram that has a line of 2nd order transitions between ordered and disordered states. Our analysis shows that this transition belongs to the universality class of the rigid 3-D Ising model, but the possibility of a slow crossover toward a first order transition cannot be completely ruled out. We also examine the distributions of interaction constants in the effective Landau-Ginzburg-Wilson Hamiltonian. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H41.00005: Transverse Magnetic Susceptibility in an Ising Spin Liquid D. M. Silevitch, C. Ancona-Torres, T. F. Rosenbaum, G. Aeppli We study the AC vector magnetic susceptibility of the dilute Ising magnet $\mathrm{LiHo_{0.045}Y_{0.955}F_4}$. Driving the system into the nonlinear regime excites coherent oscillations of hundreds of spins. We find that these clusters can be projected from the Ising axis into the transverse plane by applying a DC transverse field. The lineshape of the oscillations changes as a function of this transverse field, indicating the presence of additional excitation modes. We study as well the time dependence of the excitation spectra and find a pronounced asymmetry along and transverse to the Ising axis. The spin liquid not only permits the encoding of information labeled by frequency, but is amenable to manipulation of quantum superposed states. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H41.00006: Half-Magnetization Plateau in the Pyrochlore Spin-Peierls Model Yukitoshi Motome, Nic Shannon, Karlo Penc We present our theoretical work on the spin-Peierls model on a pyrochlore lattice under the external magnetic field. By using Monte Carlo calculations, we obtain the phase diagram in the parameter space of the temperature, the external magnetic field, and the strength of the spin-lattice coupling. We have found that a collinear trigonal phase with 3-up and 1-down spin configuration in each tetrahedral unit of the pyrochlore lattice, which has been revealed in the mean-field result for the ground state [1], remains robust in a wide regime of the finite-temperature phase diagram. This collinear phase gives rise to a half-magnetization plateau in the magnetization process. The results are compared with the recent experimental results in the chromium spinel oxides, such as CdCr$_2$O$_4$ and HgCr$_2$O$_4$ [2]. [1] K. Penc, N. Shannon, and H. Shiba, Phys. Rev. Lett. {\bf 93}, 197203 (2004). [2] H. Ueda {\it et al}, unpublished. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H41.00007: Bose Glass Phase in Heisenberg Dimer Systems Omid Nohadani, Stefan Wessel, Stephan Haas We study magnetic-field-induced antiferromagnetic order in cubic dimer systems with bond disorder. In the absence of randomness, their phase diagram features a dimer spin liquid regime at small fields $h < h_{c1}$, an antiferromagnetically ordered phase at intermediate fields $h \in [h_{c1},h_{c2}]$, and a fully polarized regime at large fields beyond $h_{c2}$. Using stochastic series expansion quantum Monte Carlo simulations, the scaling properties at the quantum critical points are shown to be mean-field-like. Furthermore, we demonstrate that in the presence of bond disorder, a new Bose Glass phase separates the dimer spin liquid regime from the antiferromagnetically ordered phase. This resembles strongly the ``triplon localization" which was recently reported for {\rm Tl$_{1-x}$K$_x$CuCl$_3$} where $K$ is randomly substituted for $Tl$.[1] [1] Y. Shindo and H. Tanaka, J. Phys. Soc. Jpn. Vol.{\bf 73} No.10 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H41.00008: NMR Investigation of 2D Frustrated Heisenberg Antiferromagnet - V.F. Mitrovi{\'c}, M-A. Vachon, A.P. Reyes, R. Coldea The \mbox{Cs$_2$CuCl$_4$} compound represents the first clean experimental realization of a 2D \mbox{spin-$1/2$} frustrated Heisenberg antiferromagnet. Detailed neutron scattering studies\footnote{R. Coldea {\it et al.}, Phys. Rev. Lett. {\bf 86}, 1335, (2001); R. Coldea, {\it et al.}, Phys. Rev. Lett. {\bf 88}, 137203, (2002).} have established that this system exhibits many exotic properties such as spin-1/2 spinon quasiparticles and unusual phases stabilized by the applied magnetic field. NMR measurements are crucial for providing insight into the microscopic mechanism governing these phase transitions, induced by low-energy processes. We report \mbox{$^{133}$Cs} spectra and \mbox{$T_1^{-1}$} rate measurements of Cs$_2$CuCl$_4$ at low temperatures in magnetic fields up to 15 T. We find that the rate exhibits a strong field dependence. Temperature and field dependence of the shift data will also be presented. Implications of these measurements to the microscopic spin structure and low energy spin dynamics in this compound will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H41.00009: $\mu$SR study of $\mathrm{LiHo_xY_{1-x}F}$ J. Rodriguez, S. Dunsiger, G.J. MacDougall, G.M. Luke, Y.J. Uemura, P. Russo, A. Savici, J. Quilliam $\mathrm{LiHo_xY_{1-x}}F$ is a dipolar ferromagnet with a complex phase diagram. The undiluted compound $\mathrm{LiHoF}$ is an Ising ferromagnet with a transition temperature of 1.53 K with a $\hat{c}$-easy axis. A very interesting feature of the phase diagram arises when $\mathrm{x \sim 0.045}$. Magnetic ac-susceptibility measurements have shown a narrowing of $\chi^{\prime \prime}$ in the low frequency side as the temperature is decreased. This narrowing is in contrast to the spin glass behavior that the material was expected to have at this doping and, due to this, this new phase is called the ``anti-glass'' phase. Even though there have been studies on this material (neutron scattering, magnetic susceptibility and specific heat) the phase diagram has not been fully mapped and some phases, like the antiglass phase, are not well understood. We have studied this system using $\mu$SR. This local magnetic probe allows to measure the microscopic distribution of magnetic fields as well as their dynamics. In this talk I will present our results on the $x=1$ (Ising ferromagnet) and $x=0.045$ (anti-glass) samples. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H41.00010: Exact Solution of the Heinsenberg-Kondo Quantum Spin Glass, Showing that an Infinite Order Quantum Phase Transition Occurs Between the Quantum Spin Glass and Kondo Spin Quenched Phases at a Temperature of Order Ten Times Lower than that Given by Mean Fi Robert Schrieffer, Bernard Coqblin We have exactly solved the Heisenberg-Kondo model of a quantum spin glass, showing that an infinite order phase transition occurs between the quantum spin glass phase and the Kondo spin quenched phase at a temperature $T_c $ of an order of magnitude smaller than that given by mean field theory, which predicts a fast order phase transition. The phase diagram will be presented as a function of the ratio of the Kondo exchange $J^K$and Heisenberg exchange $J^H$ divided by the Fermion band width $W$. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H41.00011: SU(2)-invariant spin 1/2 Hamiltonians with RVB and other valence bond phases Kumar S. Raman, Shivaji L. Sondhi, Roderich Moessner We construct a family of rotationally invariant, local, S=1/2 Hamiltonians of the Klein-AKLT variety that exhibit ground state manifolds spanned by nearest neighbor valence bond states. To these, we add perturbations which drive the systems into phases modelled by well understood quantum dimer models on the corresponding lattices. We compute within the valence bond manifold by treating the overlap between states as a small expansion parameter. We introduce a bond decoration scheme which makes this overlap expansion asymptotically exact. This leads to i) a Z$_{2}$ RVB phase on a decorated two dimensional pentagonal lattice and ii) a Cantor deconfined region on a decorated two dimensional honeycomb lattice. In case (i), we give strong evidence that the spinon and vison excitations are gapped. This work is the first demonstration of RVB liquid physics in an SU(2)-invariant spin system in the thermodynamic limit. [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H41.00012: A new method for analyzing second-order phase transitions applied to the ferromagnetic transition of a polaronic system John J. Neumeier, Y-K. Yu, J. A. Souza, H. Terashita, R. F. Jardim A new thermodynamic method for analyzing second-order phase transitions in condensed matter systems is presented. It utilizes heat capacity and thermal expansion data simultaneously and predicts the critical temperature's pressure dependence. Application to the polaronic system La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ reveals second-order behavior with an unusually large critical exponent. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H41.00013: Fermi liquid description of the helical magnetism, phase transitions, and collective modes in MnSi. Jason Jackiewicz, Kevin Bedell We look at a model for the helical magnet MnSi that incorporates the use of Fermi liquid parameters to describe the magnetic interactions. The novel phases of this material can be well described in this context, as well as the phase transitions between them. We also solve the Landau kinetic equation in the regimes of interest to develop the collective mode dispersions and show that there is interesting non-trivial behavior beyond the usual Goldstone mode. [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H41.00014: Effect of anisotropies on magnetic quantum phase transitions B. Normand, M. E. Zhitomirsky We consider the effects of superexchange anisotropy and Dzyaloshinskii--Moriya (DM) interactions on the field-- and pressure--driven magnetic phase transitions in quantum dimer systems. These two types of anisotropy term are introduced separately on the intra-- and interdimer bonds, with spatial symmetries both commensurate and incommensurate with the magnetic order, to determine the alterations of the phase diagram and magnetic excitations. Exchange anisotropy causes a general modification of the critical properties at the field--driven transition from the XY universality class (Bose--Einstein condensation) to Ising behaviour. DM interactions may act either to remove the field--driven transition completely or to create a new transition between two different antiferromagnetic states. For certain cases we compute the magnetisation and the evolution of the excitation spectra as functions of field and pressure. Our results are directly relevant to recent measurements on TlCuCl$_3$ and CuHpCl, and may assist in the further understanding of a number of other quantum dimer systems, including BaCuSi$_2$O$_6$, Sr$_2$Cu(BO$_3$)$_2$ and PHCC. [Preview Abstract] |
Tuesday, March 22, 2005 10:48AM - 11:00AM |
H41.00015: Phase transitions and symmetries of quantum dots in applied magnetic fields Wolfgang Geist, M.Y. Chou The ground state of harmonically confined two dimensional electronic systems can be changed by varying the strength of an applied external magnetic field. At certain magnetic field strength, which depends on the confinement, a maximum density droplet (MDD) is formed where all electrons are spin polarized and the system has angular momentum value $L_{MDD}=N(N-1)$, where N is the particle number. A good approximation to the MDD many body ground state is obtained by limiting single particle Fock Darwin states to the lowest Landau level (LLL). In this subspace the MDD is a single determinant with single particle angular momentum values $l_i=0\cdots N-1, i=1\cdots N$. We use variational and diffusion quantum Monte Carlo methods in order to study the instability of the MDD as well as phase transitions and pair correlation densities for several phases beyond the MDD. Monte Carlo results are compared with results obtained from diagonalizing the Hamiltonian in a subspace limited to the two Slater determinants that result in the lowest Coulomb matrix elements. We find good agreement between these two methods and unique configurations for the lowest energy determinants. [Preview Abstract] |
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