Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P32: Spin Glasses |
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Sponsoring Units: GMAG Chair: G. Luke, McMaster University Room: Baltimore Convention Center 329 |
Wednesday, March 15, 2006 11:15AM - 11:27AM |
P32.00001: Effects of high magnetic fields on the spin-glass states in disordered manganites Noboru Miura, Peter Kerschl, Nadja V. Kozlova, Konstantin Nenkov, Kathrin Doerr, Alexander Kirste, Michael von Ortenberg, Daisuke Akaboshi, Yasuhide Tomioka, Yoshinori Tokura Magnetization and magnetoresistance were measured in single crystals of random alloys RE$_{1-x}$AE$_{x}$MnO$_{3}$ (RE and AE denote the rare-earth and alkaline-earth ions at the perovskite A-site) in pulsed high magnetic fields up to 50 T with a long time duration ($\sim $10 ms) and up to 140 T with a short time duration ($\sim \mu $s). The crystals exhibit the spin glass behaviors at low temperatures in zero field. In high magnetic fields, Sm$_{1-x}$(Ba$_{1-y}$Sr$_{y})_{x}$MnO$_{3}$ showed prominent metamagnetic transitions, whereas RE$_{1-x}$Ba$_{x}$MnO$_{3}$ (RE=Sm, Eu, Gd) showed a smooth magnetization saturation with just kinks in the derivative of the magnetization. Moreover, in the metamagnetic phase transitions, peculiar time dependence and the pre-history dependence were found in the hysteresis of the magnetization. It was also found that the magnetization is accompanied with a colossal magnetoresistance. These behaviors were interpreted in terms of the developments of the clusters and the orbital orders by magnetic fields, which are dependent on the average A-site ionic radius and the randomness. [Preview Abstract] |
Wednesday, March 15, 2006 11:27AM - 11:39AM |
P32.00002: What is new for spin-glass in a quasi-2D system Wei Bao, Ying Chen, Yiming Qiu, J.E. Lorenzo, J.L. Sarrao, Derek Ho, Min Y. Lin In conventional spin glasses, magnetic interaction is not strongly anisotropic and the entire spin system is believed to be frozen below the spin-glass transition temperature. Along {\em any} direction, spin correlations are highly disordered. In La$_2$Cu$_{0.94}$Li$_{0.06}$O$_4$, for which the in-plane exchange interaction dominates the interplane one, only a fraction of spins with antiferromagnetic correlations extending to neighboring planes become spin-glass. The remaining spins with only in-plane antiferromagnetic correlations remain spin-liquid at low temperature. Spin correlations are highly disordered only along the interlayer direction, but highly ordered in-plane. Such a novel partial spin freezing out of a two-dimensional spin-liquid observed in this cold neutron scattering study is likely due to a delicate balance between disorder and quantum fluctuations in the quasi-two dimensional $S$=1/2 Heisenberg system. [Preview Abstract] |
Wednesday, March 15, 2006 11:39AM - 11:51AM |
P32.00003: Spin Glasses at the Bond Percolation Threshold Emiliano Marchetti, Stefan Boettcher Low energy excitations for the Edwards-Anderson model on hyper- cubic lattices at the bond percolation threshold $p_c$ are investigated. At $T=0$, $p_c$ separates paramagnetic and spin glass phases. At the ``edge'' of the ordered state, these excitations are characterized by a distinct scaling exponent. This exponent allows to determine the shape of the phase boundary, $T_c(p)\sim(p-p_{c})^\phi$, for $p\to p_c^+$, which is experimentally measurable in $d=3$. At $p_c$, very large spin glass systems can be studied with an {\it exact} reduction algorithm\footnote{Europhys. Lett. {\bf 67}, 453 (2004)} to produce accurate scaling behavior. For more information, see http://www.physics.emory.edu/faculty/boettcher/ [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P32.00004: Spin-glass correlations in classical dipoles Vadim Oganesyan We present analytic mean-field and high-temperature expansion results on thermodynamics of classical Ising dipoles in the strongly diluted regime. These display a broad distrubution of couplings resulting in strong enhancement of the spin-glass transition temperature from the naive estimate. We comment on reliability of these results and their relationship to other approaches. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P32.00005: Computing Barriers in Spin Glasses A. Alan Middleton The energy barriers $E_B$ between low-lying states in spin glasses are expected to scale as a power of the system size: $E_B \sim N^{\psi/d}$ for $N$ spins in a $d$-dimensional spin glass. Whether the barrier exponent $\psi$ is equal to the stiffness exponent $\theta$ (where the cost of minimal large scale excitations scales as $N^{\theta/d}$) is an unsolved question in general. In an attempt to solve this question with some rigor in large theoretical spin glass samples, numerical simulations for barriers in spin glasses on a hierarchical lattice have been carried out, using an exact algorithm for computing the barrier to the monotone growth of connected domains. The resulting $\psi$ is sensitive to the distribution of weights on the bonds between spins. These distributions give different weights to bonds that appear at different stages of the hierarchical generation of the lattice. Results for $\psi$ and $\theta$ will be presented for various lattices, including variations of Cayley trees and lattices that satisfy Migdal-Kadanoff approximations, and distributions that plausibly emulate finite-dimensional spin glasses. [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P32.00006: The End of Aging in a Spin Glass Gregory Kenning, Gilberto Rodriguez, Raymond Orbach Aging phenomena in complex systems has been used as an important tool to investigate the physics of complexity. In particular aging effects in spin glasses, measured using the Thermoremenant Magnetization (TRM) decays, have been instrumental as a probe of complex equilibrium and non-equilibrium dynamics. Current theoretical and experimental analysis suggest that the TRM decay of spin glasses is mainly composed of two terms; The ``stationary'' term which does not depend on the sample history and dominates the short time decay ($<$1s) and a long time aging term which depends on the samples history. We report finding that aging found in spin glass materials, has a finite lifetime and that after aging has ended there is a third component of the magnetization decay. This decay is independent of the waiting time, logarithmic in nature and part of the same mechanism that produces aging. Finally we find that the logarithmic decay implies a maximum aging time (MAT) that is very strongly dependent on temperature and ranges from short times near the spin glass transition temperature to many times the current best estimates of the age of the universe for low temperature. [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 12:39PM |
P32.00007: Memory and aging effect in hierarchical spin orderings of stage-2 CoCl$_{2}$ graphite intercalation compound Masatsugu Suzuki, Itsuko Suzuki, Motohiro Matsuura Stage-2 CoCl$_{2}$ graphite intercalation compound undergoes two magnetic phase transitions at $T_{cl}$ (= 7.0 K) and $T_{cu} $ (= 8.9 K). The aging dynamics of this compound is studied near $T_{cl}$ and $T_{cu}$. The intermediate state between $T_ {cl}$ and $T_{cu}$ is characterized by a spin glass phase extending over ferromagnetic islands. A genuine thermoremnant magnetization (TRM) measurement indicates that the memory of the specific spin configurations imprinted at temperatures between $T_{cl}$ and $T_{cu}$ during the field-cooled (FC) aging protocol can be recalled when the system is re-heated at a constant heating rate. The zero-field cooled (ZFC) and TRM magnetization is examined in a series of heating and reheating process. The magnetization shows both characteristic memory and rejuvenation effects. The time $(t)$ dependence of the relaxation rate $S_{ZFC}(t)=(1/H)$d$M_{ZFC}(t)$/d$\ln t$ after the ZFC aging protocol with a wait time $t_{w}$, exhibits two peaks at characteristic times $t_{cr1}$ and $t_{cr2}$ between $T_{cl}$ and $T_{cu}$. An aging process is revealed as the strong $t_{w}$ dependence of $t_{cr2}$. The observed aging and memory effect is discussed in terms of the droplet model. [Preview Abstract] |
Wednesday, March 15, 2006 12:39PM - 12:51PM |
P32.00008: Bose glass vs. Mott glass in site-diluted S=1 Heisenberg antiferromagnets Tommaso Roscilde, Stephan Haas Making use of large-scale quantum Monte Carlo simulations, we investigate the ground-state phase diagram of the square-lattice S=1 Heisenberg antiferromagnet with strong single-ion anisotropy and in presence of site-dilution of the magnetic lattice. Mapping the spins onto Holstein-Primakoff bosons, the single-ion anisotropy is seen to play the role of a repulsive on-site potential for the bosons. The clean limit of the model shows an anisotropy-driven quantum phase transition from an XY ordered (superfluid) phase to a quantum disordered (Mott insulating) phase. A similar transition is also driven by the application of a uniform field on the disordered state. Adding site dilution to the model, the non-trivial interplay between quantum fluctuations and lattice randomness gives rise to a novel quantum-disordered Mott-glass phase in zero field, with a gapless spectrum and yet a vanishing uniform susceptibility. Upon applying a field, such phase is turned into a Bose glass, with gapless spectrum and finite susceptibility. The above picture is directly relevant for experiments on doped quasi-low-dimensional Ni compounds, such as the recently investigated NiCl$_2$-4SC(NH$_2$)$_2$ (V.S. Zapf et al., condmat/0505562). [Preview Abstract] |
Wednesday, March 15, 2006 12:51PM - 1:03PM |
P32.00009: Impurity effects on frustrated ferro- and ferrimagnets in one dimension Masanori Kohno, Xiao Hu We have investigated impurity effects on magnetization for frustrated one-dimensional ferro- and ferrimagnets. Using the density-matrix renormalization group method and the exact diagonalization method, we confirmed that the magnetization decreases significantly by doping non-magnetic impurities. In a special case, the magnetization can vanish due to a single impurity in finite chains. Introducing the picture of magnetic domain inversion, we numerically investigated the impurity-density dependence of magnetization. In particular, we show that the magnetization substantially decreases down to less than 60{\%} from that of the corresponding pure system by doping an infinitesimal density of impurities. We also formulate conditions for the materials which may show this anomalous impurity effect. [Preview Abstract] |
Wednesday, March 15, 2006 1:03PM - 1:15PM |
P32.00010: Frustrating interactions in oxides induced by non-magnetic impurities Shiu Liu, Sasha Chernyshev An antiferromagnetic host material doped with non-magnetic impurities, such as Zn-doped La$_2$CuO$_4$, is generally believed to represent an excellent model case of the site-dilution of a magnetic substance. We demonstrate that there exist a significant qualitative correction to such a picture: an impurity can induce substantial frustrating interactions between spins that are nearest neighbors of the impurity site. Not only this effect explains discrepancies between experimental data and the site-dilution theory, but it could also lead to some important modification of the behavior of doped antiferromagnets close to the percolation. We study the 2D, $S=1/2$ copper-oxide plane with Zn impurities starting from the microscopic three-band Hubbard model. We show that, for a wide range of the model parameters, the substantial superexchange interactions between the next- and next-next-nearest neighbor Cu spins around the impurity site can be generated via the virtual transitions through the oxygen orbitals. Surprisingly, the interaction across the impurity $J''_{Zn}$ is greater than the next-nearest neighbor interaction $J'_{Zn}$ due to a partial cancellation of the super- and the cyclic exchanges for the latter. This study is completed by the $T$-matrix calculation of the staggered magnetization $M(x)$ as a function of Zn doping $x$. The predicted range of $J'_{Zn}$ and $J''_{Zn}$ agrees with the values needed to explain experimental deviation of $M(x)$ from the results of the site-dilution theories. [Preview Abstract] |
Wednesday, March 15, 2006 1:15PM - 1:27PM |
P32.00011: Spin Polarization Measurments of Co$_{1-x }$-- Pt$_{x}$ alloys by Point Contact Andreev Reflection Spectroscopy Muhammad Faiz, Raghava Panguluri, B. Nadgorny, Christian Kaiser, Stuart S. P. Parkin Recently Kaiser \textit{et al.,}$^{1}$ compared the spin polarization measured by spin resolved tunneling spectroscopy (Tedrow-Meservey) and the magnetic moment of Co$_{1-x }$-- Pt $_{x}$ alloys. We have measured the transport spin polarization, P$_{c}$ and magnetic properties of the same series of samples using Point Contact Andreev Reflection Spectroscopy (PCAR). All films with x varying from 0 to 100{\%} and a thickness of $\sim $1000 {\AA} were grown on Si substrates covered with $\sim $250 {\AA} of SiO$_{2}$ by magnetron sputtering. We will present a correlation between spin polarization and magnetization for this series of magnetic alloys and compare our results with the ones obtained in Ref. [1]. 1. C. Kaiser \textit{et al.}, PRL \textbf{94} 247203 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 1:27PM - 1:39PM |
P32.00012: Charge degrees of freedom in frustrated lattices Joseph Betouras, Frank Pollman, Kirill Shtengel, Peter Fulde We explore systematically the charge degrees of freedom in frustrated lattices. A model of spinless fermions on a checkerboard lattice with nearest-neighbor hopping t and Coulomb repulsion V is used at half and quarter fillings. Quantum fluctuations reduce the classical macroscopic degeneracy. For the strongly correlated limit V >> t, an added electron decays into two quasiparticles with fractional charge. We study the classical correlations and, by means of quantum field theory as well as axact diagonalisation, we also investigate the possibility of a confined or deconfined phase as well as the statistics of these quasiparticles. [Preview Abstract] |
Wednesday, March 15, 2006 1:39PM - 1:51PM |
P32.00013: Probing the Almeida-Thouless line away from the mean-field model Helmut G. Katzgraber, A. Peter Young In order to test the existence of a spin-glass phase in a field at finite temperatures, results of Monte Carlo simulations of the one-dimensional long-range Ising spin glass with power-law interactions in the presence of a (random) field are presented. By tuning the exponent of the power-law interactions, we are able to scan the full range of possible behaviors from the infinite-range (Sherrington-Kirkpatrick) model to the short-range model. A finite-size scaling analysis of the correlation length indicates that there is no transition in a field with non-mean field critical behavior at zero field. This suggests that there is no Almeida-Thouless line for short-range Ising spin glasses away from the mean-field regime. [Preview Abstract] |
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