Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A15: Focus Session: Spin Freezing on Frustrating Lattices |
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Sponsoring Units: GMAG Chair: Peter Schiffer, Pennsylvania State University Room: Colorado Convention Center Korbel 4E |
Monday, March 5, 2007 8:00AM - 8:12AM |
A15.00001: Magnetic diffuse scattering in Tb3Ga5O12 and Tb3Al5O12 garnets Kazuya Kamazawa, Taku Sato, Despina Louca In geometrically frustrated systems, long-range magnetic ordering is usually suppressed down to very low temperatures. While in the so-called spin liquid phase, magnetic diffuse scattering is observed. The physical origin of such scattering is not well understood. Using neutron scattering and DC, AC magnetic susceptibility measurements on Tb3Ga5O12 and Tb3Al5O12, we observed magnetic diffuse scattering and unusual paramagnetic behavior. The M-T curve at high magnetic field does not coincide with the one at low fields below $\sim $10 K. This indicates that magnetic correlations are gradually developing around that temperature. The nonlinear response of the AC susceptibility suggests that the diffuse scattering may originate from critical scattering, which has also been observed in frustrated spinel systems. This might indicate the realization of soft (magnon) modes in the spin liquid phase. [Preview Abstract] |
Monday, March 5, 2007 8:12AM - 8:24AM |
A15.00002: Spin glass freezing in the disorder-free pyrochlores A$_{2}$Sb$_{2}$O$_{7 }$(A = Mn, Co, Ni) Haidong Zhou, B.W. Vogt, J.A. Janik, C.R. Wiebe, A. Harter, N.S. Dalal, J.S. Gardner The pyrochlores in the series A$_{2}$Sb$_{2}$O$_{7}$ (A = Mn, Co, Ni) have been synthesized and characterized as exhibiting spin glass transitions at T$_{G }$= 41, 4.5 and 2.6 K (for A = Mn$^{2+}$ S = 5/2, Co$^{2+}$ S = 3/2, and Ni$^{2+}$ S= 1 respectively) despite the lack of chemical disorder. Since the Curie-Weiss temperature remains essentially constant for all members in the series ($\theta $ = -40 K), the frustration index for these materials increases significantly as the moment size is reduced from f = 1.1 (Mn) to 9.3 (Co) to 14.6 (Ni). There is also a corresponding change in the spin dynamics measured by the shift in the AC susceptibility signal as a function of frequency. These new materials provide an avenue to investigate the effect of quantum fluctuations on the Heisenburg pyrochlore lattice in the low spin limit, and show that there is a dramatic change in the spin dynamics as the quantum regime is approached [Preview Abstract] |
Monday, March 5, 2007 8:24AM - 8:36AM |
A15.00003: Low temperature specific heat of geometrically frustrated Gd$_2$Sn$_2$O$_7$ Kate Ross, Jeffrey Quilliam, Chas Mugford, Linton Corruccini, Jan Kycia Previous measurements of the specific heat of Gd$_2$Sn$_2$O$_7$ showed a $T^2$ power law below a strongly first-order phase transition, though there was some indication of a deviation from this power law below 500 mK\footnote{P. Bonville \emph{et al.} J. Phys.: Cond. Mat. {\bf 15}, 7777 (2003).}. Theory has predicted that anisotropy due to the dipolar interaction leads to a gapped spin wave spectrum resulting in an exponential specific heat as $T$ approaches 0.\footnote{A. G. Del Maestro and M. J. P. Gingras, J. Phys.: Cond. Mat. {\bf 16}, 3339 (2004).} We will present specific heat measurements to below 100 mK. Preliminary results show a deviation from the $T^2$ law which supports the above theoretical model. [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 9:12AM |
A15.00004: Slow spin relaxation and zero point entropy in diluted and stuffed rare earth pyrochlores Invited Speaker: Work on the cooperative paramagnet ${Tb}_{2}{Ti}_{2}{O}_{7}$, and on the spin ices ${Dy}_{2}{Ti}_{2}{O}_{7}$ and ${Ho}_{2}{Ti}_{2}{O}_{7}$ has exposed new avenues of study into the nature of excitations and ordering in geometrically frustrated magnetic systems. Here, we will present low temperature AC susceptibility, magnetization, and heat capacity data on these materials, and on their diluted and stuffed variants, formed either by replacing some magnetic rare earth ions with non-magnetic $Y$ or $Lu$, or by replacing some $Ti$ with magnetic $Ln$ ions. Experimental results on diluted ${Tb}_{2}{Ti}_{2}{O}_{7}$ and ${Dy}_{2}{Ti}_{2}{O}_{7}$ will be discussed, which indicate the existence of slow spin relaxations ($\tau\geq0.001 s$) in both systems, but with opposing characters [1,2]. For the stuffed materials, we will show that additional $Ln$ ions cause the effective magnetic interaction to become stronger and increasingly antiferromagnetic. Additionally, low temperature AC susceptibility and heat capacity data will be presented to compare the different effects of stuffing on the ground states of the materials [3]. \newline \newline [1] \textit{Phys. Rev. Lett.} \textbf{96}, 027216 (2006). \newline [2] \textit{Phys. Rev. Lett.} \textbf{91}, 107201 (2003). \newline [3] \textit{Nature Phys.} \textbf{2}, 249 (2006). [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A15.00005: Effective Thermodynamics of Artificial Spin Ice Cristiano Nisoli, RuiFang Wang, Jie Li, William F. McConville, Paul E. Lammert, Peter Schiffer, Vincent H. Crespi We analyze the effective thermodynamics of artificial spin ice: a recently realized lattice of nanoscale single-domain ferromagnetic islands that are arrayed along the edges of a square lattice[1]. After demagnetization, the moments in this model system have a static disordered configuration similar to the frozen state of the spin ice materials. We demonstrate that this athermal state has extensive degeneracy and we introduce a formalism that can predict both the entropy and an effective temperature. The theory also predicts the populations of local states and short-distance correlations of this ice-like system with no adjustable parameters. \newline \newline References: \newline [1] R.~F. Wang, C.~Nisoli, R.~S. Freitas, J.~Li, W.~McConville, B.~J. Cooley, M.~S. Lund, N.~Samarth, C.~Leighton, V.~H. Crespi and P.~Schiffer, Nature (London) {\bf 439}, 303. [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 9:36AM |
A15.00006: Dynamics of Artificial Kagome `Spin Ice' In Geometrically Frustrated Permalloy Nano Structures Yi Qi, Todd Brintlinger, John Cumings Thin films of ferro-magnetic material with lithographically designed geometries can be used as an analog for the study of spin ice or frustrated systems. Here we study the magnetic structure and magnetization dynamics of permalloy thin films in a frustrated, hexagonal geometry using Transmission Lorentz Microscopy. The permalloy films are evaporated through patterns defined by conventional electron beam lithography to form single domain elements. These elements interact absent the effects of inhomogeneity and disorder associated with multi-domain magnetic elements and their grain structures. The mapping of in-plane magnetic moments through Lorentz microscopy show these magnetic structures to be governed by spin ice rule usually found in Kagome lattice systems. The switching process of these magnetic structures is observed using in-situ application of magnetic fields. The spin ice rule is shown to be valid as the exchange energy between elements is minimized in this artificially designed geometry. [Preview Abstract] |
Monday, March 5, 2007 9:36AM - 9:48AM |
A15.00007: Quantum Antiferromagnet on a Hyper-kagome Lattice: Applications to Na$_4$Ir$_2$O$_8$ Michael Lawler, Arun Paramekanti, Hae Young Kee, Yong Baek Kim, Ashvin Vishwanath Motivated by the intriguing low temperature paramagnetic behaviour recently observed in the Mott insulator Na$_4$Ir$_2$O$_8$, we study the quantum antiferromagnet Heisenberg model on a hyper-kagome lattice. Employing a large N generalization of the Schwinger boson description of spin systems uniquely suited to frustrated lattices, we find various closely competing phases for the ground state. Implications of our results for Na$_4$Ir$_2$O$_8$ are also discussed. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:00AM |
A15.00008: Classical antiferromagnet on a hyper-kagome lattice John M. Hopkinson, Sergei V. Isakov, Hae-Young Kee, Yong Baek Kim Motivated by recent experiments on Na$_4$Ir$_3$O$_8$ [Takagi, unpublished], we study the classical antiferromagnet on a frustrated three-dimensional lattice obtained by selectively removing one of four sites in each tetrahedron of the pyrochlore lattice. This ``hyper-kagome'' lattice consists of corner-sharing triangles. We present the results of large-$N$ mean field theory and Monte Carlo computations on $O(N)$ classical spin models. We find the classical ground states to be highly degenerate. Nonetheless, at low temperatures, nematic order emerges via ``order by disorder'' in the Heisenberg model ($N$=3), representing the dominance of coplanar spin configurations. Above this transition, the spin-spin correlations show a dipolar form which can be understood to arise from a generalized ``Gauss' law'' constraint. Implications for future experiments are discussed. [Preview Abstract] |
Monday, March 5, 2007 10:00AM - 10:12AM |
A15.00009: Spin Glass Order Induced by Dynamic Frustration Raymond Osborn, Eugene Goremychkin, Brian Rainford, Devashibhai Adroja, Marek Koza It is generally believed that both frustration and disorder are essential ingredients in the formation of a spin glass ground state. It was therefore surprising that PrAu$_2$Si $_2$ was reported to show all the characteristics of a spin glass, even though it is a stoichiometric compound with a well-ordered crystal structure. We report on inelastic neutron scattering measurements of the crystal field excitations, which show that PrAu $_2$Si$_2$ has a singlet ground state and that the exchange coupling is extremely close to the critical value to induce magnetic order. We propose that fluctuations of the crystal field levels are enough to destabilize the induced moments and prevent phase-coherent long-range order, and that spin glass freezing results from this dynamic frustration rather than any intrinsic static disorder. [Preview Abstract] |
Monday, March 5, 2007 10:12AM - 10:24AM |
A15.00010: Spin dynamics in LiY$_{0.998}$Ho$_{0.002}$F$_4$ via $^{19}$F NMR and $\mu$SR M. J. Graf, A. Lascialfari, J. Lago, F. Borsa, A. Amato, J.S. Lord, S.R. Giblin, B.Z. Malkin, A.M. Tkachuk, B. Barbara The $^{3+}$Ho ion doped into the host LiYF$_4$ is a single-ion magnet demonstrating slow dynamics at the avoided level crossings (ALCs) in the hyperfine-coupled electronuclear Zeeman diagram. We have studied $^{19}$F NMR and $\mu$SR in dilute LiY$_{0.998}$Ho$_{0.002}$F$_4$ to probe these dynamics and find clear evidence for an increase in the spin-lattice relaxation rate {\it{1/T$_1$}} at field values corresponding to ALCs using both techniques. We will discuss using these signatures as probes to measure the tunnel splitting $\Delta$ and the level broadening, as well as the onset of magnet correlations for increasing Ho concentration. [Preview Abstract] |
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