Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D7: New Results in Geometrically Frustrated Magnets |
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Sponsoring Units: GMAG Chair: Arthur Ramirez, Lucent Technologies Room: Baltimore Convention Center 307 |
Monday, March 13, 2006 2:30PM - 3:06PM |
D7.00001: Spin liquids and spin dynamics in kagome antiferromagnets Invited Speaker: Among all the corner sharing highly frustrated magnets, only a few experimental systems are good candidates for a low-T fluctuating state, ie fulfilling the important conditions of the pure Heisenberg lattice with nn couplings. The combination of the weakness of the single-ion anisotropy and of a direct overlap antiferromagnetic exchange are certainly the major advantages of the chromate S=3/2 kagome bilayer Ba$_{2}$Sn$_{2}$ZnGa$_{10-7p}$Cr$_{7p}$O$_{22}$- BSZCGO(p)- and the long studied SrCr$_{9p}$Ga$_{12-9p}$O$_{19}$ - SCGO(p). Beyond the absence of ordering well below the Curie-Weiss temperature, the unusual large value of the specific heat unveils a high density of low lying excitations and its field independence suggests that the excited states are mostly singlets. Moreover, their ground state is found essentially fluctuating although an intrinsic spin glass (SG) signature is observed in susceptibility measurements. Through a review of our past years work, I'll illustrate all the potential of local studies (NMR and $\mu $SR) to reveal some key aspects of the physics of these compounds: susceptibility, fluctuations, impact of dilution defects which generate an extended response of the spin-lattice ... as well as the puzzling spin-glass state. More recently we also investigated new series of compounds, among them volborthite and delafossites which feature S=1/2 spins on a corner sharing antiferromagnetic lattice. I'll introduce these compounds and shortly discuss their relation to ideal Hamiltonians and novel features. \textit{- D. Bono et al.Phys. Rev. Lett. 93, 187201 (2004), 92, 217202 (2004)~; Cond-mat/0503496. F. Bert et al. Phys. Rev. Lett., 95, 087203 (2005). L. Limot, et al., Phys. Rev. B, 65, 132403 (2002). P. Mendels et al. Phys. Rev. Lett., 85, 3496 (2000).} [Preview Abstract] |
Monday, March 13, 2006 3:06PM - 3:42PM |
D7.00002: Frustrated dipolar interactions -- why spin ice obeys the ice rules Invited Speaker: The low temperature entropy of the the spin ice compounds, such as Ho$_2$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$, is well described by the nearest-neighbor antiferromagnetic Ising model on the pyrochlore lattice, i.e.\ by the ``ice rules''. This is surprising since the dominant coupling between the spins is their long ranged dipole interaction. We show that this phenomenon can be understood rather elegantly: one can construct a model dipole interaction, by adding terms of shorter range, which yields {\it precisely} the same ground states, and hence $T=0$ entropy, as the nearest neighbor interaction. A treatment of the small difference between the model and true dipole interactions reproduces the numerical work by Gingras et al in detail. We are also led to a more general concept of projective equivalence between interactions. S. V. Isakov, R. Moessner, S. L. Sondhi, Phys.\ Rev.\ Lett.\ {\bf 95}, 217201 (2005) [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 4:18PM |
D7.00003: Neutron studies of 3D Highly Frustrated Magnetism Invited Speaker: Because of their low propensity to order antiferromagnetic materials based on a pyrochlore lattice appear to be excellent systems for studying exotic ground states. It has also been shown that ferromagnetically coupled spins can be frustrated on such a lattice, if there is considerable local Ising anisotropy. I will review several neutron scattering studies on pyrochlore compounds done in the past couple of years, focusing on the study of spin dynamics. Evidence for large, dynamical spins at low temperatures in several antiferromagnets and a crossover from classical to quantum regime in the spin ices will be discussed. If time permits I will discuss new areas of neutron research in frustrated magnetism. \newline \newline [1] \textit{Magnetic Systems with Competing Interactions}, edited by H.T. Diep (World Scientific, Singapore, 1994), Can. J. Phys. \textbf{79}, (2001) and \textit{Frustrated Spin Systems}, ed H T Diep (World Scientific, Singapore, 2004). \newline [2] S. T. Bramwell and M. P. Gingras, Science, \textbf{294}, 1495, (2001). [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:54PM |
D7.00004: Natural and Artificial Spin Ice Invited Speaker: Geometrical frustration among spins in magnetic materials can lead to exotic low temperature states including ``spin ice'', in which the local moments mimic the frustration of hydrogen ion positions in frozen water. We have performed extensive studies of spin ice materials and related compounds, and recently have begun study of an \textit{artificial geometrically frustrated magnet} which shares many of the properties of the spin ice materials. This artificial frustrated system is based on an array of lithographically fabricated single-domain ferromagnetic islands. The islands are arranged such that the dipolar interactions between them are analogous to those in spin ice. Images of the magnetic moments of individual elements in this correlated system allow us to study the local accommodation of frustration. We see both ice-like short range correlations and an absence of long range correlations, behavior which is very similar to the low temperature state of spin ice. These results demonstrate that artificial frustrated magnets can provide a rich new arena in which to study the physics of frustration. References: J. Snyder \textit{et al.} (PRL 2003, and PRB 2004); R. F. Wang\textit{ et al.} (Nature, in press). This research was supported by the Army Research Office and the National Science Foundation. [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:30PM |
D7.00005: Colossal magnetocapacitive effects in geometrically frustrated chalcogenide spinels Invited Speaker: It is well known that the spinel structure is susceptible to the occurrence of geometrical frustration, which in the past was invoked to explain a number of unusual observations concerning the magnetic and orbital degrees of freedom in these materials. We demonstrate that several chalcogenide spinels also exhibit very unusual dielectric behavior, especially an extremely strong coupling of magnetic and dielectric properties and the simultaneous occurrence of magnetic and polar order. Especially, in CdCr$_{2}$S$_{4}$ a colossal magnetocapacitive effect is observed, which shows up as a sharp upturn of the dielectric constant \textit{$\varepsilon $}' when the sample becomes ferromagnetic and as a variation of \textit{$\varepsilon $'} up to a factor of 30 when the sample is subjected to external magnetic fields. As revealed by linear and non-linear dielectric measurements, this material shows the typical signatures of relaxor ferroelectrics, i.e. a strong increase of the static dielectric constant with decreasing temperature and considerable frequency dispersion of the complex permittivity. While in most relaxor ferroelectrics the freezing of polar moments is driven by frustrated interactions related to substitutional disorder, in the present pure system geometrical frustration seems a plausible mechanism to explain the relaxor behavior. However, one may also speculate on completely different mechanisms of ferroelectric polarization, e.g., the ordering of electronic degrees of freedom. The concomitant occurrence of polar and magnetic order makes CdCr$_{2}$S$_{4}$ another example of the rare species of multiferroic materials. In contrast to other members of this group of materials, it has sizable ordering temperatures and moments. A detailed investigation of the relaxational dynamics in this material provides clear evidence that the observed magnetocapacitive effect stems from an enormous acceleration of the relaxation dynamics induced by the development of magnetic order. In addition, recent results reveal even larger magnetocapacitive effects in In-doped CdCr$_{2}$S$_{4}$. In addition to CdCr$_{2}$S$_{4}$, we found similar effects also in ferromagnetic CdCr$_{2}$Se$_{4}$ and, most astonishing, in HgCr$_{2}$S$_{4}$, which exhibits a complex type of antiferromagnetic magnetic order at low temperatures. In the latter system, the magnetocapacitive effect, exemplified by the relative increase of \textit{$\varepsilon $'} in a field of 5~T, reaches values up to 8$\times $10$^{5}$~{\%}. [Preview Abstract] |
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