Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N9: Geometrically Frustrated Magnets I |
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Sponsoring Units: GMAG Chair: Jason Garner, NIST Room: LACC 153A |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N9.00001: Incommensurate magnetic order from a geometrically frustrated spinel CdCr$_2$O$_4$ Jae-Ho Chung, Masaaki Matsuda, Hiroaki Ueda, Yutaka Ueda, Hidenori Takagi, Kun-Pyo Hong, Sungil Park, Seung-Hun Lee In an ideal pyrochlore lattice, antiferromagnetic spins are three-dimensionally frustrated and do not order down to zero temperature. It is often found in real life, however, that a long-range order appears by a symmetry-breaking transition at an ordering temperature much lower than Curie-Weiss temperature. ZnCr$_2$O$_4$ is a well-known example showing a long-range commensurate magnetic order that is coincident with a cubic-to- tetragonal distortion with $c < a$. In this study, we have observed a closely-related compound CdCr$_2$O$_4$, which has a larger A-site ion. Interestingly, the cubic-to-tetragonal distortion occurred with $c > a$, and the coincident magnetic order turned out to be incommensurate. This indicates that the nonmagnetic A-site ions play a ciritical role in determining the ground state properties of the chromate spinels. We present the model for the spin structure, and discuss the possible mechanisms that can lead to the incommensurate order. [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N9.00002: Probing spin correlations with phonons in the strongly frustrated magnet ZnCr$_2$O$_4$ Howard Drew, Andrei Sushkov, Oleg Tchernyshyov, William Ratcliff, Sang-Wook Cheong Geometrically frustrated magnets can resist magnetic ordering and remain in a strongly correlated paramagnetic state well below the Curie-Weiss temperature. The spin-lattice coupling can play an important role in relieving the frustration in these system. In ZnCr$_2$O$_4$, an excellent realization of the Heisenberg antiferromagnet on the ``pyrochlore'' network, a lattice distortion relieves the geometrical frustration through a spin-Peierls-like phase transition at $T_c=12.5$~K. Conversely, spin correlations strongly influence the elastic properties of a frustrated magnet. By using infrared spectroscopy and published data on magnetic specific heat, we demonstrate that the frequency of an optical phonon triplet in ZnCr$_2$O$_4$ tracks the nearest-neighbor spin correlations above $T_c$. Below $T_c$, the phonon triplet splits into a singlet and a doublet, separated by 11~cm$^{-1}$. This splitting is directly proportional to the spin-Peierls order parameter. We also observed a number of weak absorption bands, arising below $T_c$, which indicates doubling of the Brillouin zone at the structural/magnetic phase transition. [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N9.00003: Nonuniform spin-Peierls distortion in the ``pyrochlore'' antiferromagnet ZnCr$_2$O$_4$ Gia-Wei Chern, Oleg Tchernyshyov The cubic spinel ZnCr$_2$O$_4$ is a nice realization of the Heisenberg antiferromagnet on the ``pyrochlore lattice'' with spins $S = 3/2$. A high degree of geometrical frustration creates a strongly correlated paramagnetic regime at low temperatures where spins remain liquid and move in a highly coordinated fashion. The frustration is eventually lifted through a spin-Peierls-like transition at 12.5 K [1]. A basic physical picture of the transition has been developed by Yamashita and Ueda [2] and Tchernyshyov, Moessner and Sondhi [3] for the cases of a uniform lattice distortion. However, accumulating experimental evidence points to the dominance of a nonuniform lattice distortion that enlarges the structural unit cell [4]. The lattice modes most likely involved in the transition are the four phonon doublets with the wavevectors $\langle \frac{1}{2} \frac{1}{2} \frac{1}{2} \rangle$. We discuss the symmetry aspects of the appropriate spin-Peierls order parameter and ways of measuring it. [1] S.-H. Lee {\em et al.}, Phys. Rev. Lett. {\bf 84,} 3718 (2000). [2] Y. Yamashita and K. Ueda, Phys. Rev. Lett. {\bf 85,} 4960 (2000). [3] O. Tchernyshyov, R. Moessner, and S. L. Sondhi, Phys. Rev. B {\bf 66,} 064403 (2002). [4] H. Ueda {\em et al.}, Bull. Amer. Phys. Soc. {\bf 48,} 826 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N9.00004: Geometrical Frustration in Rare-Earth Face-Centered Cubic Crystals Veronika Fritsch, Joe D. Thompson, John L. Sarrao In a 3-dimensional solid the simplest form of magnetic frustration is spins on a tetrahedron with antiferromagnetic coupling. A face-centered cubic ($fcc$) lattice is a simple example of a network of edge-sharing tetrahedra; however, most $fcc$ compounds exhibit well-defined magnetic order, dominated by next-neighbor ($nn$) and next-nearest-neighbor ($nnn$) interactions. To minimize the effects of $nnn$ interactions and maximize frustration, the network of edge-sharing tetrahedra has to be divided into sub-networks of corner-sharing tetrahedra, as is realized in pyrochlore and spinel structures. A further example of a $fcc$-lattice split in two sub-networks of corner-sharing tetrahedra are the intermetallic ternaries $RE$InCu$_4$ ($RE$ = heavy rare-earth). Here the rare-earth ions occupy a $fcc$-lattice, where half of the tetrahedra are filled with an In-ions and the other half with a Cu-tetrahedron. The extent of frustration in these systems is determined by the magnetic moment of the rare-earth ion and second by their separation distance, which can be tuned with chemical substitution, e.g. Ni for Cu. We present measurements of electrical resistivity, magnetic susceptibility and specific heat on single crystals of the title compounds with the trivalent rare-earth ions Gd, Dy, Ho and Er, demonstrating geometrical frustration of their spin and orbital angular momentum. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N9.00005: Field induced magnetic ordering and spin excitations in the frustrated pyrochlore Antiferromagnet Tb$_{2}$Ti$_{2}$O$_{7}$ K.C. Rule, B.D. Gaulin, S.R. Dunsiger, J.C. Ruff, J.S. Gardner, Y Qiu, J.R.D Copley Tb$_{2}$Ti$_{2}$O$_{7}$ is a rare earth titanate which is characterized by antiferromagnetically-coupled Tb moments on the geometrically frustrated pyrochlore lattice. However unlike other members of this family, Tb$_{2}$Ti$_{2}$O$_{7}$ displays no static magnetic order at temperatures down to 70mK$^{1}$. Time-of-flight neutron scattering was performed on a single crystal of Tb$_{2}$Ti$_{2}$O$_{7}$ at the NIST Center for Neutron Research. Applied magnetic fields of up to 8.5T, directed along the $<$110$>$ direction, induced two magnetically ordered states at T=1 K. This applied field clearly splits the degenerate excited state doublet, and induces a dispersive collective spin excitation which appears to go to zero energy at the 001 and, possibly, 003 zone centres. These results require continuous spin degrees of freedom to be relevant to the Tb moments. Modeling of the ordered phases will also be presented. $^{1}$Gardner \textit{et al}., Phys. Rev. Lett. \textbf{82},1012 (1999) [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N9.00006: AC susceptibility studies of the cooperative paramagnet Tb$_{2}$Ti$_{2}$O$_{7}$ B.G. Ueland, P. Schiffer, G.C. Lau, R.J. Cava The cooperative paramagnet Tb$_{2}$Ti$_{2}$O$_{7}$ has generated much interest in the frustrated magnetism community due to the lack of ordering down to temperatures well below its Curie-Weiss temperature which is $\sim$10 K. Its pyrochlore magnetic lattice and highly anisotropic g-factor suggest a similarity to spin ice (i.e. Dy$_{2}$Ti$_{2}$O$_{7}$ and Ho$_{2}$Ti$_{2}$O$_{7}$), however, the first excited crystal field level is of the same order as the exchange interaction. This presents a novel system where crystal field degeneracy should have an effect as a magnetic field is applied. We present ac susceptibility studies on single and polycrystalline samples over broad ranges of temperature, static magnetic field, and frequency. We find two peaks emerge in the complex susceptibility as a function of temperature in the presence of a static magnetic field, and we discuss the data in terms of the effects of single ion and cooperative behavior. This research is supported by the National Science Foundation. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N9.00007: Neutron studies of Yb$_2$Ti$_2$O$_7$ Jason Gardner, Georg Ehlers, Cedomir Petrovic Polarised neutron scattering experiments, including neutron spin echo has been performed on ytterbium titanate. Ytterbium titanate is an insulator that crystallizes into the cubic oxide pyrochlore structure with a lattice parameter of approximately 10.03 {\AA}. Earlier studies have shown this sample to have an effective paramagnetic moment of 3.0 $\mu_B$. During these investigations it was found that the system undergoes a phase transition at 240 mK, however it is not as simple as that reported previously in the literature. We can conclusively rule out the presence of a static, ferromagnetic state and confirm that the majority of the spin system remains dynamic below this transition. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N9.00008: Evidence for a strong coupling transition in Y$_2$Ru$_2$O$_7$ Joost van Duijn, Namjung Hur, Jon Taylor, Simon Levett, Qing Huang, Sang-Wook Cheong, Collin Broholm, Toby Perring Y$_2$Ru$_2$O$_7$ adopts the pyrochlore structure with Ru occupying the octahedral sites. Magnetization and specific heat measurements indicate that there is a magnetic phase transition at T= 78 K. Diffraction experiments have shown that the Ru-sublattice orders in a long range ordered q=0 structure where the total spin vanishes on each tetrahedron. We have performedneutron inelastic scattering experiments in order to investigate this magnetic phase transition. These experiments show what I would call a strong coupling transition within this material. There are rearrangements in the excitation spectrum at T= 78 K to energies that correspond to room temperature without an apparent change in the wave vector dependence. Light scattering experiments indicate that magneto-elastic effects are important in allowing this phase transition to proceed. From previous work on spinel’s it has been suggested that this type of transition should be considered the 3 D analogue of a spin-peierls transition in a cooperative spin system [1]. This research was funded by the U.S. Department of Energy, under Grant No. DE-FG02-02ER45983, and by the National Science Foundation, under Grant No. NSF-DMR-0103858. [1]S.-H. Lee et al., Phys. Rev. Lett. 84, 3718 (2000). [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N9.00009: Effective Low-Energy Hamiltonian for the Frustrated Tb$_2$Ti$_2$O$_7$ Pyrochlore Antiferromagnet Hamid R. Molavian, Michel J.P. Gingras The antiferromagnetic pyrochlore Tb$_2$Ti$_2$O$_7$ presents a challenging puzzle to experimentalists and theorists studying frustrated magnets. Results from muon spin resonance and neutron scattering experiments for Tb$_2$Ti$_2$O$_7$ reveal a paramagnetic structure down to $50$mK despite an antiferromagnetic Curie-Weiss temperature, $\theta_{\rm CW}=-20 $K. Crystal field calculations show that the Tb$^{3+}$ ion in Tb$_2$Ti$_2$O$_7$ is a ground state doublet with local $\langle 111 \rangle$ anisotropy and is separated from the first excited doublet state by a gap of 20K. To understand the quantum nature of Tb$_2$Ti$_2$O$_7$, we apply the Rayleigh- Schrodinger method to map the four state problem with exchange and dipole-dipole interactions onto an effective Hamiltonian with two states per ion. We give some properties of this effective hamiltonian and compare the neutron scattering results with the experimental data. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N9.00010: Magnetic Orders and Fluctuations in the Dipolar Pyrochlore Antiferromagnet Olivier Cepas, B. Sriram Shastry, A. Peter Young While the classical Heisenberg antiferromagnet on the pyrochlore lattice does not order, we will discuss, from a theoretical standpoint, possible magnetic phases induced by the dipole-dipole interactions. Such interactions play a role in systems such as Gd$_2$Ti$_2$O$_7$ or Gd$_2$Sn$_2$O$_7$ in stabilizing exotic forms of magnetic order, a subject of current debate. We will also argue that the external magnetic field induces multiple transitions, one of which is associated with no obvious broken symmetry, but can be characterized by a disorder parameter. Finally, Monte-Carlo simulations and Landau-Ginzburg expansion show that the dipolar Heisenberg model exhibits a fluctuation-induced first-order transition, thanks to the frustration and a continuous set of soft modes. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N9.00011: Ice: a strongly correlated proton system Pierre Pujol, Antonio Castro Neto, Eduardo Fradkin We discuss the problem of proton motion in Hydrogen bond materials with special focus on ice. We show that phenomenological models proposed in the past for the study of ice can be recast in terms of microscopic models in close relationship to the ones used to study the physics of Mott-Hubbard insulators. We discuss the physics of the paramagnetic phase of ice at 1/4 filling (neutral ice) and its mapping to a transverse field Ising model and also to a gauge theory in two and three dimensions. We show that H3O+ and HO- ions can be either on a confined or deconfined phase. We obtain the phase diagram of the problem as a function of temperature T and proton hopping energy t and find that there are two phases: an ordered insulating phase which results from an order-by-disorder mechanism induced by quantum fluctuations, and a disordered incoherent metal phase (or plasma). We also discuss the effects in the proton motion introduced by the lattice vibrations (phonons) and its effect on the phase diagram. Finally, we suggest that the transition from ice Ih to ice XI observed experimentally in doped ice is the confining-deconfining transition of our phase diagram. [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N9.00012: Bulk Properties and Neutron Scattering studies of LiVO2 Wei Tian, Matthew Stone, Matt Chisholm, Rongying Jin, Brian Sales, Stephen Nagler, David Mandrus LiVO2 crystallizes in the space group R-3m with V$^{3+}$ (S = 1,$t_{2g}^2 )$ ions forming a two-dimensional triangular lattice. Magnetically, the system changes from a high temperature Curie-Weiss paramagnetic state to a low-temperature non-magnetic state at T $\sim $ 500 K. It has been proposed [1] that this phase transition is associated with a peculiar frustration-related orbital ordering resulting in trimers of V$^{3+}$ ions forming a spin-singlet ground state. Single crystals of LiVO2 with typical size approximately 3 $\times $ 2 $\times $ 0.1 mm$^{3}$ have been grown by the flux method. These crystals have been studied by electron diffraction, susceptibility, and specific heat measurements. The results are largely consistent with the V$^{3+}$ trimer model picture. We also report recent powder and single crystal inelastic neutron scattering studies of the magnetic excitations in LiVO$_{2}$. [1] H. F. Pen \textit{et al}, Phys. Rev. Lett. \textbf{78} (1997) 1323. [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N9.00013: A Study of the Magnetic Properties of the Frustrated Spinels GeNi2O4 and GeCo2O4 M.K. Crawford, R.L. Harlow, R. Flippen, S. Hara, Y. Yoshida, S.I. Ikeda, Q. Huang, J.W. Lynn, Y. Qui, J.R.D. Copley, Y. Chen, R.W. Stevens, B.F. Woodfield, J. Boerio-Goates, P.L. Lee, Y. Zhang, J. Hormadaly, R.A. Fisher The spinels GeNi$_{2}$O$_{4}$ and GeCo$_{2}$O$_{4}$, in which the spin-1 Ni$^{2+}$ or spin-3/2 Co$^{2+}$ ions are located on the vertices of a lattice of corner-sharing tetrahedra, exhibit interesting magnetic and structural properties. GeNi$_{2}$O$_{4}$ has a double N\'{e}el transition (T$_{N1}$ = 12.13 K and T$_{N2}$ = 11.46 K), but the crystal structure remains cubic in the N\'{e}el state. In contrast, GeCo$_{2}$O$_{4 }$has a single N\'{e}el transition (T$_{N}$ = 20.6 K) that coincides closely with a cubic to tetragonal structural phase transition, below which $c$/$a >$ 1. In the past we have used magnetic susceptibility, heat capacity, synchrotron x-ray, and neutron powder diffraction to study these materials. In this talk we will describe selected recent results of elastic and inelastic neutron scattering measurements of polycrystalline and single crystalline samples, the latter grown by the floating zone technique at AIST. [Preview Abstract] |
Wednesday, March 23, 2005 10:36AM - 10:48AM |
N9.00014: Heat Capacity and Neutron Scattering Studies on Geometrically Frustrated Ho Double Perovskites Matthew Jacobsen, Daniel Antonio, Andrew Cornelius, Ravhi Kumar, Jason Gardner, Yiming Qiu, Cedomir Petrovic We have studied the magnetic properties of the frustrated double perovskites A$_{2}$HoSbO$_{6}$ (A = Sr or Ba) by performing heat capacity measurements in applied magnetic field and inelastic neutron scattering. The Ho nuclear contribution to the heat capacity displays a unique field dependence that can be explained by the slowing down of the Ho electron spins in an applied field. The lowest magnetic level appears to be a doublet. Inelastic neutron scattering has conclusively determined the energy of these low lying crystalline electric field levels. [Preview Abstract] |
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N9.00015: A large-N approach to two-dimensional frustrated ising antiferromagnets with transverse external and exchange fields Prashant Sharma A large-N approach is developed to treat the problem of frustrated ising antiferromagnets on two-dimensional lattices-- in particular the triangular and kagom\'e. Quantum dynamics is introduced in this model by either a transverse external field, or an exchange field that may be ferromagnetic or anti- ferromagnetic. Special emphasis is given to the Kagom\'e XXZ model in which disordered phases are found for both ferromagnetic and anti-ferromagnetic transverse exchange. [Preview Abstract] |
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