Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A18: Focus Session: Low D/Frustrated Magnetism - Pyrochlore, et al. |
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Sponsoring Units: GMAG DMP Chair: Deepak Singh, University of Maryland Room: D172 |
Monday, March 21, 2011 8:00AM - 8:36AM |
A18.00001: From Two Dimensional Correlations to a Disordered Ground State in the XY Pyrochlore, Yb$_2$Ti$_2$O$_7$ Invited Speaker: The tetrahedral geometry of the cubic pyrochlore lattice lends itself to strong geometric frustration, which has the effect of suppressing transitions to long range magnetic order (LRO) for certain types of magnetic exchange and single-ion anisotropy. Yb$_2$Ti$_2$O$_7$, whose magnetic Yb$^{3+}$ ions decorate the pyrochlore lattice, is known to have ferromagnetic exchange combined with XY single-ion anisotropy. Hence it is not expected to be frustrated, and should develop an LRO state below some Tc on the order of the exchange energy ($\theta_{CW}$ $\simeq$ 600mK). Indeed, Yb$_2$Ti$_2$O$_7$ displays a specific heat anomaly around 240mK, but this does not lead to an LRO state. Our recent triple-axis neutron scattering results have revealed that the specific heat anomaly is directly related to a change in dimensionality of the magnetic correlations, causing a transition from an unusual two-dimensionally correlated state above Tc to a short range correlated 3D state below Tc. Combined with recent specific heat results, we argue that the exact transition temperature depends on the precise level of weak structural disorder in the samples, implying that structurally perfect samples may lead to a fully developed LRO state below Tc. Furthermore, our earlier time-of-flight neutron scattering measurements revealed that even for the structurally imperfect systems, an ordered state can easily be induced by the application of a small magnetic field at low temperatures, as evidenced by the appearance of sharp spin wave excitations [1]. \\[4pt] [1] K. A. Ross, J. P. C. Ruff, C. P. Adams, J. S. Gardner, H. A. Dabkowska, Y. Qiu, J. R. D. Copley, and B. D. Gaulin. Phys. Rev. Lett. 103, 227202 (2009) [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A18.00002: Order and disorder in the local and long-range structure of the spin-glass pyrochlore, Tb$_2$Mo$_2$O$_7$ Yu Jiang, Ashfia Huq, Corwin H. Booth, Georg Ehlers, John E. Greedan, Jason S. Gardner The structure of Tb$_2$Mo$_2$O$_7$ is investigated using two techniques: the long-range lattice structure was measured using neutron powder diffraction, and local structure information was obtained from extended x-ray absorption fine structure measurements. While the long-range structure appears generally well ordered, enhanced mean-squared site displacements on the O(1) site and the lack of temperature dependence of the strongly anisotropic displacement parameters for both the Mo and O(1) sites indicates some disorder exists. Likewise, the local structure measurements indicate some Mo-Mo and Tb-O(1) nearest-neighbor disorder exists, similar to that found in the related spin-glass pyrochlore, Y$_2$Mo$_2$O$_7$. Although the freezing temperature in Tb$_2$Mo$_2$O$_7$, 25 K, is slightly higher than in Y$_2$Mo$_2$O$_7$, 22 K, the degree of local pair distance disorder is actually less in Tb$_2$Mo$_2$O$_7$. This apparent contradiction is considered in light of the interactions involved in the freezing process. [Preview Abstract] |
Monday, March 21, 2011 8:48AM - 9:00AM |
A18.00003: An Exchange Hamiltonian for Yb$_2$Ti$_2$O$_7$ Jordan Thompson, Paul McClarty, Henrik R\o{}nnow, Louis-Pierre Regnault, Andreas Sorge, Michel Gingras Yb$_2$Ti$_2$O$_7$ is a pyrochlore material with many strange properties at low temperature. Specific heat measurements on this material find evidence for a first order phase transition at a temperature of ${\rm T_c} \approx 240$ mK, but several experiments fail to find any evidence of long range order below ${\rm T_c}$. In order to understand the behaviour of the magnetic moments of the Yb$^{3+}$ ions below ${\rm T_c}$ it is necessary to quantift how they interact. I will present work based on using diffuse neutron scattering measurements to find a magnetic interaction Hamiltonian for Yb$_2$Ti$_2$O$_7$. We propose a Hamiltonian based on all of the symmetry allowed interactions on the pyrochlore lattice, along with long-range dipolar interactions. Using the energies of the symmetry allowed nearest-neighbor exchange interactions as free parameters, we perform simulated annealing to minimize the difference between experimental neutron scattering and neutron scattering computed from our exchange Hamiltonian using the random phase approximation. I will present the results of this fitting, and discuss the predictions of the resulting model for the behaviour of Yb$_2$Ti$_2$O$_7$, including calculations of the local susceptibility. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A18.00004: Low temperature freezing and dynamics in Tb2Sn2O7 Maria Matthews, Maria Dahlberg, Pawina Jiramongkolchai, Robert Cava, Peter Schiffer We have probed the low temperature magnetic behavior of the ordered spin ice material Tb2Sn2O7 through ac magnetic susceptibility measurements of both the pure material and samples with small percentages of Ti substituted on the Sn sublattice.. Our aim is to qualitatively probe the nature low temperature spin state of the stanate by slowly adjusting the chemical composition towards a known spin liquid--- terbium titanate. In pure Tb2Sn2O7, we observe a clear signature for the previously reported ordering transition at Tc = 850 mK, and we also observe evidence for dynamic freezing at temperatures well below Tc, confirming the persistence of significant magnetic fluctuations deep in the spin-ordered regime. We found that the long range ordering transition was completely suppressed by substitution with as little as 5 percent Ti, whereas larger Ti substitution resulted in a spin-glass-like spin freezing transition near 250 mK. The suppression of ordering with minimal substitution demonstrates a remarkable fragility to the spin ordering in this system. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A18.00005: Magnetic Diffuse Scattering in spinels GeCo2O4 (GCO) and GeNi2O4 (GNO) P. Manuel, M.K. Crawford, D.T. Adroja, L.C. Chapon, S. Hara, Y. Yoshida, S.I. Ikeda, J.W. Lynn, Y. Chen, R.A. Fisher Materials exhibiting geometrical magnetic frustration have been very topical in condensed matter physics due to their large ground state degeneracy usually leading to a great variety of behavior. GCO and GNO are spinels where the Co/Ni ions form a sublattice identical to the pyrochlore. This topology naturally leads to magnetic frustration which can be relieved to permit the appearance of longrange magnetic order. GCO has a N\'eel temperature of 21 K, below which a tetragonal distortion is observed. However, our heat capacity measurements show only about half of the entropy expected when integrated up to 75 K. This strongly suggests the existence of frustration which is evidenced by our neutron data where strong diffuse scattering (DF) is observed. GNO also exhibits strong DF at the same q-position but with different shape. We will present the neutron data alongside a Monte-Carlo analysis of the DF and the implications on the nature and strength of the different interactions in these two systems. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A18.00006: Dimerized Excitations in La$_{4}$Ru$_{2}$O$_{10}$ John-Paul Castellan, R. Osborn, S. Rosenkranz, M.B. Stone, S.E. Nagler, M.A. Lumsden, P. Khalifah The interplay between orbital, spin and charge degrees of freedom is at the forefront of condensed matter physics. The discovery of orbital ordering in La$_{4}$Ru$_{2}$O$_{10}$ [1] offers a unique opportunity to study orbital phenomena in a 4d transition metal oxide. La$_{4}$Ru$_{2}$O$_{10}$ undergoes a structural phase transition at $\sim $158K, below which there is a spin gap of $\sim $40 meV caused by Ru-O-Ru dimerization. We have performed single crystal time-of-flight neutron scattering measurements on the ARCS spectrometer at the SNS in ``sweep'' mode, a new technique in which neutron events are measured during continuous sample rotation, in order to quickly and efficiently map out four-dimensional volumes of S(q,w). With this method, we measured the structure factors and full dispersion of the singlet-triplet excitations as a function of \textbf{Q} and w. A model of the orbital ordering producing the magnetic excitations and the event-based technique used for their measurement will be discussed.\\[4pt] [1] P. Khalifah, et al., Science \textbf{297},2237 (2002) [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A18.00007: Strong spin-orbit coupling in ordered double perovskites Gang Chen, Leon Balents We construct and analyze a microscopic model for insulating rock salt ordered double perovskites, with the chemical formula A$_2$BB'O$_6$, where the B' atom has a 4d$^2$ or 5d$^2$ electronic configuration and forms a face centered cubic (fcc) lattice. The combination of the triply-degenerate $t_{2g}$ orbital and strong spin-orbit coupling favors an effective local spin moment $j=2$. Moreover, due to strongly orbital-dependent exchange, the effective spins have substantial biquadratic and bicubic interactions (fourth and sixth order in the spins, respectively). This leads, at the mean field level, to several interesting phases with high magnetic multipolar orders. We discovered a fundamental difference between integer spin system (considered in present work) and half-integer spin system (studied in previous work), that is, there exists a spin nematic ground state at zero temperature for integer-spin system. We also address the finite temperature properties of different phases. Existing and possible future experiments are discussed in light of these results. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A18.00008: Interplay between Lattice Distortion and Spin-Orbit Coupling in Double Perovskites Tyler Dodds, Ting-Pong Choy, Yong Baek Kim We develop anisotropic pseudo-spin nearest-neighbour antiferromagnetic Heisenberg models for monoclinically distorted double perovskites. We focus on these A$_2$BB$'$O$_6$ materials that have magnetic moments on the $4d$ or $5d$ transition metal B$'$ ions, which form a face-centred cubic lattice. In these models, we consider tetragonal distortion of B$'$-O octahedra, affecting relative occupancy of $t_{2g}$ orbitals, along with geometric effects of the distortion and spin-orbit coupling. The resulting pseudo-spin-$1/2$ models are solved in the saddle-point limit of the Sp(N) generalization of the Heisenberg model. The spin $S$ in the SU(2) case generalizes as a parameter $\kappa$ controlling quantum fluctuation in the Sp(N) case. We consider two different models that may be appropriate for these systems. In particular, using Heisenberg exchange parameters for La$_2$LiMoO$_6$ from a spin-dimer calculation [T. Aharen \textit{et al.}, Phys. Rev. B {\textbf 81}, 224409 (2010)], we conclude that this $S=1/2$ system may order, but must be very close to a disordered spin liquid state. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A18.00009: Elastic and inelastic neutron scattering study on (CuCl)LaTa$_2$O$_7$ Seunghun Lee, Kazuki Iida, Atsushi Kitada, Yoshihiro Tsujimoto, Hiroshi Kageyama, Bella Lake, Seiko Kawamura, Kazuhisa Kakurai, Yiming Qiu, Mark Green A quasi-two-dimensional frustrated spin system, (CuCl)La(Nb$_{1- x}$Ta$_x$)$_2$O$_7$, shows a quantum phase transition upon doping of Ta ions from a singlet state to an ordered state at $x\sim0.4$. (CuCl)LaNb$_2$O$_7$ has been reported as the first ferromagnetically coupled Shastry-Sutherland singlets with the triplet excitations centered at 2 meV. We report elastic and inelastic neutron scattering measurements on a powder sample of (CuCl)LaTa$_2$O$_7$ with and without an magnetic field. Our results show that upon cooling this system undergoes a magnetic ordering below 7 K with a characteristic wave vector of $Q=(1/2 \ 0\ 1/2)$. The magnetic excitations in the ordered phase are dominated by a nearly dispersionless mode centered at 2 meV similar to the triplet excitations observed in (CuCl)LaNb$_2 $O$_7$. Under field, however, the 2 meV mode in (CuCl)LaTa$_2 $O$_7$ splits into two modes, clearly indicating that it is a spin wave expected for an ordered state. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A18.00010: Structure and phase transitions of a magnetic kink crystal Sebastian Muehlbauer, Andrey Zheludev, Ekaterina Pomjakushina The insulator Ba$_2$CuGe$_2$O$_7$ crystallizes in the non-centrosymmetric space group $P-4_21m$ allowing for the Dzyaloshinskii-Moriya interaction. Below $T_N$=3.2K Ba$_2$CuGe$_2$O$_7$ exhibits an almost antiferromagnetic cycloidal magnetic order. For $H$ parallel to the c-axis, the cycloid distorts to solitions or kink domain walls. Using small angle neutron scattering and neutron diffraction the structure and phase transitions of a magnetic kink crystal have been examined in Ba$_2$CuGe$_2$O$_7$. A magnetic phase transition seen with SANS and complementary measurements of the magnetization is interpreted in terms of a transition from a Neel domain wall with the propagation vector {\it parallel} to its plane of spin rotation to a Bloch domain wall with a propagation vector {\it perpendicular} to its plane of spin rotation. Indicated by the occurrence of satellite reflections with even and odd harmonics around the AF Neel point (1,0,0) and around the FM zone center (0,0,0) it was further shown that the AF cycloidal magnetic structure of Ba$_2$CuGe$_2$O$_7$ is considerably distorted by the staggered Dzyaloshinskii vector $D_z$. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 11:00AM |
A18.00011: Neutron Scattering Study of Frustration and Magnetic Order in Spinels Invited Speaker: The `A-site spinels' are materials with magnetic cations constrained to lie on the diamond sublattice of the spinel structure. These systems have been of increasing interest, as novel theoretical and experimental results have emphasized the central role of frustration arising from competing interactions. In particular, the sub-family of diamond lattice antiferromagnets with spin-only degrees-of-freedom is predicted to exhibit novel `spiral-spin-liquid' and order-by-disorder physics (Bergman et al., Nature Physics (2007)). Real systems MnSc2S4 (Krimmel et al., PRB (2006)) and CoAl2O4 (Krimmel et al., Physica B (2006)) have been studied with powder neutron diffraction, and the results are argued to be consistent with this theory. However, as has been expressed on several occasions, studies on single-crystals are needed for a definitive answer. Here I will discuss neutron scattering results on single-crystalline CoAl2O4, which have given a much more complete picture of magnetic correlations in this material. Both elastic and inelastic measurements have been made using triple-axis and cold chopper spectrometry. With decreasing temperature, we observe intense diffuse scattering centred about locations in reciprocal space associated with collinear antiferromagnetism. At T*~6.5K, we further observe an unexpected change in the diffuse scattering lineshape, coupled with the emergence of well-defined spin-wave excitations. This temperature has been associated with an anomalous spin glass transition in the past, but we argue instead that the available data implies a first-order phase transition to an ordered state, possibly via the order-by-disorder mechanism. The ground state is degenerate, and kinetically frozen walls separating different domains give rise to the broadened scattering at magnetic wavevectors. This scenario may be present in many other frustrated systems. If time permits, I will also talk about new results on crystals of related systems MnAl2O4 and FeAl2O4. [Preview Abstract] |
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