Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session A27: Focus Session: Pyrochlores |
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Sponsoring Units: GMAG Chair: Jason Gardner, National Institute of Standards and Technology Room: Morial Convention Center 219 |
Monday, March 10, 2008 8:00AM - 8:36AM |
A27.00001: Low temperature spin dynamics and high pressure effects in frustrated pyrochlores Invited Speaker: Frustrated pyrochlores R$_{2}$M$_{2}$O$_{7}$, where R$^{3+}$ is a rare earth and M$^{4+}$ a transition or sp metal ion, show a large variety of exotic magnetic states due to the geometrical frustration of the pyrochlore lattice, consisting of corner sharing tetrahedra for both R and M ions. Neutron scattering allows one to measure their magnetic ground state as well as the spin fluctuations, in a microscopic way. An applied pressure may change the subtle energy balance between magnetic interactions, inducing new magnetic states. In this talk, I will review recent neutron results on Terbium pyrochlores, investigated by high pressure neutron diffraction and inelastic neutron scattering. Tb$_{2}$M$_{2}$O$_{7}$ pyrochlores show respectively a spin liquid state for M=Ti [1], an ordered spin ice state for M= Sn [2], and a spin glass state with chemical order for M=Mo [3]. In Tb$_{2}$Ti$_{2}$O$_{7}$ spin liquid, where only Tb$^{3+}$ ions are magnetic, an applied pressure induces long range antiferromagnetic order due to a small distortion of the lattice and magneto elastic coupling [4]. In Tb$_{2}$Sn$_{2}$O$_{7}$, the substitution of Ti$^{4+}$ by the bigger Sn$^{4+}$ ion expands the lattice, inducing a long range ordered \textit{ferromagnetic }state, with the local structure of a spin ice [2] and unconventional spin fluctuations [2,5]. The local ground state and excited crystal field states of the Tb$^{3+}$ ion were recently investigated by inelastic neutron scattering in both compounds [6]. Tb$_{2}$Mo$_{2}$O$_{7}$, where Mo$^{4+}$ ions are also magnetic, shows an even more rich behaviour, due to the complex interaction between frustrated Tb and Mo lattices, having respectively localized and itinerant magnetism. In Tb$_{2}$Mo$_{2}$O$_{7}$ spin glass, the lattice expansion induced by Tb/La substitution yields an ordered ferromagnetic state, which transforms back to spin glass under applied pressure [7]. New data about the spin fluctuations in these compounds, as measured by inelastic neutron scattering, will be presented. The talk will be dedicated to the memory of Igor Goncharenko, a renowned high pressure and neutron physicist, who died accidentally on Nov. 4$^{th}$, 2007. \newline [1] J. S. Gardner \textit{et al.}, Phys. Rev. Lett. \textbf{82}, 1012, (1999). \newline [2] I. Mirebeau \textit{et al.,} Phys. Rev. Lett. \textbf{94}, 246402, (2005). \newline [3] B. D. Gaulin \textit{et al.,} Phys. Rev. Lett. \textbf{69}, 3244, (1992). \newline [4] I. Mirebeau \textit{et al.} Nature \textbf{420}, 54 (2002); Phys. Rev. Lett. 93, 187204, (2004). \newline [5] F. Bert \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 117203, (2006)~; P. Dalmas de R\'{e}otier \textit{et al.}, Phys. Rev. Lett.\textbf{ 96}, 127202, (2006). \newline [6] I. Mirebeau, P. Bonville, M. Hennion, Phys. Rev. \textbf{76}, 184436, (2007). \newline [7] A. Apetrei et al, Phys. Rev. Lett. \textbf{97}, 206401, (2006). [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A27.00002: Spin Structure of Three Dimensional Frustrated Magnetic System of Tb$_{2}$Mo$_{2}$O$_{7}$ Deepak Singh, Joel Helton, Jeffrey Lynn, Young Lee The pyrochlore compound Tb$_{2}$Mo$_{2}$O$_{7}$ has been of interest since it exhibits spin-glass behavior despite the apparent lack of chemical disorder. This compound crystallizes in a cubic space group in which both the Tb and Mo atoms form three-dimensional networks of corner-sharing tetrahedra. Thus, each magnetic ion resides on a highly frustrated pyrochlore lattice. To clarify the nature of the spin correlations in the spin-glass state of Tb$_{2}$Mo$_{2}$O$_{7}$, we have succeeded in growing single crystal samples. Magnetic susceptibility (zero field cooled and field cooled) measurements confirm the spin-glass behavior, similar to that observed in previous powder samples. Recently we performed elastic neutron scattering measurements on a single crystal sample and identified the \textbf{Q-}vectors associated with the short-range diffuse scattering. The spin structure for this compound, consistent with elastic scattering, will be discussed. In addition, recent inelastic neutron scattering measurements will be discussed. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A27.00003: Field driven phases in the geometrically frustrated dipolar Heisenberg pyrochlore antiferromagnet Gd$_2$Ti$_2$O$_7$ Matthew Enjalran, Adrian Del Maestro, Michel J.P. Gingras The rare-earth pyrochlore gadolinium titanate, Gd$_2$Ti$_2$O$_7$, represents an excellent experimental realization of a Heisenberg antiferromagnet (AFM) in a frustrated geometry with weak long-range dipole-dipole interactions (approximately 20$\%$ of nearest neighbor AFM exchange). Experiments on Gd$_2$Ti$_2$O$_7$ in a magnetic field reveal a complex phase diagram associated with the breaking of spatial symmetries of the pyrochlore lattice as the field is applied along select symmetry directions. We study a model of classical Heisenberg spins (O(3) symmetry) on a pyrochlore lattice with exchange and dipolar interactions within mean-field theory. Using parameters relevant to the material system, we develop phase diagrams in finite magnetic fields. Our results our compared to experiments on Gd$_2$Ti$_2$O$_7$ (and Gd$_2$Sn$_2$O$_7$). [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A27.00004: Induced Magnetic Order in Yb$_2$Ti$_2$O$_7$ Carl Adams, K.A. Ross, J.P. Ruff, B.D. Gaulin, H. Dabkowska, Y. Qiu, J.R.D. Copley, J.S. Gardner Yb$_2$Ti$_2$O$_7$ is part of family of pyrochlore materials with a magnetic rare-earth and a non-magnetic transition metal. In similar compounds containing holmium, erbium, or terbium the geometric frustration results in several exotic ground states including spin-ice, spin-liquid, field-induced ordering, and low-dimensional behavior. The ytterbium-based compound has a strong anomaly in the specific heat at 240~mK [Hodges {\em et al.}, J Phys Cond Mat {\bf 13}, 9301 (2001)] but without accompanying long range order. We have made measurements of the elastic and inelastic neutron scattering on a large single crystal of Yb$_2$Ti$_2$O$_7$ over a broad range of the $(hhl)$ scattering plane using the DCS spectrometer at the NIST Center for Neutron Research. Our studies have confirmed the presence of diffuse ``rods'' of scattering along the (111) directions that persist below 100~mK. However, when the sample is field-cooled in fields as low as 0.5~T the rods of scattering are replaced by well-defined magnetic excitations. We will present the results of our studies at a variety of temperatures and fields. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A27.00005: Spin Waves and Quantum Criticality in the Frustrated XY Pyrochlore Antiferromagnet Er$_2$Ti$_2$O$_7$ Jacob Ruff, Patrick Clancy, Bruce Gaulin, Hanna Dabkowska, Mehmet Ramazanoglu, Jason Gardner, Alex Bourque, Mary Anne White, Yiming Qiu, John Copley We report recent single crystal neutron scattering measurements of the XY pyrochlore antiferromaget Er$_2$Ti$_2$O$_7$, in zero and non-zero magnetic field applied along the (110) direction. Previous results show a magnetically-ordered ground state which is suggested to be stabilized by an unusual order-by-disorder mechanism driven by quantum fluctuations [1]. We present measurements of the low-lying spin excitations of this system, revealing Goldstone modes previously thought to be absent [1]. Also, we explore the response of the ground state to applied magnetic fields, which are shown to induce a quantum phase transition separating the low-field antiferromagnetically ordered phase from the high-field polarized phase. \newline [1] J.D.M. Champion et al. ``Er$_2$Ti$_2$O$_7$: Evidence of quantum order by disorder in a frustrated antiferromagnet'' PRB 68 (2003) [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A27.00006: Lattice distortion in the spin liquid phase of Tb$_2$Ti$_2$O$_7$ Stephanie Curnoe The Tb ions in the pyrochlore antiferromagnet Tb$_2$Ti$_2$O$_7$ form a corner-shared tetrahedral network. It has been shown that the quantum anti-ferromagnetic ground state of a single tetrahedron accounts for short wavelength correlations observed in inelastic neutron scattering experiments. A bulk $k=0$ lattice distortion, specifically an $A_{2u}$ displacement of the Tb ions, can account for the isolation of single tetrahedra in the tetrahedral network. Such a distortion results in a space group reduction $Fd\bar{3}m \rightarrow F\bar{4}3m$ which is consistent with experimental observations. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A27.00007: Non-monotonic zero point entropy in diluted spin ice X. Ke, R. Freitas, B. Ueland, G. Lau, M. Dahlberg, R. Cava, R. Moessner, P. Schiffer Water ice and spin ice are important model systems in which theory can directly account for ``zero point'' entropy associated with quenched configurational disorder. Spin ice differs from water ice in the important respect that its fundamental constituents, the spins of the magnetic ions, can be removed through replacement with non-magnetic ions while keeping the lattice structure intact. In order to investigate the interplay of frustrated interactions and quenched disorder, we have performed systematic heat capacity measurements on spin ice materials which have been thus diluted up to 90{\%}. Investigations of both Ho and Dy spin ices reveal that the zero point entropy depends non-monotonically on dilution and approaches the value of Rln2 in the limit of high dilution. The data are in good agreement with a generalization of Pauling's theory for the entropy of ice. \newline X. Ke, et al, Phys. Rev. Lett. \textbf{99}, 137203 (2007). [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A27.00008: Thermodynamic and magnetic studies of novel spin ice materials M.L. Dahlberg, X. Ke, B.G. Ueland, P. Schiffer, D.V. West, R.J. Cava We report systematic low temperature measurements of the dc magnetization, ac susceptibility, and heat capacity of $Dy_{2}Sn_{2-x}Sb_{x}O_{7+x/2}$ (x=0, 0.25, and 0.5), and $Dy_{2}NbScO_{7}$. We find evidence for Ising-like single ion ground states in the $Dy_{2}Sn_{2-x}Sb_{x}O_{7+x/2}$ materials, and these materials possess nearly the same zero-point entropy as the canonical spin ices $Ho_{2}Ti_{2}O_{7}$ and $Dy_{2}Ti_{2}O_{7}$. These results strongly suggest that the $Dy_{2}Sn_{2-x}Sb_{x}O_{7+x/2}$ materials studied have spin-ice states at low temperatures despite the cation disorder on the B sites of the pyrochlore lattice We also observe a somewhat reduced zero-point entropy in $Dy_{2}NbScO_{7}$, which is possibly associated with a higher level of cation disorder [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A27.00009: Direct observation of the ice rule in demagnetized artificial kagome spin ice Yi Qi, Todd Brintlinger, John Cumings, Paula Mellado, Oleg Tchernyshyov Artificially designed magnetic structures have proven to be good analogs for the study of spin ice.~ We studied an artificial kagome spin ice, which is lithographically patterned using permalloy elements $\sim $23nm thick, $\sim $100nm wide, and $\sim $500nm long.~ Lorentz TEM is used to unambiguously determine each local pseudospin.~Even with a rigorous randomizing demagnetization process, we directly observe an exclusive ice rule and ice-like short range correlations. This is the first time that the ice rule has been confirmed by direct counting of the nearest-neighbors in any ice system, real or artificial. The further-neighbor correlations we observe are larger in magnitude than predicted for a random ice-rule ensemble, and we attribute this to the effects of dipolar interactions. Further, we investigate the entropy associated with artificial kagome spin ice based on the general concept of Shannon entropy [1].~ These results will be presented and compared with relevant theoretical studies. [1] C.E. Shannon, Bell System Technical Journal, 27, 379 (1948) [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A27.00010: Magnetization reversal in artificial kagome ice. Paula Mellado, Oleg Tchernyshyov, Yi Qi, Todd Brintlinger, John Cumings We study magnetization dynamics in an artificial kagome spin ice realized as a honeycomb network of connected ferromagnetic nanowires [1]. Our model is focused on magnetic charges defined as the flux of magnetization into a network site. In this system, the allowed values of magnetic charge are $\pm 1$ and $\pm 3$, while in the original square ice [2] they are 0, $\pm 2$, and $\pm 4$. The ice rule is equivalent to the minimization of the absolute value of magnetic charge. In our model, magnetization reversal in a given link is triggered when the total magnetic field at one of its sites reaches a critical value and the site emits a domain wall with charge $\pm 2$, which propagates the entire length of the link reversing the magnetization. The resulting redistribution of magnetic charges provides a positive feedback by increasing the local values of the magnetic field at neighboring sites. A sufficiently strong feedback triggers an avalanche-like reversal observed experimentally. That and an inherently dissipative character of the reversal process suggest interesting parallels with granular materials and sheds light on the physics of rotational demagnetization of spin ice [3]. Supported by NSF Grant DMR-0348679. [1] M. Tanaka \textit{et al.}, Phys. Rev. B \textbf{73}, 052411 (2006). [2] R. F. Wang \textit{et al.}, Nature \textbf{439}, 303 (2006). [3] C. Nisoli \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 217203 (2007). [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 11:00AM |
A27.00011: Neutron Scattering from Magnetically Frustrated Ruthenium Pyrochlores Invited Speaker: Spin-1 ruthenium pyrochlores feature strong exchange interactions and deeply suppressed N\'{e}el ordering. In addition a doping induced metallic phase has been demonstrated. I discuss neutron scattering experiments that explore the strongly frustrated quantum magnetism of Y$_{2}$Ru$_{2}$O$_{7}$ [1] and Pr$_{2-x}$Bi$_{x}$Ru$_{2}$O$_{7}$ [2]. In Y$_{2}$Ru$_{2}$O$_{7}$ ($\Theta _{CW}$= --1100 K, T$_{N}$=77 K) much of the magnetic spectral weight is concentrated in a 20 meV spectral peak above an 11 meV low temperature gap in the excitation spectrum. In Pr$_{2-x}$Bi$_{x}$Ru$_{2}$O$_{7}$ magnetic order which occurs for T$<$T$_{N}$=160 K when $x$=0, is suppressed when the material becomes a metal for $x\approx $1. I discuss the anomalous low energy spin dynamics associated with praseodymium for $x$=0 and $x$=1 and relate the results to other rare-earth pyrochlores. \newline \newline [1] J. van Duijn, N. Hur, J. W. Taylor, Y. Qiu, Q. Z. Huang, S.-W. Cheong, C. Broholm, and T. G. Perring, Submitted to Phys. Rev. B Rapid Communications, cond-mat 0707.2678v1 (2007). \newline [2] J. van Duijn, K. H. Kim, N. Hur, D. Adroja, M. A. Adams, Q. Z. Huang, M. Jaime, S.-W. Cheong, C. Broholm, and T. G. Perring, Phys. Rev. Lett.\textbf{ 94}, 177201 (2005). [Preview Abstract] |
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