Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y29: Focus Session: Disordered and Glassy Magnets 
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Sponsoring Units: GMAG DMP Chair: Daniel Silevitch, University of Chicago Room: 206A 
Friday, March 6, 2015 8:00AM  8:12AM 
Y29.00001: Barkhausen noise in the Random Field Ising Magnet NdFeB Jian Xu, Daniel Silevitch, Thomas Rosenbaum With the application of a magnetic field transverse to the magnetic easy axis, sintered blocks of the rareearth ferromagnet Nd$_{\mathrm{2}}$Fe$_{\mathrm{14}}$B form a realization of the RandomField Ising Model at room temperature. We study domain reversal and avalanche dynamics through an analysis of the Barkhausen noise. Powerlaw behavior with a cutoff is observed in the avalanche energy spectrum, consistent with theoretical predictions for disordered materials. Two regimes of behavior are found, one at low temperature and high transverse field where the system shows behavior consistent with randomnessdominated dynamics, and a hightemperature, lowtransversefield regime in which thermal fluctuations dominate the dynamics. In the randomnessdominated regime, the critical exponents are consistent with meanfield predictions for heavily disordered system, whereas in the thermalfluctuation regime, the critical exponents differ substantially from the meanfield predictions. [Preview Abstract] 
Friday, March 6, 2015 8:12AM  8:24AM 
Y29.00002: Interplay of disorder and geometrical frustration in Gadolinium Gallium Garnet Nayoon Woo, D.M. Silevitch, T.F. Rosenbaum We study the effects of disorder on the geometrically frustrated Heisenberg antiferromagnet Gadolinium Gallium Garnet (GGG) using neodymium doping (0.1 to 1\%) in combination with linear and nonlinear ac magnetic susceptibility. The Nd doping actually alleviates the effects of disorder due to excess Gd ions occupying Ga sites. The linear, frequencydependent susceptibility reveals that 1\% Nd doping suppresses the appearance of any longrange order from approximately 80mK to below 30mK. The dynamics of isolated, correlated spin clusters were studied as a function of doping level using nonlinear susceptometry. In this regime, both the aggregate moment of the clustered spins and the activation field required to excite a nonlinear response were inversely correlated with the dopant density. [Preview Abstract] 
Friday, March 6, 2015 8:24AM  8:36AM 
Y29.00003: Spin Glass Dynamics: Finite Size Effects Samaresh Guchhait, Raymond Orbach Because the spin glass lower critical dimension $2 
Friday, March 6, 2015 8:36AM  9:12AM 
Y29.00004: Random Fields, Topology, and Glassy States of Matter Invited Speaker: Eugene Chudnovsky The debate goes on for more than forty years whether weak static random fields destroy the longrange order in condensed matter systems. A recently found answer depends on the topology of the order parameter. The $n$component order parameter in $d$ dimensions exhibits glassy behavior at $n < d + 1$, characterized by pinning of topological defects with singularities: vortices, strings, etc. At $n = d + 1$, the presence of nonsingular topological objects, such as kinks and~skyrmions, leads to a weak metastability. At $n > d + 1$ topological objects are absent and the behavior of the system is fully reversible, characterized by the exponential decay of correlations in quantitative agreement with the LarkinImryMa argument. These findings have been confirmed numerically on lattices of up to one billion sites. (Research supported by the DOE Grant DEFG0293ER45487.)\\[4pt] Reference: T. C. Proctor, D. A. Garanin, and E. M. Chudnovsky, Physical Review Letters \textbf{112}, 097201 (2014). [Preview Abstract] 
Friday, March 6, 2015 9:12AM  9:24AM 
Y29.00005: Improved fair sampling of ground states in Ising spin glasses Helmut G. Katzgraber, Zheng Zhu, Andrew J. Ochoa Verifying that an optimization approach can sample all solutions that minimize a Hamiltonian is a stringent test for any newlydeveloped algorithm. While most solvers easily compute the minimum of a cost function for small to moderate input sizes, equiprobable sampling of all groundstate configurations (within Poissonian fluctuations) is much harder to obtain. Most notably, methods such as transversefield quantum annealing fail in passing this test for certain highlydegenerate problems. Here we present an attempt to sample ground states for Ising spin glasses based on a combination of lowtemperature parallel tempering Monte Carlo combined with the cluster algorithm by Houdayer. Because the latter is rejection free and obeys details balance, the groundstate manifold is efficiently sampled. We illustrate the approach for Ising spin glasses on the DWave Two quantum annealer topology, known as the Chimera graph, as well as twodimensional Ising spin glasses. [Preview Abstract] 
Friday, March 6, 2015 9:24AM  9:36AM 
Y29.00006: The cumulative overlap distribution function in spin glasses: mean field vs. three dimensions David Yllanes, Alain Billoire, Andrea Maiorano, Enzo Marinari, Victor MartinMayor We use a sampledependent analysis, based on medians and quantiles, to analyze the behavior of the overlap probability distribution in spin glasses. Using analytical and numerical meanfield results for the SherringtonKirkpatrick model, as well as data from toy models, we show that this approach is an effective tool to distinguish the lowtemperature behavior of replica symmmetry breaking systems from that expected in the droplet picture. An application of the method to the threedimensional EdwardsAnderson models shows agreement with the replica symmetry breaking predictions. [Preview Abstract] 

Y29.00007: ABSTRACT WITHDRAWN 
Friday, March 6, 2015 9:48AM  10:00AM 
Y29.00008: Ground States of a Disordered Frustrated Quantum Dimer Magnet Alexander Hristov, Maxwell Shapiro, Ian Fisher, Minseong Lee, Linsey Rodenbach, Ashley Bernheisel, Eun Sang Choi, JuHyun Park, Leonardo Civale, Tim Munsie, Graeme Luke We present results of thermodynamic measurements of the sitediluted spindimer magnet Ba$_3($Mn$_{1x}$V$_x)_2$O$_8$, including magnetization, torque magnetometry, and AC susceptibility. The parent compound Ba$_3$Mn$_2$O$_8$ is a frustrated $S=1$ quantum dimermagnet with a singlet ground state, and triplet and quintuplet excitations. A magnetic field can be used to tune the energy spectrum of this system, yielding successive triplet and quintuplet condensates at low temperatures. Site substitution with $S=0$ V breaks Mndimers, introducing site disorder into the highfield ordered states. This substitution also introduces unpaired $S=1$ Mn ions, and it has been an open question whether such spins order at low temperatures. Here, we present evidence of the spinfreezing of unpaired Mn ions below 240mK for all compositions measured, from $x$=0.05 to 0.85. We also present the evolution of the high field ordered state with increasing disorder. [Preview Abstract] 
Friday, March 6, 2015 10:00AM  10:12AM 
Y29.00009: Absence of Magnetic Order and Persistent Spin Dynamics in Tb$_2$Ge$_2$O$_7$ Alannah Hallas, Angel ArevaloLopez, Murray Wilson, Lian Liu, J. Paul Attfield, Yasutomo Uemura, Chris Wiebe, Graeme Luke The terbium pyrochlores exhibit many unique magnetic properties, which has generated significant interest in this family of frustrated materials. A candidate spin liquid, Tb$_2$Ti$_2$O$_7$ fails to magnetically order, despite strong antiferromagnetic correlations. The application of external pressure has been found to produce partial antiferromagnetic order in Tb$_2$Ti$_2$O$_7$. Recently, we synthesized a new member of this family, Tb$_2$Ge$_2$O$_7$. Due to the large ionic radii decrease from titanium to germanium, Tb$_2$Ge$_2$O$_7$ can be considered a chemical pressure analog of Tb$_2$Ti$_2$O$_7$. However, neutron scattering measurements revealed an absence of magnetic order in Tb$_2$Ge$_2$O$_7$ down to 20 mK and dominant ferromagnetic correlations. Now, we have investigated the low temperature magnetism of Tb$_2$Ge$_2$O$_7$ with muon spin rotation. Our zero field $\mu$SR measurements confirm an absence of static order in Tb$_2$Ge$_2$O$_7$. We find a sharp increase in magnetic correlations below 10 K and persistent spin dynamics down to 25 mK. Our longitudinal field $\mu$SR measurements on Tb$_2$Ge$_2$O$_7$ at 25 mK are consistent with a system of fluctuating moments, with a fluctuation rate of 11 MHz. This fluctuation rate is nearly temperature independent below 2.5 K. [Preview Abstract] 
Friday, March 6, 2015 10:12AM  10:24AM 
Y29.00010: Local Probe Study of S=1 Spin Liquid Candidate Ba$_3$NiSb$_2$O$_9$ Jeffrey Quilliam, Fabrice Bert, Philippe Mendels, JeanChristophe Orain, Anthoni Manseau, C\'{e}line Darie, Christophe Payen, Catherine GuillotDeudon The family of hexagonal perovskites, Ba$_3M$Sb$_2$O$_9$, has attracted a considerable amount of attention in recent years, with the discovery of several spin liquid candidates. For $M=$Cu, the material is fairly disordered and likely exhibits a honeycomblike lattice whereas in other cases the structure consists of triangular planes of spins. Three different structural phases of Ba$_3$NiSb$_2$O$_9$ have been discovered, depending on synthesis pressure [1]. Two of these phases (6HA and 6HB) consist of triangular planes of $S=1$ moments, and differ primarily by the stacking of these planes. Here, we present muon spin rotation ($\mu$SR) and $^{121}$Sb nuclear magnetic resonance (NMR) results on a highpressure synthesis of this material, 6HBBa$_3$NiSb$_2$O$_9$. Most importantly, we demonstrate that there are no signs of magnetic ordering or spin freezing down to temperatures as low as 20 mK, making this material a plausible spin liquid candidate. Furthermore our NMR results are indicative of gapless excitations, consistent with previous specific heat and magnetic susceptibility results [1].\\[4pt] [1] J. G. Cheng \emph{et al.}, PRL {\bf 107}, 197204 (2011). [Preview Abstract] 
Friday, March 6, 2015 10:24AM  11:00AM 
Y29.00011: Giant anisotropic interactions in frustrated quantum magnet BiCu2PO6 Invited Speaker: Kemp Plumb I will discuss a series of comprehensive inelastic neutron scattering measurements which uncover the full magnetic excitation spectrum in the valence bond ordered compound BiCu$_2$PO$_6$. Owing to its frustrated geometry and potential to realize unique quantum phase transitions in high magnetic fields, BiCu$_2$PO$_6$ has received significant attention in recent literature. However, the true nature of the magnetic Hamiltonian responsible for the high field phenomena has not been known until now. I will present measurements of the spin excitation spectrum in BiCu$_2$PO$_6$ from which the magnetic Hamiltonian is elucidated. The spectrum is unique to the frustrated twoleg ladder geometry in BiCu$_2$PO$_6$ and we have been able to correctly describe the lowest energy excitations within the framework of a bondoperator theory, incorporating anisotropic magnetic exchange interactions which are comparable to the Heisenberg exchange terms. The anisotropic exchange interactions originate from spin orbit coupling and are of an unexpectedly large magnitude for a Cu based magnetic compound, potentially indicating the relevance of Bismuth in the superexchange pathway. BiCu$_2$PO$_6$ is a complex and unique quantum magnet combining frustration and anisotropic exchange; the discovery of such large anisotropic interactions in BiCu$_2$PO$_6$ hints at new routes for incorporating spin anisotropies in 3d transition metal based magnets. [Preview Abstract] 
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