Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y34: Focus Session: Frustrated and Low-D Magnetism -- Quantum Magnetism II |
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Sponsoring Units: DMP GMAG Chair: Alexander Chernyshev, University of California, Irvine Room: E144 |
Friday, March 19, 2010 8:00AM - 8:12AM |
Y34.00001: Order by static disorder in the Ising chain magnet Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ V. Kiryukhin, S. Lee, H.T. Yi, Y.J. Choi, S-W. Cheong, W. Ratcliff II, Q. Huang Ising chain compound Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ exhibits up-up- down-down long-range magnetic order (LRO) in a broad range of 0.75 $<$$x$$<$1. The LRO is abruptly lost in the narrow vicinity of $x$=1, and the magnetic state becomes incommensurate. The commensurate state (but not the LRO) is recovered for larger $x$. This is surprising because the stoichiometric $x$=1 state exhibits the best Co/Mn ionic order, and the magnetic LRO appears only in the samples with reduced ionic order. We argue that this ``order-by-static-disorder'' phenomenon may be related to the disruption of the long-range magnetic interactions by the magnetic-site disorder, reducing magnetic frustration. [Preview Abstract] |
Friday, March 19, 2010 8:12AM - 8:24AM |
Y34.00002: Inelastic neutron scattering studies of magnetism in A-site spinels S.E. Nagler, G.J. MacDougall, D. Gout, G. Ehlers, M.E. Hagen, J. Niedziela, T. Hong, D. Mandrus The A-site spinels, CoAl$_{2}$O$_{4}$ and FeAl$_{2}$O$_{4}$, have magnetic cations residing solely on the diamond sublattice. There is strong interest in these materials due to a the central role of frustration in determining physical properties. Despite large negative Curie-Weiss constants of $\vert \Theta \vert >$100K, in neither of these systems has long range order been reported. Instead, anomalous glass transitions are seen with T$^{\ast }$=6K and 12K, respectively. In CoAl$_{2}$O$_{4}$, the emergence of short-range spin-spin correlations has led to speculations of novel `spiral spin liquid' behaviour and perhaps an order-by-disorder transition. In FeAl$_{2}$O$_{4}$, alternate theories involving strongly entangled spin and orbital degrees of freedom may be more appropriate. In order to explore the nature of the magnetism in these systems, we have studied their excitation spectra with inelastic neutron using the HB-1 and CNCS instruments at the HFIR and SNS facilities, respectively. We will present the results of these studies, and discuss implications for the magnetic ground state. [Preview Abstract] |
Friday, March 19, 2010 8:24AM - 8:36AM |
Y34.00003: Short-range correlations in single-crystalline CoAl$_{2}$O$_{4}$ G.J. MacDougall, D. Gout, J.L. Zarestky, D. Mandrus, S.E. Nagler The A-site spinels, where magnetic cations reside on a diamond sublattice, have been receiving much attention in recent years. Order can be frustrated in these systems due to competing nearest and further neighbor exchange, and theoretical studies suggest a number of interesting ground states and a central role for fluctuations. One such spinel, CoAl$_{2}$O$_{4}$, has been held up as a material of particular interest. Despite a large Curie-Weiss constant of $\Theta \sim $104K, only a glassy transition is reported at T$^{\ast }\sim $4-10K. Preliminary results on powders has been associated with novel spin liquid behavior, but careful studies of single crystals are needed. To this end, we have grown several large single crystals of CoAl$_{2}$O$_{4}$ at ORNL via the floating zone method, and studied them with neutron scattering using the HB-1a beamline at the High Flux Isotope Reactor. We will present the results of these neutron scattering experiments, and discuss both the evolution of magnetic properties in this system and implications for existing theories. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 8:48AM |
Y34.00004: Diffuse Magnetic Scattering in GeNi$_{2}$O$_{4}$ M.K. Crawford, P. Manuel, D.T. Adroja, S. Hara, Y. Yoshida, S.I. Ikeda, J.W. Lynn, Y. Chen, R.A. Fisher The cubic spinel GeNi$_{2}$O$_{4}$, in which the spin-1 Ni$^{2+}$ ions are located at the vertices of a lattice of corner-sharing tetrahedra, exhibits interesting magnetic properties. For example, GeNi$_{2}$O$_{4 }$has two N\'{e}el transitions (T$_{N1}$ = 12.1 K and T$_{N2}$ = 11.4 K) that are not associated with any obvious structural phase transition. In the past we have reported the results of magnetic susceptibility, heat capacity, synchrotron x-ray powder diffraction, and neutron powder diffraction measurements for this material. In this talk we will describe the results of diffuse magnetic scattering measurements of the magnetic correlations in GeNi$_{2}$O$_{4}$ made at temperatures above and below T$_{N1}$ and T$_{N2}$. The samples were single crystals grown by the traveling solvent floating zone technique at AIST. The measurements were made using the PRISMA spectrometer at ISIS and a triple-axis spectrometer at NIST. [Preview Abstract] |
Friday, March 19, 2010 8:48AM - 9:00AM |
Y34.00005: Thermodynamic Spin Glass Transition in a Pyrochlore Heisenberg Model with Weak Disorder Ka-Ming Tam, Michel Gingras The conditions for spin glassy behaviors are frustration and disorder, with almost all cases the frustration originating from competing random antiferromagnetic and ferromagnetic couplings. The glassy behavior observed experimentally in the pyrochlore magnet, Y$_{2}$Mo$_{2}$O$_{7}$, where the magnetic Mo$^{4+}$ ions interact predominately via nearest neighbor antiferromagnetic exchange with weak disorder presents itself as a new class of spin glass material where the origin of the frustration is mostly geometrical. We provide numerical data obtained from advanced spin glass simulation techniques to show that weak disorder can generate a rugged energy landscape, and lead to a finite temperature spin glass transition in the antiferromagnetic Heisenberg model in pyrochlore lattice in presence of weak disorder. [Preview Abstract] |
Friday, March 19, 2010 9:00AM - 9:12AM |
Y34.00006: Long-Range Magnetic Ordering in Pyrochlore Iridate Eu$_2$Ir$_2$O$_7$ S. Zhao, J.M. Mackie, D.E. MacLaughlin, O.O. Bernal, Y. Ohta, S. Nakatsuji In the pyrochlore iridate~Eu$_2$Ir$_2$O$_7$ [1,\,2] Eu$^{3+}$ is nonmagnetic ($L = S$, $J = 0$) and $S(\mbox{Ir$^{4+}$}) = 1/2$~[1], so that it is a rare example of a frustrated $S=1/2$ pyrochlore. Spin-glass-like behavior at the metal-insulator transition (MIT) and no magnetic ordering down to 0.3~K have been reported for this compound~[2,\,3]. We discuss $\mu$SR measurements on Eu$_2$Ir$_2$O$_7$ polycrystalline samples that yield strong evidence for long-range magnetic ordering. We observe well-defined muon spin precession frequencies below $T_M \approx 120$~K, consistent with the MIT temperature~[2] but indicating long-range ordering instead of a spin-glass like transition. Significant dynamic muon spin relaxation persists to low temperatures, as is often the case in frustrated antiferromagnets. \par \noindent Work supported by NSF (U.S.), Grants~0801407 (UCR) and 0604105 (CSULA), and MEXT (Japan), Grants-in-Aid Nos.~17071003 and 19052003. \frenchspacing \par \noindent [1] B. J. Kim et al., Phys. Rev.Lett. 101, 076402 (2008). \par \noindent [2] N. Taira et al., J. Phys.: Condens. Matter 13, 5527 (2001). \par \noindent [3] C. L. Chien and A. W. Sleight, Phys. Rev. B 18, 2031 (1978). [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:24AM |
Y34.00007: Study of Magnetic Frustration in SrHo$_{2}$O$_{4}$ Sanhita Ghosh, Haidong Zhou, Christopher Wiebe, J.S. Gardner, Y. Qiu Geometrically frustrated systems are of interest because they exhibit novel behaviour due to lack of long range ordering at low temperatures. We have synthesized and measured the magnetic properties of lanthanide based single crystals of the formula SrL$_{2}$O$_{4}$ ; L=lanthanide. We report results obtained from magnetic susceptibility, heat capacity and neutron scattering measurements carried out on SrHo$_{2}$O$_{4}$ single crystals synthesized by the floating zone method. Analysis of elastic and inelastic neutron scattering data show evidence of short range ordering at low temperatures and carry the signatures of magnetic frustration. Experimental observations from elastic neutron scattering measurements show agreement with simulation results for a model based on nearest neighbour interactions between Ho spins. [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y34.00008: Dilatometric Study of LiHoF$_{4}$ In a Transverse Magnetic Field John Dunn, Claudia Stahl, Robert Hill Theoretical and experimental work have not presented a consistent picture of the phase diagram of the nearly ideal Ising ferromagnet LiHoF$_{4}$ in a transverse magnetic field. Using a capacitive dilatometer, we have investigated the thermal expansion and magnetostriction of LiHoF$_{4}$ in magnetic fields applied perpendicular to the Ising direction. Critical points for the ferromagnetic phase transition have been determined at low fields close to the classical phase transition. Excellent agreement has been found with existing experimental data suggesting that, in this regime, the current theoretical calculations have not entirely captured the physics of this interesting model system. [Preview Abstract] |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y34.00009: Supersolid phases in a realistic three-dimensional spin model Luis Seabra, Nic Shannon Supersolid phases, in which a superfluid component coexists with conventional crystalline long range order, have recently attracted a great deal of attention in the context of both helium physics and quantum spin systems. Motivated by recent experiments on AgNiO$_2$~[$1$], we study the magnetic phase diagram of a realistic three-dimensional spin model with single-ion anisotropy and competing interactions on a layered triangular lattice, using classical Monte Carlo simulation techniques, complemented by spin-wave calculations and Ginzburg-Landau theory. For parameters relevant to AgNiO$_2$~[$2$], we find a cascade of different phases as a function of magnetic field, including two phases which are supersolids the sense of Liu and Fisher [$3$]. One of these phases is continuously connected with the collinear ground state of AgNiO$_2$, and is accessible at relatively low values of magnetic field. The nature of this low-field transition, and the possibility of observing this new supersolid phase in AgNiO$_2$, are discussed. \\ \\ $[$1$]$ A. Coldea {\it et al.}, arXiv:0908.4169v1.\\ $[$2$]$ E. Wheeler {\it et al.}, Phys Rev B \textbf{79}, 104421, (2009).\\ $[$3$]$ K.S. Liu and M. E. Fisher, J Low Temp Phys \textbf{10}, 655 (1973). [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:00AM |
Y34.00010: Interplay of Quantum Criticality and Geometric Frustration in Columbite SungBin Lee, Ribhu Kaul, Leon Balents Motivated by CoNb$_2$O$_6$ (belonging to the columbite family of minerals), we theoretically study the physics of quantum ferromagnetic Ising chains coupled anti-ferromagnetically on a triangular lattice in the plane perpendicular to the chain direction. We combine exact solutions of the chain physics with perturbative approximations for the transverse couplings. When the triangular lattice has an isosceles distortion (which occurs in the real material), the $T=0$ phase diagram is rich with five different states of matter: ferrimagnetic, N\'eel, anti-ferromagnetic, paramagnetic and incommensurate phases, separated by quantum phase transitions. [Preview Abstract] |
Friday, March 19, 2010 10:00AM - 10:12AM |
Y34.00011: Voltage induced control and magnetoresistance of magnetically frustrated systems A. Kalitsov, M. Chshiev, B. Canals, C. Lacroix The discovery of giant magnetoresistance [1] (GMR) in magnetic nanostructures has generated a new field of spin-based electronics (spintronics) [2]. This advent has considerably increased an interest in related phenomenon in bulk materials, colossal magnetoresistance [3] (CMR), which is several orders higher than GMR, and can be viewed as an ``intrinsic'' property of material. The CMR is typically observed in certain manganite compounds with characteristic magnetic fields of several Tesla. Such fields make them inappropriate for use in spintronic applications where appropriate scale should be about Oersteds. Here we promote magnetically frustrated (MF) bulk materials [4] as a possible alternative for spintronic applications with high magnetoresistance (MR) which can be controlled with relatively small voltages. We demonstrate that MR of MF systems may reach extremely high values and their magnetic configuration may be controlled by applied voltage. The proposed phenomenon is the bulk material analog of spin transfer torque [5] used in spin-valve structures. This work was supported by Nanosciences Foundation (France). [1] M. Baibich et al, Phys. Rev. Lett. 61, 2472 (1988); [2] S. Wolf, Science, 294, 1488 (2001); [3] S. Jin et al, Science, 264, 413 (1994); [4] J. Gardner et al, arXiv:0906.3661; [5] J. Slonczewski, JMMM 159, L1 (1996). [Preview Abstract] |
Friday, March 19, 2010 10:12AM - 10:24AM |
Y34.00012: Cluster dynamics in pure and doped GGG D.M. Silevitch, M. Schmidt, S. Ghosh, G. Aeppli, T.F. Rosenbaum The Heisenberg spin system Gadolinium Gallium Garnet (GGG) exhibits a strong geometrical frustration, giving rise to local regions of correlated spins which largely decouple from the background spin bath. We discuss the effects of introducing low quantities ($\sim$1\%) of impurities, which relieve the frustration and hence act to set an upper bound on the characteristic cluster size. AC magnetic susceptibility is used to study the evolution of the system dynamics for a range of dopant concentrations, and the effects on cluster sizes and characteristic timescales are discussed. [Preview Abstract] |
Friday, March 19, 2010 10:24AM - 10:36AM |
Y34.00013: Can FeCrAs be understood in terms of frustrated classical spins? John Hopkinson, Travis Redpath, Alton Leibel, Hae-Young Kee Recently W. Wu {\it{et al.}} (EPL {\bf{85}} 17009 (2009)) have observed unusual non-Fermi liquid physics in a magnetically frustrated iron pnictide, FeCrAs. In this material, magnetic Cr atoms form distorted kagome planes which are coupled through planes of decoupled Fe trimers. We study a $J_1$ (Cr-Cr) - $J_2$ (Cr-Fe) classical extended Heisenberg model on this lattice by numerical minimization and classical Monte Carlo simulation. In our simplified model the Fe trimers are treated as single spins. In agreement with experiment, we find magnetic order for a wide range of parameters with wavevector k=(1/3,1/3,0). We derive exact expressions for the spin structure of the ground state. We calculate the heat capacity and magnetic susceptibility within this model and map out the phase diagram, which shows an unusual double transition in the vicinity of $|J_1/J_2|$ =0.4. [Preview Abstract] |
Friday, March 19, 2010 10:36AM - 10:48AM |
Y34.00014: Field-Induced Orbital Antiferromagnetism in Mott Insulators Khaled Al-Hassanieh, Cristian Batista, Gerardo Ortiz, Lev Bulaevskii We report on a new electromagnetic phenomenon that emerges in Mott insulators, i.e., materials that do not conduct electricity because of strong electronic Coulomb repulsion. The phenomenon manifests as antiferromagnetic ordering due to orbital electric currents which are spontaneously generated from the coupling between spin currents and an external homogenous magnetic field. This novel spin-charge current effect provides the mechanism to detect the so far elusive spin currents by means of unpolarized neutron scattering, nuclear magnetic resonance or muon spectroscopy. We illustrate this mechanism by solving a half-filled Hubbard model on a frustrated ladder, a simple but nontrivial case of strongly interacting electrons. [Preview Abstract] |
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