Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Z15: Focus Session: Frustrated Systems: Artificial and Disordered |
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Sponsoring Units: GMAG DMP Chair: John Cumings, University of Maryland Room: 317 |
Friday, March 22, 2013 11:15AM - 11:27AM |
Z15.00001: Two-Dimensional Magnetic Correlations and Partial Long-Range Order in Geometrically Frustrated Sr$_2$YRuO$_6$ Eduardo Granado, Jeffrey W. Lynn, Renato F. Jardim, Milton S. Torikachvili Geometrically frustrated magnets are fascinating materials displaying a rich variety of physical states. The simplest three-dimensional structure leading to frustrated magnetism and the first one to be investigated is the face-centered cubic (FCC) lattice with antiferromagnetic nearest-neighbor interactions. Sr$_2$YRuO$_6$ is a particular example of this, crystallizing in the ordered double perovskite structure with the Ru$^{5+}$ ions defining an FCC magnetic network. Neutron diffraction experiments were performed on this material, revealing planar magnetic correlations that condense into a partial long-range ordered state with coupled alternate antiferromagnetic (AFM) YRuO$_{4}$ square layers coexisting with the short-range correlations below $T_{N1}=32$ K. A second transition to a fully ordered AFM state below $T_{N2}=24$ K is observed. The reduced dimensionality of the spin correlations in an FCC lattice is arguably due to a cancellation of the magnetic coupling between consecutive AFM square layers. The interesting magnetic phenomena observed here in Sr$_2$YRuO$_6$ are entirely driven by its lattice geometry, and may also occur in other FCC antiferromagnets. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z15.00002: Neutron Scattering Study of the Double Perovskite FCC Antiferromagnet Ba$_2$YRuO$_6$ J.P. Carlo, J.P. Clancy, K. Fritsch, C.A. Marjerrison, G.E. Granroth, H.A. Dabkowska, B.D. Gaulin Magnetic cations in the rock-salt ordered double perovskite structure comprise a geometrically frustrated FCC network of edge-sharing tetrahedra. Previous measurements of the $4d^3$ Ru$^{5+}$ system Ba$_2$YRuO$_6$ [1] indicated the existence of long-range commensurate antiferromagnetic order below $T_N$ = 36K, a factor f $\sim$ 15 lower than the Curie-Weiss temperature $\Theta_W$ = -522K. We report time-of-flight neutron spectroscopy of Ba$_2$YRuO$_6$ confirming the existence of the long-range ordered state below $T_N$. The magnetic inelastic scattering extends over a bandwidth of $\sim$ 15 meV, and develops a $\sim$ 5 meV gap at the [100] magnetic ordering wavevector at and below $T_N$. Strong spin-orbit coupling in this $4d^3$ system is expected to result in a $j_{eff}$=3/2 magnetic moment. This is distinct from the $4d^1$, $j_{eff}$=3/2 moment which arises in its sister antiferromagnetic FCC compound, Ba$_2$YMoO$_6$, which displays an apparent singlet ground state and a $\sim$ 28 meV singlet-tripet gap at low temperatures [2], matching its |$\Theta_W$| $\sim$ 300K. [1] T. Aharen et al. Phys. Rev. B 80, 134423 (2009). [2] J. P. Carlo et al. Phys. Rev. B 84, 100404(R) (2011). [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z15.00003: Phase diagram of split 2D dipolar spin ice Tommaso Roscilde, Louis-Paul Henry Long-ranged dipolar interactions, which are very natural in artificial square-lattice spin ice, can mask some of the most relevant aspects of spin-ice physics, as they remove the extensive degeneracy of the ground state manifold to give a unique ground state, and they bind monopole pairs into localized spin flips. Following an earlier idea of G. M\"oller and R. Moessner [Phys. Rev. Lett. 96, 237202 (2006)] we investigate how adding a third direction to square ice allows to recover fundamental traits of spin-ice physics even in the presence of dipolar interactions. Using Monte Carlo simulations based on a generalized loop algorithm, we explore the phase diagram of square dipolar spin ice in which horizontal and vertical dipoles are spatially separated in a third direction (split 2D spin ice). As a function of the splitting we recover a two-fold degenerate staggered state for coplanar dipoles, and a four-fold degenerate ``Manhattan'' state for strongly split dipoles, separated by a first order transition. The competition between the two states at intermediate splitting leads to a strong suppression of the ordering transition temperatures, and makes space for the observation of a hallmark of spin-ice physics in the paramagnetic phase: pinch points in the static structure factor. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z15.00004: Phase diagram of quantum square ice Louis-Paul Henry, Peter Holdsworth, Frederic Mila, Tommaso Roscilde We have investigated the ground-state and finite-temperature phase diagram of quantum square ice - realized by the transverse-field Ising model on a checkerboard lattice - using both linear spin-wave (LSW) theory and quantum Monte Carlo (QMC). We generalize the model with different couplings between nearest ($J_1$) and next-to-nearest ($J_2$) neighbors on the checkerboard lattice. Our QMC approach generalizes the loop algorithm - very efficient in the study of constrained classical systems - to a ``brane algorithm'' for quantum systems. At the LSW level the vast degeneracy of the ground-state for $J_1=J_2$ and $J_2>J_1$ remains intact; moreover LSW theory breaks down in extended regions of the phase diagram, pointing at non-classical states [1]. Our QMC study goes beyond perturbative schemes and addresses directly the nature of the low-temperature phases. We have critically examined the possibility of a resonating-plaquette state for $J_1=J_2$, suggested by degenerate perturbation theory on the ice-rule manifold for weak fields. Our QMC results for finite fields confirm the absence of N\'eel or collinear order, but they do not confirm the presence of resonating-plaquette order, pointing at a possibly more complex non-classical state.\\[4pt] [1] L.-P. Henry et al., PRB 85, 134427 (2012). [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z15.00005: Resonant dynamics of Dirac monopoles and strings in an artificial spin-ice lattice Olle Heinonen, Sebastian Gliga, Attila Kakay, Riccardo Hertel Spin ices can occur in atomic structures, in which the magnetic interaction with neighboring vertices cannot be simultaneously minimized at each atomic vertex, leading to frustration. Artificial spin-ice lattices are arrays of patterned elements geometrically arranged to mimic the frustration in such atomic lattices, and have the advantage that their properties are directly observable using microscopy techniques. Artificial spin-ices can support topological defects, such as Dirac monopoles and Dirac strings connecting the monopoles. We present micromagnetic simulations of the resonant dynamics of a square spin-ice lattice. The simulations predict that topological defects give rise to specific signatures in the excitation spectrum of the lattice and that, moreover, the amplitude of a defect-specific resonant peak increases linearly with the number of defects or length of the Dirac line. A measured spectrum therefore allows to both identify the defects in an array as well as to determine their number. In addition, we observe that the main bulk-like FMR spectral peak is split in the presence of defects in the lattice, compared to a defect-free reference state. This splitting is caused by Dirac strings, in which the FMR frequency is increased due to the different magnetostatic coupling of the elements within a string as compared to the rest of the lattice. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z15.00006: Magnon lifetime in the easy-plane antiferromagnets Alexander Chernyshev, Michael Zhitomirsky Considering a prototypical 2D easy-plane antiferromagnet on a square lattice in which gapped excitations coexist with acoustic spin waves, we find that random disorder induces a relaxation of the gapped magnon with the rate that greatly exceed the effect of conventional magnon-magnon scattering. Anomalous disorder-induced $T$-dependence of the energy gap of the optical magnon and of the scattering rate of the acoustic magnon are also discussed. These can be readily probed by the high-resolution neutron-resonance spin-echo technique. Implications for other systems are discussed and comparison with the available experimental data is presented. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z15.00007: Magnetic enhancement and cluster-glass behavior in (Sc$_{1-x}$Er$_x$)$_{3.1}$In Eteri Svanidze, Emilia Morosan Sc$_3$In is a weak itinerant ferromagnet with no magnetic constituents. In this talk, we will present DC and AC magnetization data on Sc$_{3.1}$In doped with Er$^{3+}$ local moment ions. As x increases in (Sc$_{1-x}$Er$_x$)$_{3.1}$In, the Weiss temperature nearly triples up to $x \leq 0.1$. The effective moment per formula unit is larger than the simple sum of the itinerant moment in pure Sc$_{3.1}$In and the Er$^{3+}$ local moment. Moreover, Er doping of as little as $x = 0.02$ induces a cluster-glass state. The glassy behavior persists up to $x = 0.1$, and a structural transition likely occurs for higher doping levels. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z15.00008: Spin Glass Phase in the Disordered Spin Systems Nvsen Ma, Dao-Xin Yao, Anders Sandvik We use quantum Monte Carlo simulations to study a glassy ground state of S = 1/2 quantum spins by using a dimerized J1-J2-J3 Heisenberg model on the square lattice. J1 corresponds to weak bonds, and J2 and J3 are stronger bonds which are randomly distributed on columnar rungs forming coupled 2-leg ladders. By tuning the average value of J2 and J3, the system undergoes Neel glass paramagnetic quantum phase transition. The size of the glass region is affected by the value of the disorder strength. In the glass phase, we find that the uniform susceptibility decreases with T according to $exp(1/T^a)$ with $a<1$; thus the state is incompressible at $T = 0$ and classified as a Mott glass (MG). At the Neel-MG transition, the susceptibility behaves as $T^{2/z-1}$, where z is the dynamical exponent and it is close to 1. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z15.00009: Microscopic Coexistence of Antiferromagnetic and Spin-Glass States Shravani Chillal, Matthias Thede, Fred Jochen Litterst, Severian Gvasaliya, Tatiana Shaplygina, Sergey Lushnikov, Andrey Zheludev The disordered antiferromagnet PbFe1/2Nb1/2O3 (PFN) is investigated in a wide temperature range by combining Mossbauer spectroscopy and neutron diffraction experiments. It is demonstrated that the magnetic ground state is a microscopic coexistence of antiferromagnetic and spin-glass orders. This speromagnet-like phase features frozen-in short-range fluctuations of the Fe3$+$ magnetic moments that are transverse to the long-range ordered antiferromagnetic spin component. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z15.00010: Randomness effects on the distorted triangular-lattice antiferromagnets Toru Sakai, Hiroki Nakano The quantum spin liquid-like behaviors were observed on some distorted tirangular-lattice antiferromagnet organic compounds[1]. If the lattice vibration is much slower than the spin dynamics, the lattice distortion possibly plays a role of the bond randomness in the spin system. Thus inorder to explain the observed spin liquid behavior of the organic compound, we consider the antiferromagnetic Heisenberg model on some distorted triangular lattices with the bond randomness. Using the numerical exact diagonalization we calculated the standard spin glass order parameter, as well as the ordinary Neel order parameter. The present study suggested that the spin glass order can survive in some region where the Neel order vanishes. We will discuss a scenario of the observed spin liquid behavior of the distorted triangular-lattice antiferromagnets, based on the result. [1] Y. Shimizu, K. Miyagawa, K. Kanoda, M. Maesato and G. Saito, Phys. Rev. Lett. 91 (2003) 107001. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z15.00011: Frustrated antiferromagnetism in bulk Ti-doped BiFeO$_3$ ceramics Miguel Angel Garcia, Mara Bernardo, Teresa Jardiel, Marco Peiteado, Federico Mompean, Mar Garcia-Hernandez, Marina Villegas, Amador Caballero We present here a magnetic characterization of Ti-doped BiFeO$_3$ prepared by a ceramic route. A detailed analysis of the microstructure revealed that Ti$^{+4}$ is not homogeneously distributed but partially segregated towards Ti enriched grain boundary regions that define Ti-deficient domains with a size distribution of the order of tens of nanometers. Since the size of these domains is smaller than the spin cycloid wavelength (64 nm) they hold a net magnetic moment. Consequently the material exhibits frustrated antiferromagnetism with hysteresis, coercivity and remanance. This ferromagnetic-like behavior vanishes at the Neel temperature of the BiFeO$_3$. The small magnetic moment per domain (M$_{\mathrm{S}}$ $\sim$0.2 emu/g) yields very large coercive fields of 27 KOe at 5 K and 17 KOe at room temperature. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z15.00012: Modeling geometric frustration with magnetic colloids Gabi Steinbach, Sibylle Gemming, Artur Erbe, Dennis Nissen, Manfred Albrecht The implementation of artificial frustrated spin systems can give insight into the mechanisms which lead to the different equilibrium configurations in geometrically frustrated magnetic materials. Prominent artificial systems are the patterns of superconducting rings or nanometer-sized ferromagnetic islands. These are Ising systems allowing two possible in-plane orientations for each macroscopic moment. Here we present an alternative method using magnetically interacting colloids. The spherical shape of the particles allows the modeling of spin systems with continuous symmetry. Micrometer-sized silica particles are half capped with a Co/Pd multilayer thin film. Such thin films on curved surfaces have a predefined net magnetic moment called macrospin. The interaction of such particles in the self-arranged close-packed 2D structure is frustrated. Using video microscopy, the direction of the individual macrospins can be visualized by the spatial orientation of the magnetic caps. This setup allows us to investigate geometric frustration in static systems and in dynamic processes, e.g. during the particle-wise cluster growth. Further, we evaluate the impact of intrinsic defects and control defect formation extrinsically e.g. by varying the growth conditions. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z15.00013: Effect of proton irradiation on the magnetic and magneto-transport properties of TbFeCo metglass Nattawut Anuniwat, Manli Ding, Joseph Poon, Jiwei Lu, Brad Weaver The ferrimagnetism in amorphous rare-earth transition metal alloys is well known, and has recently been investigated for applications in perpendicular magnetic random access memory (p-MRAM), which is considered to be a universal memory technology due to the low power dissipation and the non-volatility. The amorphous TbFeCo thin films were deposited by rf magneton sputtering. The as-deposited film exhibited a low saturation magnetization ($M_{S}\sim $100 emu/cc) and a high perpendicular anisotropy ($K_{U}\sim $10$^{6}$ erg/cc). Hall-bar devices were fabricated for characterizing the magneto-transport behaviors. Both thin film samples and Hall bar devices were exposed to 2 MeV-energy protons with incremental fluences. Magneto-transport and standard magnetic measurements are employed to investigate defects/displacement damages. The magneto-transport suggests that compensation temperature of the film decreases after irradiations. The changes in saturation moments and coercive fields will be discussed as a function of total fluence, which may be related to structural damages. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z15.00014: Heat Capacity and Magnetic Properties of NiMnIn, NiCoMnIn and NiCoMnSn Metamagnetic Materials Jing-Han Chen, Joseph H. Ross, Jr., N. Bruno, J. Monroe, I. Karaman, Jianguo Li NiMnIn and NiMnSn Heusler materials feature coupled glassy magnetic and martensitic structural transformations. Co substitution can induce a large magnetocaloric effect near room temperature with little hysteresis, leading to interest for solid-state refrigeration as well as energy recovery. Recent work has also identified NiMnIn compositions with extremely sharp coupled magnetic-structural transformations. We report thermodynamic measurements for a number of these systems. NiMnCoSn melt-spun tapes can be processed to exhibit more well-defined martensitic transformations, however we find the magnetic contributions to the heat capacity to be similar to that of bulk materials. In both cases the magnetic entropy agrees with what is expected for $J$ = 2 Mn ions according to the alloy composition. A significant difference in electronic $\gamma$ points to electronic differences despite the same $e/a$ ratio. NiMnIn alloys include compositions exhibiting sharp $\lambda$-like transitions, while in bulk NiCoMnIn compositions the magnetic contribution is increased over the Mn-only value. The kinetic arrest behavior reduces the total magnetic entropy in these materials, and we also examine low-temperature anomalies as further indications of the glassy properties of these materials. [Preview Abstract] |
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