Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W36: Focus Session: Frustrated and Low-D Magnetism -- Spin Ice |
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Sponsoring Units: DMP GMAG Chair: Xianglin Ke, Oak Ridge National Laboratory Room: E146 |
Thursday, March 18, 2010 11:15AM - 11:51AM |
W36.00001: Coulomb Phases in the spin ices Ho$_{2}$Ti$_{2}$O$_{7}$ and Dy$_{2}$Ti$_{2}$O$_{7}$ Invited Speaker: Recent experiments on the spin ices Ho$_{2}$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$ using polarized neutron scattering have revealed the pinch point scattering characteristic of dipolar, or ice rule, spin correlations. Such scattering has not previously been observed in the zero field spin ice state, but is strongly anticipated in theories of spin ice, where it is characteristic of the Coulomb phase which supports emergent magnetic monopole excitations. The scattering compares well with simple ice rule models, but there are extra contributions implying a modification of the pure ice rule constraint. I will discuss this comparison and its implication for the projective equivalence of near neighbour and dipolar spin ice Hamiltonians. Finally the effects of ice rule defects are clearly visible in the data, I will illustrate how their behaviour supports the picture of magnetic monopoles in spin ice. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W36.00002: Spin ice on the trillium lattice Travis Redpath, John Hopkinson We study a local ferromagnetic Ising model for classical spins on the trillium lattice. The ground state of this model features two spins out(/in) and one spin in(/out) on each triangle, and leads to a macroscopic ground state degeneracy. Our Monte Carlo simulations find a ground state entropy intermediate to that of spin ice on the kagome and pyrochlore lattices, suggesting that trillium spin ice is highly frustrated. To motivate the search for trillium spin ice, we calculate the magnetic susceptibility and structure factor. We note the qualitative resemblance of the susceptibility to previously published work on EuPtSi, which features local moments on the trillium lattice. [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W36.00003: Manifestations of monopole physics in spin ice: Thermal quenches Claudio Castelnovo, Roderich Moessner, Shivaji Sondhi We study the diffusion annihilation process which occurs when spin ice is quenched from a high temperature paramagnetic phase deep into the spin ice regime, where the excitations - magnetic monopoles - are sparse. We find that due to the Coulomb interaction between the monopoles, a dynamical arrest occurs, in which non-universal lattice-scale constraints impede the complete decay of charge fluctuations. This phenomenon is outside the reach of universal mean-field theory for a two-component Coulomb liquid. We identify the relevant timescales for the dynamical arrest and propose an experiment for detecting monopoles and their dynamics in spin ice based on this non-equilibrium phenomenon. [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W36.00004: Quantum melting of spin ice Shigeki Onoda, Yoichi Tanaka A quantum melting of the spin ice is proposed for pyrochlore-lattice magnets Pr2TM2O7 (TM =Ir, Zr, and Sn). The quantum pseudospin-1/2 model is derived from the strong-coupling perturbation of the f-p electron transfer in the basis of atomic non-Kramers magnetic doublets. The ground states are characterized by a cooperative ferroquadrupole and pseudospin chirality in the cubic unit cell, forming a magnetic analog of smectic liquid crystals. Then, pinch points observed in spin correlations for dipolar spin-ice systems are replaced with the minima. The relevance to experiments is discussed. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 1:03PM |
W36.00005: Dirac Strings and Magnetic Monopoles in the Spin Ice, Dy$_{2}$Ti$_{2}$O$_{7}$ Invited Speaker: Recent proposals in condensed matter physics that magnetic monopoles can appear as emergent quasiparticles have attracted wide levels of interest. Dirac's original picture of magnetic monopoles had them connected to strings through which magnetic flux flowed. Here we report studies into a system called Spin Ice. Spins on magnetic ions mimic the hydrogen bonds in water ice, obeying ``ice rules'' of 2 spins into and 2 spins out of their tetrahedron. In these materials it has been predicted that strings of spins form screening the applied magnetic field via a 3D Kasteleyn transition [1]. The geometry of spin-ice allows for net magnetic charge (magnetic monopoles) to form where ``ice rules'' are broken at the tips of the strings [2]. Here we present three experimental pieces of evidence for these strings and magnetic monopoles [3]. Magnetization data confirms the Kasteleyn transition leads to the formation of strings of spins. Neutron scattering then is used to measure the field dependent behavior of these strings, along which \textbf{B} flows towards the monopoles analogous to Dirac strings and therefore $\nabla \cdot \underline{B}=0$ remains intact. Finally heat capacity results can be described by a gas of magnetic monopoles interacting via the magnetic Coulomb interaction. \\[4pt] [1] L.D.C. Jaubert, J.T. Chalker, P.C.W. Holdsworth {\&} R. Moessner. \textit{Phys Rev. Lett}. 100, 067207 (2008) \\[0pt] [2] C. Castelnovo, R. Moessner {\&} S.L. Sondhi. \textit{Nature} 451, 42 (2008) \\[0pt] [3] D.J.P. Morris, D.A. Tennant, S.A. Grigera, B. Klemke, C. Castelnovo, R. Moessner, C. Czternasty, M. Meissner, K.C. Rule, J.-U. Hoffmann, K. Kiefer, S. Gerischer, D. Slobinsky {\&} R.S. Perry. \textit{Science} 326, 411 (2009) [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W36.00006: High Resolution Neutron Scattering Study of Ho$_{2}$Ti$_{2}$O$_{7}$ Jason Gardner I will review recent neutron scattering work on the pyrochlore oxides Ho$_{2}$Ti$_{2}$O$_{7}$, looking at the slow spin dynamics in the system. For many years now, the nuclear spin system was held responsible for the persistent (electronic) spin dynamics in Ho$_{2}$Ti$_{2}$O$_{7}$ at mK temperatures. We can now measure both spin systems directly with the improved signal to noise ratio seen at modern back scattering instrumentation. I hope to show this is not the case. To complicate the story further propagating magnetic ``monopoles'' have now been observed in this and other spin ice materials. I will comment on these exotic excitations and what our data can say about them. This work was performed in collaboration with Georg Ehlers and other at the SNS, in Oak Ridge. \\[4pt] [1] ``Direct observation of a nuclear spin excitation in Ho2Ti2O7'' G Ehlers, E Mamontov, M Zamponi, K C Kam and J S Gardner, Phys. Rev. Lett. \textbf{102}, 016405 (2009). \\[0pt] [2] ``Observation of Magnetic Monopoles in Spin Ice'' Kadowaki et al., J. Phys. Soc Japan \textbf{78} 103706 (2009) and Signature of magnetic monopole and Dirac string dynamics in spin ice'' Jaubert and Holdsworth, Nature Phys. 5, 258 (2009). [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W36.00007: Magnetic Monopole Dynamics in Artificial Kagome Ice Stephen Daunheimer, Yi Qi, John Cumings Using Lorentz-force TEM, we have directly observed the existence of magnetic monopoles in artificial kagome ice, as have been recently seen in the rare-earth titanate spin ices [1]. Monopoles arise in this system from the unique geometrical interactions that result~from~nanoscale patterning of a NiFe thin film. The monopoles act as independent magnetic charges, free to move through the kagome lattice. We will present empirical guidelines for the monopole dynamics and make connections with relevant theories.\\[4pt] [1] C. Castelnovo, R. Moessner, and S. L. Sondhi, Nature \textbf{451}, 42 (2008). [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W36.00008: Relating local correlations to interaction energies between neighboring ferromagnetic islands in a triangular-lattice geometry Sheng Zhang, Jie Li, Jason Bartell, Cristiano Nisoli, Paul Lammert, Vincent Crespi, Peter Schiffer We have studied geometrically frustrated arrays of single domain ferromagnetic islands in triangular lattices, in which the frustration comes from magnetostatic interactions between neighboring islands. The islands are elongated and aligned so that the magnetic moments are uniaxial. We fabricated several groups of these arrays with different lattice spacing, varying both the distance along the short-axis direction of islands and the distance along the long-axis direction. By analyzing the local correlations between neighboring islands, we observe effects of both direct pair-wise interactions and indirect interactions via a third island. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W36.00009: Comparing frustrated and non-frustrated clusters of single-domain ferromagnetic islands Jie Li, Sheng Zhang, Jason Bartell, Cristiano Nisoli, Paul Lammert, Vincent Crespi, Peter Schiffer We have studied the magnetic moment configurations of different geometry clusters of single-domain ferromagnetic islands. The microstates of small clusters can be exhaustively enumerated, which opens the system to additional modes of analysis, as compared to extended systems, both in terms of effective thermodynamic treatments and models of the kinetics of the annealing. Each of these clusters consisted of 4 islands arranged on the two perpendicular sides of a square, taken from the lattice of artificial spin ice [1, 2]. The magnetic moment configurations are imaged by magnetic force microscopy after effectively annealing through ac demagnetization. We then compared the results for cluster geometries with and without frustration of the magnetostatic interactions between the island moments. We find that non-frustrated clusters more readily achieve their lowest energy states than frustrated clusters and their moment configurations can be better described by a simple Boltzmann distribution. \\[4pt] [1] R. F. Wang et al., Nature 439, 303 (2006). \\[0pt] [2] X. Ke et al., Phys. Rev. Lett. 101, 037205 (2008). [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W36.00010: Towards an effective Hamiltonian for Tb$_{2}$Ti$_{2}$O$_{7}$ Paul McClarty, Hamid Molavian, Michel Gingras Tb$_{2}$Ti$_{2}$O$_{7}$ is a pyrochlore antiferromagnet that has dynamical spins and only short-range correlations even at $50$ mK $-$ the lowest temperature explored so far $-$ although the Curie-Weiss temperature is $\theta_{{\rm CW}} = -14$ K. The absence of long-range order in this material is not understood. Recently, magnetic quantum fluctuations have been shown to be significant in Tb$_{2}$Ti$_{2}$O$_{7}$. We present an effective Hamiltonian that takes into account virtual crystal field excitations (VCFEs). The semiclassical ground state phase diagram of this model allows one to see how the physics of spin ice is connected to the possible physics of Tb$_{2}$Ti$_{2}$O$_{7}$ and Tb$_{2}$Sn$_{2}$O$_{7}$. In addition to the dipolar spin ice model ground states, there is a $\mathbf{q}=0$ ordered ice state over a large part of the phase diagram $-$ ferromagnetic correlations being preferred by quantum corrections in spite of an antiferromagnetic nearest neighbor exchange in the microscopic model. Frustration is hence seen to arise from VCFEs over and above the effect of dipolar interactions in spin ice in inducing ice-like correlations. [Preview Abstract] |
Thursday, March 18, 2010 2:03PM - 2:15PM |
W36.00011: Magnetization dynamics in an artificial spin ice on kagome Olga Petrova, Paula Mellado, Oleg Tchernyshyov We study magnetization dynamics in an artificial spin ice on kagome realized as a honeycomb network of connected ferromagnetic nanowires studied recently by several experimental groups [1]. The sites of the honeycomb network carry magnetic charge, defined as the source of the magnetic field $\mathbf H$, of strength $\pm 1$ in suitably chosen units. Magnetization reversal in individual wires under the action of an applied magnetic field is mediated by the emission of a domain wall carrying magnetic charge $\pm 2$ at one of the wire's ends, its propagation along the wire and its absorption at the other end. We include the effects of quenched disorder, arising from lattice imperfections, domain wall's inertia, observed recently in permalloy nanowires, and magnetostatic interactions between magnetic charges [2]. The inertia and magnetostatic repulsion between like charges are responsible for avalanches in magnetization reversal observed experimentally [1]. That and an inherently dissipative character of the magnetization dynamics suggest interesting parallels with granular materials [3]. [1] Y. Qi, T. Brintlinger, and J. Cumings, Phys. Rev. B \textbf{77,} 094418 (2008). [2] E. Saitoh \textit{et al.}, Nature \textbf{432,} 203 (2004). [3] X. Ke \textit{et al.}, Phys. Rev. Lett. \textbf{101,} 037205 (2008). [Preview Abstract] |
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