Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P31: Focus Session: Spin Ice |
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Sponsoring Units: GMAG Chair: Oleg Tchernyshyov, Johns Hopkins University Room: 335 |
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P31.00001: Magnetothermodynamics of spin ice and related compounds Invited Speaker: Xianglin Ke Geometrically magnetic frustration, which results from the competition of spin-spin interactions of magnetic ions on a regular magnetic lattice, leads to a variety of exotic low temperature states including ``spin ice.'' ``Spin ice'' refers to a magnetic state wherein the two-in/two-out spin configurations of rare earth pyrochlore compounds mimic the proton positions in the water ice, characterized by the ``zero point entropy'' of (R/2) ln(3/2). In this study, we examine how structural disorder affects spin dynamics and the magnetic ``zero point entropy.'' By diluting the ``spin ice'' materials with nonmagnetic ions on the rare earth sites, we have found that the entropy of the diluted species depends non-monotonically on the dilution concentration, and we explain this behavior using a generalized Pauling approximation. Nonmagnetic doping on B sites leads to only a small decrease of the ``zero point entropy,'' indicating the robust nature of ``spin-ice.'' We have also studied Dy2Ge2O7, which has the same chemical formula as ``spin ice'' materials and Ising-like spins but a tetragonal structure. Dy2Ge2O7 undergoes a long range antiferromagnetic ordering transition, but the spin dynamics at temperatures above the order transition is similar to that observed in the canonical ``spin ice'' systems, suggesting that such dynamics are generic to a broader class of Ising-like rare earth systems. \\[4pt] References: \\[0pt] [1] X. Ke \textit{et al}., Phys. Rev. Lett. \textbf{99}, 137203 (2007).\\[0pt] [2] X. Ke \textit{et al}., Phys. Rev. B. \textbf{76}, 214413 (2007).\\[0pt] [3] X. Ke \textit{et al}., Phys. Rev. B. 7\textbf{8}, 104411 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P31.00002: Magnetic charge order in kagome spin ice with dipolar interactions Gia-Wei Chern, Paula Mellado, Oleg Tchernyshyov Dipolar interactions in spin ice are described most effectively in terms of magnetic charges residing on the dual lattice [1]. While spin ice on the pyrochlore lattice contains no magnetic monopoles at low temperatures, spin ice on kagome [2] contains a unit magnetic charge ($\pm 1$) on every triangle. With the aid of Monte-Carlo simulations, we show that long-range Coulomb interaction between the monopoles lifts the degeneracy of the spin-ice states and induces a phase transition into a state with ordered magnetic charges but no spin order. The residual entropy is reduced from the spin-ice value but remains extensive. The phase transition is continuous with critical exponents close to the two-dimensional Ising universality class. [1] C. Castelnovo, R. Moessner, and S. L. Sondhi, Nature {\bf 451,} 42 (2008). [2] A. S. Wills, R. Ballou, and C. Lacroix, Phys. Rev. B {\bf 66}, 144407 (2002). [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P31.00003: Comparing artificial frustrated magnets: geometric effects in nanomagnet arrays Jie Li, Xianglin Ke, Cristiano Nisoli, Paul Lammert, Vincent Crespi, Peter Schiffer We have studied arrays of single-domain ferromagnetic islands arranged on lattices such that the magnetostatic interactions between the islands are frustrated by the geometry of the arrays. We compare results for three different lattice geometries: the previously studied square ``artificial spin ice'' lattice[1,2], a hexagonal lattice, and a ladder lattice which is topologically-equivalent to the former one. After the ac demagnetization the magnetic moment configurations are imaged via Magnetic Force Microscopy (MFM). We find that the ladder lattice shows local correlations which are similar to those of the square lattice, suggesting it as a basis for comparison of the energetics of the other two lattices. The normalized magnetostatic energy of all three geometries decreases with decreasing demagnetization step size, but the lattices approach their ground states at different rates. 1. R. F. Wang, C. Nisoli, R. S. Freitas, J. Li, W. McConville, B. J. Cooley, M. S. Lund, N. Samarth, C. Leighton, V. H. Crespi, and P. Schiffer, Nature 439, 303 (2006). 2. X. Ke, J. Li, C. Nisoli, P. E. Lammert, W. McConville, R. F. Wang, V. H. Crespi, and P. Schiffer, Phys. Rev. Lett. 101, 037205 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P31.00004: Cooperative Paramagnetic Behavior in the Stuffed Pyrochlore Tb$_{2+ \textit{x}}$Ti$_{2-2 \textit{x}}$Nb$_{ \textit{x}}$O$_{7}$ B.G. Ueland, J.S. Gardner, M.L. Dahlberg, P. Schiffer, A.J. Williams, J.G. Kim, R.J. Cava The pyrochlore Tb$_{2}$Ti$_{2}$O$_{7}$ is a cooperative paramagnet that has generated much interest in the frustrated magnetism community due to the presence of persistent short range spin-spin correlations and its apparent lack of long range magnetic order down to temperatures below $\textit{T}$ = 0.05 K, despite an effective spin-spin interaction strength given by \textit{$\theta_{W}$} $\sim$ 10 K. Motivated by recent work on the stuffed spin ices, we have begun investigations into stuffed variants of Tb$_{2}$Ti$_{2}$O$_{7}$, in which we replace some of the nonmagnetic Ti$^{4+}$ with magnetic Tb$^{3+}$, thus gradually changing the geometry of the magnetic sublattice from a lattice of corner sharing tetrahedra to a disordered lattice of side sharing tetrahedra. Here, we present results from magnetization and neutron scattering studies on powder samples of Tb$_{2+ \textit{x}}$Ti$_{2-2 \textit{x}}$Nb$_{ \textit{x}}$O$_{7}$, where $\textit{x}$ = 0.2, 0.4, 0.6, or 1, where diffraction data indicate that the $\textit{x}$ = 1 material has a disordered fluorite lattice. Preliminary results indicate that short range magnetic correlations similar to those present in Tb$_{2}$Ti$_{2}$O$_{7}$ exist in all of the materials studied, and that long range antiferromagnetic order may exist in the $\textit{x}$ = 1 material. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P31.00005: Quantized Berry phase and entanglement entropy for a spin ladder system Isao Maruyama, Shou Tanaya, Mitsuhiro Arikawa, Yasuhiro Hatsugai We have demonstrated that quantized Berry phases and entanglement can be used as a new tool for exploring gapped systems which do not exhibit symmetry breaking. Especially, using quantized Berry phases in several gapped systems we identified location of singlet pairs successfully, which illustrate the valence bond solid (VBS)[1], the itinerant singlet [2], the Kondo singlet. Recently, we have studied four-spin ring exchange interaction in a spin-1/2 two-leg ladder, which introduces frustration and generates various phases, such as the dominant vector-chirality (DVC) phase. In the DVC phase, the Berry phase detects the plaquette singlet[3] while entanglement entropy shows that degree of freedom of the edge state is non-zero. It means that singlets are localized at every links and separated as a free spin by adopting the boundary as in the VBS phase. In fact, under the open boundary condition, we found S=1 three-fold degenerated excited states with the small gap depends on the system-size as in the Kennedy triplet. [1] T.Hirano, H.Katsura, Y.Hatsugai, PRB 77 (2008) 094431 [2] I.Maruyama, Y.Hatsugai, JPSJ 76 (2007) 113601 [3] I.Maruyama, T.Hirano, Y.Hatsugai, AX.0806.4416 [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P31.00006: Tuning magnetic frustration of nanomagnets in triangular-lattice geometry X. Ke, J. Li, S. Zhang, C. Nisoli, V. Crespi, P. Schiffer We study the configuration of magnetic moments on triangular lattices of single-domain ferromagnetic islands, examining the consequences of magnetostatic interactions in this frustrated geometry. By varying the island-island distance along one direction, we are able to tune the ratio of different interactions between neighboring islands, resulting in a corresponding variation in the local correlations between the island moments. Unlike other artificial frustrated magnets, this lattice geometry displays regions of ordered moment orientation, possibly resulting from a higher degree of anisotropy leading to a reduced level of frustration. Reference: X. Ke \textit{et al}., Appl. Phys. Lett., in press (2008). [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P31.00007: Magnetization Reversal in Artificial Kagome Ice Stephen Daunheimer, Yi Qi, Todd Brintlinger, Paula Mellado, Oleg Tchernyshyov, John Cumings Lorentz-force transmission electron microscopy (LTEM) and ferromagnetic resonance (FMR) have been used to examine magnetization reversal of artificial kagome ice in detail. Previously, no ice rule violations have been observed in this novel magnetic metamaterial [1], in contrast to artificial square ice, where ice rule violations appear to be a robust feature [2]. Theoretical considerations support this observation but predict the generation of transient defects violating the ice rules during magnetization reversal. Such defects are expected to self-annihilate upon completion of the reversal process or removal of the applied magnetic field. We will present results on the experimental exploration for ice-rule violations in this system both during field rotation and uniaxial field reversal using both FMR and LTEM. [1] Y. Qi et al., Phys. Rev. B 77, 094418 (2008) [2] X. Ke et al., Phys. Rev. Lett. 101, 037205 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P31.00008: Equilibration and response properties in spin ice systems Claudio Castelnovo, Roderich Moessner, Shivaji Sondhi It was recently argued that magnetic monopoles emerge in a class of exotic magnets known as spin ice: the dipole moment of the underlying electronic degrees of freedom fractionalises into deconfined monopoles. Here we investigate analytically and numerically the effects that these peculiar excitations have on the equilibration and response properties of a system. In particular, we study temperature quenches in exhaustive detail. The implications of these results on the possibility of finding new experimental signatures of magnetic monpole excitations in rare earth titanates (Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$) are discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P31.00009: Monopole and Dirac string constrained Dynamics in Spin Ice Ludovic Jaubert, Peter Holdsworth Since its discovery approximately 10 years ago [1], spin ice has proved to be an abounding source of exotic collective phenomena, in particular long-range dipolar correlations and unconventional phase transitions. But its most impressive property is undoubtedly the occurrence of 3d fractionalisation through excitations out of its topological ground state, behaving like effective \textit{magnetic monopoles} [2]. I shall present an experimental signature of their influence in magnetic relaxation measurements for a spin ice material Dy$_{2}$Ti$_{2}$O$_{7}$ [3] and show that the observed low temperature dynamical slow down can be explained quantitatively by the Coulomb interactions between monopoles and the overlapping of Dirac strings filling the quasi-particle vacuum [4]. \\[3pt] [1] M. Harris \& al. Phys. Rev. Lett. \textbf{79}, 2554 (1997). \\[0pt] [2] C. Castelnovo \& al. Nature \textbf{451}, 42-45 (2008). \\[0pt] [3] J. Snyder \& al. Phys. Rev. B \textbf{69}, 064414 (2004). \\[0pt] [4] L.D.C. Jaubert \& P.C.W. Holdsworth (submitted) [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P31.00010: Slow spin relaxation in dipolar spin ice. Martin Orendac, Lucia Sedlakova, Alzbeta Orendacova, Peter Vrabel, Alexander Feher, Daniel M. Pajerowski, Justin D. Cohen, Mark W. Meisel, Masae Shirai, Steven T. Bramwell Spin relaxation in dipolar spin ice Dy$_{2}$Ti$_{2}$O$_{7}$ and Ho$_{2}$Ti$_{2}$O$_{7}$ was investigated using the magnetocaloric effect and susceptibility. The magnetocaloric behavior of Dy$_{2}$Ti$_{2}$O$_{7 }$at temperatures where the orientation of spins is governed by ``ice rules`` (T $<$ T$_{ice})$ revealed thermally activated relaxation; however, the resulting temperature dependence of the relaxation time is more complicated than anticipated by a mere extrapolation of the corresponding high temperature data [1]. A susceptibility study of Ho$_{2}$Ti$_{2}$O$_{7 }$ was performed at T $>$ T$_{ice }$and in high magnetic fields, and the results suggest a slow relaxation of spins analogous to the behavior reported in a highly polarized cooperative paramagnet [2]. [1] J. Snyder et al., Phys. Rev. Lett. 91 (2003) 107201. [2] B. G. Ueland et al., Phys. Rev. Lett. 96 (2006) 027216. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P31.00011: Quantum Spin Ice for Pr Pyrochlore Magnets Shigeki Onoda, Yoichi Tanaka We theoretically propose a new state comprising a quantum-mechanical analogue of the spin ice for pyrochlore magnets. In classical spin-ice systems like Dy$_2$Ti$_2$O$_7$, the ice rule is mainly driven by the magnetic dipolar interaction, which is proportional to the square of the total angular momentum $J$. Therefore, for Pr$^{3+}$ ions having two $f$ electrons forming the $J=4$ localized moment, the dipolar interaction becomes an order of magnitude smaller than that for Dy ions. Then, the magnetic superexchange interaction should play a important role. In fact, the form of the exchange interaction is nontrivial because of the highly relativistic nature of $f$ electrons with strong LS coupling and crystal-field effect. Here, we present a microscopic derivation of the effective relativistic spin-orbital Hamiltonian for the pyrochlore magnets Pr$_2TM_2 $O$_7$ with a transition-metal element $TM$. Then, it is shown that the nearest-neighbor exchange interaction is significantly modified from antiferromagnetic to ferromagnetic by quantum- mechanical processes through excited states split by the crystal field. This bears a quantum-mechanical formation of the ice rule for Pr magnetic moments. Solving the Hamiltonian for a Pr$_4$O tetrahedral cluster, we obtain a further small energy splitting of the low-energy states, leaving doubly degenerate ground states per tetrahedron. Implications for the lattice model and possible relevance to experiments are also discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P31.00012: SrEr$_2$O$_4$ in an applied magnetic field --a quantum phase transition? O.A. Petrenko, G. Balakrishnan, T.J. Hayes, P. Manuel, D.T. Adroja, L.C. Chapon SrEr$_2$O$_4$ belongs to a family of materials with the formula Sr$Ln_2$O$_4$, where $Ln=$ Gd, Dy, Ho, Er, Tm and Yb. In these compounds the magnetic $Ln$ ions are linked through a network of triangles and hexagons~[1]. Despite the strong exchange interaction ($\Theta_{CW}\!\! \approx \! -12$~K), long range ordering develops in SrEr$_2$O$_4$ only at 0.75~K~[2]. The structure consists of FM chains running along the {\it c}~axis, two adjacent chains being stacked antiferromagnetically. The moments point along the {\it c} direction, but only one of the two Er sites has a sizeable moment of 4.5~$\rm \mu_B$. An unusual behaviour in SrEr$_2$O$_4$ is observed in an applied field, where for $H\!\! \parallel \! c$ axis, a field of 0.5~T completely destroys long range magnetic order and introduces instead some sort of state with short range magnetic correlations. This conclusion is reached on the basis of neutron diffraction experiment at ISIS, where a replacement of the sharp Bragg peaks by broad diffuse scattering features is observed. A further increase in magnetic field causes a restoration of the long range order and a disappearance of the diffuse scattering. These observations resemble the behaviour seen around a quantum critical phase transition, although additional investigations are required to prove the presence of a QCP in SrEr$_2$O$_4$. [1]. H.~Karunadasa {\it et al.}, Phys. Rev. B {\bf 71}, 144414 (2005). [2]. O.A.~Petrenko {\it et al.}, Phys. Rev. B {\bf 78}, 184410 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P31.00013: Dynamic spin ice: Pr$_{2}$Sn$_{2}$O$_{7}$ Christopher Wiebe, Haidong Zhou, John Janik, Luis Balicas, Youn-joon Jo, Yiming Qiu, John Copley, Jason Gardner In this presentation, we report a new spin ice - Pr$_{2}$Sn$_{2}$O$_{7}$ - which appears to have enhanced residual entropy due to the dynamic nature of the spins. Neutron scattering experiments show that at 200 mK, there is a significant amount of magnetic diffuse scattering which can be fit to the dipolar spin ice model. However, these short-ranged ordered spins have a quasielastic response that is atypical of the canonical spin ices, and suggests that the ground state is dynamic (ie. composed of locally ordered 2-in, 2-out spin configurations that can tunnel between energetically equivalent orientations). We report this as an example of a \textit{dynamic} spin ice down to 200 mK. [Preview Abstract] |
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