Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q4: Focus Session: Pyrochlore Magnets: Ordering and Freezing |
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Sponsoring Units: GMAG Chair: Michel Gingras, University of Waterloo Room: 112/110 |
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q4.00001: Effects of interaction strength on the ground states of magnetically frustrated pyrochlores Alannah Hallas, Angel Arevalo Lopez, Haidong Zhou, Graeme Luke, Christopher Wiebe The pyrochlore oxides are ubiquitous in frustrated magnetism because of their diverse array of novel magnetic ground states. The magnetism in these materials can be probed by the application of chemical pressure, that is, by varying the size of the non-magnetic ion. Germanium and lead are, respectively, the smallest and largest possible B-site cations for the pyrochlore lattice. We present thermodynamic and magnetization measurements on five new materials A$_2$Ge$_2$O$_7$ (A = Yb, Er, Tb) and A$_2$Pb$_2$O$_7$ (A = Pr, Nd). The strength of the magnetic interactions in these materials is strongly enhanced for the germanium pyrochlores and significantly reduced in the case of the lead pyrochlores. Comparison to the well-studied titanium and tin pyrochlores (B = Ti, Sn) provide context for our results. Our measurements reveal that, in some cases, the magnetic ground state for a given magnetic ion is stable against changes in chemical pressure. In other cases, varying the chemical pressure for a given magnetic ion gives rise to a distinctly different magnetic ground state. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q4.00002: Physical Properties of Ni$_{2}$GeO$_{4}$ Spinel Perturbed by Magnetic Dilution and Applied Pressure Jory Korobanik, Fereidoon Razavi Geometrically frustrated magnetic systems have yielded an interesting and rich playground for physicists. Recently, a new disordered low temperature state was discovered in the frustrated pyrochlore type Ho$_{2}$Ti$_{2}$O$_{7}$ which is termed spin ice [1]. This phase is the magnetic analog to water ice with local spin disorder replacing proton disorder. Geometric frustration arises when nearest neighbor exchange interactions cannot be simultaneously satisfied resulting in large macroscopic degeneracy. This has the effect of suppressing Neel ordering temperature [2]. This work seeks to understand the effects of applied pressure and magnetic dilution to the frustrated spinel Ni$_{2}$GeO$_{4}$. The parent material undergoes two closely spaced ordering events at T$_{1}=$12.1K and T$_{2} =$ 11.4K. [3] Upon dilution a downward shift in the ordering temperatures is observed with a destruction of the lower T2 transition. Heat capacity, AC and DC magnetometry are used to probe the changes in physical properties. \\[4pt] [1] Harris \textit{et al}, \textit{Phys. Rev. Lett. }\textbf{\textit{79,}}\textit{ 2554 (1997).}\\[0pt] [2] J. Greedan, \textit{J. Mater. Chem}., 11, 37-53 (2001).\\[0pt] [3] Lashley \textit{et al}, \textit{Phys. Rev. B} \textbf{78}, (2008). [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q4.00003: Half-Magnetization Plateau of a Dipolar Spin Ice in a [100] Field Ying-Jer Kao, Sheng-Ching Lin We report here numerical results of the low-temperature behavior of a dipolar spin ice in a magnetic field along the [100] direction. Tuning the magnetic field, the system exhibit a half-magnetization plateau at low temperature. This half-polarized phase should correspond to a quantum solid phase in an effective 2D quantum boson model, and the transition from the Coulomb phase with a power-law correlation to this state can be regarded as a superfluid to a quantum solid transition. We discuss possible experimental signatures of this half-polarized state. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q4.00004: Geometric Frustration with Disorder Nayoon Woo, Daniel M. Silevitch, Thomas F. Rosenbaum We study the effects of Nd doping on the geometrically-frustrated Heisenberg antiferromagnet Gadolinium Gallium Garnet (GGG), using linear and nonlinear ac magnetic susceptibility. Doping levels from 0.1 to 1 percent Nd alleviate the intrinsic frustration of pure GGG and elevate the ordering temperature compared to the pure material. We use nonlinear pump-probe magnetic susceptometry to examine cluster dynamics for both the pure and the doped series. At low frequency ($\sim$10 Hz), spectral hole burning is possible, indicating the presence of spin clusters with discrete energy levels largely decoupled from the overall spin bath. At kHz, we find a Fano resonance, revealing scattering pathways between spin cluster excitations and the bath. We trace the evolution of this resonance behavior as a function of dopant concentration. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q4.00005: ${\rm LiHo_xY_{1-x}F_4}$ in the highly-diluted limit Juan Carlos Andresen, Moshe Schechter, Vadim Oganesyan, Helmut G. Katzgraber The rare-earth material ${\rm LiHo_xY_{1-x}F_4}$ has attracted much attention recently, not only because it is well described by a long-range dipolar Ising model, but also because it has a rich phase diagram in the temperature--concentration plane that makes it especially interesting to explore exotic magnetic phenomena. The existence of a spin-glass phase in this material has been a long-standing controversy. In particular, it is unclear if the spin-glass phase extends to the low-concentration limit, or if an exotic anti-glass state emerges. Using large-scale Monte Carlo simulations we probe this difficult regime of the phase diagram. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q4.00006: Study on the magnetic properties of ${\mathrm{Y}_{2-x}\mathrm{Bi}_{x}\mathrm{Ir}_{2}\mathrm{O}_{7}}$ by ${\mu}$SR and DC susceptibility T. Medina, T.J. Williams, T.J. Munsie, R.M. D'Ortenzio, L. Liu, B.J. Frandsen, Y.J. Uemura, M.C. Shapiro, S.C. Riggs, M.B. Stone, I.R. Fisher, C. Thompson, C. Marjerrison, H.A. Dabkowska, G.M. Luke Pyrochlore iridates have received considerable attention recently as they possess strong electron correlation and spin orbit coupling, giving rise to a finite temperature metal-insulator transition (MIT). The nature of this MIT transition is related to the magnetic order of the Ir atoms which also experience frustration. By doping ${\mathrm{Y}_{2}\mathrm{Ir}_{2}\mathrm{O}_{7}}$ with Bi we are trying to elucidate the magnetic configuration of the iridium ions. Here we present a study on the magnetic properties of the ${\mathrm{Y}_{2-x}\mathrm{Bi}_{x}\mathrm{Ir}_{2}\mathrm{O}_{7}}$ system using ${\mu}$SR and DC susceptibility. Our results show that pure ${\mathrm{Y}_{2}\mathrm{Ir}_{2}\mathrm{O}_{7}}$ has a magnetic transition to long-range order. Substituting the Bi by Y results in a lower temperature transition with a less homogeneous spin-glass like order with increasing {\it x}. We will show the resulting magnetic phase diagram for this system to understand how its magnetic properties behave near the metal insulator phase boundary. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q4.00007: Effective spin-1/2 exchange model for Tb$_2$Ti$_2$O$_7$: beyond the independent tetrahedra approximation Soumya Mukherjee, Stephanie Curnoe In the pyrochlore crystal terbium titanate (Tb$_2$Ti$_2$O$_7$) the magnetic Tb$^{3+}$ ions form a network of corner-sharing tetrahedra. The edges of the tetrahedra are nearest-neighbour exchange paths. The tetrahedra occur in two different orientations, therefore they can be divided into two sets. The independent tetrahedra approximation includes exchange interactions on only one set of tetrahedra and neglects interactions on the other. Although this approach can reproduce the main features of diffuse neutron scattering intensity patterns, it cannot describe any long range effects. In this work we look beyond the independent tetrahedra approximation by considering exchange paths on both sets of tetrahedra. Second order perturbation theory is used to find an effective spin-1/2 exchange model for Tb$_2$Ti$_2$O$_7$. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q4.00008: Thermal Order-by-Disorder at Criticality in XY Pyrochlore Magnets Michel Gingras, Behnam Javanparast, Alexandre Day, Zhihao Hao We investigate analytically and numerically the problem of long-range order selection via thermal fluctuations close to the critical region of the paramagnetic phase to long-range order transition in a system of interacting XY spins on the pyrochlore lattice and for which we consider the most general bilinear anisotropic nearest-neighbor spin Hamiltonian. At the standard mean-field theory (s-MFT) level, in a certain region of the parameter space of this Hamiltonian, the ordered state displays an accidental $U(1)$ degeneracy. This degeneracy is lifted by fluctuations beyond s-MFT and a certain form of order-by-disorder near criticality is thus fund to be at play. We analytically explore this selection at the microscopic level by using an extension of the method originally developed by Thouless, Anderson and Palmer (TAP) to study the effect of fluctuations in spin glasses. These TAP calculations provide an insight into the long-range order fluctuation-induced selection mechanism in terms of the spin-spin coupling constants of the microscopic Hamiltonian. We also employ a cluster mean field theory (c-MFT) to further explore numerically this problem. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q4.00009: RKKY interactions and anomalous Hall effect in metallic rare-earth pyrochlores SungBin Lee, Arun Paramekanti, Yong Baek Kim Motivated by experiments on Pr$_2$Ir$_2$O$_7$, we consider metallic pyrochlore systems A$_2$B$_2$O$_7$, where the A-sites are occupied by rare-earth local moments and the B-sites host 5d transition metal ions with itinerant strongly spin-orbit coupled electrons. Assuming non-Kramers doublets on the A-site, we derive the RKKY interaction between them mediated by the B-site itinerant electrons and find extended non-Heisenberg interactions. Analyzing a simplified model of the RKKY interaction, we uncover a local moment phase with coexisting spiral Ising-like magnetic dipolar and XY-like quadrupolar ordering. This state breaks time-reversal and lattice symmetries, and reconstructs the B-site electronic band structure, producing a Weyl Metallic phase with an intrinsic anomalous Hall effect and an undetectably small magnetization. We discuss implications of our results for Pr$_2$Ir$_2$O$_7$. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q4.00010: Competing order in spin ice Patrik Henelius, Taoran Lin, Zhihao Hao, Michel Gingras The spin ice family of materials is one of the foremost realizations of a frustrated system displaying a macroscopic ground state degeneracy down to very low temperatures. With the strongest interactions frustrated, small perturbations may eventually cause non-trivial and exotic ordering, which would be hidden in an unfrustrated system. In this study we find that Dy$_2$Ti$_2$O$_7$, a member of the spin ice family, is a prime example of this phenomenology. The value of the weak third neighbor interaction determines which ordered state is eventually selected. We calculate the phase diagram and explore the implications of recent zero-field specific heat measurements and previous neutron scattering data. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q4.00011: Chemical pressure effects on magnetism in the quantum spin liquid candidates Yb$_{2}$B$_{2}$O$_{7}$ (B = Sn, Ti, Ge) Zhiling Dun, Minsong Lee, Eunsang Choi, Alannah Hallas, Chris Wiebe, Alannah Hallas, Jason Gardner, Everton Arrighi, Ricardo Freitas, Angel Lopez, Haidong Zhou, Jinguang Cheng The linear and nonlinear AC susceptibility measurements of Yb-pyrochlores, Yb$_{2}$B$_{2}$O$_{7}$ (B = Sn, Ti and Ge), show a ferromagnetic ordering at 0.13 K but with short range ordering nature for Yb$_{2}$Sn$_{2}$O$_{7}$, a ferromagnetic ordering at 0.25 K for Yb$_{2}$Ti$_{2}$O$_{7}$, and an antiferromagnetic ordering at 0.62 K for Yb$_{2}$Ge$_{2}$O$_{7}$. These systematical results (i) clarified the nature of the controversial magnetic ground state in Yb$_{2}$Ti$_{2}$O$_{7}$; (ii) realized a distinct antiferromagnetic ordering state in Yb$_2$Ge$_2$O$_7$; and (iii) demonstrated that the application of chemical pressure through the series of Yb-pyrochlores can efficiently perturb the fragile quantum spin fluctuations of the Yb$^{3+}$ ions and lead to very different magnetic ground states. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q4.00012: Spin liquids and magnetic ordering in pyrochlores Ludovic Jaubert, Han Yan, Owen Benton, Nic Shannon By their diversity, rare earth pyrochlores have proven to be a very fertile testing ground for exotic phenomena in magnetism, ranging from monopoles in spin ice (Dy2Ti2O7), to textbook order-by-disorder transitions (Er2Ti2O7), Higgs mechanism in quantum spin ice (Yb2Ti2O7), potential spin liquid phases (Er2Sn2O7) mediated by lattice fluctuations (Tb2Ti2O7), and many more. In this talk, I will give a brief overview of this pyrochlore diversity, illustrated by direct comparison with experiments, both from the point of view of spin liquid stabilization and magnetic ordering processes. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q4.00013: Flucutation driven selection at crticality: the case of multi-k partial order on the pyrochlore lattice Zhihao Hao, Behnam Javanparast, Matthew Enjalran, Michel Gingras We study the problem of partially ordered phases with periodically arranged disordered sites on the pyrochlore lattice. The periodicity of the phases is characterized by one or more wave vectors $k = \{\frac{1}{2}\frac{1}{2}\frac{1}{2}\}$. Starting from a general microscopic Hamiltonian including anisotropic nearest-neighbor exchange, long-range dipolar interactions and second- and third-nearest neighbor exchange, we identify using standard mean-field theory (s-MFT) an extended range of interaction parameters that support partially ordered phases. We demonstrate that thermal fluctuations beyond s-MFT are responsible for the selection of one particular partially ordered phase, e.g. the ``4-$k$'' phase over the ``1-$k$'' phase. We suggest that the transition into the 4-$k$ phase is continuous with its critical properties controlled by the cubic fixed point of a Ginzburg-Landau theory with a 4-component vector order-parameter. By combining an extension of the Thouless-Anderson-Palmer method originally used to study fluctuations in spin glasses with parallel-tempering Monte-Carlo simulations, we establish the phase diagram for different types of partially ordered phases. Our result reveals the origin of 4-$k$ phase observed bellow 1K in Gd2Ti2O7. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q4.00014: High Magnetic Field Phase of the Pyrochlore Quantum Magnet Tb$_2$Ti$_2$O$_7$ Liang Yin, Jian-sheng Xia, Yasu Takano, Neil Sullivan, Eun-sung Choi, Qiu Ju Li, Xuefeng Sun By means of ac magnetic-susceptibility and vibrating-sample magnetization measurements in magnetic fields along [111], we find a new magnetic phase of Tb$_2$Ti$_2$O$_7$ between 15T and 16T and below 2.5K with the existing magnetic transition at 1~2T and below 0.6mK. This new magnetic transition implies that the long-range ordering, which has been discussed in other reports, exists between 1T and 15T at low temperatures. Above the critical field of this transition, Tb$_2$Ti$_2$O$_7$ tends to be fully-polarized up to 35T or form an unknown spin-configuration. A new temperature-field phase diagram is established below 4~K and in a field up to 35T. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q4.00015: Quantum critical dynamics of LiHoF4 Vadim Oganesyan, Haifu Ma We study spin-wave dynamics in the extended transverse Ising model appropriate for LiHoF4 in a field. The model includes electron and nuclear spins, as well as lattice deformations. Structure factors are computed and compared against neutron scattering data. [Preview Abstract] |
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