Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E24: 3D Frustrated Spin Systems: Ising Pyrochlores and Spin IceFocus
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Sponsoring Units: GMAG DMP Chair: Bruce Gaulin, McMaster Univ Room: LACC 403A |
Tuesday, March 6, 2018 8:00AM - 8:12AM |
E24.00001: Magneto-elastic coupling induced vibronic bound state in the spin ice pyrochlore Ho2Ti2O7 Jonathan Gaudet, Alannah Hallas, Connor Buhariwalla, Matthew Stone, Makoto Tachibana, Marisa Sanders, Robert Cava, Bruce Gaulin The holmium pyrochlore Ho2Ti2O7 is renowned as an exemplary realization of dipolar spin ice physics. While the Ising anisotropy that originates from crystal electric field (CEF) effects is a key ingredient to the spin ice state, it has only been the subject of a select few investigations [1,2]. Here, taking advantage of recent advances in the instrumentation of time-of-flight neutron spectroscopy, we take a closer look at the crystal electric field scheme of Ho2Ti2O7. In doing so, we observe the splitting of a high energy CEF excitation, a feature that could not be observed in previous neutron scattering works due to lower energy resolution. We show that this split excitation cannot be accounted for by either a pure CEF excitation or by a pure phonon excitation. After ruling out several conventional origins for the splitting, we show that its origin is a magneto-elastic coupling induced vibronic bound state, which is a hybridized excitation resulting from the entanglement of a phonon and a crystal field excitation [3]. [1] S. Rosenkranz et al., J. Appl. Phys. 87, 5914 (2000), [2] M. Ruminy et al., Phys. Rev. B 94, 024430 (2016), [3] P.Thalmeier and P.Fulde, Phys. Rev. Lett. 49, 1588(1982) |
Tuesday, March 6, 2018 8:12AM - 8:24AM |
E24.00002: Neutron Investigations of Thin Film Ho2Ti2O7 William Ratcliff, Colin Heikes, Kevin Barry, Christianne Beekman The spin-ice Ho2Ti2O7 has been the subject of study [1-2] for several years as a magnetic analogue of the water ice system. Excitations in this material have been found to map onto magnetic monopoles[3]. Recently, groups have started to explore the effects of strain [4-5] on this system by growing the material in thin film form. They found that at low temperatures, strain allowed the spin ice to fully order at low temperatures. We have recently investigated these films through neutron diffraction measurements performed on the SPINS, BT-7, and MACS instruments at the NCNR. We couple these diffraction studies with capacitive torque magnetometry measurements to explore the effects of size on the magnetic ordering in these materials. |
Tuesday, March 6, 2018 8:24AM - 8:36AM |
E24.00003: Critical magneto-electric dynamics of pyrochlore spin ice Puhan Zhang, Jing Luo, Gia-Wei Chern Spin-ice materials such as Ho$_2$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$ are a class of geometrically frustrated ferromagnets that retain an extensive residual entropy even at very low temperatures. Magnetic ions in these materials form a three-dimensional network of corner-sharing tetrahedra. Spins in the disordered yet highly correlated ground states obey the two-in-two-out ice rules in every tetrahedra, similar to the Bernal-Fowler rules in water ice. More importantly, topological defects that violate the ice-rules, i.e. 3-in-1-out or 1-in-3-out tetrahedra, behave as emergent magnetic monopoles. Recently, it has been shown that these emergent monopoles also possess an electric dipole moment~[1]. This observation opens the possibility of monitoring and even controlling the spin dynamics with an electric field. Here we present a detailed numerical investigation of the magneto-electric dynamics of spin-ice systems using extensive Monte Carlo simulations. Our results shed light on the recent experiment~[2] in spin-ice Dy$_2$Ti$_2$O$_7$ that shows a critical speeding up of dielectric relaxation near the monopole liquid-gas transition. |
Tuesday, March 6, 2018 8:36AM - 9:12AM |
E24.00004: Beller Lectureship: Magnetic field tuning of order by disorder in frustrated Ising pyrochlores Invited Speaker: Santiago Grigera We demonstrate the appearance of thermal order by disorder in Ising pyrochlores with staggered antiferromagnetic order frustrated by an applied magnetic field. We use a mean-field cluster variational method, a low-temperature expansion, and Monte Carlo simulations to characterize the order-by-disorder transition. By direct evaluation of the density of states, we quantitatively show how a symmetry-broken state is selected by thermal excitations. We discuss the relevance of our results to experiments in 2D and 3D samples and evaluate how anomalous finite-size effects could be exploited to detect this phenomenon experimentally in two-dimensional artificial systems, or in antiferromagnetic all-in–all-out pyrochlores like Nd2Hf2O7 or Nd2Zr2O7 , for the first time. |
Tuesday, March 6, 2018 9:12AM - 9:24AM |
E24.00005: Dy2ScNbO7: a new spin ice candidate? Megan Rutherford, Cole Mauws, Casey Marjerrison, Sara Haravifard, James Beare, Graeme Luke, Jamie Ritch, David Herbert, Christopher Wiebe Using standard solid state methods, Dy2ScNbO7, a pyrochlore oxide, was synthesized. While the A-site is occupied by the magnetic Dy3+ cation, the B site is split into a mixture of disordered non-magnetic Sc3+ and Nb5+ cations. It appears that Dy2ScNbO7 has a spin ice ground state that is similar to the titanate analogue, Dy2Ti2O7. Despite its similarities, Dy2ScNbO7 exhibits much faster low temperature spin dynamics than any other dysprosium spin ice candidate. Attempts to grow single crystals of Dy2ScNbO7 have been successful using the floating zone image furnace. Recent characterization results will be presented. |
Tuesday, March 6, 2018 9:24AM - 9:36AM |
E24.00006: AC Susceptibility Measurements of the Spin-Ice Materials Dy2Ti2O7 and Ho2Ti2O7: Measuring Magnetic Monopole Transport James Beare, Graeme Luke, Rabaya Bagnum Dy2Ti2O7 and Ho2Ti2O7 are two spin-ice compounds on the pyrochlore lattice whose ground states are highly degenerate. Magnetic exitations in these compounds may be treated as effective magnetic monopoles which move freely around the lattice. Application of an alternating magnetic field allows the in-phase and out-of-phase alternating current (AC) susceptibility to be measured in a Superconducting Quantum Interference Device (SQUID) magnetometer. Recent work on Dy2Ti2O7 tori[1] and Ho2Ti2O7 needles[2] indicate that the behaviour of the magnetic monopoles may be modeled as a supercooled magnetic liquid approaching a glass transition. In this work, samples were cut into spheres to account for the demagnetizing field produced by the large magnetic moment of the rare-earth ions. The real and imaginagy AC susceptibilites are fit to a generalized Havriliak–Negami (HN) magnetic susceptibility and the relaxation rate is found. A divergence in the miscropic relaxation rate is found in Dy2Ti2O7 indicating this compound exhibits the characteristics of a supercooled magnetic liquid. [1] Kassner et al., PNAS 112.28 (2015), [2] Quilliam et al., PRB 83, 094424 (2011) |
Tuesday, March 6, 2018 9:36AM - 9:48AM |
E24.00007: Long-wavelength Correlations in Ferromagnetic Titanate Pyrochlores as Revealed by Small Angle Neutron Scattering Connor Buhariwalla, Qianli Ma, Lisa DeBeer-Schmitt, Kevin Xie, David Pomaranski, Hanna Dabkowska, Jan Kycia, Bruce Gaulin We have carried out small angle neutron scattering measurements on single crystals of two pyrochlores that display ferromagnetic Curie-Weiss susceptibilities, Yb2Ti2O7 and Ho2Ti2O7 . Ho2Ti2O7 is established as displaying a prototypical classical dipolar spin ice ground state, while Yb2Ti2O7 has been purported as a candidate for a quantum spin ice ground state. Our results for Yb2Ti2O7 show distinct SANS features below its ΘCW ∼ 0.50 K, with rods of diffuse scattering along 111 directions, off-rod scattering which peaks in temperature near ΘCW , and quasi-Bragg scattering at very small angles which correlates well with TC ∼ 0.26 K. The quasi-Bragg scattering corresponds to finite extent ferromagnetic domains ∼ 140Å across, at the lowest temperatures. We interpret the 111 rods of diffuse scattering as arising from domain boundaries between the finite-extent ferromagnetic domains. In contrast the SANS signal in Ho2Ti2O7 is isotropic within the (HHL) plane around Q=0. However the strength of this overall SANS signal has a temperature dependence resembling that of the magnetic heat capacity, with a peak near 3 K. Below the break between the field-cooled and the zero-field cooled susceptibility in Ho2Ti2O7 at ∼ 0.60 K, the SANS signal is very low, approaching zero. |
Tuesday, March 6, 2018 9:48AM - 10:00AM |
E24.00008: Thermal and Magnetic Probes of the Monopole Picture in Spin Ice via SQUID Magnetometry Kenneth Schlax, Laura Troyer, Alexander Thaler, Greg MacDougall, Dale Van Harlingen Dysprosium titanate (Dy2Ti2O7) exhibits several low temperature frustrated magnetic states, including a spin ice state at low fields. Single spin-flip excitations above the spin ice configuration manifest as a dilute gas of pairs of oppositely charged Dirac monopole quasiparticles on the pyrochlore lattice. We probe the density, dynamics, and transitional behavior of thermal and magnetic effects of the monopole picture by measuring the non-equilibrium magnetization using a superconducting quantum interference device (SQUID) magnetometer under an applied field and thermal gradient to create a local imbalance of opposite-polarity monopole quasiparticles. We have grown both oriented single crystals and thin films of Dy2Ti2O7 to facilitate this measurement, and to understand the effects of growth conditions and crystal defects on the magnetic landscape of Dy2Ti2O7. |
Tuesday, March 6, 2018 10:00AM - 10:12AM |
E24.00009: Proposal for the Detection of Magnetic Monopoles in Spin Ice via Nanoscale Magnetometry Franziska Kirschner, Felix Flicker, Amir Yacoby, Norman Yao, Stephen Blundell Since the origin of the “monopole” concept in spin ice, there has been an intensive search for a direct experimental signature of these emergent quasiparticles. Despite various attempts, to date no such signature has been found. We present a proposal for applying nanoscale magnetometry to the search for magnetic monopoles in the spin ices holmium and dysprosium titanate. Employing Monte Carlo simulations of the dipolar spin ice model, we find that when cooled to below 1.5K these materials exhibit a sufficiently low monopole density to enable the direct observation of magnetic fields from individual monopoles. We demonstrate that noise spectroscopy can capture the intrinsic fluctuations associated with monopole dynamics, allowing one to isolate the qualitative effects associated with both the Coulomb interaction between monopoles and the topological constraints implied by Dirac strings. We describe three different nanoscale magnetometry platforms (muon spin rotation, nitrogen vacancy defects, and nanoSQUID arrays) that can be used to detect monopoles in these experiments, and analyze the advantages of each. |
Tuesday, March 6, 2018 10:12AM - 10:24AM |
E24.00010: Effects of magnetisation patterns and emergent monopoles on the electronic structure of spin ice Dy$_2$Ti$_2$O$_7$ in photoabsorption measurements Attila Szabó, Claudio Castelnovo Spin ice, a frustrated magnetic system, exhibits several unusual features, including a highly degenerate (spin liquid) ground state and emergent magnetic monopole excitations. While experimental signatures of these properties have been observed in magnetisation and neutron scattering measurements, their effect on the electronic structure has not been explored. In this project, a simplified model for the electronic structure of spin ice Dy$_2$Ti$_2$O$_7$ was developed to investigate the effects of the magnetic pattern of the localised Dy f-moments on the (insulating) band structure of the Ti d-electrons. Whereas a number of effects are potentially at play, including Aharonov-Bohm and Berry phases, we find that most of them have a largely negligible effect, with the exception of scattering due to the induced electric dipole moment on tetrahedra that host monopoles. This results in a perturbation of the band structure that is linear in the density of monopoles and may be observable in an experimentally measurable change in the tail of the photoabsorption spectrum. |
Tuesday, March 6, 2018 10:24AM - 10:36AM |
E24.00011: Magnetic Flux Spectroscopy of Classical Spin Ice Ho2Ti2O7 Christopher Watson, Ilya Sochnikov, John Kirtley, Robert Cava, Kathryn Moler Classical spin ices such as Ho2Ti2O7 have generated intense interest, both theoretical and experimental, in the last decade. Despite the success of magnetic monopole models, however, a full understanding of the energetics and relaxation rates in these materials has remained elusive. Additionally, several recent studies have demonstrated that defects including oxygen vacancies and stuffed spins (i.e. additional spins from Ho atoms occupying Ti sites) must be accounted for in understanding spin ice magnetic dynamics. We use a scanning SQUID microscope to measure the temperature dependence of the magnetic flux noise spectrum in three regions with different defect densities, taken over two samples from a single Ho2Ti2O7 crystal. We find qualitative deviations from Arrhenius behavior, including evidence of screening at low frequencies and high temperatures, and compare these observations to expectations from monopole and defect models. Magnetic flux spectroscopy is found to be a powerful tool for studying systems with complex magnetic dynamics. |
Tuesday, March 6, 2018 10:36AM - 10:48AM |
E24.00012: Quantum Dynamics in Kagome Ice Ho3Mg2Sb3O14 Zhiling Dun, XIAOJIAN BAI, Joseph Paddison, Clarina Dela Cruz, Matthew Stone, Tao Hong, Nicholas Butch, Martin Mourigal, Haidong Zhou A promising route to realize entangled magnetic states combines geometrical frustration with quantum-tunneling effects. Spin-ice materials are canonical examples of frustration, and Ising spins in a transverse magnetic field are the simplest many-body model of quantum tunneling. In this talk, I shall show that the recently discovered tripod kagome lattice material Ho3Mg2Sb3O14 unites an ice-like magnetic degeneracy with quantum-tunneling terms generated by an intrinsic splitting of the Ho3+ ground-state doublet. Using neutron scattering and thermodynamic experiments, we observe a symmetry-breaking transition at T* ~ 0.32 K to a remarkable quantum state with three peculiarities: a macroscopic degeneracy of ice-like microstates; a fragmentation of the spin into periodic and aperiodic components; and persistent spin fluctuations down to 0.12 K. A model incorporating the interplay of frustration and quantum dynamics is necessary to explain our scattering data. Our results establish Ho3Mg2Sb3O14 realizes quantum kagome ice, a frustrated Ising model with an intrinsic homogeneous transverse field. |
Tuesday, March 6, 2018 10:48AM - 11:00AM |
E24.00013: Classical Spin Liquid State in a Quantum Kagome Ice Kai-Hsin Wu, Ying-Jer Kao We study spin-1/2 XYZh model on a kagome lattice using quantum Monte-Carlo (QMC) simulations. Recent QMC studies show that there exists a quantum Kagome ice state with no magnetic orders. Combined with the theoretical study, the state is proposed to be a Z2 quantum spin liquid (QSL). The transition to the nearby FM phase has been identified as a first-order phase transition. However, no direct evidences of a Z2 QSL are shown. Here we present results of thermal entropy and topological entanglement entropy that suggest the system remains classical. We conclude that down to the lowest temperature reached, the state is not a Z2 quantum spin liquid. |
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