Showing papers by "J. S. Gardner published in 2009"

Two-dimensional kagome correlations and field induced order in the ferromagnetic XY pyrochlore Yb2Ti2O7.

Abstract: Neutron scattering measurements show the ferromagnetic XY pyrochlore Yb2Ti2O7 to display strong quasi-two-dimensional (2D) spin correlations at low temperature, which give way to long range order (LRO) under the application of modest magnetic fields. Rods of scattering along 111 directions due to these 2D spin correlations imply a magnetic decomposition of the cubic pyrochlore system into decoupled kagome planes. A magnetic field of approximately 0.5 T applied along the [110] direction induces a transition to a 3D LRO state characterized by long-lived, dispersive spin waves. Our measurements map out a complex low temperature-field phase diagram for this exotic pyrochlore magnet.

Revisiting static and dynamic spin-ice correlations in Ho 2 Ti 2 O 7 with neutron scattering

Abstract: Elastic and inelastic neutron-scattering studies have been carried out on the pyrochlore magnet ${\text{Ho}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$. Measurements in zero applied magnetic field show that the disordered spin-ice ground state of ${\text{Ho}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ is characterized by a pattern of rectangular diffuse elastic scattering within the [HHL] plane of reciprocal space, which closely resembles the zone-boundary scattering seen in its sister compound ${\text{Dy}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$. Well-defined peaks in the zone-boundary scattering develop only within the spin-ice ground state below $\ensuremath{\sim}2\text{ }\text{K}$. In contrast, the overall diffuse-scattering pattern evolves on a much higher-temperature scale of $\ensuremath{\sim}17\text{ }\text{K}$. The diffuse scattering at small wave vectors below [001] is found to vanish on going to $\mathbf{Q}=0$, an explicit signature of expectations for dipolar spin ice. Very high energy-resolution inelastic measurements reveal that the spin-ice ground state below $\ensuremath{\sim}2\text{ }\text{K}$ is also characterized by a transition from dynamic to static spin correlations on the time scale of ${10}^{\ensuremath{-}9}\text{ }\text{s}$. Measurements in a magnetic field applied along the $[1\overline{1}0]$ direction in zero-field-cooled conditions show that the system can be broken up into orthogonal sets of polarized $\ensuremath{\alpha}$ chains along $[1\overline{1}0]$ and quasi-one-dimensional $\ensuremath{\beta}$ chains along [110]. Three-dimensional correlations between $\ensuremath{\beta}$ chains are shown to be very sensitive to the precise alignment of the $[1\overline{1}0]$ externally applied magnetic field.

Direct observation of a nuclear spin excitation in Ho2Ti2O7.

Abstract: A single nondispersive excitation is observed by means of neutron backscattering, at E_{0}=26.3 microeV in the spin ice Ho2Ti2O7 but not in the isotopically enriched 162Dy2Ti2O7 analogue. The intensity of this excitation is rather small, less, similar0.2% of the elastic intensity. It is clearly observed below 80 K but resolution limited only below approximately 65 K. The application of a magnetic field up to micro_{0}H=4.5 T, at 1.6 K, has no measurable effect on the energy or intensity. This nuclear excitation is believed to perturb the electronic, Ising spin system resulting in the persistent spin dynamics observed in spin ice compounds.

Chemical pressure induced spin freezing phase transition in kagome Pr langasites

Abstract: The 2D kagome system ${\mathrm{Pr}}_{3}{\mathrm{Ga}}_{5}{\mathrm{SiO}}_{14}$ has been previously identified as a spin-liquid candidate in zero field, displaying no magnetic long-ranged order down to at least 35 mK. Perturbations upon such systems, either under applied fields or applied pressure, should induce a spin freezing phase transition, but there are very few experimental realizations of this phenomena other than the well-studied 3D pyrochlore ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$. In this Letter, we report the observation of a spin freezing phase transition in ${\mathrm{Pr}}_{3}{\mathrm{Ga}}_{5}{\mathrm{SiO}}_{14}$ through the application of chemical pressure---that is, through a systematic substitution on the Si site with larger ions and an elongation of the nearest-neighbor Pr-Pr distance in the kagome lattice. This results in a suppression of the ${T}^{2}$ component of the heat capacity, and the reduction of the exchange constant eventually leads to dipolar-driven spin freezing.

Neutron scattering investigations of the partially ordered pyrochlore Tb2Sn2O7

Abstract: Neutron scattering measurements have been performed on polycrystalline Tb2Sn2O7 at temperatures above and below that of the phase transition, TN = 0.87 K, to investigate further the spin dynamics in the magnetically ordered state. In particular, new neutron spin echo results are presented showing a dependence on Q in the dynamics. We show evidence of the coexistence of static ferromagnetism and dynamically fluctuating spins down to 30 mK and we make a comparison of this partially ordered system to the spin liquid Tb2Ti2O7.

Magnetic order and crystal field excitations in Er2Ru2O7: a neutron scattering study

Abstract: The magnetic pyrochlore Er2Ru2O7 has been studied with neutron scattering and susceptibility measurements down to a base temperature of 270 mK. For the low temperature phase in which the Er sublattice orders, new magnetic Bragg peaks are reported which can be indexed with integer (hkl) for a face centered cubic cell. Inelastic measurements reveal a wealth of crystal field levels of the Er ion and a copious amount of magnetic scattering below 15 meV. The three lowest groups of crystal field levels are at 6.7, 9.1 and 18.5 meV.

On the nature and anisotropy of $Q \neq 0$ correlations in Tb_2Ti_2O_7 under the application of magnetic fields

Abstract: We report an analysis of neutron diffraction from single crystals of the spin-liquid pyrochlore Tb2Ti2O7 under the application of magnetic fields along the crystallographic $[110]$ direction. Such a perturbation has been shown to destroy the spin liquid ground state and induce long-range order, although the nature of the ordered state was not immediately determined. Recently, it has been proposed that the ordered state is characterized by spin-ice-like correlations, evincing an emergent ferromagnetic tendency in this material despite the large negative Curie-Weiss constant. Here, we argue instead that the ordered state is dominated by $Q eq 0$ correlations that emerge either from strong antiferromagnetism or magnetoelastic distortion of the crystal. In contrast to previous reports, we observe no evidence for re-entrant behaviour in the high field limit. Extreme sensitivity of the ordered state to the alignment of the applied field is suggested to account for these discrepancies.