We report measurements performed on a polycrystalline sample of the pyrochlore compound . It undergoes a second order magnetic phase transition at K to a noncoplanar all-in–all-out magnetic structure of the magnetic moments. The thermal behavior of the low temperature specific heat fingerprints excitations with linear dispersion in a three-dimensional lattice. The temperature independent spin-lattice relaxation rate measured below and the anomalously slow paramagnetic spin dynamics detected up to are suggested to be due to magnetic short-range correlations in unidimensional spin clusters, i.e., spin loops. The observation of a spontaneous field in muon spin relaxation measurements is associated with the absence of a divergence-free field for the ground state of an all-in–all-out pyrochlore magnet as predicted recently.
Bulk measurements for a powder sample. (a) Specific heat versus temperature. Our results are compared with those in Ref. [18]. (b) Temperature dependence of the inverse magnetic susceptibility expressed in SI units and measured in a field of 0.95 mT. Inset: Low-temperature data. The sample used for this measurement was a thin pellet and the field was applied in its plane so as to minimize demagnetization field effects. In both panels, solid lines are results of fits as explained in the text.
Neutron diffraction results for a powder sample of . (a) Nuclear pattern at = 15 K. (b) Magnetic diagram given by the difference between 60 mK and 1.2 K data sets. Experimental points near = and are not shown because they are strongly influenced by neutrons scattered from the copper container. The observed magnetic reflections are labeled with Miller indices. (a, b) Solid lines result from Rietveld analyses. Solid lines at the bottom give the difference between fits and data. Vertical markers indicate the positions of Bragg peaks. (c) The corresponding magnetic structure. Spheres represent ions, and arrows their magnetic moments oriented along the local trigonal axes of the cubic crystal structure. Two corner-sharing tetrahedra are shown, one with the magnetic moments pointing inwards and another, adjacent tetrahedron with moments pointing outwards. (d) Diffractogram recorded at 1.2 K and difference from the data set recorded at 50 K. No diffuse scattering intensity is observed at the approach of the magnetic transition. Note that for the sake of clarity, the most intense Bragg peaks present at 1.2 K have been truncated.
Inelastic neutron scattering results for a powder sample. (a) Wave-vector integrated backscattering spectrum at 0.06 K. The instrument resolution determined from an independent measurement on vanadium is shown by the dotted line. (b) Wave-vector dependence of the quasielastic half-width at half-maximum (HWHM) of the magnetic scattering in the paramagnetic phase at 1.2 K. In both panels, solid lines are results of fits as explained in the text.
results. (a) An asymmetry spectrum recorded deep in the ordered state (LTF spectrometer). The solid line results from a phenomenological fit. Inset: The short-time part of the spectrum. (b) Temperature dependence of the spin-lattice relaxation rate in zero field (open symbols) and for mT (filled symbols). Data were recorded at different spectrometers as indicated. The value is shown as the dotted line and the solid line emphasizes the temperature-independent zero-field at low temperatures.