We report the results of neutron scattering on a powder sample of Gd3Ga5O12 at high magnetic fields. We find that in high fields (B>1.8 T) the system is not fully polarized, but has a small canting of the moments induced by the dipolar interaction. W
e show that the degree of canting is accurately predicted by the standard Hamiltonian which includes the dipolar interaction. The inelastic scattering is dominated at large momentum transfers by a band of almost dispersionless excitations. We show that these correspond to the spin waves localized on ten site rings, expected for a system described by a nearest neighbor interaction, and that the spectrum at high fields B>1.8 T is well-described by a spin wave theory. The phase for fields <1.8 T is characterized by an antiferromagnetic Bragg peak at (210) and an incommensurate peak.
The magnetic properties of Co3V2O8 have been studied by single-crystal neutron-diffraction. In zero magnetic field, the observed broadening of the magnetic Bragg peaks suggests the presence of disorder both in the low-temperature ferromagnetic and in
the higher-temperature antiferromagnetic state. The field dependence of the intensity and position of the magnetic reflections in Co3V2O8 reveals a complex sequence of phase transitions in this Kagome staircase compound. For H//a, a commensurate-incommensurate-commensurate transition is found in a field of 0.072 T in the antiferromagnetic phase at 7.5 K. For H//c at low-temperature, an applied field induces an unusual transformation from a ferromagnetic to an antiferromagnetic state at about 1 T accompanied by a sharp increase in magnetisation.
Gd3Ga5O12, (GGG), has an extraordinary magnetic phase diagram, where no long range order is found down to 25 mK despite Theta_CW approx 2 K. However, long range order is induced by an applied field of around 1 T. Motivated by recent theoretical devel
opments and the experimental results for a closely related hyperkagome system, we have performed neutron diffraction measurements on a single crystal sample of GGG in an applied magnetic field. The measurements reveal that the H-T phase diagram of GGG is much more complicated than previously assumed. The application of an external field at low T results in an intensity change for most of the magnetic peaks which can be divided into three distinct sets: ferromagnetic, commensurate antiferromagnetic, and incommensurate antiferromagnetic. The ferromagnetic peaks (e.g. (112), (440) and (220)) have intensities that increase with the field and saturate at high field. The antiferromagnetic reflections have intensities that grow in low fields, reach a maximum at an intermediate field (apart from the (002) peak which shows two local maxima) and then decrease and disappear above 2 T. These AFM peaks appear, disappear and reach maxima in different fields. We conclude that the competition between magnetic interactions and alternative ground states prevents GGG from ordering in zero field. It is, however, on the verge of ordering and an applied magnetic field can be used to crystallise ordered components. The range of ferromagnetic and antiferromagnetic propagation vectors found reflects the complex frustration in GGG.
The spin-liquid phase of two highly frustrated pyrochlore magnets Gd2Ti2O7 and Gd2Sn2O7 is probed using electron spin resonance in the temperature range 1.3 - 30 K. The deviation of the absorption line from the paramagnetic position u =gamma H obser
ved in both compounds below the Curie-Weiss temperature Theta_CW ~ 10 K, suggests an opening up of a gap in the excitation spectra. On cooling to 1.3 K (which is above the ordering transition T_N ~ 1.0 K) the resonance spectrum is transformed into a wide band of excitations with the gap amounting to Delta ~ 26 GHz (1.2 K) in Gd2Ti2O7 and 18 GHz (0.8 K) in Gd2Sn2O7. The gaps increase linearly with the external magnetic field. For Gd2Ti2O7 this branch co-exists with an additional nearly paramagnetic line absent in Gd2Sn2O7. These low lying excitations with gaps, which are preformed in the spin-liquid state, may be interpreted as collective spin modes split by the single-ion anisotropy.
