Andrew Boothroyd

Abstract:

An ac photopyroelectric calorimeter has been used to measure the thermal diffusivity and specific heat of the perovskite manganites RMnO3 (R ≤ La, Pr, Nd) close to their magnetic transitions. Taking into account that the inverse of the thermal diffusivity has the same critical behaviour as the specific heat, the critical exponent α of the magnetic transition has been obtained by means of both magnitudes. The results in all cases are consistent with the Heisenberg model (α ≤ -0.11), irrespective of the rare-earth ion. © 2005 IOP Publishing Ltd.

Abstract:

We present two experiments that provide information on spin anisotropy and the magnetic structure of NaxCoO2. First, we report low-energy neutron inelastic scattering measurements of the zone-center magnetic excitations in the magnetically ordered phase of Na0.75CoO2. The energy spectra suggest the existence of two gaps, and are very well fitted by a spin-wave model with both in-plane and out-of-plane anisotropy terms. The gap energies decrease with increasing temperature and both gaps are found to have closed when the temperature exceeds the magnetic ordering temperature T_m~22 K. Secondly, we present neutron diffraction studies of Na0.85CoO2 with a magnetic field applied approximately parallel to the c axis. For fields in excess of ~8T a magnetic Bragg peak was observed at the (0,0,3) position in reciprocal space. We interpret this as a spin-flop transition of the A-type antiferromagnetic structure, and we show that the spin-flop field is consistent with the size of the anisotropy gap.