Microwave investigation of the phase diagram of hexagonal multiferroic HoMnO3

Using a microwave cavity perturbation technique at 16.5 GHz, we have investigated the complex dielectric and magnetic susceptibilities of the hexagonal multiferroic manganite HoMnO3. The c-axis and ab-plane magnetic susceptibilities both appear to be dominated by the contribution of the Ho3+ moments below 100 K. Along the same crystal orientations, Debye relaxation effects are found on the dielectric and magnetic-susceptibility functions above 100 K in a temperature range where unconventional magnetic fluctuations associated to the Mn3+ moments are known to exist. At low temperatures, anomalies at the various phase transitions are observed on both susceptibility functions. These were investigated in magnetic fields up to 16 T and an H-T phase diagram is deduced for the two crystal directions. Along the c axis, the diagram is compared with the one found in the literature.