Spin flop transitions and complex magnetic behaviour in CoMoO4 powders and β-CoMoO4 thin films

Magnetic measurements were made on alpha-CoMoO4 powder and beta-CoMoO4 thin film samples. alpha-CoMoO4 powder showed antiferromagnetic ordering below 13 K and two sharp spin flop transitions with complete magnetic saturation after the second transition (>5 T). A magnetic phase diagram for alpha-CoMoO4 was constructed, revealing three low-temperature magnetic phases: an antiferromagnetic phase at low field, an intermediate spin-flop phase, and a paramagnetic phase at high field. Two well-defined magnetisation plateaus were observed either side of the spin-flop phase: the first occurred with a moment per Co ion of 1/3 of spin saturation (where Co moments are completely aligned to the applied magnetic field), whereas the second occurred at a moment per Co ion matching calculations for spin saturation (3 mu(B) per Co). No geometric origin for this factor of 1/3 could be found. Hysteresis was observed around the first spin-flop transition, indicating the presence of spin canting or magnetic domains under an applied field. The first successful fabrication of CoMoO4 thin films was achieved via RF magnetron sputtering, yielding beta phase films which remain in the beta phase upon cooling. The beta-CoMoO4 thin films demonstrated antiferromagnetic ordering below 10 K. While two spin-flop transitions are seen in the beta phase films (with a hysteresis around the first), spin saturation is not reached in fields up to 6 T, suggesting the existence of a third spin reorientation at higher field. A magnetic phase diagram was also constructed for the beta-CoMoO4 film, which demonstrated the same three low-temperature magnetic phases, though with lower critical fields and transition temperatures. Comparisons with isostructural Ni and Mn molybdates showed that the key differences in the alpha and beta phases arise from the different coordination of the Mo tetramers, indicating differences in magnetic properties in both phases arise from variations in the inter-tetramer exchange coupling.