Elastic neutron scattering study of magnetic phase transitions in DyPO₄ and HoPO₄ 

Neutron diffraction measurements have been made on single crystals of the two metamagnets DyPO₄ and HoPO₄ to locate the tricritical points in these substances. In the process of the experiments an effect has been discovered which allows the accurate determination of these points. This effect is the sensitivity of the extinction of magnetic reflections to the changing domain configurations in the mixed phase below the tricritical point. Relaxation effects have been observed in the antiferromagnetic domain formation below the first order phase transition. For DyPO₄ the tricritical temperature is found to be 2.07±0.05 K. In HoPO₄ these extinction effects were found to be small and the deviation of the up‐moment sublattice magnetization from the mean‐field‐theory result was used to locate the tricritical temperature at 1.0±0.1 K.