Neutron Diffraction Study of Tb and Ho under High Pressure

The pressure dependence of the helical turn angle $\omega$ of Tb and Ho single crystals was measured by neutron diffraction up to 6 kbar at temperatures above 80°K. At constant $T-T_N$, the relative changes of $\omega$ with pressure $\left(\frac{1}{\omega }\right)\frac{d\omega }{\mathrm{dp}}$ were slightly temperature-dependent, but representative average values were 20×${10}^{-3\mathrm{}}$/kbar for Tb and 1.2×${10}^{-3\mathrm{}}$/kbar for Ho. These values were explained satisfactorily by a calculation based on Miwa's theory. The values for the pressure coefficients of the Néel temperature of Tb and Ho, as well as of the ferromagnetic transition temperature of Tb, were also obtained and were in reasonable agreement with the reported values. The spiral temperature range in Tb was found to increase with pressure at the rate of 0.3±0.12°K/kbar.