Effect of Pressure and Temperature on the Magnetic Hyperfine Interaction of Fe(II) in CoO

Mössbauer spectra of ${\mathrm{Fe}}^{57}$ in cobalt oxide have been measured at pressures up to 150 kbar and at temperatures between 4.2 and 300 K. At constant pressures, the internal magnetic field at the Fe(II) ions reaches a maximum at temperatures of about 180 K. On increasing the pressure from 0 to 150 kbar, the internal field is raised from 120 to 170 kOe at 4.2 K and from 182 to 228 kOe at 195 K. This variation of the internal field is described within a crystal-field model for $3d^6$ ions, attributing the increase of the magnetic field with pressure at low temperatures primarily to changes in the radial distribution of the singly occupied $t_{2g}$ orbital.