High-pressure studies on ferrites

High-pressure studies to 150 kbar have been made on four orthoferrites, on Ni${\mathrm{Fe}}_2$ ${\mathrm{O}}_4$, and on ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ using Mössbauer resonance. For the last two compounds data were obtained at both 22 and 147 °C. The major features observed for the four rare-earth orthoferrites include an increase in the magnetic field with pressure which can be interpreted in terms of an increase in $T_c$ with pressure, and a modest change in the quadrupole splitting, which depends on the size of the rare-earth ion. For Ni${\mathrm{Fe}}_2$ ${\mathrm{O}}_4$ the changes in the magnetic field with pressure and temperature were complex and could best be interpreted in terms of a decrease in the saturation field $H_0$ with pressure. $H_0$ decreases more rapidly for the tetrahedral than for the octahedral site. For ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$, with one tetrahedral and two octahedral sites, the behavior of the magnetic field at the tetrahedral site and that of the average value for the two octahedral sites is qualitatively similar to Ni${\mathrm{Fe}}_2$ ${\mathrm{O}}_4$. At 1 atm and 22 °C the isomer shifts at the two octahedral sites in ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ are measurably different; however, by 40 kbar these isomer shifts become identical. At 147 °C and 1 atm there is a small difference in isomer shift between the two octahedral sites which disappears at modest pressure. These observations can be interpreted in terms of the rate of electron exchange between the sites.