Effect of Pressure on the Néel Temperature of Magnetite

The effect of pressure up to ≃45 kbar on the Néel temperature $T_N$ of magnetite (${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$) was measured. $T_N$ increases linearly over this range with slope $\frac{\partial T_N}{\partial p}=2.05\mathrm{}\pm 0.10°$K/kbar, or $\frac{\partial \ln {\mathrm{T}}_N}{\partial p}=2.42\mathrm{}\times {10}^{-3\mathrm{}}$ ${\mathrm{kbar}.\mathrm{}}^{-1\mathrm{}}$ To a good approximation, this latter value is a measure of the effect of pressure on the $A-B$ super-exchange interaction, i.e., $\frac{\partial \ln J_{\mathrm{AB}}}{\partial p}$. On the basis of a model due to Weisz, which assumes that the spin interaction in the inverse spinels is inversely proportional to the interionic distance, the present results, along with the known linear compressibility, indicate that the oxygen parameter $u$ increases with pressure; the slope is $\frac{\partial \ln u}{\partial p}=1.5\mathrm{}\times {10}^{-4\mathrm{}}$ ${\mathrm{kbar}}^{-1\mathrm{}}$.