Study of the Low-Temperature Transition in Magnetite and the Internal Fields Acting on Iron Nuclei in Some Spinel Ferrites, Using Mössbauer Absorption

A study has been made of the internal fields acting on ${\mathrm{Fe}}^{57}$ nuclei in some spinel ferrites, with particular reference to the low-temperature order-disorder transition in magnetite, using the techniques of Mössbauer absorption. For the ${\mathrm{Fe}}^{3+}$ ions at both the octahedral and tetrahedral sites in nickel ferrite (Ni${\mathrm{Fe}}_2$ ${\mathrm{O}}_4$) at 300°K, $\gamma {\mathrm{Fe}}_2{\mathrm{O}}_3$ at 85° and 300°K, and magnetite (${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$) at 85°K, the effective magnetic field at the ${\mathrm{Fe}}^{57}$ nuclei is the same and equal to about 5.1×${10}^5$ oe. In magnetite, the value of $H_{\mathrm{eff}}$ in the ${\mathrm{Fe}}^{2+}$ ions is about 4.5×${10}^5$ oe at 85°K. Measurements on ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ at room temperatures provide a microscopic confirmation of Verwey's hypothesis that above the transition temperature of magnetite there is a fast exchange between the ferrous and ferric ions in the octahedral sites.