Magnetic Field at the Nucleus in Spinel-Type Crystals

Measurements of the magnetic field at the ${\mathrm{Fe}}^{57}$ nucleus were obtained on powder samples at room temperature using the Mössbauer effect for the spinel-type ferrites $\gamma$-${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$, "ordered" lithium ferrite ${(\mathrm{L}\mathrm{i}}_{0.5}$ ${\mathrm{Fe}}_{2.5}$ ${\mathrm{O}}_4$), and "disordered" lithium ferrite giving values of|496±20|koe,|508±20|koe, and|510±20| koe. These compounds contain only trivalent and no divalent iron. The Mössbauer spectra of all of these compounds were very similar and no difference could be detected between the "ordered" and "disordered" compounds. Only one set of lines was observed indicating that the fields at the octahedral and tetrahedral sites are about the same value. The value of the hyperfine interaction constant $A$ obtained from electron paramagnetic resonance spectrum of the divalent ${\mathrm{Mn}}^{55}$ (isoelectronic with trivalent Fe) impurity in single crystals of the isomorphous spinel-type crystal "disordered" lithium aluminate (${\mathrm{Li}}_{0.5}$ ${\mathrm{Al}}_{2.5}$ ${\mathrm{O}}_4$) was found to be|77.2±1.0| ×${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The angular variation of the spectrum indicated that the divalent ${\mathrm{Mn}}^{55}$ ions were located substantially on octahedral sites. The corresponding magnetic field for the ${\mathrm{Mn}}^{55}$ nucleus is around 550 koe, which is close to the value obtained elsewhere for the ${\mathrm{Mn}}^{55}$ nucleus located on a tetrahedral site in a spinel-type aluminate. This is in good agreement with the Mössbauer results.