Neutron Study of the Crystal and Magnetic Structures ofMnFe2−tCrtO4

Powder neutron diffraction measurements have been performed at temperatures down to 4.2°K on three compositions in the mixed spinel series $\mathrm{Mn}{\mathrm{Fe}}_{2-t}{\mathrm{Cr}}_t{\mathrm{O}}_4$ ($0\le t\le 2$). Separation of the nuclear and magnetic scattering was accomplished both by applying an external field and by measuring the neutron intensities above the Curie temperature. Analysis of the nuclear scattering indicates that all the compounds investigated have essentially the normal spinel structure, the fraction of divalent atoms on the tetrahedral sites being about 0.8, 0.9, and 1.0 for $t=0.5, 1.0, \mathrm{and} 1.5$, respectively, while in every case the Cr atoms are located on the octahedral sites. The magnetic scattering is consistent with antiparallel $A$- and $B$-site moments, with no evidence being found for ordered triangular configurations. The magnitude of the $A$-site moments in each instance is approximately the moment value of the cations located there, while the $B$-site moments are significantly lower than expected on this basis. These results can be attributed to the occurrence within the $B$ sublattice of spinquenched cations, random antiparallel spins, or short-range ordered triangular configurations, since any of these alternatives lowers the sublattice moment on the average.