Cationic Distribution in Cobalt Spinels

Abstract

The molecular field treatment of the cobalt spinel CoxmN(1−xm)[Co(1−x)m N2−(1−x)m]O4 yields, for the high-temperature approximation, Curie—Weiss constants of the form μ2=xμA2+ (1−x)μB2 and θμ2/m = A+Bx+Cx2, where μA and μB are the magnetic moments of Co2+ on tetrahedral and octahedral sites, respectively, and A, B, and C are related to the exchange integrals. Magnetic measurements were made in Co2GeO4 and CoAl2O4, whose cationic distributions were determined by x-ray diffraction, to establish values of μA and μB as 4.31β and 4.88β, respectively. These values are in agreement with those expected from crystal field considerations and were confirmed by measurements on Co2TiO4, whose distribution is known. The cation distribution parameter x was then determined from the magnetic moment measurements of the series ComZn2−mTiO4 and ComMg2−mTiO4. It was found that Zn2+ replaces Co2+ exclusively on A sites, whereas Mg2+ substitutes equally on A and B sites, both as expected. The experimental values of θ, μ, and x for all samples followed the expression given above, with the constants A, B, and C yielding values of the exchange integrals for A-A, A-B, and B-B interactions.