MOSSBAUER STUDIES OF C03O4 - BULK MATERIAL AND ULTRAFINE PARTICLES

Mössbauer spectra of bulk CO3O4, doped with 57Co, show the two spinel sites. Above the Néel temperature, TN, the spectra are composed of two quadrupole-split lines corresponding to Fe3+ at both the tetrahedral (A) sites and octahedral (B) sites. The isomeric shifts relative to Fe at the A and B sites are (-0.316 ±0.010) and (-0.393 ±0.005) mm{plus 45 degree rule}sec, respectively. A comparison with other spinels indicates a higher electronic charge density at the nucleus which is attributed to the smaller interatomic distances in Co3O4. Below TN, the A site Fe3+ spectrum is magnetically split due to the antiferromagnetic Co2+ sublattice. The Fe3+ spectrum of the B site remains as a single somewhat broadened line that can be partially split by an external field of 6 kOe. This effect is attributed to slow relaxation of the Fe spin which results from a slight canting of the spins at the A sites. Experimentally one finds TN = (33.0 1.0)°K, and the field at the nucleus for the A sites is (43 8±3) kOe at 0°K. This value for Fe3+ in tetrahedral coordination is lower than that found in other spinels. In one sample prepared from different starting materials, some evidence was found for Fe2+ at the A sites. Above TN, the spectra of ultrafin CO3O4 particles, with a diameter of 100 Å, are essentially the same as those of the bulk material. Below TN, the ultrafine particles display superparamagnetic behavior and the B site spectrum broadening is enhanced. The anisotropy constant estimated from the superparamagnetic-antiferromagnetic transition is 4 × 104 erg{plus 45 degree rule}cm3. © 1969.