The heat capacity of nickel ferrite was measured as a function of temperature over the range from 50 to 1200 C using a differential scanning calorimeter. A thermal anomaly was observed at 584.9 C, the expected Curie temperature, T(sub c). The observed behavior was interpreted by recognizing the sum of three contributions: (1) lattice (vibrational), (2) a spin wave (magnetic) component and (3) a (lambda)-transition (antiferromagnetic-paramagnetic transition) at the Curie temperature. The first was modeled using vibrational frequencies derived from an experimentally-based ir absorption spectrum, while the second was modeled using a spin wave analysis that provided a T(sup 3/2) dependency in the low temperature limit, but incorporated an exchange interaction between cation spins in the octahedral and tetrahedral sites at elevated temperature, as first suggested by Grimes. The (lambda)-transition was fitted to an inden-type model which consisted of two truncated power law series in dimensionless temperature (T/T/(sub C)). Exponential equality was observed belwo and above T(sub c), indicating symmetry about the Curie temperature . Application of the methodology to existing heat capacity data for other transition metal ferrites (AFe(sub 2)O(sub 4), A=Fe,Co)revealed the same exponential equality, i.e., m=n=5.
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