A comparative study of low temperature heat capacities between zinc ferrite and magnesium ferrite nanoparticles

Calorimetric measurements between 1 and 40 K have been made on zinc ferrite (ZnFe2O4) and magnesium ferrite (MgFe2O4) nanoparticles prepared from an aerogel process. For ZnFe2O4 the expected X-type heat capacity peak near 10 K, which corresponds to a long-range antiferromagnetic transition in the bulk material, is greatly suppressed. Broad peaks rise after the sample is annealed at 500 degrees C or 800 degrees C. Low temperature magnetic entropies thus obtained account for 40-60% of 2Rln(2S+1) for Fe3+ ions with S = 5/2. In contrast, heat capacities of MgFe2O4 fine particles exhibit only minor anomalies corresponding to less than 10% of 2Rln6, which further diminishes after the sample is annealed at 500 degrees C or 800 degrees C. Such observations can be explained by considering the relative distribution of Fe3+ among the tetrahedral A and the octahedral B sites in the spinel type lattice. Bulk form ZnFe2O4 has Fe3+ ions preferring the B sites, whereas MgFe2O4 is largely an inversion spinel with the Fe3+ ions distributed in A and B sites. The aerogel process disturbed significantly these equilibrium conditions, which were thermally adjusted through annealing. While the ferrimagnetic ordering would be observed only at much higher temperatures, the heat capacity anomaly near 10 K is associated with the weak B-B type magnetic interaction. Additional information from this study includes enhanced lattice heat capacity for both fine particles, yielding reduced Debye temperatures.