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 fe rrite (ZnFe2O4) and magnesium ferrite (MgFe2O4) nanoparticles prepared from an aerogel process. For ZnFe2O4 the expected X-type heat capacit y peak near 10 K, which corresponds to a long-range antiferromagnetic transition in the bulk material, is greatly suppressed. Broad peaks ri se after the sample is annealed at 500 degrees C or 800 degrees C. Low temperature magnetic entropies thus obtained account for 40-60% of 2R ln(2S+1) for Fe3+ ions with S = 5/2. In contrast, heat capacities of M gFe2O4 fine particles exhibit only minor anomalies corresponding to le ss than 10% of 2Rln6, which further diminishes after the sample is ann ealed at 500 degrees C or 800 degrees C. Such observations can be expl ained by considering the relative distribution of Fe3+ among the tetra hedral A and the octahedral B sites in the spinel type lattice. Bulk f orm ZnFe2O4 has Fe3+ ions preferring the B sites, whereas MgFe2O4 is l argely an inversion spinel with the Fe3+ ions distributed in A and B s ites. The aerogel process disturbed significantly these equilibrium co nditions, which were thermally adjusted through annealing. While the f errimagnetic ordering would be observed only at much higher temperatur es, the heat capacity anomaly near 10 K is associated with the weak B- B type magnetic interaction. Additional information from this study in cludes enhanced lattice heat capacity for both fine particles, yieldin g reduced Debye temperatures.