LOW-TEMPERATURE CALORIMETRIC PROPERTIES OF ZINC FERRITE NANOPARTICLES

Calorimetric measurements between 1 and 40 K by a thermal relaxation technique have been made on zinc ferrite nanoparticles prepared from an aerogel process. The expected lambda-type heat-capacity peak near 10 K, which corresponds to a long-range antiferromagnetic transition in the bulk form of this material, is greatly suppressed. Broad peaks begin to prevail after the sample is annealed at 500 or 800 degrees C, but ball milling of the nanoparticles leads to almost complete disappearance of the low-temperature ordering. In all cases, calorimetrically based magnetic entropy at 40 K accounts for only a fraction of 2R In(2S + 1) with S = 5/2 for Fe3+ These results are corroborated by magnetic data, which also indicate magnetic ordering at high temperatures. Such observations can be understood by considering the relative distribution of Fe3+ between two nonequivalent (A and B) sites in the spinel-type lattice. In particular, the as-prepared fine particles show large Fe3+ occupancy of the A sites, whereas these ions prefer the B sites in bulk zinc ferrite. Meanwhile, the lattice heat capacity is enhanced, yielding effective Debye temperatures of 225, 285, 345, and 360 K for the as-prepared, 500 degrees C-annealed, 800 degrees C-annealed, and ball milled sample, respectively, in contrast to 425 K for the bulk material.