LOW-TEMPERATURE CALORIMETRIC PROPERTIES OF ZINC FERRITE NANOPARTICLES

Calorimetric measurements between 1 and 40 K by a thermal relaxation t echnique 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 t he bulk form of this material, is greatly suppressed. Broad peaks begi n to prevail after the sample is annealed at 500 or 800 degrees C, but ball milling of the nanoparticles leads to almost complete disappeara nce of the low-temperature ordering. In all cases, calorimetrically ba sed 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 distributi on of Fe3+ between two nonequivalent (A and B) sites in the spinel-typ e lattice. In particular, the as-prepared fine particles show large Fe 3+ occupancy of the A sites, whereas these ions prefer the B sites in bulk zinc ferrite. Meanwhile, the lattice heat capacity is enhanced, y ielding effective Debye temperatures of 225, 285, 345, and 360 K for t he as-prepared, 500 degrees C-annealed, 800 degrees C-annealed, and ba ll milled sample, respectively, in contrast to 425 K for the bulk mate rial.