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 λ-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 °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 ln(2S+1) with S=5/2 for ${\mathrm{Fe}}^{3+}$. 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 ${\mathrm{Fe}}^{3+}$ between two nonequivalent (A and B) sites in the spinel-type lattice. In particular, the as-prepared fine particles show large ${\mathrm{Fe}}^{3+}$ occupancy of the A sites, whereas these ions prefer the B sites in bulk zinc ferrite. Meanwhile, the lattice heat capacity is enhanced, yielding effective Debye temperatures of 225, 285, 345, and 360 K for the as-prepared, 500 °C-annealed, 800 °C-annealed, and ball milled sample, respectively, in contrast to 425 K for the bulk material.