Jc. Ho et al., LOW-TEMPERATURE CALORIMETRIC PROPERTIES OF ZINC FERRITE NANOPARTICLES, Physical review. B, Condensed matter, 52(14), 1995, pp. 10122-10126
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.