Magnetic properties of ultrafine clusters of Fe5HO8·4H2O (ferrihydrite, FH), isolated in pores of polysorb, were studied by Mossbauer spectroscopy and by thermodynamical analysis. Thermodynamical analysis allowed the conclusion that magnetic properties of ultrafine clusters cannot be interpreted in terms of a second-order magnetic phase transition or of superparamagnetic behavior alone but require the consideration of a jump-like first order magnetic phase transition (JMT). The critical radius Rcr below which the JMT is to be expected in clusters was derived from thermodynamic criteria. It was determined as Rcr = 2 αβη/(1 - Tcc/To), where α, β and η are constants derived from surface energy, magnetostriction, compressibility and Tcc = 3/2 NkBTo 2ηβ2 (N is the number of iron atoms, kB is the Boltzmann constant, To is the Curie temperature of the clusters). For the smallest FH clusters isolated in pores of polysorb, the critical radius and the JMT temperature were estimated by Mossbauer spectroscopy to be Rcr ∼ 1.5-2.0 nm and TJMT ∼ 4.2-6 K, respectively. Satisfactory agreement between the value Rcr, estimated from the experimental data and the one derived by thermodynamical analysis was achieved. Interfacial (cluster-surface) and intercluster interactions were found to destroy the JMT effect and to give rise to a second-order magnetic phase transition.
|Zeitschrift fur Physik D-Atoms Molecules and Clusters
|Seiten (von - bis)
|Veröffentlicht - 01.04.1996