MAGNETIC-PROPERTIES OF ULTRAFINE FERRIHYDRITE CLUSTERS STUDIED BY MOSSBAUER-SPECTROSCOPY AND BY THERMODYNAMICAL ANALYSIS

Magnetic properties of ultrafine clusters of Fe5HO8 . 4H(2)O (ferrihyd rite, FH), isolated in pores of polysorb, were studied by Mossbauer sp ectroscopy and by thermodynamical analysis. Thermodynamical analysis a llowed the conclusion that magnetic properties of ultrafine clusters c annot be interpreted in terms of a second-order magnetic phase transit ion or of superparamagnetic behavior alone but require the considerati on of a jump-like first order magnetic phase transition (JMT). The cri tical radius R(cr) below which the JMT is to be expected in clusters w as derived from thermodynamic criteria. It was determined as R(cr) = 2 alpha beta eta(1 - T-cc/T-o), where alpha,beta and eta are constants derived from surface energy, magnetostriction, compressibility and T-c c = 3/2 Nk(B)T(o)(2) eta beta(2) (N is the number of iron atoms, k(B) is the Boltzmann constant, T-o is the Curie temperature of the cluster s). For the smallest FH clusters isolated in pores of polysorb, the cr itical radius and the JMT temperature were estimated by Mossbauer spec troscopy to be R(cr) similar to 1.5-2.0 nm and T-JMT similar to 4.2-6 K, respectively. Satisfactory agreement between the value R(cr), estim ated 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.