Magnetic properties of ultrafine ferrihydrite clusters studied by Mossbauer spectroscopy and by thermodynamical analysis

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 R_cr below which the JMT is to be expected in clusters was derived from thermodynamic criteria. It was determined as R_cr = 2 αβη/(1 - T_cc/T_o), where α, β and η are constants derived from surface energy, magnetostriction, compressibility and T_cc = 3/2 Nk_BT_o ²ηβ² (N is the number of iron atoms, k_B is the Boltzmann constant, T_o 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 R_cr ∼ 1.5-2.0 nm and T_JMT ∼ 4.2-6 K, respectively. Satisfactory agreement between the value R_cr, 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.