MOSSBAUER STUDY OF MIXED FERRITE FERROFLUIDS

摘要

Mossbauer spectroscopy was applied to study the magnetic properties of ferrofluids consisting of ultrafine Co_xZn_(1_x)Fe_2O_4 particles (x = 0.7, d = 10.2 nm; x = 0.5, d = 7.2 nm; x = 0.3, d = 5.2 nm) dispersed in Isopar-M as in a carrier liquid and coated by oleic acid as a surfactant. The distribution of Co and Zn ions in sublattices A and B of the spinel structure was determined. The blocking temperatures T_b of superparamagnetic transitions were estimated by the comparison between the Mossbauer subspectrum areas associated with paramagnetic and ferromagnetic fractions. The blocking temperatures above room temperature were determined by the linear extrapolation of experimental points as T_b = 400, 183, and 83 K for particle sizes of 10.2, 7.2, and 5.2 nm, respectively. These values of Tb were used to calculate the magnetic anisotropy constants as 2.5·10~5, 3·10~5, and 9·10~5 J·m~(-3), respectively.