Magnetic properties of magnetic liquids with iron-oxide particles - the influence of anisotropy and interactions

Magnetic liquids containing iron-oxide particles were investigated by magnetization and Mossbauer measurements. The particles were shown to be maghemite with a spontaneous saturation magnetization M-s = 320 kA m(-1) at 200 K and a normalized high-held susceptibility chi/M-0 = 5.1 x 10(-6) m kA(-1), practically independent of temperature. M-s increases with decreasing temperature according to an effective Bloch law with an exponent larger than 1.5, as expected for fine magnetic particles. The model of magnetic particles with uniaxial anisotropy and the actual size distribution gives a consistent description of independent measurements of the temperature dependence of the hyperfine held and the isothermal magnetization versus field. From this an effective anisotropy constant of about 4.5 x 10(4) J m(-3) is estimated for a particle with diameter 7.5 nm. The magnetic relaxation, as observed in zero-held-cooled magnetization and isothermal remanence decay, is influenced by interactions and strongly dependent on the applied magnetic field.