High field magnetization of the colloidal Mn-Zn ferrite

The magnetic properties of a hydrocarbon based colloid (stabilized by oleic acid) of polydisperse chemically co-precipitated Mn_0.5Zn_0.5Fe₂O₄ particles are discussed in the paper. Two fractions are separated from an initial ferrofluid sample by centrifugation. X-ray diffraction measurements show the crystal structure of the particles and a broadened shape of the diffraction lines. Mean particle diameters 6.2 nm (the light fraction) and 9.7 nm (the heavy one) were determined by TEM measurements. The magnetization of the colloids is measured by VSM up to 12 T. A significant slope in the magnetization curves in the high field region is displayed for both samples in the temperature interval from 100 to 295 K. Two methods are used to estimate the spontaneous magnetization of the material: a high field approximation of the magnetization curve by M(H) = M_s(1 -kT/mH) + xH and a decomposition of the magnetization curve in a spectrum of Langevin functions of varying magnetic moments. Both methods give practically identical results. Evaluation of the spontaneous intrinsic magnetization shows a slight dependence of the relative pyromagnetic coefficient M^-1_s∂M_s/∂T on particle size.