Remanence curves of cobalt ferrite powders obtained by fractionation of a suspension through a silica gel column

摘要

A magnetic ink has been prepared which consists of a colloidal suspension of cobalt particles of sim;10hyphen;nm median diameter of the volume fraction. Gel filtration techniques have been used to separate the particles into a number of fractions of different particle size. Isothermal remanent magnetization (IRM) curves have been plotted for each fraction in the dried state. The maximum value ofIR/IS(remanence/saturation magnetization) is 0.5 equal to a value for a distribution of uniaxial particles with easy axes randomly oriented. This value is well below the value of 0.8 expected for cobalt ferrite which has cubic crystalline anisotropy. All fractions, except for those containing the largest particles, have values ofIR/IS0.5 due to the presence of superparamagnetic particles. The experimental points on the IRM curves have been fitted to theoretical curves derived from a model by Chantrelletal.7for a system of uniaxial particles with axes randomly oriented and containing a lognormal distribution of particle volume. Using a value of anisotropy constantK=1.8times;105Jthinsp;mminus;3obtained from measurement of the decay of remanence with temperature, satisfactory agreement between particle size determined experimentally and that used in the model has been obtained.