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

Abstract

A magnetic ink has been prepared which consists of a colloidal suspension of cobalt particles of ∼10-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 of IR/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 of IR/IS <0.5 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 Chantrell et al.7 for a system of uniaxial particles with axes randomly oriented and containing a lognormal distribution of particle volume. Using a value of anisotropy constant K=1.8×105 J m−3 obtained 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.