The low temperature magnetisation of a system of fine cobalt particles

The low temperature magnetisation of a system of fine cobalt particles dispersed in toluene has been measured. The remanence decays logarithmically with temperature. From these results a value of the effective anisotropy constant K = (1.9±0.5) × 106ergs/cc (at 40K) is obtained. Theory is presented to show that the magnetisation on approach to remanence is linear with H. From the gradient a second estimate of the anisotropy constant can be obtained at 77K having a value of K = (1.6±0.5) × 106erg/cc. These values which are much larger than would be expected for f.c.c, cobalt are consistent with the presence of dominant uniaxial shape anisotropy. This conclusion is supported by the observed remanence to saturation ratio of 0.5 at 0 K.