Contribution of domain rotation to the magnetic susceptibility of spherical two-domain particles of magnetite

Summary. Starting with a simplified picture of the presumed magnetization configuration in a two-domain sphere of magnetite, the initial susceptibility due to rotation of the magnetization in the domains has been calculated. In particles between 100 and 300 nm in diameter the calculated susceptibility for randomly oriented particles drops rapidly from about 6 to a little over 3, which is the value to be expected for large multidomain particles. The results agree well with the available experimental results and imply that in magnetite particles in this size range the low field susceptibility is dominated by the domain rotation and that wall motion is effectively inhibited.