Absence of anisotropy effects in the magnetization of ferrofluids

Existing magnetization measurements on magnetic colloids known as ferrofluids have been analyzed using the Langevin theory for spherical particles with no anisotropy energy. Deviations from simple Langevin theory are interpreted as being a reflection of a distribution of particle sizes. We consider the more general situation of ellipsoidal particles with a general anisotropy energy which depends only upon the relative orientation of the magnetic moment and the particle. By assuming that the orientation of the particle is in thermodynamic equilibrium, we show that the anisotropy has no effect on the magnetization of the colloid and that the Langevin theory obtains. Physically this result can be thought of as follows. The magnetic field tends to align the magnetic moment along that field. The anisotropy energy, E/sub a/, then favors aligning the easy axis of the particle along that moment and this second alignment has no effect on the moment