Experimental Determination of the Anisotropy Distribution in Ferromagnetic Powders

Abstract

A technique to determine the anisotropy distribution in a ferromagnetic powder consisting of uniaxial particles has been developed. This technique can be used for samples with randomly oriented particles as well as with oriented samples. In principle it eliminates incoherent magnetization reversals which led to errors in previous methods. It involves saturating the material in a magnetic field and then making torque measurements at an angle several degrees away from the saturating direction. In these measurements the torque on the specimen as a function of magnetic field is recorded as the field is cycled from zero to a specified field and then reduced to zero. This cycling procedure is continued, gradually increasing the maximum applied field upon subsequent cycles until the magnetic field exceeds the highest anisotropy field of the particles in the distribution. For elongated γ-Fe2O3 particles this technique gives length-to-width distributions which approach those observed by the electron microscope technique. These results are compared to those obtained by remanence measurements which give smaller values of anisotropy constants, indicating the importance of incoherent magnetization reversals, when the remanence is acquired.