Effect of a Cavity on a Single-Domain Magnetic Particle

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Abstract
It has been suggested that the observed low coercive forces of some ferromagnetic powders, as compared with predictions of single-domain-particle theory, may be due to cavities in the particles. Here this possibility is investigated theoretically. Some general formulas are established for a particle of arbitrary shape and for an ellipsoidal particle containing an ellipsoidal cavity; attention is then turned to the case in which both ellipsoids are spheroids, the particle prolate, the cavity either prolate or oblate. Several special cases are studied. In some cases the cavity increases the coercive force, in others it decreases it; for an originally large coercive force, the usual effect is a decrease. It is concluded that the proposed mechanism is capable of accounting, at least in part, for the observed decrease.