Influence of dipolar collective effects on coercivity and demagnetizing factors in hard magnetic materials

Magnetization reversal in systems made of high coercivity exchange-decoupled grains is modeled. Dipolar collective effects, negligible for a sample in the form of a foil with out-of-plane magnetization, become more and more significant as the sample shape evolves toward three-dimensional geometry, and reach their maximum in case of a foil with in-plane magnetization. These collective effects are manifested by coercivity variations, which allow differences in coercivity for samples of the same material but having different shapes to be understood. Taking collective effects into account requires the introduction of a correction term to the demagnetizing factor. The numerical results obtained are experimentally confirmed for NdFeB permanent magnets.