Self-pinning: Dominant coercivity mechanism in exchange-coupled permanent/composite magnets

Our micromagnetic calculation demonstrates that the dominant coercivity mechanism is self-pinning in most exchange-coupled permanent and composite magnets. Such a pinning is attributed to the change of the intrinsic parameters associated with the phase change at the interface. From this self-pinning some more specific formulas on pinning field can be derived. In particular, for sufficiently large soft grains/defects, the pinning field can be expressed as HP=αHK, where HK=2k∕MS is the anisotropy field and α depends on the material parameters and micromagnetic structures. For an exchange-coupled Nd2Fe14B–αFe system with abrupt change of parameters in the interface, α≈0.1. Reducing the size of the soft grain will increase the coercivity, while the smooth change of the parameters in the interface will lead to a reduction in the coercivity. Comparison with experimental data justifies our calculation.