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

The thickness dependent nucleation and pinning fields have been obtained for an exchange-coupled hard/soft/hard magnetic layered system based on a micromagnetic calculation. The calculation reveals that the coercivity mechanism is nucleation for small soft layer thickness while it is pinning for large one. The critical thickness at which the coercivity mechanism changes is generally very small. Thus the dominant coercivity mechanism in such a magnetic system is pinning rather than nucleation. Such a pinning, however, is attributed to the change of the intrinsic parameters associated with the phase change at the interface and has both attributes of the traditional nucleation and pinning. Analysis shows that this pinning mechanism is the dominant coercivity mechanism in most exchange-coupled permanent and composite magnetic materials, which is called as self-pinning in this paper. From this self-pinning some specific formulae on pinning field can be derived. In particular, for sufficiently large soft grains/defects, the pinning field can be expressed as HP = αHK, where H K = 2 K M S is the anisotropy field and α depends on the material parameters and micromagnetic structures. These results are consistent with available experimental data.