The Magnetic Microstructure of Bulk Ferromagnets

A review will be given on the magnetic microstructure of bulk ferromagnetic materials, the character of which is mainly determined by anisotropy, surface orientation and microstructure. The review will start with an analysis of surface and internal domains in Fe-like materials with cubic anisotropy and NdFeB as an example for strong uniaxial materials. For strongly misoriented surfaces, domain branching is the characteristic feature in each case.

It will then be shown how the domain structure changes when the structural length (grain size) decreases down to the nanometer regime. For soft magnetic nanocrystalline material, the exchange coupling between the nano-grains leads to a seemingly uniform magnetic material. Comparing samples with weak and strong induced anisotropy, significant differences in the magnetic microstructure are found, reflecting the interplay of the uniform, field-induced and the random magnetocrystalline anisotropy. In particular for weak induced anisotropy an irregular patchy modulation of magnetization on the scale of a few micrometers is found within otherwise regular domains, which can be interpreted in terms of the random anisotropy model. These differences are also reflected in the magnetization processes and in the domain behaviour at elevated temperatures. In high-anisotropy materials with grain sizes in the 100 nm regime (generally known as nanocrystalline permanent magnets), so-called interaction domains are observed due to the predominance of magnetostatic interactions between the grains.