Nucleation fields and grain boundaries in hard magnetic materials

Numerical micromagnetic calculations of the nucleation field of two-dimensional magnetic microstructures reveal the effects of interparticle interactions on the magnetization reversal mechanism. The results show that the magnetic behavior of interacting particles drastically changes with the grain size. For an average grain diameter D>100 nm, the particles of magnetic microstructures are strongly coupled by long range magnetostatic interactions. Therefore, the reduced nucleation field of misoriented grains determines the coercive field of large grained particle ensembles irrespective of the grain boundary type. In smaller grains long-range demagnetizing fields become less effective. Nonmagnetic boundary phases prevent the expansion of a reversed domain nucleus into the neighboring grains. If the average grain diameter approaches the domain wall width, short-range exchange interactions between neighboring grains increase the nucleation field