Magnetization process and magnetic losses in field-annealed amorphous and nanocrystalline ribbons

The phenomenology of magnetization process and losses in amorphous and nanocrystalline ribbons and its dependence on the anisotropy induced by field annealing is presented and discussed for a wide range of measuring conditions. Attention is focused on the evolution of the domain structure with the direction (longitudinal/transverse) and intensity of the uniaxial macroscopic anisotropy Ku and the ensuing effects on the magnetization reversal mechanisms. Such an evolution is directly observed by means of Kerr effect experiments. It is shown that the balance between domain wall displacements and moment rotations determines shape and area of the quasi-static hysteresis loop, while imposing a specific dependence of the energy loss on the magnetizing frequency. With transverse domains, rotations predominantly contribute to the magnetization reversal, but the total energy losses can approach the correspondingly expected classical limit only when attaining the MHz range. Nanocrystalline ribbons realize the best combination of low losses and high permeability at all frequencies, but high permeability may eventually lead to skin effect. It is shown that, in all cases, the frequency dependence of the excess loss component follows a power law close to f 1/ 2 , suffering from specific anomalies at high frequencies. The origin of such anomalies is envisaged as due to drifting balance between rotations and domain wall displacements, skin effects, and resonance phenomena.