Hysteresis and nucleation field in two-dimensional magnetization process

We have investigated the vector nature of the magnetization process in disk-shaped Co-based amorphous samples, providing a joint description of spin rotation and domain nucleation processes. Thermo-magnetic treatments have been exploited to induce a macroscopic uniaxial anisotropy in the specimen plane. An alternating field Ha has been applied to the specimens along a direction forming with the easy axis an angle yHa between 0! and 90!: The two orthogonal components of the magnetization Mk and M? (along the easy axis and perpendicularly to it), and of the effective field Hk and H?; have been measured by means of a compensated winding and a many-turn H-coil placed on the sample surface. We have studied the evolution of the Mk vs. M? and Hk vs. H? loops as a function of yHa : In particular we have pointed out the condition leading to the nucleation of domains in a two-dimensional magnetization process, and obtained the nucleation field values HN for any yHa : These results have been interpreted proposing a model accounting for nucleation in the framework of the Stoner–Wohlfarth theory.