Development of a pinned wall sensor using cobalt-rich, near-zero magnetostrictive amorphous alloys

A novel type of a harmonic sensor that-is based on the concept of the domain wall pinning is described. Two-step annealing of the cobalt;rich amorphous precursors gives rise to the unique signature, which is attributable to the hysteresis loop with a step change in flux. The first annealing under a magnetic field causes a field-induced uniaxial anisotropy in M-H loop whose origin is believed to be due to structural inhomogeneities produced by internal segregation of oxygen atoms on cobalt {111} lattice planes. Subsequent annealing without magnetic field gives rise to the pinned wall stepped hysteresis characteristic, which is indicative of the useful markers. During annealing, it is observed that the surface and bulk of the amorphous material undergo substantial oxidation and crystallization. Consequently, microstructural features affect tag-performance significantly. Those features, including (1) Crystal structure of the cobalt layer (fee or hcp), (2) oxygen faulting in cobalt layer, (3) surface oxide, and (4) magnetic state of the cobalt layer, are discussed. In addition, a surface wall pinning model is suggested since we are convinced that wall pinning is most effectively achieved at the interface between the amorphous and its semihard cobalt layer. It is also observed that there is a good correlation between domain wall pinning field and thickness of crystalline Co layer.