The relationship between microstructure and magnetoelastic response in the magneto strictive amorphous alloy Co47.4Fe31.6Si2B19 after magnetic field annealing

Activation of the magnetostrictive sensors is achieved by magnetization of the ferromagnetic biasing element. This bias field causes the magneto strictive element to operate near the so-called “knee” of its M-H curve. However, magnetic biasing element complicates the mechanical design and adds cost to the marker structure. It is, therefore, of primary concern to provide a magnetomechanical marker which avoids the need to use a separate biasing element. In order to make a “self-biased” sensor, two-step magnetic annealing of the magnetostrictive amorphous alloy with Co47.4Fe31.6Si2B19 composition was devised. After annealing, the surface was fully crystallized and formation of α-iron crystallites was observed in the bulk of the amorphous ribbon. With this surface layer and the low density of α-iron particles in the bulk of the ribbon, a reasonably long ring-down time was achieved. The low acoustic and eddy current loss appear to be attributable to the low density of α-iron particles in the bulk of the ribbon. It appears that this crystallized surface layer can replace a bias magnet since the semi-hard surface layer fixes the magnetic domain configuration which is necessary for the operation of the sensor material. It was also found that microstructural changes during magnetic field annealing appear to be critical to the tag performance.