Defect study of crystallized amorphous Co-rich magnetic alloy

Microstructure of Co-rich amorphous magnetic alloy, Co74.26Fe4.74B18.9Si2.1, formed by planar casting process was characterized using transmission electron microscopy (TEM) and auger electron spectroscopy (AES) to determine the cause of deterioration of the magnetic properties during extended manufacturing trial of the strip as domain wall pinned sensors. TEM analysis showed that micro-crystalline SiC particle contamination has caused the metastable B4Si crystallites to nucleate at the wheel (cold) side of the as-cast film. The B4Si crystals depleted the metalloids at the interface, which in turn, resulted in premature precipitation and excessive growth of Co grains at the perimeter of B4Si particles. Such microstructural inhomogeneity remained on the wheel side of the film even after field-annealing, which adversely affected the domain pinning. AES depth analysis also indicated that the as-cast film had a steep concentration gradient with the wheel side, being depleted of B and Si. The gradient flattened slightly during annealing. As the microstructural and compositional inhomogeneities disrupt the layered structure essential for the magnetic sensor, it is critical to control the processing parameters to eliminate the sources of such defects discussed above.