Microstructural origin of the ultra-low coercivity in bulk Fe65.5Cr4Mo4Ga4P12B5.5C5 metallic glasses

Bulk metallic glasses (BMGs) have an extremely low coercivity, suggesting that the BMGs must have unique magnetic microstructures. These magnetic microstructures, however, are largely unknown. We prepared Fe65.5Cr4Mo4Ga4P12B5.5C5 BMGs by a flux-melting and water-quenching technique. We then studied the topographical and magnetic microstructures of as-quenched and annealed BMGs by atomic force microscopy (AFM) and magnetic force microscopy (MFM), respectively. The microstructural results were compared with those achieved in rapidly quenched glassy ribbons. Magnetic domains are pinned inside the BMGs due to residual stress, most of which can be removed by annealing. This is also applicable to the glassy ribbons. However, when compared to the glassy ribbons, our BMGs are with a significantly smaller surface roughness, a much smaller free volume, and free from non-magnetic inclusions. It is these unique microstructural factors and the stress release by annealing that greatly improve the soft ferromagnetic properties of our iron-based BMGs.