Magnetic aspects of the ferromagnetic “bulk metallic glass” alloy system Nd–Fe–Al

Detailed structural and magnetic characterization was carried out on nominally amorphous melt-spun ribbon based on the composition Nd60Fe30−xAl10+x to pursue the origins of its anomalously large coercivity values. The combined structural and magnetic analyses point to the existence of antiferromagnetic motes within the melt-spun ribbon. The average diameter of these motes is 1.2±0.5 nm and their crystal structure is related to the Nd6Fe13−xAl1+x, δ-phase structure, which under equilibrium conditions is the primary solidification phase near the Nd60Fe30Al10 composition. These motes are hypothesized to strongly couple to the ferromagnetic matrix phase in an “exchange-bias”-type manner and confer extremely high values of coercivity at low temperature. The composition dependence of the δ-phase Néel temperature is reflected in the composition dependence of the onset of large values of coercivity in the melt spun ribbons. The results obtained from melt-spun ribbons are compared to those obtained from a 10 gm drop-cast ingot 6 mm diameter of the same nominal composition and are found to be internally consistent. Delineation of the solidification pathway in the vicinity of the composition RE60Fe30Al10 provides a sound physical explanation of the variety of microstructures and associated variety of magnetic behavior found in cast and melt-spun forms of this material.