Magnetic Characteristics for the Mould-Cast Hard Magnetic Nd70–xFe30Alx ( –10) Alloys

Bulk amorphous or nanocrystalline Nd-Fe-based alloys show surprising hard magnetic properties. Its pronounced room temperature coercivity results from the high random anisotropy of ferromagnetic nanoclusters. In this paper, the magnetic characteristics of the mould-cast Nd_70-xFe₃₀Al_x (x=0 -10) alloys were studied in detail. The substitution of Nd by Al could improve the glass forming ability, reduce the coercivity and Curie temperature, and suppress the formation of Nd₂Fe₁₇ phase. The microstructure consisting of 5-20 nm-sized dHCP Nd phases embedded in the amorphous phase was evident. The room temperature coercivities decreased from 340 to 270 kA/m with increasing Al content. The domain structure was investigated by magnetic force microscopy. The temperature dependence of the coercivity could be well explained by the strong pinning model of domain walls of Gaunt. The double layer hexagonal-close-packed (dHCP) Nd phase works as the pinning center. The Hopkinson peak was also investigated using the mathematical formalism based on the Stoner-Wohlfarth model. The hard magnetic properties of the alloys disappeared after heat treatment at 873 K, since part of the amorphous phase and the ferromagnetic clusters transferred into Th₂Zn₁₇-type phases and some unknown phase. This paper provides further insights into the origin of the hard magnetism for the amorphous and nanocluster alloys.