Bulk Nd–Fe–Al Amorphous Alloys with Hard Magnetic Properties

Abstract

An amorphous phase in Nd–Fe–Al system was formed in an extremely wide composition range of 0 to 90 at% Fe and 0 to 93 at% Al by melt spinning. Based on the information on the amorphous formation, ferromagnetic Nd90−xFexAl10 bulk amorphous alloys with high coercive force at room temperature were obtained by a copper mold casting method. The maximum diameter of the cylindrical amorphous samples with a length of 50 mm is about 7 mm for the 20%Fe alloy and about 4 mm for the 30%Fe alloy. Neither glass transition nor supercooled liquid region is observed in the temperature range before crystallization, being different from previous bulk glassy alloys exhibiting a wide supercooled liquid region before crystallization. The onset temperature of crystallization (Tx) and melting temperature (Tm) are measured to be 778 and 863 K, respectively, for the Nd70Fe20Al10 alloy. The resulting reduced ratio of Tx⁄Tm is as high as 0.90 and the temperature interval between Tx and Tm is as small as 85 K. The extremely high Tx⁄Tm and small ΔTm(=Tm−Tx) values are the reason for the achievement of the large glass-forming ability. The bulk amorphous Nd70Fe20Al10 alloy has a ferromagnetism with the Curie temperature (Tc) of about 600 K which is much higher than the highest Tc (about 480 K) for the Nd–Fe binary amorphous alloy ribbons. The remanence (Br) and intrinsic coercive force (iHc) for the bulk Nd60Fe30Al10 alloy are 0.122 T and 277 kA/m, respectively, in the as-cast state and 0.128 T and 277 kA/m, respectively, in the annealed state for 600 s at 600 K. The Br and iHc decrease to 0.045 T and 265 kA/m, respectively, for the crystallized Nd60Fe30Al10 sample consisting of Nd+Al2Nd+δ phases and the maximum hard magnetic properties are achieved in the amorphous state. The hard magnetic properties for the bulk amorphous alloys are presumably due to the homogeneous development of ferromagnetic clusters with large random magnetic anisotropy. The finding of the bulk amorphous alloys exhibiting hard magnetic properties at room temperature is promising for the future development as a new type of metallic amorphous permanent magnet.