Synthesis of Fe-Co-Nd-B amorphous alloys with glass transition and their crystallization-induced hard magnetic properties

The glass transition followed by a supercooled liquid region was observed in the composition range of 5 to 57%Co, 2 to 4%Nd and 18 to 30 at%B in melt-spun Fe-Co-Nd-B amorphous alloys. The largest value of the supercooled liquid region defined by the difference between the glass transition temperature (T-g) and the crystallization temperature (T-x), Delta T-x(= T-x - T-g), was 40 K for Fe66.5Co10Nd3.5B20 and the T-g and T-x values are 802 K and 842 K, respectively. The crystallized structure consists of Fe3B, Nd2Fe14B, alpha-Fe and Nd2Fe23B3 phases and their grain sizes after annealing for 420 s at 933 K are 20, 20, 70 and 10 nm, respectively. The interparticle spacing of the Nd2Fe14B phase are evaluated to be less than 50 nm far the 10%Co alloy. The magnetization at an applied field of 1256 kA/m (I-1256), remanence (B-r), intrinsic coercive force (iH(c)) and maximum energy product (BH)(max) of the Fe66.5Co10Nd3.5B20 alloy subjected to an optimum annealing treatment (933 K, 420 s) are 1.46 T, 1.25 T, 187 kA/m and 86 kJ/m(3), respectively. The rather good hard magnetic properties are interpreted to result from the exchange magnetic coupling among Nd2Fe14B, Fe3B and alpha-Fe phases. The achievement of the rather good hard magnetic properties in the crystallized state of the Fe66.5Co10Nd3.5B20 amorphous alloy with the large Delta T-x of 40 K is expected to enable the future fabrication of a bulk hard magnetic material by the simple process of the formation of a bulk amorphous alloy followed by optimum crystallization.