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

Abstract

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 (Tg) and the crystallization temperature (Tx), ΔTx(=Tx-Tg), was 40 K for Fe66.5Co10Nd3.5B20 and the Tg and Tx values are 802 K and 842 K, respectively. The crystallized structure consists of Fe3B, Nd2Fe14B, α-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 for the 10%Co alloy. The magnetization at an applied field of 1256 kA/m (I1255), remanence (Br), intrinsic coercive force (iHc) 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/m3, respectively. The rather good hard magnetic properties are interpreted to result from the exchange magnetic coupling among Nd2Fe14B, Fe3B and α-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 ΔTx 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.