FERROMAGNETIC BULK AMORPHOUS-ALLOYS

This article reviews our recent results on the development of ferromag netic bulk amorphous alloys prepared by casting processes. The multico mponent Fe-(Al,Ga)-(P,C,B,Si) alloys are amorphized in the bulk form w ith diameters up to 2 mm, and the temperature interval of the supercoo led liquid region before crystallization is in the range of 50 to 67 K . These bulk amorphous alloys exhibit good soft magnetic properties, i .e., high B-s of 1.1 to 1.2 T, low H-c of 2 to 6 A/m, and high mu(e) o f about 7000 at 1 kHz. The Nd-Fe-Al and Pr-Fe-Al bulk amorphous alloys are also produced in the diameter range of up to 12 mm by the copper mold casting process and exhibit rather good hard magnetic properties, i.e., B-r of about 0.1 T, high H-c of 300 to 400 kA/m, and rather hig h (JH)(max) of 13 to 20 kJ/m(3). The crystallization causes the disapp earance of the hard magnetic properties. Furthermore, the melt-spun Nd -Fe-Al and Pr-Fe-Al alloy ribbons exhibit soft-type magnetic propertie s. Consequently, the hard magnetic properties are concluded to be obta ined only for the bulk amorphous alloys. The bulk Nd-Fe-Al and Pr-Fe-A l amorphous alloys have an extremely high T-x/T-m, of about 0.90 and a small Delta T-m (= T-m - T-x) of less than 100 K and, hence, their la rge glass-forming ability is due to the steep increase in viscosity in the supercooled liquid state. The high T-x/T-m enables the developmen t of a fully relaxed, clustered amorphous structure including Nd-Nd an d Nd-Fe atomic pairs. It is, therefore, presumed that the hard magneti c properties are due to the development of Nd-Nd and Nd-Fe atomic pair s with large random magnetic anisotropy. The Nd- and Pr-based bulk amo rphous alloys can be regarded as a new type of clustered amorphous mat erial, and the control of the clustered amorphous structure is expecte d to enable the appearance of novel functional properties which cannot be obtained for an ordinary amorphous structure.