Melt-spun yttrium-iron alloys: Magnetic properties and crystallization kinetics

Abstract

Magnetic and crystallization data are presented for a series of X-ray amorphous Y1−xFex (0.4≤x≤0.79) alloys prepared by melt-spinning. Magnetization measurements, made from 4.2 to 600 K in fields up to 100 kOe, are interpreted on the basis of asperomagnetic structures in which the Fe moment directions are distributed over a decreasing portion of the unit sphere with increasing Fe concentration. In general, the ratio of spontaneous to saturation magnetization, and ordering temperatures, are higher than found for sputtered Y-Fe alloys, which may indicate a higher degree of compositional short-range order in these melt-spun alloys. The thermal stability of the alloys was investigated by differential scanning calorimetry and X-ray diffraction. Crystallization temperatures varied from 640 K at x=0.4 to 945 K at x=0.79. For the Y-rich (0.4≤x≤0.5) alloys, crystallization occurs by a multiple stage process, which includes the formation of a metastable Y-Fe intermetallic phase. Apparent activation energies for the transformations averaged 2.4–3.6 eV, in the range of moderate to high stability glasses. The kinetics of the x=0.67 and 0.75 compositions fit the Johnson-Mehl-Avrami relationship with a time dependence of t3–4 up to 65% conversion and t1.7–2.3 thereafter, indicative of a two-step crystallization mechanism. The coercive force of these melt-spun alloys was low (200–400 Oe) at all temperatures and, moreover, remained so during crystallization.