Moessbauer study of a binary amorphous ferromagnet: Fe80B20

Binary amorphous ferromagnetic Fe80B20 has been studied by 57Fe Moessbauer spectroscopy from 4.2 to 1050 K. In the amorphous state at temperatures less than the Curie temperature T/sub C/ = 685 K, the spectra of Fe80B20 exhibit well-defined and essentially symmetric hyperfine-field distributions P (H). The analyses of P (H) were obtained by deconvoluting the spectra via a Fourier series. The shape of P (H) is practically independent of temperature. The values of the effective hyperfine field H/sub eff/(T) show a temperature dependence of H/sub eff/(0)(1-BT/sup 3/2/-CT/sup 5/2/xxx), indicative of spin-wave excitation. The values of B = (22 +- 2) x 10-6 K/sup -3/2/ and C = (1.4 +- 0.4) x 10-8 K/sup -5/2/ have been determined. The value of B/sub 3/2/ = BT/sub C//sup 3/2/ = 0.4 +- 0.05 is four times larger than those of crystalline Fe and Ni. rf sideband measurements of Fe80B20 at several frequencies under various field strengths indicate that Fe80B20 is highly magnetostrictive with a magnetostriction constant of about 45 x 10-6, which is comparable to the value of 30 x 10-6 obtained from static measurements. The amorphous state of Fe80B20 can not be preserved near T/sub C/ for more than a few minutes. The value of T/sub C/ = 685 K has been determined by the frquency-counting method. A very unusual crystallization behavior, which it both temperature and time dependent, has been observed. The onset of crystallization has been found at T/sub CR/ = 715 K under heating rates of 20 and 40 K/min. A highly unstable crystalline phase of Fe3B with T/sub C/ = 820 K forms after crystallization. This Fe3B phase transforms completely into Fe2B and α-Fe after heating at 1050 K for only 30 min