Evidence for phase selectivity in excimer-laser-induced amorphization of thermally annealed Fe66Co18B15Si glassy ferromagnet

Pulsed-excimer-laser irradiation effects (lambda=308 nm, tau=10 ns) on the magnetic properties and phase composition of Fe66Co18B15Si metallic glass have been studied by transmission and conversion electron Mossbauer spectroscopy, scanning electron microscopy, and energy-dispersive x-ray analysis. Depending on the number of applied pulses, excimer laser irradiation of amorphous Fe66Co18B15Si was found to induce controlled changes in the magnetic anisotropy without onset of bulk crystallization. Excimer-laser-induced amorphizatian was observed in partially crystallized Fe66Co18B15Si samples (T-a=648 K, t(a)=1 h) and was found to be accompanied by the development of laser-induced magnetic anisotropy. In completely crystallized Fe66Co18B15Si specimens (T-a=723 K, t(a)=1 h), which contained two crystalline phases of alpha-(FeCo) and (FeCo)(3)(BSi), the effect of excimer-laser-induced amorphization exhibited phase selectivity with respect to the crystalline components of the alloy system. By a combination of complementary methods, the laser-induced phase transformation of thermally annealed Fe66Co18B15Si specimens was shown to consist of partial amorphization and surface oxidation of the irradiated material. When compared to bulk data, a different behavior of the surface magnetic texture and relative abundance of alloy phases was observed. In both as-quenched and excimer-laser-irradiated amorphous Fe66Co18B15Si, the hyperfine field H-hf(T) exhibited a temperature dependence of H-hf(0)[1-B-3/2(T/T-C)(3/2)], indicative of spin-wave excitations. The value B-3/2 =0.32+/-0.05 of the temperature coefficient for the amorphous specimen is twice that obtained for the laser-irradiated Fe66Co18B15Si system. It is concluded that laser-induced quenching stresses reduce the fluctuations of exchange interactions in the irradiated sample. This study suggests that excimer-laser irradiation of glassy ferromagnets offers the intriguing opportunity of designing materials with specific combinations of magnetic and structural properties, not obtainable by conventional methods.