The Effect of Heat Treatment on Local Magnetic Interactions in Amorphous Fe-Based Ribbons Studied Using Mössbauer Spectroscopy

The results of a study of Fe-based alloy ribbons in the as-quenched state and after isothermal annealing are presented. Heat treatment was carried out in a vacuum at T = 723 K for 30 min; this temperature is above the Curie temperature of the initial amorphous alloy but below its reported crystallization-onset temperature. The objective was to assess the influence of crystallization on the local Fe environment and magnetic interactions using X-ray diffraction, Mössbauer spectroscopy, and magnetization measurements. In the as-quenched state, the ribbon exhibits a diffuse X-ray diffraction maximum and a broadened magnetic Mössbauer spectrum characteristic of an amorphous ferromagnet. After annealing, distinct Bragg reflections and better-defined magnetic spectral components indicate that the ribbon has become almost entirely crystalline. The X-ray diffraction pattern of the as-prepared alloy exhibits a broad amorphous halo, which transforms into a nanocrystalline structure upon thermal treatment. The crystallized state comprises a body-centered cubic α-Fe(Si) solid solution and a metastable tetragonal Fe3B phase, as confirmed by their respective diffraction planes: (110), (200), (211) and (411), (321), (213). The hyperfine-field distributions remain bimodal, but the dominant peak (maximum) shifts from approximately 25 to 32 T and becomes narrower, consistent with the development of well-defined crystalline Fe environments. These structural changes are discussed in the context of approaching to ferromagnetic saturation.