Magnetic microstructure of amorphous/nanocrystalline Fe-Mo-Cu-B alloys

Structure-to-magnetic arrangement relationship is investigated in NANOPERM-type Fe76Mo8Cu1B15 metallic ribbons prepared by planar flow casting. Its particular composition proved to be suitable for model case studies of the complexity of magnetic interactions frequently observed in nanocrystalline alloys. As-quenched as well as annealed samples are characterized by Mossbauer spectroscopy, transmission electron microscopy (TEM), high resolution TEM, X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The Curie temperature of the as-quenched alloy is about 40 degrees C. As a consequence, Mossbauer spectra sensitively reflect structural changes which are induced by annealing at moderate temperatures (below the first crystallization stage) when the investigated samples are still amorphous. The effects of structural rearrangement proposed from the DSC results are recognized both at room and low (77 K) temperature Mossbauer spectra. Annealing performed within the temperature range of the first crystallization stage provided different amounts of nanocrystallites. Nevertheless, the total amount of crystals towards the end of the first DSC peak was surprisingly low reaching only about 35%. XRD and TEM confirmed a bcc arrangement of Fe atoms without any distortion in lattice constant. Although the diameter the grains reaches barely 10 nm, the magnetic interactions among them and their impact on the surrounding amorphous residual matrix are striking. They are thoroughly discussed by the help of Mossbauer effect spectra taken at room temperature and at 77 K. Modifications of surface properties achieved by laser treatment are discussed using conversion electron Mossbauer spectrometry.