Microwave power absorption analysis in the devitrification process of Co-based amorphous ribbons

Melt-spun Co₆₆Fe₄B₁₂Si₁₃Nb₄Cu soft magnetic ribbons were devitrified at low annealing temperatures (623 K), for times 5-20 min. Microwave power absorption measurements at 9.4 GHz (X-band) were carried out in two geometries. In geometry 1, the ribbon's plane was oriented parallel to AC magnetic field. For the orientation 2, the ribbon's plane was normal to the AC magnetic field. In both cases, the ribbon's axis was parallel to the DC magnetic field. For both orientations, two absorptions were observed: the first corresponds to a low field microwave absorption (LFA) centered in zero dc magnetic field, and a higher field absorption corresponding to the ferromagnetic resonance (FMR). In the geometry 1, a single FMR spectrum was observed for all the samples, with a shift in resonant field as annealing increased. For geometry 2, evidence of the superposition of two FMR signals was observed. FMR spectra are therefore due to a combination of two different magnetic phases corresponding to the amorphous matrix and nanocrystallites. Deconvolution calculations were carried out on FMR spectra to separate the contributions. Their behavior as a function of annealing time was in good agreement with the magnetic softening, also obtained with LFA results. The differences in microwave absorption, for both geometries, can be explained by differences in the electromagnetic wave propagation volume.