Influence of Annealing Temperature and Atmosphere on Surface Microstructure and Magnetism in FINEMET-Type FeSiNbCuB Ribbons

Paper is primarily devoted to the surface investigations of the FINEMET-type
${\rm Fe}_{73.5}{\rm Si}_{13.5}{\rm Nb}_{3}{\rm Cu}_{1}{\rm B}_{9}$
ribbons. As-quenched (AQ) samples were subsequently annealed at different temperatures from 733 K to 923 K in vacuum and hydrogen in order to follow the influence of both treatment parameters on microstructure and magnetic properties. Two-phase magneto-optical (MO) hysteresis loops observed at AQ ribbons correspond to an amorphous structure composed of clusters of the size in units of nm (calculated from XRD). The annealing temperatures are responsible for the origin of the
$\alpha$
-Fe and/or
$\alpha$
-Fe(Si) nanocrystals of various size formed at the surface and consequently penetrating into the ribbon volume while the atmospheres influence surface oxidation. An interesting occurrence of the asymmetric reversal of longitudinal magnetization, clearly observed after annealing at 733 K and 743 K, is discussed from the viewpoint of quadratic magneto-optical effects. The loop asymmetry decreases with increasing annealing temperature and disappears at 923 K.