IMPROVEMENT OF SOFT MAGNETIC PROPERTIES IN FE-BASED RAPIDLY QUENCHED ALLOYS AND FESI ELECTRICAL STEELS BY MAGNETIC FIELD ANNEALING

Abstract

In this work, a controllable field-induced magnetic anisotropy is produced in series of Fe-Co-Mo-B type amorphous and nanocrystalline alloys with different amount of crystalline phase. The amorphous melt-spun ribbons were isothermally annealed under a high vacuum at temperatures 713 K ≤ Ta ≤ 823 K in the presence of transverse or longitudinal static magnetic field, which was sufficient to reach their full magnetic saturation. The reference samples were annealed and cooled in a zero magnetic field. The structural analysis has revealed formation of a two-phase nanocrystalline microstructure with ultrafine BCC-FeCo grains embedded in an amorphous matrix. The zero field annealed specimens show an appreciable increase of the coercivity with nanocrystallization. Sheared loops with good field linearity and improved magnetic softness were achieved for all investigated alloys after transverse field annealing. Heat treatment in longitudinal field results in squared hysteresis loops is characterized by extremely low coercive field values in the range of 3-8 A/m. These values are superior for HITPERM-type alloys and they remain fairly stable also at elevated temperatures. The magnetic field annealing effects were investigated also in the series of non-oriented electrical steel sheets containing about 2.9 wt.% Si characterized by a different degree of deformation. Here, the observed improvement of soft magnetic properties due to heat treatment in static magnetic field is less significant mainly because of the higher magnetocrystalline anisotropy. The development of induced anisotropy in the field-annealed alloys is discussed in the frame of the magnetic atoms pair ordering theory.