Loss improvement in Finemet by surface modification

Finemet is a nanocrystalline material with excellent soft magnetic properties. Though the losses are very low compared to other crystalline metallic soft magnets the fraction of the frequency dependent dynamic losses in Finemet is high and therefore offers a large potential for further improvement. Different surface defects like scratches and micro hardness indentations were produced on the surface of amorphous Fe73.5Cu1Nb3Si13.5B9 ribbons. Afterwards these and also untreated ribbons were nanocrystallised. It was found that the surface treatments can substantially reduce the power losses. This effect is already known for commercial grain oriented silicon steels, where the decrease of the power losses is attributed to domain refinement and the activation behaviour of 180 ° domain walls. The frequency dependence of the relative loss change indicates that domain refinement also occurs in surface treated Finemet. The treatment increases the power loss at very low frequencies. This can be attributed to the defects acting as obstacles for domain wall movement under nearly static conditions. For higher frequencies the dynamic loss becomes dominant and decreases with increasing number of movable domain walls.