Modern soft magnets: Amorphous and nanocrystalline materials

This article surveys amorphous and nanocrystalline alloys for soft magnetic applications. Both materials have much in common, starting from the technique of production and including the key factors that determine their properties. Thus the magneto-crystalline anisotropy randomly fluctuates on a scale much smaller than the domain wall width and, as a consequence, is averaged out by exchange interactions so that there is no net anisotropy effect on the magnetization process, the prerequisite for good soft magnetic behaviour. Superior soft magnetic properties additionally require low magnetostriction, which is true of amorphous Co-based alloys and, more recently, nanocrystalline Fe-based alloys, but at a significantly higher saturation induction and with better thermal stability. Both materials reveal low losses of up to several hundred kilohertz and their B–H loop can be tailored by magnetic field annealing according to the demands of the application.