Tailoring soft and hard magnets by annealing Co-based metallic glass

Co 67Fe 4Mo 2Si 17B 11 metallic glass ribbon has been subjected to the isothermal annealing at temperatures in the range 250–600°C so as to produce a series of samples with gradually coarser microstructure. For this series of samples a giant increase of the coercivity, exceeding five orders of magnitude, is observed. It shows a possibility to tailor soft or hard magnets using the same parent material. An abrupt increase of the coercivity occurs in a relatively small range of annealing temperatures between 480 and 520°C, and is mainly due to a strengthening of the pinning effect of the precipitates (fine crystalline structure) on the domain walls. Samples annealed at higher temperatures become fully crystallized. First, the metastable phase(s) is created which decomposes to the stable phases at still higher temperature. Coercivity for fully crystallized samples shows first a narrow plateau and afterwards a gradual decrease of its value with increasing temperature of annealing. Magnetic and microstructural properties of the samples, annealed at various temperatures, were investigated applying a number of complementary techniques including DSC and TGM methods, X-ray diffraction, TEM, strain-modulated FMR spectroscopy as well as conventional magnetic measurements.