The influence of microstructure on magnetic properties of nanocrystalline Fe-Pt-Nb-B permanent magnet ribbons

摘要

A FePt-based hard-magnetic nanocomposite of exchange spring type was prepared by isothermal annealing of melt-spun Fe_(52)Pt_(28)Nb_2B_18 (atomic percent) ribbons. The relationship between microstructure and magnetic properties was investigated by qualitative and quantitative structural analysis based on the x-ray diffraction, transmission electron microscopy, and ~(57)Fe Mossbauer spectrometry on one hand and the superconducting quantum interference device magnetometry on the other hand. The microstructure consists of Ll_o-FePt hard-magnetic grains (15-45 nm in diameter) dispersed in a soft magnetic medium composed by Al FePt, Fe_2B, and boron-rich (FeB)PtNb remainder phase. The ribbons annealed at 700 ℃ for 1 h exhibit promising hard-magnetic properties at room temperature: M_r/M_s=0.69; H_c=820 kA/m and (BH)_(max) = 70 kJ/m3. Strong exchange coupling between hard and soft magnetic phases was demonstrated by a smooth demagnetizing curve and positive δM-peak in the Henkel plot. The magnetic properties measured from 5 to 750 K reveals that the hard characteristics remains rather stable up to 550 K, indicating a good prospect for the use of these permanent magnets in a wide temperature range.