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

A FePt-based hard-magnetic nanocomposite of exchange spring type was prepared by isothermal annealing of melt-spun $Fe52Pt28Nb2B18$ (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 $F57e$ Mössbauer spectrometry on one hand and the superconducting quantum interference device magnetometry on the other hand. The microstructure consists of $L10-FePt$ hard-magnetic grains (15–45 nm in diameter) dispersed in a soft magnetic medium composed by A1 FePt, $Fe2B$⁠, and boron-rich (FeB)PtNb remainder phase. The ribbons annealed at $700°C$ for 1 h exhibit promising hard-magnetic properties at room temperature: $Mr/Ms=0.69$⁠; $Hc=820kA/m$ and $(BH)max=70kJ/m3$⁠. 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.