No abstract

The crystallization behaviour in Fe44.5Co44.5Zr7B4 alloy is investigated by DSC‐calorimetry, transmission electron microscopy and magnetization measurements. Two distinct crystallization steps, which correspond to the formation of α′‐FeCo phase (Tx1 = 774 K) and the crystallization of the remaining amorphous matrix (Tx2 = 957 K) are observed. Typical size of the nanograins in the samples that show advanced degree of crystallization is about 25 nm. The measurements of the hysteresis loops are performed in the temperature range 300 K–700 K. The specimens in the optimum heat treated stage exhibit fairly low values of coercive field (Hc < 35 Am–1) in whole temperature range investigated. These values of coercive field are lower than those reported for the original HITPERM alloys.

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=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.