The structure and magnetic properties of nanocrystalline Co21Fe64 − xNbxB15 alloys

The crystallization behaviour of amorphous Co21Fe64 xNbxB15 (x = 3, 5 and 7) ribbons, prepared by the melt-spinning technique, was investigated by DSC, by X-ray diffraction and by the temperature dependence of saturation magnetization. The DSC measurement revealed two well-pronounced exothermic peaks, corresponding to two different steps of crystallization. With increasing Nb content, x the onset of the both peaks shifted to higher temperatures and the separation between peaks increased slightly. It was proved by TEM that in the alloys with 5 and 7 at.% of Nb the first crystallization step leads to a nanocrystalline structure with fine bcc Fe-Co grains embedded in a residual amorphous matrix. For example, after annealing for 1 h at 600 °C the mean grain size was about 12 nm for x = 7. The magnetic measurements have shown that the first crystallization generally leads to the increase of the saturation magnetization Js, to the decrease of saturation magnetostriction constant λs and to the increase of the coercive field Hc. The nanocrystalline Co21Fe57Nb7B15 alloy exhibits moderate soft magnetic properties with Js = 1.42 T, λs = 21.4 ppm and Hc = 27 A m−1.