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 investiga ted by DSC, by X-ray diffraction and by the temperature dependence of saturation magnetization. The DSC measurement revealed two well-pronou nced exothermic peaks, corresponding to two different steps of crystal lization. With increasing Nb content, x the onset of the both peaks sh ifted to higher temperatures and the separation between peaks increase d 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 struct ure with fine bcc Fe-Co grains embedded in a residual amorphous matrix . For example, after annealing for I h at 600 degrees 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 J(s), to the decrease of saturation magnetost riction constant lambda(s) and to the increase of the coercive field H -c. The nanocrystalline Co21Fe57Nb7B15 alloy exhibits moderate soft ma gnetic properties with J(s) = 1.42 T, lambda(s) = 21.4 ppm and H-c = 2 7 A m(-1). (C) 1997 Elsevier Science S.A.