Magnetic phase transitions in Fe-(Nb, Zr)-B-Al amorphous and nanocrystalline alloys

Abstract

Nanocrystalline Fe±(Nb, Zr)±B alloys, prepared by primary crystallization of melt-spun amorphous ribbons, are new soft magnetic materials with a low coercivity and a high saturation magnetization [1]. The microstructure consists of a large proportion of body-centred cubic (b.c.c.) a-Fe nanograins embedded in a residual amorphous matrix. It was reported that both the reduced magnetocrystalline and magnetoelastic anisotropies provide the basis for the magnetic softness of these materials [2]. The nanocrystallization mechanism was described by the nucleation-and-growth and grain-growth models [3, 4]. In this letter we report the magnetic phase transitions in Fe83:5Nb7B8:5Al1 and Fe85:5Nb4Zr4 B5:5Al1 (compositions given in atomic per cent) amorphous and nanocrystalline alloys. The amorphous alloys were prepared by the single-roller melt-spinning method. The thickness and width of the resulting ribbons were about 30 im and 1.5 mm, respectively. The amorphous ribbons were annealed in a conventional furnace under an argon atmosphere. The magnetic phase transitions were followed by thermogravimetric analysis (TGA) (Perkin±Elmer 7) by heating from room temperature to about 800 8C at a heating rate of 10 8C miny1 under a small applied magnetic ®eld. The microstructure was observed by transmission electron microscopy (TEM) using a JEM-2010 operating at 200 kV. Fig. 1a, b and c shows the TGA curves of Fe83:5Nb7B8:5Al1 alloy as quenched, annealed for 1 h at 460 8C and annealed for 1 h at 655 8C, respectively.