Time-resolved synchrotron x-ray diffraction studies of the crystallization of amorphous Co(80-x)FexB20

This paper addresses the time-dependent crystallization process occurring in ‘bulk’ amorphous Co80-xFexB20 (x = 20, 40) metallic ribbons by means of synchrotron x-ray diffraction (SXRD) and transmission electron microscopy (TEM). Metallic ribbons, produced via melt-spinning technique, were annealed in-situ, with SXRD patterns collected every 60 seconds. SXRD reveals that Co40Fe40B20 alloys crystallize from an amorphous structure to a primary bcc α-(Co,Fe) phase, whereas Co60Fe20B20 initially crystallizes into the same bcc α-(Co,Fe) but exhibits cooperative growth of both stable and metastable boride phases later into the hold. Johnson-Mehl-Avrami-Kolmogorov (JMAK) statistics was used on post annealed samples to determine the mechanisms of growth, and the activation energy (Ea) of the α-(Co,Fe) phase. Results indicate that the growth mechanisms are similar for both alloy compositions for all annealing temperatures, with an Avrami exponent of n = 1.51(1) and 2.02(6) for x = 20, and 40 respectively, suggesting one-dimensional growth, with a decreasing nucleation rate. Activation energy for α-(Co,Fe) was determined to be 2.7(1) eV and 2.4(3) eV in x = 20 and 40 respectively suggesting that those alloys with a lower Co content have a stronger resistance to crystallization. Based on these results, fabrication of CoFeB magnetic tunnel junctions via depositing amorphous layers and subsequently annealing to induce lattice matching presents itself as a viable and efficient method, for increasing the giant magnetoresistance in magnetic tunnel junctions.