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. Metallic ribbons, produced via melt-spinning technique, were annealed in-situ, with SXRD patterns collected every 60 s. SXRD reveals that Co40Fe40B20 alloys crystallize from an amorphous structure to a primary bcc alpha-(Co, Fe)phase, whereas Co60Fe20B20 initially crystallizes into the same bcc alpha-(Co, Fe)but exhibits cooperative growth of both stable and metastable boride phases later into the hold. Johnson-Mehl-Avrami-Kolmogorov statistics was used on post annealed samples to determine the mechanisms of growth and the activation energy (E-alpha)of the alpha-(Co, Fe)phase. Results indicate that the growth mechanisms are similar for both alloy compositions for all annealing temperatures, with the 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 alpha-(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. (C) 2014 AIP Publishing LLC.