Abstract
The partitioning and segregation of B during the crystallization of amorphous FeCoB in Ta/FeCoB/MgO layered structures is investigated by atom probe tomography to obtain a better understanding of the beneficial impact of Ta capping layers on FeCoB/MgO/FeCoB magnetic tunneling junctions. Boron, initially uniformly dissolved in the amorphous FeCoB layer, is rejected from FeCo grains on crystallization and first segregates at the interface to the Ta layer where it prevents nucleation of crystalline FeCo. Only later, it is fully absorbed by the Ta layer. In the studied thin film structures, B neither segregates at nor dissolves into the MgO barrier even after prolonged heat treatment. Important kinetic parameters are derived from detailed isochronal annealing series. They allow the quantitative modelling of the observed process. The combination of high affinity to B but low diffusivity makes Ta the unique capping material to achieve an optimum performance.
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