Crystallization of amorphous CoFeB ferromagnetic layers in CoFeB/MgO/CoFeB magnetic tunnel junctions

We demonstrate that the crystallization of ferromagnetic CoFeB layers originates at the interface with a MgO layer in CoFeB/ MgO/CoFeB magnetic tunnel junctions by annealing using cross-sectional transmission electron microscopy and electron diffraction. The CoFeB layers, which are amorphous in the as-deposited state, crystallize with a (001) out-of-plane texture by annealing at 360 degrees C. Crystal grains of 15-20 nm in the CoFeB layers are observed at the interface with the MgO layer, but not at the interface with a Ta or Ru layer. Much smaller crystal grains with random crystal orientations are formed in a region away from the MgO interface in the CoFeB layers. The depth profiles obtained by X-ray photoelectron spectroscopy show that boron diffuses from the crystallized region at the interface into the MgO layer and the rest of the region in the CoFeB layers during crystallization, indicating that crystal grains have much lower B contents than the original composition.