Structures, magnetic properties and thermal stability of CoFeB/MgO films

AbstractThe CoFeB/MgO films with various boron (B) contents were prepared by sputtering deposition and post annealed with different temperature. It was found that the films become amorphous and coercivity (HC) decreases with the increase of B content for the as-deposited CoFeB/MgO films. After vacuum annealing, the HC decreases because of the film stress release. The HC decreases more obviously in the CoFeB/MgO films with low B contents than that in the films with high B contents. The reason is that the sample with 12.1% B is amorphous before annealing and it becomes crystalline after annealing at 400°C and the sample with 6.7% B is always amorphous before and after annealing, the competition of the effect of film stress and the crystal anisotropy on the HC makes the slightly variation of HC. Partial crystallization can be achieved in the samples by vacuum annealing. With the increase of the B content, higher annealing temperature is needed to crystallize the sample. The average grain size decreases with the increase of B content in the CoFeB/MgO films. B atoms diffuse to the interface of CoFeB/MgO films and form boron oxide there after annealing, which makes the saturation magnetization (Ms) of the films decrease