Magnetic Properties of CoFeB-MgO Stacks with Different Buffer-Layer Materials (Ta or Mo)

We investigate the effect of the buffer-layer materials and their crystallographic structures on the magnetic properties of the CoFeB-MgO stacks. Amorphous Ta, amorphous Mo, and crystalline Mo are used as the buffer layer of CoFeB-MgO. The CoFeB-MgO stacks on both the Mo buffer layers show higher perpendicular anisotropy than that on Ta after 400-°C annealing. Difference is also seen between the samples with amorphous and crystalline Mo in the intermixing property between Mo and CoFeB; intermixing is much more unlikely for the crystalline sample. Magnetic damping constant is also evaluated from ferromagnetic resonance measurements. Samples with crystalline Mo buffer layer show smaller damping constant than those with Ta buffer layer after 400-°C annealing. This paper clarifies that the magnetic properties of CoFeB-MgO depend on the buffer-layer materials and their crystallinities.