Tuning Surface Spin Polarization of CoFeB by Boron Diffusion Detected by Spin Resolved Photoemission

Research of spin polarization of magnetic CoFeB thin films is of practical importance in spintronic applications. Here, using a direct characterization technique of spin-resolved photoemission spectroscopy, we obtain the surface spin polarization of amorphous Co₄₀Fe₄₀B₂₀ thin films with different annealing temperatures from 100 °C to 500 °C prepared by magnetron sputtering. After high annealing temperature, a quasi-semiconductor state is gradually formed at the CoFeB surface due to the boron diffusion. While the global magnetization remains almost constant, the secondary electrons’ spin polarization, average valence band spin polarization and the spin polarization at Fermi level from spin-resolved photoemission spectroscopy show a general trend of decreasing with the increasing annealing temperature above 100 °C. These distinct surface properties are attributed to the enhanced Fe–B bonding due to the boron segregation upon surface after annealing as confirmed by x-ray photoelectron spectroscopy and scanning transmission electron microscopy with energy dispersive spectroscopy. Our findings provide insight into the surface spin-resolved electronic structure of the CoFeB thin films, which should be important for development of high-performance magnetic random-access memories.