Influence of perpendicular magnetic anisotropy on spin-transfer switching current in CoFeB/MgO/CoFeB magnetic tunnel junctions

We investigated the spin-torque diode effect in submicron-scale Co(60)Fe(20)B(20)/MgO/(Co(x)Fe(1-x))(80)B(20) (0 <= x <= 0.9) magnetic tunnel junctions (MTJs) under perpendicular magnetic fields H(ext) up to 10 kOe. Asingle peak was clearly observed in every spin-torque diode spectrum and the dependence of resonant frequency f(res) on H(ext) was well explained by using Kittel's formula. It was found that effective demagnetizing fields in the perpendicular-to-plane direction of the Fe-rich CoFeB free layers obtained from the spectra were considerably smaller than those expected from the magnetizations of the free layers. This suggested that the Fe-rich CoFeB free layers exhibited a perpendicular magnetic anisotropy, which agreed well with the reduced switching current density in the MTJs. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3057974]