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

Abstract

We investigated the spin-torque diode effect in submicron-scale Co60Fe20B20∕MgO∕(CoxFe1−x)80B20 (0⩽x⩽0.9) magnetic tunnel junctions (MTJs) under perpendicular magnetic fields Hext up to 10kOe. A single peak was clearly observed in every spin-torque diode spectrum and the dependence of resonant frequency fres on Hext 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.