Current-induced magnetization switching in MgO barrier based magnetic tunnel junctions with CoFeB/Ru/CoFeB synthetic ferrimagnetic free layer

We report the intrinsic critical current density (J(c0)) in current-induced magnetization switching and the thermal stability factor (E/k(B)T, where E, k(B), and T are the energy potential, the Boltzmann constant, and temperature, respectively) in MgO based magnetic tunnel junctions with a Co40Fe40B20(2 nm)/Ru(0.7-2.4 nm)/Co40Fe40B20(2 nm) synthetic ferrimagnetic (SyF) free layer. We show that J(c0) and E/k(B)T can be determined by analyzing the average critical current density as a function of coercivity using the Slonczewski's model taking into account thermal fluctuation. We find that high antiferromagnetic coupling between the two CoFeB layers in a SyF free layer results in reduced J(c0) without reducing high E/k(B)T.