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

We investigated the effect of using a synthetic ferrimagnetic (SyF) free layer in MgO-based magnetic tunnel junctions (MTJs) on current-induced magnetization switching (CIMS), particularly for application to spin-transfer torque random access memory (SPRAM). The employed SyF free layer had a Co40Fe40B20/Ru/Co40Fe40B20 and Co20Fe60B20/Ru/Co20Fe60B20 structures, and the MTJs (100 x (150-300) nm(2)) were annealed at 300 degrees C. The use of SyF free layer resulted in low intrinsic critical current density (J(c0)) without degrading the thermal-stability factor (E/k(B)T, where E, k(B), and Tare the energy potential, the Boltzmann constant, and temperature, respectively). When the two CoFeB layers of a strongly antiferromagnetically coupled SyF free layer had the same thickness, J(c0) was reduced to 2-4 X 10(6) A/cm(2). This low J(c0) may be due to the decreased effective volume under the large spin accumulation at the CoFeB/Ru. The E/k(B)T was over 60, resulting in a retention time of over ten years and suppression of the write current dispersion for SPRAM. The use of the SyF free layer also resulted in a bistable (parallel/antiparallel) magnetization configuration at zero field, enabling the realization of CIMS without the need to apply external fields to compensate for the offset field.