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/Co₄₀Fe₄₀B₂₀and Co₂₀Fe₆₀B₂₀/Ru/Co₂₀Fe₆₀B₂₀structures, and the MTJs (100 times (150-300) nm²) were annealed at 300 ^degC. 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 T are 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 times10⁶ A/cm². 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.