Current-Induced Magnetization Switching in MgO Barrier Magnetic Tunnel Junctions With CoFeB-Based Synthetic Ferrimagnetic Free Layers

摘要

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 Co$_{40}$Fe $_{40}$B$_{20}$ /Ru/Co$_{40}$Fe$_{40}$B $_{20}$and Co$_{20}$ Fe$_{60}$B$_{20}$/Ru/Co $_{20}$Fe$_{60}$ B$_{20}$structures, and the MTJs (100 $times$ (150–300) nm$^{2}$) were annealed at 300 $^{circ}$C. The use of SyF free layer resulted in low intrinsic critical current density $(J_{rm c0})$ without degrading the thermal-stability factor ($E/k_{rm B}T$, where $E, k_{rm B}$, 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, $J_{rm c0}$ was reduced to $2hbox{–-}4 times 10^{6}$ A/cm$^{2}$ . This low $J_{rm c0}$ may be due to the decreased effective volume under the large spin accumulation at the CoFeB/Ru. The $E/k_{rm B}T$ 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.