  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(100x(150-300) nm^2) were annealed at 300oC. The use of SyF free layer
resulted in low intrinsic critical current density (Jc0) without degrading the
thermal-stability factor (E/kBT, where E, kB, 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, Jc0 was reduced to 2-4x10^6 A/cm^2. This low Jc0 may be due to the
decreased effective volume under the large spin accumulation at the CoFeB/Ru.
The E/kBT 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.
