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 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
^deg
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
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 times10
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.