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The inherent switching current asymmetry (beta) pertaining to ordinary magnetic tunnel junctions (MTJs) significantly reduces the write margin of spin-transfer-torque magnetoresistive random access memory (STT-MRAM). In this letter, we report how beta can be controlled by the effective magnetostatic field (H e) that develops along the easy axis of the MTJs. As H e is varied by 24% of the effective uniaxial anisotropy field, beta measured at 100 ns is reduced from 1.51 to 1.04. The results suggest that tuning the magnetostatic offset field (H off) arising from the dipolar coupling between adjacent ferromagnetic layers can control beta and improve the write margin of STT-MRAM.