Ultralow-Voltage Spin-Transfer Switching in Perpendicularly Magnetized Magnetic Tunnel Junctions with Synthetic Antiferromagnetic Reference Layer

We developed top-free-type perpendicularly magnetized magnetic tunnel junctions (p-MTJs) with a synthetic antiferromagnetic (SAF) bottom reference layer consisting of [Co/Pt] superlattice and Ru spacer layers. We successfully demonstrated practical properties such as a low resistance–area ( RA ) product (<3 Ω µm 2 ), ultralow writing voltage (<200 mV) for spin-transfer-torque (STT) switching, and high annealing stability (up to 350 °C) in the same p-MTJ cells. Moreover, the p-MTJs showed clear bi-stable states even at zero external magnetic fields, thanks to the low stray field from the SAF structure. The results are promising for both high integration and ultralow-voltage operation of STT magnetoresistive random access memory (MRAM).