Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) have been developed for decades for spin-transfer-torque magnetic random-access memory. Common stack designs use a hard layer (HL) with strong PMA to pin the reference layer (RL) by forming a synthetic antiferromagnet through a thin nonmagnetic coupling layer. Compared to bottom-pinned MTJs, very limited progress has been made to top-pinned MTJs, especially on the RL pinning due to its inferior thermal robustness. Herein, an alternative stack design is proposed for top-pinned MTJs, i.e., a synthetic ferromagnet (SFM). In the SFM, the RL is coupled with the HL ferromagnetically through a coupling layer. Micromagnetic simulations predict the advantage of the SFM design to stabilize the RL at scaled critical dimension (CD), which is experimentally proven by the observation of an increased RL pinning field on the device level. Because of the RL stray field acting on the free layer (FL), a compensation magnet (CM) is inserted below the FL to form a top-pinned MTJ stack without compromising magnetotransport properties. Devices with centered FL switching loops can be obtained after a two-step field setting. The stray field of CM has limited impact on the RL due to the large distance in between, thus keeping the RL’s pinning field larger than 150 mT down to devices with 20-nm CD. Finally, current switching is realized in devices with SFM and CM, showing critical current density around 5–8 ${\mathrm{MA}/\mathrm{cm}}^2$ and an averaged thermal stability as high as 50. Thus, the SFM pinning layer design shows great potential in stabilizing top-pinned devices and paves the way for multiple future spintronic applications requiring a top-pinned stack design.

• Received 16 July 2018
• Revised 10 October 2018

DOI:https://doi.org/10.1103/PhysRevApplied.10.054054