Micromagnetic Calculation for Magnetic Tunnel Junctions With Equal Parallel and Antiparallel Thermal Stability

In magnetic tunnel junctions (MTJs) with perpendicular magnetic easy axes, the retention times for parallel and antiparallel states are different because of dipolar coupling with the reference layer (RL). To make retention times symmetrical, we model three types of MTJs that have single or synthetic ferrimagnetic (SyF) RLs with stepped structures. Micromagnetic calculations are performed on the basis of the Landau-Lifshitz-Gilbert equation of motion to evaluate the stray fields from the RLs of each design. A SyF structure with a step on the bottom magnetic layer has the smallest stray field of 2 mT at lateral dimensions of 30 nm × 30 nm in the layer. Symmetric thermal stability for parallel and antiparallel states should help to realize high-density nonvolatile magnetic random-access memory and logic chips.