Ultra-Low Write Energy Composite Free Layer Spin–Orbit Torque MRAM

Abstract

A highly efficient exchange-coupled free-layer spin–orbit torque (SOT) magnetic random access memory cell is proposed for ultra-high-density memory. By exploiting typically unrealized benefits of SOT—in particular, its compatibility with low-damping magnetic insulators and the energy efficiencies associated with exchange coupling of hard/soft composite structures—a write energy of 18 aJ/bit for 1 ns switching is achieved. Furthermore, high magnetocrystalline anisotropy materials such as L1 ₀ -FePt or L1 ₀ -FePd are employed not only to facilitate achievement of ultra-high-density memory but also to allow for reduction of heavy metal layer volume and a reduction in write energy not seen in previous CoFeB-based cells. This energy is within a factor 72 of the theoretical limit of $60~k_{B}T$ . It also represents a factor of >500 improvement relative to state-of-the-art DDR4 DRAM cells.