Voltage-controlled magnetic tunnel junctions with synthetic ferromagnet free layer sandwiched by asymmetric double MgO barriers

Memory and computing applications utilizing voltage-controlled magnetic random-access memory (MRAM) require perpendicular magnetic tunnel junctions (pMTJs) capable of high thermal stability and large write efficiency at advanced technology nodes. We first discuss the scaling requirements for voltage-controlled MRAM to replace various existing memory applications at an advanced CMOS technology node, including cell size, thermal stability, interfacial perpendicular magnetic anisotropy (PMA), and voltage-controlled magnetic anisotropy (VCMA). For replacing volatile memories including SRAM, eDRAM, and DRAM, we employ the retention relaxation technique along with a DRAM-style refresh scheme in the analysis. To enhance both PMA and VCMA at scaled nodes, we explore pMTJs with asymmetric double MgO barriers sandwiching a synthetic ferromagnet (SyF) free layer. In a thin MgO/SyF free layer/thick MgO/fixed layer MTJ stack, the SyF free layer is realized in various ways: single layer (X  =  Ta, W, Mo, Ir) and bilayers (X  =  Ta/W, Mo/Ir) of heavy metals insertions in CoFeB/X/CoFeB and Co₂FeAl/X/Co₂FeAl structures, and multiple insertions in [CoFeB/X]_n/CoFeB structures for n  =  1–4. A VCMA coefficient of 36 fJ (V m)⁻¹ is achieved in a MgO/CoFeB/Ta/CoFeB/MgO-based MTJ which is comparable to that of the single MgO barrier MTJ. We find PMA enhancement, although with VCMA reduction for an increasing number n of [CoFeB/X]_n repetitions. In addition, we demonstrate large TMR of 78%, large interfacial PMA of 1.45 mJ m⁻², and VCMA of 65 fJ (V m)⁻¹ in a MgO/Co₂FeAl/W/Co₂FeAl/MgO-based MTJ annealed above 400 °C. (paper)