An important task of micro- and nanoelectronics is to establish a new universal memory type in the near future. Magnetoresistive Random Access Memory with Spin Transfer Torque (STT-MRAM) is a promising candidate for future universal memory. Perpendicular Magnetic Tunnel Junctions (p-MTJs) with interface-induced uniaxial magnetic anisotropy show potential, but still require damping reduction and thermal stability increase. Therefore, research of new materials and architectures for MTJ-based structures is intensifying.
We propose using MTJs with a composite free layer. The free magnetic layer of such a structure consists of two half-ellipses separated by a non-magnetic spacer (figure 1 right). In contrast to p-MTJ, the magnetization of the magnetic layers lies in-plane. This allows substantially broadening the scope of the magnetic materials suited for constructing such MTJs and boosting the thermal stability factor while maintaining fast switching.
A penta-layer MTJ with composite free layer demonstrates a substantial decrease of the switching time (figure 2) and current reduction. As for a physical explanation of this effect, we found that the switching processes of the left and right part of the composite free layer occur in opposite senses to each other. Each half of the free layer generates a stray magnetic field that influences the other half and helps accelerate switching. It is important that the switching mostly occurs in the x-y plane when under the influence of the magnetic fields generated by either half of the composite free layer. This fact means that, similar to p-MTJs, the switching barrier in an MTJ with a composite free layer is practically equal to the thermal stability barrier defined by shape anisotropy. At the same time MTJs with composite free layer have good thermal stability factor scalability. An MTJ with 52.5nm x 10nm cross- section and 5nm thickness of the free layer has a thermal stability factor of 60kT, which exceeds that of the p-MTJ demonstrated so far.