Control of Interlayer Exchange Coupling and Its Impact on Spin–Torque Switching of Hybrid Free Layers With Perpendicular Magnetic Anisotropy

To ensure data retention in high-density spin-transfer-torque magnetic random access memory (STT-MRAM), a [Co/Ni]-CoFeB hybrid free layer (HFL) with a high energy barrier was proposed for small-dimension magnetic tunnel junctions. Its behavior on device level, however, needs further research. In particular, the impact of the interlayer that provides magnetic coupling between the [Co/Ni] multilayers and the CoFeB on the spin-torque switching is not known. In this paper, both macrospin modeling and micromagnetic simulations are used to study the influence of interlayer exchange coupling (IEC) on the switching behavior of the HFL for different device sizes. Both methods provide the optimal value for the coupling constant ( $J_{{\mathrm{ ex}},o}$ ) and switching current ( $J_{{\mathrm{ sw}},o}$ ), as well as their size dependence, to realize HFL switching with the lowest energy consumption. In addition to the simulations, control of the IEC in the HFL by introducing a TaCoFeB interlayer and a Co termination layer on the [Co/Ni] part is shown, and a broad $J_{{\mathrm{ ex}},o}$ range is achieved without compromising the magnetotransport properties of the stacks. Both simulations and experimental work show that the HFL design with adequate interlayer engineering can be a viable route for high-density STT-MRAM devices and other spintronic applications.