Top-Pinned STT-MRAM Devices With High Thermal Stability Hybrid Free Layers for High-Density Memory Applications

The hybrid free layer (HFL) design consisting of a single layer of CoFeB coupled via a spacer to [Co/Ni] multilayers was proposed previously to increase perpendicular magnetic anisotropy in the free layer of top-pinned magnetic tunnel junction stacks. It is thus expected that a high thermal stability factor (Δ) can be achieved in devices with small dimensions, which is required for data retention in high-density memory applications. Here, we report on the electrical and magnetic properties of the HFL in devices with various critical dimensions (CD), in which case the [Co/Ni] multilayers in the HFL have only one Co/Ni bilayer on Pt seed. It is shown first that the HFL can provide larger coercive field (H
_c
) than dual-MgO FL counterparts. μ
₀
H
_c
around 125 mT is achieved in sub-20 nm devices with the HFL. To derive A, the field sweep method was applied. The fitting of the switching probability versus applied field was compared for a macrospin reversal and a domain wall mediated reversal (DWMR). The DWMR model is valid when the CD is larger than around 40 nm in both dual-MgO FL and HFL cases, where the switching is realized through domain wall motion as shown via micromagnetic simulations. In both regimes, the HFL provides a higher A than the dual-MgO FL.