Enhancement of magnetic coupling and magnetic anisotropy in MTJs with multiple CoFeB/MgO interfaces for high thermal stability

Magnetic coupling between two CoFeB layers through the W insertion layer is important in the conventional double CoFeB/MgO interface, magnetic tunneling junctions (MTJs) (double-MTJs) with MgO/CoFeB/W/CoFeB/MgO free layer stack because it increases the effective magnetic volume of the free layer. The magnetic coupling energy constant per unit area, J(cpl), between two CoFeB layers through the W layer and the effective perpendicular magnetic anisotropy (PMA) energy constant per unit area, K(eff)t(*), were investigated for conventional double-MTJs with various W insertion layer thicknesses. As the W layer thickness increased, K(eff)t(*) increased and J(cpl) decreased. There exists a trade-off relationship between J(cpl) and K(eff)t(*). In conventional double-MTJs with a single W insertion layer, large values for J(cpl) and K(eff)t(*) were difficult to obtain simultaneously. To improve this tradeoff, we employed a free layer stack with a thin ferromagnetic layer (ferromagnetic bridge layer: FBL) located in the W insertion layer. In the double-MTJs with FBL annealed at 400 degrees C, a large J(cpl) value of 0.37 mJ/m(2) was achieved while maintaining the maximum values of K(eff)t*. Accordingly, the MTJ with FBL provides an MTJ stack structure for obtaining high thermal stability.

Automated summary

The research shows that the thickness of the W insertion layer affects the magnetic coupling energy and effective perpendicular magnetic anisotropy in conventional double-MTJs. There is a trade-off relationship between J(cpl) and K(eff)t(*), but adding a ferromagnetic bridge layer can improve J(cpl) while maintaining the maximum value of K(eff)t(*).