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

摘要

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 °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.