Effect of Magnetic Coupling Between Two CoFeB Layers on Thermal Stability in Perpendicular Magnetic Tunnel Junctions With MgO/CoFeB/Insertion Layer/CoFeB/MgO Free Layer

We investigated the effect of magnetic coupling (the energy constant of unit area J(cpl)) between CoFeB free layers on thermal stability factor Delta in magnetic tunnel junction (MTJ) with CoFeB/MgO double interfacial perpendicular magnetic anisotropy (IPMA). We newly introduced Jcpl in models for calculations, which were based on magnetic domain propagation model when MTJ diameter d is larger than critical diameter dc and magnetization coherent rotation model when d <= d(c). With increasing J(cpl), Delta increases and saturates when Jcpl is over a critical value J(cpl_c). Magnetostatic coupling constants (J(stat)) between the free layers were also calculated. Jstat is much smaller than J(cpl_c) and cannot maximize Delta by itself, so an interlayer exchange coupling (the critical energy constant of unit area J(ex_c)) is required to cover the difference between J(cpl_c) and Jstat. Jex_ c also rapidly increases with the decrease in d and reaches a plateau of 0.15 mJ/m(2) in d <= 30 nm. MTJ devices with the smallest Jex (= 0.01mJ/m(2)) and a moderate PMA (constant per unit area K-eff t*) were made and Delta s were evaluated by experiments. The experimental Delta s were fitted by the calculations with adjusting stiffness constants As. The calculated Delta s and the experimental Delta s were in good agreement when As had a smaller value than that of the blanket film. Since the experimentally used J(ex) value was almost 0, Delta values were estimated when J(ex) was a greater value of 0.15 mJ/m(2). As a result, 30% enhancement of Delta was obtained at the smallest d = 20 nm.