We investigated the effect of magnetic coupling (the energy constant of unit area Jcpl ) between CoFeB free layers on thermal stability factor Δ 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 ≤ dc. With increasing Jcpl , Δ increases and saturates when Jcpl is over a critical value Jcplc. Magnetostatic coupling constants ( Jstat ) between the free layers were also calculated. Jstat is much smaller than Jcplc and cannot maximize Δ by itself, so an interlayer exchange coupling (the critical energy constant of unit area Jexc ) is required to cover the difference between Jcplc and Jstat. Jexc also rapidly increases with the decrease in d and reaches a plateau of 0.15 mJ/m2 in d ≤ 30 nm. MTJ devices with the smallest Jex(=0.01 mJ/m^2 ) and a moderate PMA (constant per unit area Keff t^∗ ) were made and Δ s were evaluated by experiments. The experimental Δ s were fitted by the calculations with adjusting stiffness constants A_s. The calculated Δ s and the experimental Δ s were in good agreement when A_s had a smaller value than that of the blanket film. Since the experimentally used Jex value was almost 0, Δ values were estimated when Jex was a greater value of 0.15 mJ/m2. As a result, 30% enhancement of Δ was obtained at the smallest d = 20 nm.
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