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 Jcpl_c. Magneto-static coupling constants (Jstat) between the free layers were also calculated. Jstat is so smaller than Jcpl_c and cannot maximize Δ by itself, that an interlayer exchange coupling (the critical energy constant of unit area: Jex_c) is required to cover the difference between Jcpl_c and Jstat. Jex_c also rapidly increases with the decrease of d and reaches a plateau of 0.15 mJ/m2 in d ≤ 30 nm. MTJ devices with a smallest Jex (=0.01 mJ/m2) and a moderate PMA (constant per unit area: Keff t*) were made and Δs were evaluated by experiments. The experimental Δs were fitted by the calculations with adjusting stiffness constants As. The calculated Δs and the experimental Δs were in good agreement when the As was a smaller value than that of the blanket film. Since the experimentally used Jex value was almost zero, Δ values were estimated when Jex was a greater value of 0.15mJ/m2. As a result, 30% enhancement of Δ was obtained at the smallest d = 20 nm.
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