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We investigated magnetic anisotropy and tunnel magnetoresistance (TMR) properties in MgO/(Co0.25Fe0.75)100-xBx stack structures with x=0, 15, 20, and 25 (in at.%). After annealing at 350°C, the easy axis of magnetization switches from in-plane to perpendicular direction in 1.5-nm-thick CoFeB with the B composition near x=15 . The effective magnetic anisotropy energy density (Keff) shows a maximum of 1.9×105 J/m3 in the 1.5 nm-thick CoFeB film with x=20 annealed at 350 °C. Keff is determined by the competition between contributions of interface anisotropy energy per effective CoFeB thickness (Ki/t*, where t* is the effective CoFeB layer thickness) and demagnetization energy (-MS2/2μ0) . Bulk magnetic anisotropy energy (Kb) is negligibly small with comparison to those two terms. To obtain MgO/ferromagnetic stack structure with a high Keff, materials and structures that reduce demagnetization energy while maintaining a high Ki and a thin t* have to be explored. In MTJs with the higher B compositions, high TMR ratio is obtained at higher annealing temperature. High TMR ratio of 136% is observed in a MTJ with x=25 annealed at 350 °C.