We investigated magnetic anisotropy and tunnel magnetoresistance (TMR) properties in MgO/(Co-0.25Fe-0.75-100-xB x 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 (K-eff shows a maximum of 1.9× 10 5J/m 3 in the 1.5 nm-thick CoFeB film with x=20 annealed at 350 °C. K effis determined by the competition between contributions of interface anisotropy energy per effective CoFeB thickness (K i/t * , where t * is the effective CoFeB layer thickness) and demagnetization energy (-M 2 s/2μ-0. Bulk magnetic anisotropy energy (K-b is negligibly small with comparison to those two terms. To obtain MgO/ferromagnetic stack structure with a high K eff , materials and structures that reduce demagnetization energy while maintaining a high K i 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.
- CoFeB composition
- MgO barrier
- perpendicular magnetic anisotropy
- tunnel magnetoresistance