Boron Composition Dependence of Magnetic Anisotropy and Tunnel Magnetoresistance in MgO/CoFe(B) Based Stack Structures

We investigated magnetic anisotropy and tunnel magnetoresistance (TMR) properties in MgO/(Co0.25Fe0.75)100-xBxstack structures withx=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 nearx=15 . The effective magnetic anisotropy energy density (Keff) shows a maximum of 1.9×105J/m3in the 1.5 nm-thick CoFeB film withx=20 annealed at 350 °C.Keffis determined by the competition between contributions of interface anisotropy energy per effective CoFeB thickness (Ki/t*, wheret*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 highKeff, materials and structures that reduce demagnetization energy while maintaining a highKiand a thint*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 withx=25 annealed at 350 °C.