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/(Co 0.25 Fe 0.75 ) 100- x B 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 5 J/m 3 in the 1.5 nm-thick CoFeB film with x =20 annealed at 350 °C. K eff is 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 S 2 /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.