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_0.25Fe_0.75)_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 (K_eff) shows a maximum of 1.9×10⁵ J/m³ 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_S²/2μ₀) . 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.