Composition dependence of the second-order interfacial magnetic anisotropy for MgO/CoFeB/Ta films

The CoFeB thickness, t dependence of the effective first- and second-order magnetic anisotropy, K1eff and K2, for MgO/(Co1-xFex)80B20/Ta films (x=0.3-1.0) is investigated. As Co40Fe40B20 thickness decreases, K1eff increases and shows a perpendicular magnetic anisotropy for t=1.2 nm. On the other hand, in-plane magnetic anisotropy is observed for t≥1.4 nm. Also, a 1.3-nm-thick CoFeB sample demonstrates an easy-cone behavior, which suggests that the magnitude of K1eff and K2 becomes comparable. By plotting the product of K2 and t-td as a function of t-td, where td is a magnetic dead layer thickness, linear dependences with negative y-axis intercepts are displayed for all ranges of x. The extracted interfacial K2, Ki(2) are varied depending on the compositions in the range of-0.024 to −0.042 erg/cm2 for x=100% and 30, 50%, respectively. A magnetic phase diagram summarizing the results of K1-2πMs2 and K2 suggests that the ratio of K2 against K1-2πMs2 is varied depending on the compositions. These results give us a guideline to achieve the desired magnetic properties of CoFeB for spintronic applications.