Perpendicular Magnetic Anisotropy and Easy Cone State in Ta/Co60Fe20B20/MgO

We used broadband ferromagnetic resonance (FMR) spectroscopy to measure the second- and fourth-order perpendicular magnetic anisotropies in Ta/(t) Co60Fe20B20/MgO layers over a Co60Fe20B20 thickness range of 5.0 nm >= t >= 0.8 nm. For t > 1.0 nm, the easy axis is in the plane of the film, but when t < 1.0 nm, the easy axis is directed perpendicular to the surface. However, the presence of a substantial higher order perpendicular anisotropy results in an easy cone state when t = 1.0 nm. Angular-dependent FMR measurements verify the presence of the easy cone state. Measurement of the spectroscopic g-factor via FMR for both the in-plane and out-of-plane geometries suggests a significant change in electronic and/or physical structure at t approximate to 1.0 nm thickness.