Mechanism of higher order contributions in interface anisotropy of thin films: FeCo/MgO case

Current state of the art perpendicular magnetic tunnel junctions (pMTJs) rely upon a large uniaxial interface anisotropy to overcome the demagnetizing field. Experimental measurements have found that as well as the expected K1sin2(θ) anisotropy energy, a higher order K2sin4(θ) contribution is present. In thin films with bulk anisotropies K2 can arise from a distribution of the anisotropy axes. In the course of constructing a model of FeCo/MgO we unexpectedly found a K2sin4(θ) dependence in the free energy, even without any disorder in the anisotropy axes or higher order magneto-crystalline anisotropy.