The role of inhomogeneity of perpendicular anisotropy in magnetic properties of ultra thin CoFeB film

In this work, the hysteresis loops of ultra thin CoFeB films with perpendicular anisotropy are simulated using a micromagnetic model with microstructure included. An inhomogeneous region of the perpendicular uniaxial anisotropy constant K is introduced in the model, and this is the key factor to explain why the in-plane saturation field is several thousand Oersted, while the perpendicular coercivity can be on the order of 100 Oersted. Tendency of the simulated loops with increasing total simulation size reveals that this model is reliable, and the calculated loops basically agree with experiments. The effects of different K distribution, defect sizes, defect patterns, and higher order magnetic anisotropy on simulated M-H loops are also discussed in the paper.