Enhancement of perpendicular magnetic anisotropy in FeB free layers using a thin MgO cap layer

We prepared magnetic tunnel junction films with PtMn/CoFe/Ru/CoFeB/MgO tunnel barrier/FeB free layer/MgO cap layer/Ta multilayers using sputtering and measured magnetic and magnetoresistive properties of the films at room temperature. The magnetization curves of the FeB plane film measured under perpendicular-to-plane magnetic fields showed much smaller saturation fields ( H s ) than those expected from the demagnetizing field. H s decreased from 4 to 0.4 kOe with increasing MgO cap layer thickness. The small H s is due to the perpendicular magnetic anisotropy (PMA) induced at both MgO barrier-FeB and FeB-MgO cap interfaces. After microfabrication, the small free layer cells having a 1.6 nm thick MgO cap layer showed a magnetization easy axis in the perpendicular-to-plane direction. By inducing PMA from both upper and lower interfaces, we can stabilize the magnetization of the relatively thick (2 nm) FeB free layer in the perpendicular-to-plane direction.