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 (Hs) than those expected from the demagnetizing field. Hs decreased from 4 to 0.4 kOe with increasing MgO cap layer thickness. The small Hs 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.