  Understanding the magnetic anisotropy at ferromagnetic metal/oxide interface
is a fundamental and intriguing subject. Here we propose an approach to
manipulate the strength of perpendicular magnetic anisotropy (PMA) by varying
MgO thickness in the MgO/CoFeB/Ta thin films. We identify that the PMA at the
MgO/CoFeB interface is tuned by the crystalline structure of bulk MgO layer and
decreases dramatically due to the onset of crystalline MgO forming with the
increase of MgO thickness. Our work opens an avenue to manipulate the magnetic
anisotropy by the modification of the ferromagnetic metal/oxide interface.
