  Recently, perpendicular magnetic anisotropy (PMA) and its voltage control
(VC) was demonstrated for Cr/Fe/MgO (Physical Review Applied 5, 044006 (2016)).
In this study, we shed a light on the origin of large voltage-induced
anisotropy change in Cr/Fe/MgO. Analysis of the chemical structure of Cr/Fe/MgO
revealed the existence of Cr atoms in the proximity of the Fe/MgO interface,
which can affect both magnetic anisotropy (MA) and its VC. We showed that PMA
and its VC can be enhanced by controlled Cr doping at the Fe/MgO interface. For
Cr/Fe (5.9 {\AA})/Cr (0.7 {\AA})/MgO with an effective PMA of 0.8 MJ/m3, a
maximum value of the voltage-controlled magnetic anisotropy (VCMA) effect of
370 fJ/Vm was demonstrated.
