  Despite the crucial role of interfacial perpendicular magnetic anisotropy in
Co(Fe)/MgO based magnetic tunnel junction, the underlying mechanism is still
being debated. Here, we report an anatomical study of oxygen and Mg effect on
Pt/Co bilayers through repeated in-situ anomalous Hall effect measurements,
controlled oxygen exposure and Mg deposition in an ultrahigh vacuum system. We
found that chemisorbed oxygen not only quenches the effective magnetic moment
of the Co surface layer, but also softens its magnetic anisotropy. However, a
subsequent Mg dusting on the oxygen pre-exposed Pt/Co surface can recover the
magnetic anisotropy. The ab initio calculations on the exchange splitting and
orbital hybridization near the Fermi level give a clear physical explanation of
the experimental observations. Our results suggest that Co(Fe)-O-M bond plays a
more important role than the widely perceived Co(Fe)-O bond does in realizing
interfacial perpendicular magnetic anisotropy in Co(Fe)/MgO heterostructures.
