  We have studied the magnetic layer thickness dependence of the orbital
magnetic moment in magnetic heterostructures to identify contributions from
interfaces. Three different heterostructures, Ta/CoFeB/MgO, Pt/Co/AlO$_x$ and
Pt/Co/Pt, which possess significant interface contribution to the perpendicular
magnetic anisotropy, are studied as model systems. X-ray magnetic circular
dichroism spectroscopy is used to evaluate the relative orbital moment, i.e.
the ratio of the orbital to spin moments, of the magnetic elements constituting
the heterostructures. We find that the relative orbital moment of Co in
Pt/Co/Pt remains constant against its thickness whereas the moment increases
with decreasing Co layer thickness for Pt/Co/AlO$_x$, suggesting that a
non-zero interface orbital moment exists for the latter system. For
Ta/CoFeB/MgO, a non-zero interface orbital moment is found only for Fe. X-ray
absorption spectra shows that a particular oxidized Co state in Pt/Co/AlO$_x$,
absent in other heterosturctures, may give rise to the interface orbital moment
in this system. These results show element specific contributions to the
interface orbital magnetic moments in ultrathin magnetic heterostructures.
