  The strong perpendicular magnetic anisotropy of $L{\rm1_0}$-ordered FePt has
been the subject of extensive studies for a long time. However, it is not known
which element, Fe or Pt, mainly contributes to the magnetic anisotropy energy
(MAE). We have investigated the anisotropy of the orbital magnetic moments of
Fe 3$d$ and Pt 5$d$ electrons in $L{\rm1_0}$-ordered FePt thin films by Fe and
Pt $L_{2,3}$-edge x-ray magnetic circular dichroism (XMCD) measurements for
samples with various degrees of long-range chemical order $S$. Fe
$L_{2,3}$-edge XMCD showed that the orbital magnetic moment was larger when the
magnetic field was applied perpendicular to the film than parallel to it, and
that the anisotropy of the orbital magnetic moment increased with $S$. Pt
$L_{2,3}$-edge XMCD also showed that the orbital magnetic moment was smaller
when the magnetic field was applied perpendicular to the film than parallel to
it, opposite to the Fe $L_{2,3}$-edge XMCD results although the anisotropy of
the orbital magnetic moment increases with $S$ like the Fe edge. These results
are qualitatively consistent with the first-principles calculation by Solovyev
${\it et\ al.}$ [Phys. Rev. B $\bf{52}$, 13419 (1995).], which also predicts
the dominant contributions of Pt 5$d$ to the magnetic anisotropy energy rather
than Fe 3$d$ due to the strong spin-orbit coupling and the small spin splitting
of the Pt 5$d$ bands in $L{\rm1_0}$-ordered FePt.
