  Ferromagnetism was observed in a Pt(100) ultrathin film deposited on a
SrTiO3(100) substrate. The ferromagnetism, which appears in films with
thicknesses of 2.2-4.4 nm, periodically changes with a period of approximately
1 nm (5-6 ML) depending on the film thickness. This is consistent with the
period derived from the quantum-well states formed in the thin film. X-ray
magnetic circular dichroism measurements were conducted to understand the
intrinsic nature of the ferromagnetism in the Pt(100) ultrathin films, and
contrary to our expectations, the orbital magnetic moment of pure Pt is much
smaller than that of the Pt/ferromagnetic multilayer system. These results
suggest that the origin of the large magnetic anisotropy in Pt components
cannot be explained only by the amount of spin-orbit coupling in Pt.
