Physica B 248 (1998) 9—13 Measurement of the magnetism of a single atomic plane with X-ray diffraction S. Ferrer*, J. Alvarez, X. Torrelles, E. Lundgren, P. Fajardo ESRF, BP 220, 38043 Grenoble Cedex, France Abstract Surface X-ray diffraction measurements have been done in cobalt ultrathin films (2—12 atomic layers) grown on Pt(1 1 1). Cobalt grows, at room temperature, in an imperfect layer by layer mode with its own lateral lattice spacing. The packing of the atomic planes is predominantly FCC but it changes to HCP upon annealing to & 450 K. Resonant magnetic diffraction experiments in the L  absorption edge of Pt allow to determine the magnetism of the Pt atoms at the interface. By measuring magnetic crystal truncation rods, it has been found that only the Pt atoms in contact with the Co overlayer are magnetised ( & 0.2  ) being the magnetisation of the second Pt layer about ten times smaller. The induced magnetic moment of Pt interface has been found to vary rather linearly with the thickness of the Co film, similarly to the magnetisation of the overlayer. The transformation FCC to HCP of the cobalt stacking results in a &25% enhancement of the magnetisation at the interface.  1998 Elsevier Science B.V. All rights reserved. Keywords: Surfaces; Magnetism The interaction of X-rays with matter is usually dominated by the Thomson scattering which re- sults from the coupling of the electric field of the X-rays with the charge of the electrons in the atoms. Moreover, the interaction of X-rays with atoms possessing a magnetic moment may result in a measurable coupling between the magnetic field of the radiation and the magnetic moment of the atoms. This magnetic effect which is much weaker than the charge scattering, was brought to evidence more than 20 years ago [1] and since then it has * Corresponding author. Fax: (33)4 76 88 2160; e-mail: ferrer@esrf.fr. been used to study bulk magnetic materials. The application of this phenomenon to surface magnet- ism was, however, hopeless even with synchrotron X-ray sources, since surfaces, basically two dimen- sional objects, are very diluted systems compared with bulk materials which have three dimensions. Recently, it was discovered [2] that under some resonant conditions (the photon energy has to co- incide with the energy of some atomic absorption edges), the magnetic coupling of the X-rays with the magnetic moment of the atoms in resonant condi- tion is largely enhanced. This important result opened the possibility of performing surface mag- netic diffraction experiments aimed at probing the magnetism of a single atomic plane. Furthermore, 0921-4526/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 1 9 4 - X