* Corresponding author. Tel.: #49-30-8062-2058; fax: #49- 30-8062-2523. E-mail address: maletta@hmi.de (H. Maletta). Journal of Magnetism and Magnetic Materials 226 }230 (2001) 1675}1677 A circularly polarized X-ray study of the temperature-dependent spin-reorientation transition of thin Co "lms J. Langer, R. Sellmann, J. Hunter-Dunn, A. Hahlin, O. Karis, D. Arvanitis, H. Maletta* Department SF2, Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, D-14109 Berlin, Germany Stanford Synchrotron Radiation Laboratory, Stanford, CA 94309, USA University of Uppsala, Box 530, S-75121 Uppsala, Sweden Abstract Utilizing vector magnetometry by means of X-ray circular dichroism, the temperature-driven spin reorientation from an in-plane to out-of-plane orientation in thin Co "lms is studied in an epitaxially grown Au/Co/Au trilayer with decreasing temperature. Probing the remanent state by changing the photon helicity provides evidence for a smooth transition and the formation of a metastable magnetic multi-domain state at temperatures below the transition temperature.  2001 Elsevier Science B.V. All rights reserved. Keywords: X-ray dichroism; Spin-reorientation; Thin "lms With the reduction of the dimensionality in thin "lms interface phenomena become more important and may predominate over bulk properties. One prominent example is the spin-reorientation transition for magnetic systems. In the case of ultrathin Co "lms sandwiched between Au, the magnetocrystalline anisotropy is increased at the interfaces due to symmetry breaking and forces the magnetization in an out-of-plane orientation by overcoming the shape anisotropy which prefers an in-plane orientation for thicker "lms. The transition depends both on the thickness of the Co layer and the temperature as well as on the properties of the adjacent layers the magnetic "lm is sandwiched between. On the system Co/Au experimental work exists already on the thickness-driven transition [1}4]. However, less work is done in the "eld of the temperature-driven reorientation, to the exception of a study utilizing polarized neutron re#ectometry [5]. In the present work as a further mag- netometry tool, X-ray magnetic circular dichroism (XMCD) [6] at the L  -Co edges was employed. Since XMCD probes the projection of the magnetization along the k-vector in the direction of the incident photons, this allows to do temperature-dependent vector mag- netometry by varying the angle between the photon beam and the sample surface: At normal X-ray incidence one measures the out-of-plane magnetization compon- ent, while the in-plane component is probed at grazing X-ray incidence. The sample was prepared by e-beam evaporation and structurally characterized in situ in a UHV chamber. On a (11}20) saphire substrate and a 15 nm W-BCC (1 1 0) bu!er, 5 nm Au-FCC (1 1 1) and 1.9 nm Co-HCP (0 0 0 1) were deposited and capped with 2.0 nm Au-FCC (1 1 1) (see Ref. [5]). The XMCD measurements were done at the undulator beamline 5.2 at the Stanford Synchrotron Radiation Laboratory (SSRL) for a description of experi- mental setup, see Ref. [6] in the X-ray absorption mode by recording the sample drain current. The spectra were saturation corrected after recording spectra at several angles as shown earlier [6,7]. 0304-8853/01/$ - see front matter  2001 Elsevier Science B.V. All rights reserved. PII:S0304-8853(00)01017-9