Separate Curie temperatures in magnetic trilayers and the effect of spin fluctuations

Co/Cu/Ni/Cu(0 0 1) trilayers are studied in situ by X-ray magnetic circular dichroism at temperatures between 25 and 300 K. The ultrathin magnetic lay ers (1-2 monolayers (NIL) of Co and 3-5 ML monolayers of Ni) are strongly c oupled through the Cu spacer (similar to2-4 ML thick). However, they presen t the onset of ferromagnetic order at separate temperatures. Interlayer exc hange coupling increases the temperature of ferromagnetic order of Ni by si milar to 100 K for Ni thicknesses as small as 3 ML. The influence of spin-f luctuations and the reduced dimensionality is discussed. Finally, it is sho wn that crossing magnetization curves may be recorded when the ordering tem perature of Co is smaller than the one of Ni. (C) 2001 Elsevier Science B.V . All rights reserved.