Sudden jump of the Curie temperature at the coalescence of Co islands on Cu(001)

The ferromagnetism of CoCu(0 0 1) at the transition from islands to closed layer growth is studied by temperature-dependent magnetization and suscepti bility measurements in situ in UHV. The Curie temperatures above and below a critical coverage d(c) approximate to 1.8 ML for percolation were measure d. Near this particular thickness a discontinuous jump of the Curie tempera ture of more than 100 K is observed. At d(c) the Curie temperature is metas table and varies strongly with time and heat treatment. Below d(c) the samp le remains ferromagnetic. We find a remanent magnetization and finite coerc ive fields, which clearly excludes the possibility of superparamagnetic beh aviour. (C) 1999 Elsevier Science B.V. All rights reserved.