``Live'' Surface Ferromagnetism in Fe Dot Multilayers on Cu(111)*

We investigate the crossover behavior from two-dimensional (2D) to three dimensional (3D) in multilayers of magnetic nanodots grown by stacking 2D Fe nanodot assemblies on Cu(111) single crystal substrate with a Cu spacing layer. Using in-situ magneto-optical Kerr effect, we have observed a striking ferromagnetic to spin-glass phase transition with increasing number of Fe dot layers. The topmost layer of the Fe dots survives the phase transition and remains ferromagnetic. This unusual surface ferromagnetism is likely caused by a stronger surface state-mediated intralayer dot coupling which is stronger than the interlayer dot coupling, as confirmed by the fact that the critical temperature of the surface ferromagnetism is considerably higher than that of the bulk spin glass phase in the system.