2. Why is the ferromagnetic arrangement of the magnetic moments the most favorable for the O-excess Cu 2 O surfaces? We assume that unpaired electrons at the surface interact with each other and that this exchange interaction causes an energetic splitting of the spin-up/spin-down states (see Figure ). According to the Stoner theory, such an energetic splitting leads to a redistribution of spinup electrons into spin-down states and, therefore, an increase of kinetic energy for the redistributed electrons. In some metals, however, this increase in kinetic energy is countered by a decrease in potential energy due to the ferromagnetic exchange interaction of parallel aligned electron spins. This leads to an overall decrease in energy and therefore a ferromagnetic ground state. We suggest that the observed surface ferromagnetism of O-excess Cu 2 O surfaces can be explained in a similar way. Yu et al. and Soon et al. also proposed that the origin of the surface ferromagnetism of Cu 2 O can be understood within the framework of Stoner's theory.