Coexistence of superconductivity and ferromagnetism in layered systems

It is shown for layered systems that when the electronic concentration varies monotonically from layer to layer the homogeneous superconducting electron pairing of adjacent layers with antiparallel spins is accompanied by a spontaneous ferromagnetic ordering (the SF-phase), while in the case of nonequivalent alternating layers a paramagnetic superconducting state (the SP-phase) is generated for certain magnetic fields H and chemical potential differences δμ between adjacent layers. The paramagnetic limit grows with increasing δμ, and the transition from the normal to the superconducting (SF or SP) state is a first order phase transition both in the field H and in δμ.