Influence of superconductivity on magnetic properties of superconductor/ferromagnet epitaxial bilayers

Abstract Single crystal Fe/Nb (110) bilayers with a Nb thickness d Nb in the range from 250 to 650 A and with a Fe thickness d Fe in the range from 7 to 27 A were prepared using MBE techniques. Magnetization measurements showed the existence of a magnetically `dead' Fe-interface-layer with a thickness d NM varying strongly with minor modifications of the growth conditions. For bilayers with a small magnetic layer thickness d M and a small d NM the FMR measurements revealed an anomalous decrease of the effective magnetization 4 πM eff of the ferromagnetic Fe-layer below the superconducting transition temperature T c of Nb. Additional magnetization measurements using a SQUID magnetometer indicated that the anomalous decrease is indeed due to a decrease of the magnetization M of the Fe layer. The absolute change of 4 πM eff below T c depends strongly on the actual values of d M and d NM : upon decreasing both d M and d NM the effect increases considerably. We discuss two possible mechanisms for the anomalous temperature dependence of 4 πM eff below T c : The first mechanism is a new type of cryptoferromagnetism, i.e. a spatial modulation of ferromagnetic order due to a modification of the RKKY-interaction in the superconducting state. The second mechanism, which at the moment cannot be excluded, is a spatial modulation of the spin direction in the Fe-layer caused by the demagnetizing field in the geometrically rough interface.