Search for ferromagnetic order in overdoped copper-oxide superconductors

In copper-oxides that show high-temperature superconductivity (HTS), the critical temperature (T_c) has a dome-shaped doping dependence. The cause of demise of both T_c and superfluid density n_s on the overdoped side is a major puzzle. A recent study of transport and diamagnetism in a large number of overdoped La_2-xSr_xCuO₄ (LSCO) films shows that this cannot be accounted for by disorder within the conventional Bardeen-Cooper-Schrieffer theory. This brings to focus an alternative explanation - competition of HTS with ferromagnetic order, fluctuating in superconducting samples and static beyond the superconductor-to-metal transition. Here, we examine this proposal by growing single-crystal LSCO thin films with doping on both sides of the transition by molecular beam epitaxy, and using polarized neutron reflectometry to measure their magnetic moments. In a heavily overdoped, metallic but non-superconducting LSCO (x = 0.35) film, the spin asymmetry of reflectivity shows a very small static magnetic moment (~2 emu/cm³). Less-doped, superconducting LSCO films show no magnetic moment in neutron reflectivity, both above and below T_c. Therefore, the collapse of HTS with overdoping is not caused by competing ferromagnetic order.