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-xSrxCuO4 (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 singlecrystal 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/cm(3)). Less-doped, superconducting LSCO films show no magnetic moment in neutron reflectivity, both above and below Tc. Therefore, the collapse of HTS with overdoping is not caused by competing ferromagnetic order.