  The doping of charge carriers into the CuO2 planes of copper oxide Mott
insulators causes a gradual destruction of antiferromagnetism and the emergence
of high-temperature superconductivity. Optimal superconductivity is achieved at
a doping concentration p beyond which further increases in doping cause a
weakening and eventual disappearance of superconductivity. A potential
explanation for this demise is that ferromagnetic fluctuations compete with
superconductivity in the overdoped regime. In this case a ferromagnetic phase
at very low temperatures is predicted to exist beyond the doping concentration
at which superconductivity disappears. Here we report on a direct examination
of this scenario in overdoped La2-xSrxCuO4 using the technique of muon spin
relaxation. We detect the onset of static magnetic moments of electronic origin
at low temperature in the heavily overdoped nonsuperconducting region. However,
the magnetism does not exist in a commensurate long-range ordered state.
Instead it appears as a dilute concentration of static magnetic moments. This
finding places severe restrictions on the form of ferromagnetism that may exist
in the overdoped regime. Although an extrinsic impurity cannot be absolutely
ruled out as the source of the magnetism that does occur, the results presented
here lend support to electronic band calculations that predict the occurrence
of weak localized ferromagnetism at high doping.

Superconductivity