  Superconductors are a striking example of a quantum phenomenon in which
electrons move coherently over macroscopic distances without scattering. The
high-temperature superconducting oxides(cuprates) are the most studied class of
superconductors, composed of two-dimensional CuO2 planes separated by other
layers which control the electron concentration in the planes. A key unresolved
issue in cuprates is the relationship between superconductivity and magnetism.
In this paper, we report a sharp phase boundary of static three-dimensional
magnetic order in the electron-doped superconductor La2-xCexCuO4-d where small
changes in doping or depth from the surface switch the material from
superconducting to magnetic. Using low-energy spin polarized muons, we find
static magnetism disappears close to where superconductivity begins and well
below the doping where dramatic changes in the transport properties are
reported. These results indicate a higher degree of symmetry between the
electron and hole-doped cuprates than previously thought.
