  We report a detailed study, using neutron scattering, transport and
magnetization measurements, of the interplay between superconducting (SC) and
spin density wave (SDW) order in La2CuO(4+y). Both kinds of order set in below
the same critical temperature. However, the SDW order grows with applied
magnetic field, whereas SC order is suppressed. Most importantly, the field
dependence of the SDW Bragg peak intensity has a cusp at zero field, as
predicted by a recent theory of competing SDW and SC order. This leads us to
conclude that there is a repulsive coupling between the two order parameters.
The question of whether the two kinds of order coexist or microscopically phase
separate is discussed.
