  Static spin-stripe order and superconductivity were systematically studied in
La_2-xBa_xCuO_4 (0.11 - 0.17) at ambient pressure by means of magnetization and
muSR experiments. We find that all the investigated La_2-xBa_xCuO_4 samples
exhibit static spin-stripe order and that the quasi two-dimensional
superconducting (SC) transition temperature T_c1 and the static spin-stripe
order temperature T_so have very similar values throughout the phase diagram.
Moreover, the magnetic and the SC properties of the x = 0.155 (LBCO-0.155) and
x = 0.17 (LBCO-0.17) samples were studied under hydrostatic pressure. As a
remarkable result, in these bulk cuprate superconductors the three-dimensional
SC transition temperature T_c and T_so nearly coincide at all pressure
investigated (0 - 2.3 GPa). We also observed a pressure induced transition from
long-range spin stripe order to a disordered magnetic state at 1.6 GPa in
LBCO-0.155, coexisting with a SC state with substantial superfluid density. In
LBCO-0.17 a disordered magnetic state is present at all p. The present results
indicate that static magnetic order and SC pairing correlations develop in a
cooperative fashion in La_2-xBa_xCuO_4, and provide a new route of
understanding the complex interplay between static magnetism and
superconductivity in the stripe phase of cuprates.
