The magnetic ordering in the spin ladder compound Sr_2.5Ca_11.5Cu₂₄O₄₁

μSR and neutron experiments on Sr_2.5Ca_11.5Cu₂₄O₄₁ single crystal have been performed. The time-dependenet muon polarization shows a significant change at around T_N ≃ 2.1 K indicating the onset of magnetic long-range order. Considering both the μSR and neutron scattering data, a possible magnetic structure on the chain and ladder sites has been proposed. However, the lack of coherent μSR precession signal strongly suggests that the distribution of the internal field is not homogeneous. This can be interpreted in line with the scenario that the singlet state and antiferromagnetic order coexist at ambient pressure and these localized hole pairs begin to move under pressure, resulting in the occurrence of superconductivity.