The magnetic ordering in the spin ladder compound Sr2.5Ca11.5Cu24O41

mu SR and neutron experiments on Sr2.5Ca11.5Cu24O41 single crystal have been performed. The time-dependent muon polarization shows a significant change at around T-N similar or equal to 2.1 K indicating the onset of magnetic long-range order. Considering both the mu SR and neutron scattering data, a possible magnetic structure on the chain and ladder sites has been proposed. However. the lack of coherent mu SR 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. (C) 2000 Elsevier Science B.V. All rights reserved.