  The influence of bond randomness on long range magnetic ordering in the
weakly coupled S = 1/2 antiferromagnetic spin chain materials
Cu(py)2(Cl1-xBrx)2 is studied by muon spin rotation and bulk measurements.
Disorder is found to have a strong effect on the ordering temperature TN, and
an even stronger one on the saturation magnetization m0, but considerably more
so in the effectively lower-dimensional Br-rich materials. The observed
behavior is attributed to Random Singlet ground states of individual spin
chains, but remains in contradiction with chain mean field theory predictions.
In this context, we discuss the possibility of a universal distribution of
ordered moments in the weakly coupled Random Singlet chains model.
