Ordering in weakly coupled random singlet spin chains

Abstract

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. © 2012 American Physical Society.