Quantum Monte Carlo Simulations on S=1/2 antiferromagnetic-Ferromagnetic   Random Alternating spin chain
  The S=1/2 Heisenberg chain with bond alternation and randomness of antiferromagnetic (AFM) and ferromagnetic (FM) interactions is investigated by quantum Monte Carlo simulations of loop/cluster algorithm. Our results have shown interesting finite temperature magnetic properties of this model. The relevance of our study to the observed results of the material (CH$_3$)$_2$CHNH$_3$Cu(Cl$_x$Br$_{1-x}$)$_3$ is discussed. 
Peng Zhang, Zhaoxin Xu, Heping Ying and Jianhui Dai