A possibility of the uniform antiferromagnetic order is pointed out in an S = 1/2 ferromagnetic-antiferromagnetic (F-AF) random alternating Heisenberg quantum spin chain compound: (CH3)2CHNH 3Cu(Clx Br1-x)3. The system exhibits bond alternation of strong random bonds that take ±2J and weak uniform AF bonds of -J. In pure concentration limits, the model reduces to the AF-AF alternation chain at x = 0 and to the F-AF alternation chain at x = 1. By using the nonequilibrium relaxation method of quantum Monte Carlo simulations, it is found that the uniform AF order exhibits critical behaviors in the intermediate concentration region. This explains the experimental observation of the magnetic phase transition. The present results suggest that the uniform AF order survives even in the presence of randomly located ferromagnetic bonds.
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