Antiferromagnetic order in an ferromagnetic-antiferromagnetic random alternating quantum spin chain: (CH₃)₂CHNH ₃Cu(Cl_x Br_1-x)₃

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(Cl_x Br_1-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.