Possible AF order in F-AF random alternating Heisenberg chain

The S = 1/2 ferromagnetic (F) - antiferromagnetic (AF) random alternating Heisenberg quantum spin chain is investigated by the quantum Monte Carlo method. The randomness is only present on the strong bonds taking values of ±2J randomly, and the weak bonds are uniformly antiferromagnetic with J. The model corresponds to the quasi one-dimensional compound, (CH₃)₂CHNH₃Cu(Cl_x, Br_1-x)₃, whose ground state interpolates the Haldane state of the F-AF alternating spin chain at x = 1 and the singlet-dimer state of the AF-AF alternating spin chain at x = 0. The nonequilibrium relaxation function of the staggered magnetic susceptibility exhibits algebraic divergence in the intermediate region of 0.44 < x < 0.87, which explains the experimental observation of the magnetic phase transition. The results suggest that the antiferromagnetic order becomes critical by the randomness of the bond distributions even in the presence of the rich ferromagnetic bonds.