Effect of bond randomness on magnetic states in ferromagnetic-antiferromagnetic alternating chains (CH3)2CHNH3CuCl3

The compound (CH3)2CHNH3CuCl3 consists of ferromagnetic and antiferromagnetic alternating Heisenberg chains with S=1/2; a pair of ferromagnetically coupled spins behaves as a spin with S=1. Consequently, it has the Haldane gap in its energy spectrum at T=0 K. We investigated an issue what will happen when Cl ions are randomly replaced by other kind of anions, for instance by Br ions. Performing experiments on single crystals of mixed compounds (CH3)2CHNH3Cu(ClxBr1-x)3 in which Cl ions are randomly replaced by Br ions, we found that the energy gap disappeared over the region 0.44<x<0.87. The experimental data, which prove the disappearance of the energy gap, are presented and discussed