Doping effects of magnetic impurities on the spin-Peierls (p-CyDOV) and dimer (p-BrDOV) systems

The magnetic susceptibilities of the mixed radical crystals, (p-CyDOV)(1-x)(p-BrDOV)(x) (x = 0-1), were measured to study the doping effect of magnetic impurities on the spin-Peierls (SP) transition of p-CyDOV (3-(4-cyanophenyl)-1,5-dimethyl-6-oxoverdazyl) and on the dimer state of p-BrDOV (3-(4-bromophenyl) 1,5-dimethyl-6-oxoverdazyl) radical crystals. p-BrDOV and p-CyDOV, which have van der Waals radii and unpaired spin distributions similar to each other and S = 1/2 spin, were chosen as dopants in the above SP and dimer systems, respectively. The effect of doping on the SP system is very remarkable. The SP transition temperature (T-SP = 15.0 K) decreases rapidly upon p-BrDOV doping and the SP state almost disappears in the lightly doped sample with x = 0.05, although the effect of p-BrDOV doping was not observed in the powder X-ray diffraction pattern and the ESR spectrum of p-CyDOV at the low concentration region (x 0-0.09) of p-BrDOV. The susceptibilities of mixed crystals with x = 0.05 and 0.07 can be well explained by the sum of the susceptibilities of one-dimensional (1D) Heisenberg antiferromagnet (chi(ID-HAF)) With intrachain exchange interaction (2J/k(B)) similar to that of pure p-CyDOV and Curie impurity (X-Curie) Similarly, the temperature dependence of the susceptibilities for the mixed crystals with x = 0.01 and 0.03 above T-SP is described by the above two contributions (chi(ID-HAF) and chi(Curie)) However, the susceptibilities below TSP can be explained by considering the contribution from the SP state (chi(SP)) in addition to the above two contributions. On the other hand, the dimer state in p-BrDOV survives in the heavily doped sample with x = 0.85. The intradimer exchange interaction 2J/k(B) remains constant between x = 0.85 and 1, suggesting that the substituted p-CyDOV molecules play a role as p-BrDOV ones in the mixed crystals with x = 0.85-1. This work is the first report of the effects of magnetic impurities on the organic SP transition system.