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 magneti c impurities on the spin-Peierls (SP) transition of p-CyDOV (3-(4-cyanophen yl)-1,5-dimethyl-6-oxoverdazyl) and on the dimer state of p-BrDOV (3-(4-bro mophenyl) 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 di mer systems, respectively. The effect of doping on the SP system is very re markable. The SP transition temperature (T-SP = 15.0 K) decreases rapidly u pon 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 observe d in the powder X-ray diffraction pattern and the ESR spectrum of p-CyDOV a t 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 o f 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 de pendence 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 co nsidering the contribution from the SP state (chi(SP)) in addition to the a bove two contributions. On the other hand, the dimer state in p-BrDOV survi ves in the heavily doped sample with x = 0.85. The intradimer exchange inte raction 2J/k(B) remains constant between x = 0.85 and 1, suggesting that th e substituted p-CyDOV molecules play a role as p-BrDOV ones in the mixed cr ystals with x = 0.85-1. This work is the first report of the effects of mag netic impurities on the organic SP transition system.