Heat capacity study of the doping effect of paramagnetic radicals on the organic spin-Peierls system (p-CyDOV radical crystal)

The doping effect of paramagnetic organic radical impurities (p-CyDTV) on the organic spin-Peiels (SP) system of p-CyDOV (3-(4-cyanophenyl)-1,5-dimethyl- 6-oxoverdazyl; the SP transition temperature T_SP = 15.0 K) was studied by the heat capacity measurement down to 0.1 K. At low temperatures below 1 K, quite a sharp heat capacity peak was detected for each system with small impurity concentration (x) at the temperature T_N(x); T _N(0) = 0.135 K (±0.002 K), T_N(0.01) = 0.290 K and T_N(0.07) = 0.164 K. The appearance of the peak for the system with x = 0 is considered to be due to minute and inevitable impurities in the pure p-CyDOV. From the dependence of T_N(x), T_N(0) < T _N(0.01), and the heat capacity behavior on x, the peaks for x = 0 and 0.01 are explained to be accompanied with the three-dimensional (3D) antiferromagnetic (AFM) transition in the SP phase. While the decrease of T _N(x) for higher impurities x = 0.07 than that for x = 0.01 may be interpreted as merely a 3D AFM transition from the paramagnetic state in the quasi-1D systems with the interchain interaction |zJ′/J| = 6 × 10^-4. Although no anomaly of the heat capacity was found around T_SP(x) = 13-15 K in the present systems, where the characteristic decrease of the magnetic susceptibility is reported, we observed a distinct hump of the heat capacity at T′_SP(0) = 5.6 K and T′_SP(0.01) = 5.3 K with magnetic entropy consumption up to 3%, suggesting that this transition may relate to SP transition.