Heat Capacity and Ferromagnetic Phase Transition of the Organic Radical Crystal DNPNN with Chiral Helicity of Molecular Packing

Heat capacities of the organic radical crystal DNPNN, which has chiral helicity of the molecular packing, were measured by adiabatic calorimetry and relaxation method under magnetic fields. A heat capacity peak due to magnetic phase transition was observed at 1.08 K. Magnetic field dependence of the magnetic phase transition temperature revealed that the observed magnetic phase transition is ferromagnetic. From the zero-field magnetic heat capacity, the magnetic enthalpy and entropy were evaluated to be 24.6 J mol-1 and 5.72 J K-1 mol-1, respectively. The experimental magnetic entropy agrees well with the expected value Rln2 (= 5.76 J K-1 mol-1) for S = 1/2 spin systems. The zero-field magnetic heat capacity above the magnetic phase transition temperature is reproduced well by the theoretical heat capacity curve for the high-temperature expansion of the S = 1/2 one-dimensional ferromagnetic Heisenberg model with the intrachain magnetic interaction J/kB = +5.6 K. From the mean-field approximation, the interchain magnetic interaction was estimated to be zJ′/kB ≈ +0.83 K. The magnetic heat capacity proportional to T 3/2 at very low temperatures suggests that DNPNN orders three-dimensional ferromagnetically.

(by Y. Miyazaki)

	Fig. 1. Molecular structure of DNPNN.
	Fig. 2. Heat capacities of DNPNN by adiabatic calorimetry and relaxation method under magnetic fields. Red curve indicates the normal heat capacity.
	Fig. 3. Magnetic heat capacities of DNPNN under magnetic fields. Blue curve indicate the theoretical heat capacity for high-temperature expansion of S = 1/2 one-dimensional ferromagnetic Heisenberg models with J/kB = +5.6 K. Red curve shows the heat capacity expected by the spin wave theory for three-dimensional ferromagnets.