Heat Capacity and Magnetic Phase Transition of the Hydrogen-Bonded One-Dimensional Organic Ferromagnet
BImNN

Heat capacities of the hydrogen-bonded one-dimensional organic ferromagnet BImNN were measured by adiabatic calorimetry and relaxation method under magnetic fields. A heat capacity peak due to magnetic phase transition was found at 1.15 K. Magnetic field dependence of the magnetic phase transition temperature revealed that the observed magnetic phase transition is ferromagnetic. To estimate its lattice heat capacity, heat capacity measurement of non-magnetic BImNNH3, which is a precursor of BImNN, was also performed. From the zero-field magnetic heat capacity calculated by the estimated lattice heat capacity, the magnetic entropy was evaluated to be 4.64 J K−1 mol−1, which is about 80% of 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 expressed roughly by the theoretical heat capacity curve for the high-temperature expansions of S = 1/2 one-dimensional ferromagnetic Heisenberg model.

(by Y. Miyazaki)

	Fig. 1. Molecular structures of ImNN, BImNN, F4BImNN, and BImNNH3.
	Fig. 2. Heat capacities of BImNN and BImNNH3 by adiabatic calorimetry and relaxation method at zero magnetic field.
	Fig. 3. Heat capacities (upper) and magnetic heat capacities (lower) of BImNN under magnetic fields. For the sake of clarity, the magnetic heat capacities except for the zero-field magnetic heat capacities are shifted upwards. Red, green and blue curves indicate the theoretical heat capacities for high-temperature expansions of S = 1/2 one-dimensional ferromagnetic Heisenberg models with J/kB = +10 K, J/kB = +14 K, and J/kB = +17 K, respectively.