Insight into Magnetocaloric Properties of Mn2Nb Molecular Magnet by Relaxation Calorimetry

Magnetocaloric effect in [Nb^IV{[(μ-CN)₄Mn^II(H₂O)₂]}₂·4H₂O]_n molecular magnet is thoroughly reported. The compound crystallizes in the tetragonal I4/m space group. It exhibits a phase transition to a long-range ferrimagnetically ordered state at T_c = 47.0(2) K. Relaxation calorimetry measurements are performed and a self-consistent scheme based on the magnetic entropy counting for the baseline determination is developed. Temperature dependence of the magnetic entropy change ΔS_M as well as the adiabatic temperature change ΔT_ad due to the applied field changes μ₀ΔH = 0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 7, and 9 T is evaluated. The maximum value of |ΔS_M| for μ₀ΔH = 5 T is 5.03 J K^–1 mol^–1 (9.07 J K^–1 kg^–1) at 49.5 K. The corresponding maximum value of ΔT_ad = 1.7 K is attained at 49.0 K. The molecular field model is used to simulate the temperature and field dependence of the magnetic entropy change. The exchange coupling constant between the Mn^II and Nb^IV ions is estimated to be equal to −10.26 K. At the lowest temperatures and for the lowest applied field change values the inverse magnetocaloric effect is revealed, which seems to be characteristic for systems with antiferromagnetic coupling. Temperature dependence of exponent n quantifying the field dependence of ΔS_M is calculated on the basis of the experimental results and within the mean-field model. Its predicting power for the universality class of the critical behavior is discussed. Finally, the studied compound is employed as the working substance in the two most natural refrigeration cycles, i.e., the regenerative Brayton cycle and the regenerative Ericsson cycle, to assess its cooling effectiveness. A cascade system is suggested for the most efficient cooling performance.