Insight into Magnetocaloric Properties of Mn₂Nb Molecular Magnet by Relaxation Calorimetry

Magnetocaloric effect in [Nb$^\mathrm{IV}${($μ$-CN)$_4$Mn$^\mathrm{II}$(H$_2$O)$_2$]}$_2\cdot$4H$_2$O]$_n$ molecular magnet is reported. The compound crystallizes in the tetragonal I4/m space group. It exhibits a phase transition to a long-range ferrimagnetically ordered state at $T_\mathrm{c}$ = 47.0(2) K. In order to calculate magnetocaloric properties relaxation calorimetry measurements are performed and a self-consistent scheme based on the magnetic entropy counting for the baseline determination is developed. 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$^\mathrm{II}$ and Nb$^\mathrm{IV}$ 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. The temperature dependence of exponent $n$ quantifying the field dependence of $ΔS_\mathrm{M}$ 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 Brayton cycle and the Ericsson cycle, to assess its cooling effectiveness. A cascade system is suggested for the most efficient cooling performance.