Nonequilibrium Magnetothermal Effects in Anisotropic 3d-Metal Complexes with Arbitrary Spins

In this article, we extend the recently proposed theoretical framework for nonequilibrium magnetothermal effects induced by a sudden magnetic field quenching to anisotropic 3d-metal complexes with arbitrary spins. The formalism is applicable not only to the case of complete magnetic field switching off, but also to the case of partial field quenching. A simple and universal semiquantitative rule is formulated, which allows for the prediction of the sign of a thermal effect (that means heat absorption or heat release) from the magnetic field dependencies of the spin energy levels. In many specific cases, this rule can be used to predict the sign of the magnetothermal effect prior to calculations, based on an analysis of the field dependencies of the spin levels of the complexes under study. According to this rule, each excited state contributes to cooling or heating depending on whether it becomes destabilized or stabilized as the field decreases. The performed numerical analysis of the specific heat release, as a function of temperature and initial and final magnetic fields for complexes with spins S = 1, 3/2, 2, and 5/2, demonstrates that systems with easy-axis magnetic anisotropy (D < 0) exhibit heat absorption in cases of complete and incomplete field quenching, with the effect being strongly enhanced in the latter case. In contrast, in complexes with easy-plane-type anisotropy (D > 0), the sign of the thermal effect is shown to be dependent on the temperature, the initial and final values of the magnetic field, and also on whether the spin of the complex is integer or half-integer. These results provide clear and practical guidelines for the design of low-temperature molecular magnetic refrigerants operating in fast field-quenching regimes.