Modelling the non-equilibrium low-temperature magnetic cooling effect in Mn12 clusters

Abstract

Based on the theoretical framework recently developed by some of us, we predict and justify the possibility of a nonequilibrium magnetic cooling effect in the Mn₁₂ family of clusters by considering a monocrystalline sample of the prototypical single-molecule magnet Mn₁₂Ac as a representative example. In contrast to the quasi-static processes underlying the conventional magnetocaloric effect (MCE), we address a dynamic regime involving sudden magnetic field quenching. The proposed cooling mechanism is determined by the relaxation kinetics arising after a sudden change in the spin Hamiltonian that generates a nonequilibrium population distribution within the spin subsystem and therefore does not rely on the standard equilibrium entropy cycles. During the subsequent restoration of thermal equilibrium, heat is redistributed between the phonon bath and the spin degrees of freedom. Under appropriate conditions, this relaxation-driven process results in cooling of the lattice. The central result is that the strong easy-axis magnetic anisotropy associated with a significant magnetization reversal barrier, features typically considered detrimental to conventional magnetocaloric cooling, becomes advantageous in the nonequilibrium regime. These properties enhance both the magnitude of the cooling effect and the practical feasibility of the sudden-quench approach. This study therefore broadens the potential cryogenic applicability of single-molecule magnets by identifying a cooling mechanism that operates precisely in the parameter range where classical magnetocaloric approaches are least efficient.