Through quantum tunneling to dipolar order: the effect of varying magnetic anisotropy in three structurally related Mn4 molecular clusters

Abstract

By means of time-dependent specific heat experiments, we have studied the long-time electron spin-lattice relaxation mechanisms of three tetranuclear mixed-valence clusters of manganese ions (Mn4Cl, Mn4Ac and Mn4Me), which have the same magnetic core but different ligand molecules, and thus varying anisotropy. The relaxation of the spins towards equilibrium takes place, below 1 K, via an incoherent tunneling mechanism in the magnetic ground doublet, in which process energy is exchanged with the phonon bath. It is shown that the spin-lattice relaxation rate increases when the tunneling probabilities increase, thus indicating that quantum tunneling brings the spins towards equilibrium. For Mn4Me, this fast relaxation enables the magnetic system to undergo a transition to a long-range ordered state below TC=0.21 K.