Magnetic dipolar ordering and relaxation in the high-spin molecular cluster compound Mn6

TL;DR

This study investigates the magnetic dipolar order and relaxation phenomena in the high-spin Mn6 molecular cluster, revealing a ferromagnetic transition at very low temperature and analyzing nuclear relaxation dynamics.

Contribution

It provides the first detailed experimental and theoretical analysis of dipolar magnetic ordering and nuclear relaxation in a high-symmetry Mn6 molecular cluster.

Findings

Transition to ferromagnetic dipolar order at 0.16 K

Good agreement between Monte-Carlo simulations and experiments

Identification of nuclear relaxation contributions in high magnetic fields

Abstract

Few examples of magnetic systems displaying a transition to pure dipolar magnetic order are known to date, and single-molecule magnets can provide an interesting example. The molecular cluster spins and thus their dipolar interaction energy can be quite high, leading to reasonably accessible ordering temperatures, provided the crystal field anisotropy is sufficiently small. This condition can be met for molecular clusters of sufficiently high symmetry, as for the Mn6 compound studied here. Magnetic specific heat and susceptibility experiments show a transition to ferromagnetic dipolar order at T_{c} = 0.16 K. Classical Monte-Carlo calculations indeed predict ferromagnetic ordering and account for the correct value of T_{c}. In high magnetic fields we detected the contribution of the ^{55}Mn nuclei to the specific heat, and the characteristic timescale of nuclear relaxation. This was compared with results obtained directly from pulse-NMR experiments. The data are in good mutual agreement and can be well described by the theory for magnetic relaxation in highly polarized paramagnetic crystals and for dynamic nuclear polarization, which we extensively review. The experiments provide an interesting comparison with the recently investigated nuclear spin dynamics in the anisotropic single molecule magnet Mn12-ac.