Quantum tunnelling of magnetization in Mn12-ac studied by 55Mn NMR

TL;DR

This study investigates quantum tunnelling of magnetization in Mn12-ac using 55Mn NMR at ultra-low temperatures, revealing temperature-independent nuclear spin relaxation driven by electronic spin tunnelling.

Contribution

It provides the first detailed nuclear magnetic resonance analysis of quantum tunnelling effects in Mn12-ac at millikelvin temperatures.

Findings

Nuclear spin-lattice relaxation becomes temperature-independent below 0.75 K.

Quantum tunnelling of electronic spins influences nuclear relaxation.

Fast-tunnelling molecules and nuclear spin diffusion significantly affect relaxation processes.

Abstract

We present an ultra-low temperature study (down to T = 20 mK) of the nuclear spin-lattice relaxation (SLR) in the 55Mn nuclei of the molecular magnet Mn12-ac. The nuclear spins act as local probes for the electronic spin fluctuations, due to thermal excitations and to tunnelling events. In the quantum regime (below T = 0.75 K), the nuclear SLR becomes temperature-independent and is driven by fluctuations of the cluster's electronic spin due to the quantum tunnelling of magnetization in the ground doublet. The quantitative analysis of the nuclear SLR shows that the presence of fast-tunnelling molecules, combined with nuclear intercluster spin diffusion, plays an important role in the relaxation process.