Quantum Spin Dynamics in Single-Molecule Magnets

TL;DR

This thesis investigates quantum spin dynamics in single-molecule magnets Mn12-ac and Mn6, exploring quantum tunneling effects, nuclear spin interactions, and magnetic ordering using ultra-low temperature NMR experiments.

Contribution

It provides new experimental insights into nuclear spin behavior and proposes a unifying model for quantum tunneling effects in single-molecule magnets.

Findings

Quantum tunneling affects nuclear spin relaxation and diffusion.

Long-range ferromagnetic order observed in Mn6 due to dipolar interactions.

A theoretical model unifies experimental observations of spin dynamics.

Abstract

This thesis contains a thorough investigation of the quantum spin dynamics in Mn12-ac and Mn6 Single-molecule magnets. In particular, we have investigated the interplay between quantum tunneling of magnetization and nuclear spin dynamics in Mn12-ac by ultra-low temperature NMR experiments. We discuss the effect of quantum tunneling on the nuclear spin-lattice relaxation, the nuclear spin diffusion, the thermalization of the nuclear spins, the isotope effect, the influence of strong perpendicular fields, and we propose a model to unify all our observations. For the isotropic Mn6 molecule, we report the observation of long-range ferromagentic ordering of the cluster spins, due to dipolar interactions only. The nuclear spin dynamics provides an interesting comparison with the case of Mn12-ac. Two large introductory chapters describe the design of our ultra-low temperature setups, and the theoretical background necessary to understand the interplay between nuclear spins and quantum tunneling, with emphasis on the Prokof'ev-Stamp theory of the "spin-bath".