Introduction to Single-Molecule Toroics

This book deals with molecular systems which host toroidal magnetic states, referred in the literature as single-molecule toroics (SMTs). The study of such molecules finds its origin in the broader context of molecular magnetism (MM) whose birth and rich history are detailed in Magneto-Structural Correlation in Exchange Coupled Systems, a book written by Willet, Gatteschi and Kahn in 1985. For further developments in MM, please refer to Kahn’s Molecular Magnetism first published in 1993; for recent developments, to the book Molecular Nanomagnets by Villain, Sessoli and Gatteschi published in 2006; and Introduction to Molecular Magnetism: From Transition Metals to Lanthanides by Benelli and Gatteschi from 2015. In recent decades, molecular-based magnetic materials have attracted a great deal of attention due to their potential applications in magnetic storage devices, molecular spintronics and quantum information technologies based on molecular qubits [1–3]. These magnetic materials are composed of molecules which, like traditional bulk magnets, are able to retain their magnetisation even after the removal of a magnetic field. Early dilution studies of this effect, however, readily demonstrated that molecular magnetism is resultant from the intrinsic magnetic anisotropy of the individual molecules and that, in fact, intermolecular interactions between constituent molecules are not necessary to observe the phenomenon. Considerable growth in molecular magnetism has led to the discovery of single-molecule magnets (SMMs), single-chain magnets (SCMs) and spin crossover (SCO) molecules [1–3]. So too has the SMT subdiscipline grown out of MM, in this case from the study of polynuclear molecular rings comprised of strongly, magnetically anisotropic molecules whose magnetic axes adopt a characteristic, non-collinear vortex configuration [4–7].