Single-molecule magnets: Iron lines up.

T he bottom-up synthesis of magnetic materials is a key aim of research in molecular magnetism. The goal is not to compete with bulk magnets, such as those decorating the fridge door at home or operating valves in your car, whose magnetism depends on the collective contributions of neighbouring magnetic atoms arranged with long-range order. Instead, molecules that are intrinsically magnetic — single-molecule magnets (SMMs) — are attractive because of their small size, versatile chemical composition and adaptable fabrication methods. They hold promise for spin-dependent electronics or magnetic information storage, with fields that put forth devices such as reading heads for hard disks or non-volatile computer memory components. In particular, SMMs that contain just one spin centre should be ideal for future miniaturized spin-based computational devices that can store and process information by making use of the fascinating fundamental phenomena of quantum mechanics 1. Jeffrey R. Long and his collaborators from spectroscopy and theoretical chemistry have now described 2 in Nature Chemistry a 'simple' mononuclear, two-coordinate iron(i) compound with impressive SMM properties. Their findings outline a few remarkable design principles for future advances. The SMM nature of a compound manifests itself, among other features, through magnetic hysteresis: magnetization induced by an external field relaxes only slowly after the field is removed. Below a certain 'blocking temperature' , the magnetization may even be stable for a period of time, from minutes to years, before the compound relaxes. This slow magnetic relaxation arises from coupling of the electronic spins with the orbital moments of the atoms. Among other effects, this spin–orbit coupling causes zero-field splitting (ZFS) of the atomic electronic ground states, which generates directionality for the molecular magnetic moments (magnetic anisotropy) and creates an energy barrier towards reversal of the magnetization. To be a good SMM, a compound must combine a high-spin ground state and a ZFS that is large and negative and provides a well-defined axis for easy magnetization. An established strategy for building oligonuclear SMMs relies on clusters of transition metal ions with suitable local spins and large ZFS. To optimize the global cluster spin, ferro-or ferrimagnetic spin coupling is required between the metal centres, and these centres need to be organized in a geometry that favours build-up of large magnetic anisotropy. The latter in particular is not trivial, but successful schemes have been derived from the principles of classical magnetochemistry 3. Recently, however, mononuclear complexes of terbium and …