The structure of a mineral has been validated, ending the controversy about its potential usefulness as a model of an unusual magnetic lattice. This model might provide insight into superconductivity.
The mineral herbertsmithite has been hailed as a rare model of an unconventional type of magnetism thought to have a key role in the mechanism of high-temperature superconductivity (a form of superconductivity that occurs above 30 kelvin). But concerns have been raised that chemical disorder in this material could produce defects in the array of magnetic atoms, disturbing or even destroying the properties that make it a useful model. In the Journal of the American Chemical Society, Daniel Nocera and his group1 now report that they have clearly identified the type of disorder present, and have found it to have little influence on the magnetic lattice of the mineral. This represents a crucial step in establishing a simple, clean system to provide unambiguous insight into this important form of magnetism.
A material's magnetism is ultimately derived from unpaired electrons, each of which has a property called spin (S), with a value of ½, that bestows on each of them a magnetic moment. In insulators, such moments or spins are localized on atoms, and commonly interact with their closest neighbours so that specific spin orientations are preferred. In most cases, an antiparallel (antiferromagnetic) configuration of nearest-neighbour spins is favoured so that, when a lattice of such atoms is cooled, their spins usually freeze to form an ordered array (Fig. 1a). However, under certain circumstances — for some magnetic lattices, for instance — quite different behaviour can ensue. One such case is the kagome antiferromagnet, which is formed from corner-sharing triangles (Fig. 1b). In these systems, it is impossible to arrange each near-neighbour pair of spins so that they are all antiparallel, and the system is said to be geometrically frustrated.