Angewandte Chemie International Edition
Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 1999, 38 (15), 2264 - 2266
Molecules containing iron ions
display tunable magnetic properties
Magnetism is a phenomenon that has amazed many of us, even as children. This amazement still lingers in some researchers today, because some details of magnetism's origin have not yet been completely explored. A team of scientists working with Andrea Cornia at the University of Modena and Reggio Emilia (Italy) has specialized in creeping up on the molecular mechanism of magnetism with small, iron-containing molecules. The latest highlight: a ring of six iron atoms changes its magnetic properties if its chemical composition is altered slightly.
Iron is magnetic because the atoms of this element behave like little individual magnets. If they are aligned parallel to each other in a magnetic field, for example, they maintain their orientation - the bulk material is then also externally magnetic. The question that Cornia and his co-workers asked themselves is this: how can we finely tune the magnetic properties of small arrays of iron atoms? A molecule consisting of six iron ions arranged in a ring around a central lithium ion is a suitable subject for studying this question. The metal ions within this ring structure are very close to each other, whereas interference from iron ions in neighboring molecules can be blocked out, because they are held at a distance by a shell of nonmagnetic molecular moieties.
A detailed examination of this ring-shaped molecule in a sensitive measuring device showed that this compound lies on the boundary between a "real" magnet and a nonmagnetic material. Normally, the magnetic contributions of the iron ions in the ring cancel each other exactly. However, if the molecule is put into a strong magnetic field, some of the elemental magnets flip over and the molecule becomes magnetic. The strength of the applied field can indeed be used to tune the magnetism of the cluster, that is, the number of flips in the ring.
However, the researchers got an additional surprise when they removed the lithium ion from the center of the ring and replaced it with a sodium ion: the resulting molecule had quite different magnetic properties. For instance, the larger sodium ion strengthens magnetic interactions between the iron atoms, so that switching on the magnetism of the ring required higher fields. Perhaps a similar approach can someday be used to tailor special magnetic materials for highly dense information storage or magneto-optical switches.