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Unformatted text preview: The Drosophila of single-molecule magnetism: [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ] Rashmi Bagai and George Christou* Received 14th July 2008 First published as an Advance Article on the web 23rd February 2009 DOI: 10.1039/b811963e Single-molecule magnets (SMMs) are individual molecules that can function as nanoscale magnetic particles. The [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ] (Mn 12 ; R = Me, Et, etc. ) family of SMMs was the first one discovered; it is also the one whose study has provided the majority of current knowledge on this interesting magnetic phenomenon, prompting its description here as the Drosophila of the field. This tutorial review will survey the various chemical studies that have been carried out to date on this family. This will include a discussion of methods that have been developed for their structural and redox transformation, and the effect of the latter on the magnetic and SMM properties. 1. Introduction Magnets are a multi-billion dollar annual industry with a host of uses such as in switches, computer hard drives, credit/debit/ ATM cards, televisions, audio devices, motors, and highly specialized instruments such as medical MRI equipment, among many others. As industry continues the push towards greater miniaturization, smaller devices, and more digital information storage in a computer hard drive or iPod r memory, the need for smaller and smaller magnets increases. Magnetism has consequently become a major sub-division of Nanoscience. The need is for nanoscale magnets of identical size and behaviour, and the standard approach to this has been to make smaller and smaller pieces of traditional magnets, which are composed of metals, metal alloys, metal oxides, or similar. This so-called top-down approach is reaching its limits as the ability to fabricate nanoscale magnets that are of identical size ( i.e. monodisperse) becomes increas- ingly more dicult with decreasing size. The ability of a single molecule to function as a magnet is thus of great importance to the field of nanomagnetism because it represents an alternative, bottom-up route to nanoscale magnetic materials. Indeed, it brings to the area all the advantages of molecular chemistry, including mono- dispersity, crystallinity, true solubility (rather than colloid formation), protection by a shell of organic groups that prevents close contact of a molecules magnetic core with those of neighbouring molecules, and the ability to vary this organic shell at will using standard chemical methods. Such molecules are called single-molecule magnets (SMMs). 14 They have also occasionally been called molecular nanomagnets. To function as an SMM, a molecule must display slow magnetization relaxation below a characteristic blocking temperature, T B . This behaviour results from a large ground spin state ( S ) ( i.e. , lots of unpaired electrons) com- bined with a large and negative Ising (or easy-axis) type of magnetoanisotropy, as measured by the axial zero-field...
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