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Single-molecule magnets based on the orbitally-degenerate spin centers and stable organic radicals complexes

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Higher operating temperature for miniature magnets

Single-molecule magnets (SMMs) could lead to high-density information storage based on spin flips in single molecules. Scientists developed new magnetic materials rationally designed to overcome current bottlenecks to widespread implementation.

Energy

Magnets are widely used in numerous fields — the market for them is bigger than that for semiconductors. The discovery that some polynuclear metal complexes consisting of an organic and a metal part behave like magnets on the molecular level has sparked tremendous interest. One of the barriers to application has been the low blocking temperatures at which single-molecule magnetisation has been observed. The blocking temperature in essence sets the upper temperature limit of operation of the magnet. Low blocking temperature, much less than ambient temperature, makes application in real devices challenging. A major goal of the EU-funded project VOLURAD was to determine rules for rational design of polynuclear cluster systems with higher blocking temperatures. Researchers used an approach based on heterospin systems. In heterospin systems, two spin orientations (up–down) are separated by an energy barrier, U. If the barrier is higher, magnetisation blocking or maintenance of the magnetised state could be longer. This means that the system would be suitable for information storage. Scientists thus sought to increase the blocking temperature by increasing the U. Investigators focused on the coordination chemistry of heterospin systems that have not yet been well studied. Polynuclear complexes contain two or more central metal atoms or ions surrounded by non-metal atoms or groups of atoms called ligands in a single coordination sphere. The team synthesised several new polynuclear complexes. They consisted of 4d metal centres (ruthenium) or 5d metal centres (osmium or rhenium) with 3d metal ions as well as 3d metal ions with nitroxides (stable free radicals). The building blocks were chosen to meet the prerequisites for SMM behaviour. These are strong negative (perpendicular) anisotropy and high-spin ground state (relatively many unpaired electrons). Most of the new compounds have been structurally characterised with single-crystal X-ray diffraction. Their magnetic properties have also been determined and publications are in various stages of review. Results have multidisciplinary significance, contributing to areas including molecule-based materials, chemistry and nanotechnology. Perhaps the most immediate tangible impact will be on information processing, contributing to European leadership in a booming technology sector.

Keywords

Single-molecule magnets, polynuclear metal complexes, blocking temperature, heterospin, energy barrier

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