A cluster formed by calixarenes coordinated to a mixed metal manganese–gadolinium magnetic core has excited states that are populated at low temperature, and shows promise for magnetic refrigeration systems.
Single-molecule magnets (SMMs), which combine properties explained by classical and quantum physics, are attractive not just for fundamental studies, but also for various applications. They are used, for example, in magnetic refrigeration, which relies on materials that alter their temperature under a changing magnetic field — the so-called magnetocaloric effect. SMMs have been built by encapsulating metal clusters within large rigid shells, such as calixarenes.
Now, Scott Dalgarno, Euan Brechin and colleagues in Edinburgh and Zaragoza have constructed1 a calixarene-based mixed 3d/4f metal cluster that shows promise for magnetic refrigeration at low temperatures. Salts of gadolinium — widely used in cooling systems — and manganese were reacted in solution with calixarenes to form a cluster with a magnetic core. Structural studies showed that it consisted of a square of gadolinium(III) ions inside a square of manganese(III)–calixarene building units.
The magnetism of the cluster was highly isotropic, and exhibited excited spin states that were populated even at low temperature. This resulted in a large magnetocaloric effect and these states only became depopulated under a high external magnetic field. This high ferromagnetic limit, together with the low-lying excited states and the high isotropy, makes this 3d/4f cluster a good candidate to be a low-temperature magnetic refrigerant.
References
Karotsis, G., Evangelisti, M., Dalgarno, S. J. & Brechin, E. K. A calix[4]arene 3d/4f magnetic cooler. Angew. Chem. Int. Ed. 10.1002/anie.200905012 (2009).
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Pichon, A. Gadolinium keeps its cool. Nature Chem (2009). https://doi.org/10.1038/nchem.509
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DOI: https://doi.org/10.1038/nchem.509
