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Cool Magnetic Molecules Print

Certain materials are known to heat up or cool down when they are exposed to a changing magnetic field. This is known as the magnetocaloric effect. All magnetic materials exhibit this effect, but in most cases, it is too small to be technologically useful. Recently, however, the search for special molecules with a surprisingly large capacity to keep cool has heated up, driven by environmental and cost considerations as well as by recent improvements in our ability to design, assemble, and probe the structure and chemistry of small molecules. An international collaboration of researchers from Spain, Scotland, and the U.S. has utilized ALS Beamline 11.3.1 (small-molecule crystallography) to characterize the design of such "molecular coolers." The work targets the synthesis of molecular cluster compounds containing many unpaired electrons ("nanomagnets") for applications involving enhanced magnetic refrigeration at very low temperatures.

Not Your Mother's  Refrigerator Magnets

Some day soon, magnets could do more than clutter up the front of your refrigerator with notes and pictures. In molecular form, "nanomagnets" might just be the key to refrigeration that uses less power, runs more quietly, and doesn't require potentially harmful refrigerants—substances that have powerful greenhouse effects if allowed to escape into the atmosphere. Given that a significant chunk of global energy use is related to cooling, any improvement in the efficiency of the process would help decrease our overall energy consumption as well.

Interest in magnetic refrigeration has grown in recent years since the discovery of a  "giant magnetocaloric effect" (a surprisingly large change in temperature for a given applied magnetic field), particularly in certain alloys of gadolinium (Gd). While some of this research is focused on materials that would operate near room temperature, Karotsis et al. here investigate the magnetic cooling properties of molecules containing Gd (or its closest neighbors in the periodic table) for use at very low, cryogenic temperatures. Helium-3, the coolant currently used in this regime, is becoming rare and expensive, and valid alternatives are therefore desirable.

Artist rendition of a frozen cluster

The "frozen" [Mn4Gd4] cluster pictured is a promising candidate for refrigeration in the ultra-low-temperature region, providing for example a valid alternative to the use of helium-3, which is becoming rare and expensive.

Underlying the magnetocaloric effect is the idea that magnetism is the result of the ordering of electronic spins in the material. It is already well known that heating a magnet to its Curie temperature will thermally disrupt this order and cause the magnetism to vanish. Conversely, introducing a magnetic field to certain materials will cause the material to heat up. This happens because, as the spins in such (paramagnetic) materials align with the magnetic field, the magnetic entropy (disorder) of the system decreases; if the system is isolated (insulated), the total entropy must remain constant, and the thermal entropy (temperature) increases to compensate. The process is reversed for a decreasing magnetic field: magnetic entropy (disorder) increases, and thermal entropy (temperature) decreases.

Single-molecule magnets are metal–organic compounds that show such behavior at the molecular level, so that collective long-range magnetic ordering of magnetic moments is not necessary. The researchers' idea was to use molecular cluster compounds containing paramagnetic metal ions. The most suitable metal ions for the job are those with isotropic electronic configurations, such as Gd(III), and these must be bound together within a molecule through the use of bridging ligands, such as calixarenes. The word calixarene is derived from calix, the latin word for "chalice," because these organic molecules have bowl-shaped cavities. Their rigid conformations are helpful in self-assembly, and the polyphenolic pocket at the lower rim is an attractive feature for metal complexation.

By employing calix[4]arenes (calixarene with 4 repeating units), it was possible to build heterometallic transition-metal/lanthanide (3d/4f) cluster molecules whose structures describe a square of Mn ions surrounding a square of ions from the lanthanide series. The flexibility offered by the similarity of chemistry across the lanthanide series means that the isotropy or anisotropy of the resulting clusters can be tuned at will, by using either Gd(III), Tb(III), or Dy(III) as the lanthanide-series ion. The structures were determined crystallographically at ALS Beamline 11.3.1, and magnetic measurements down to 80 mK and specific heat measurements down to 0.3 K were carried out on powdered crystalline samples for magnetic fields of 0–9 T.

Molecular structures of C4 and [Mn4Gd4]

Left: Bowl-shaped calix[4]arene (C4) molecule. Right: Molecular structure of the [Mn4Gd4] cluster, showing the four C4 molecules (gray) at the perimeter, the four Mn ions (purple) at the C4 lower rims, and the four Gd ions (green) nearest the center.

Clusters with Tb(III) and Dy(III) both exhibited superparamagnetic behavior but performed poorly in terms of magnetic refrigeration. However, replacement with isotropic Gd(III) resulted in a large number of molecular spin states being populated even at the lowest investigated temperatures. This, combined with the high magnetic isotropy, makes it an excellent magnetic refrigerant for low-temperature applications.

Very low temperature experiments are of great importance because they provide us with a detailed fundamental understanding of the physical properties of materials (be they magnetic, electrical, etc.) and thus they are indispensible for the design of new materials. This work shows how lanthanide polymetallic molecule clusters, which are otherwise chemically isostructural, can have dramatically different magnetic properties depending on the lanthanide.


 

Research conducted by G. Karotsis and E.K. Brechin (University of Edinburgh, UK); S. Kennedy and S.J. Dalgarno (Heriot-Watt University, UK); S.J. Teat and C.M. Beavers (ALS); D.A. Fowler (University of Missouri); and J.J. Morales and M. Evangelisti [Spanish National Research Council (CSIC) and Universidad de Zaragoza, Spain].

Research funding: Engineering and Physical Sciences Research Council (UK), Leverhulme Trust, and the Spanish Ministry for Science and Innovation. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: G. Karotsis, S. Kennedy, S.J. Teat, C.M. Beavers, D.A. Fowler, J.J. Morales, M. Evangelisti, S.J. Dalgarno, and E.K. Brechin, "[MnIII4LnIII4] calix[4]arene clusters as enhanced magnetic coolers and molecular magnets," J. Am. Chem. Soc. 132, 12983 (2010).

ALS Science Highlight #228

 

ALSNews Vol. 320