Molecular coolers

The basics

Magnetic refrigeration exploits the magnetocaloric effect (MCE), which describes the changes of magnetic entropy ΔSm and adiabatic temperature ΔTad, following a change in applied magnetic field ΔH. This standard technique in cryogenics is useful to cool down from a few Kelvin. Applications include, among others: superconducting magnets, helium liquefiers, medical instrumentation, in addition to many scientific researches.

 

Adiabatic demagnetization refrigerators are used as low-temperature platforms in research laboratories and in space-borne missions, where the absence of gravity hinders cooling by methods based on the 3He-4He dilution. Some six years ago, the number of adiabatic demagnetization refrigerators being sold worldwide was circa 30 per annum, making this market too small and thus not very attractive from an economic perspective. However, a sea-change took place and several companies are currently operating in this market. The driving factor was the increasing cost of helium, especially of the rarer 3He isotope.

 

The molecules

All magnetic materials show the MCE, although the intensity of the effect depends on the properties of each material. Molecular magnetic coolers are privileged candidates, chiefly because (i) their quantum characteristics can be controlled and optimised, (ii) they often are soluble in common organic solvents, hence allowing the transfer of their functionality. What used to be an unexplored and emerging research field has entered into adulthood during the last few years. In fact, more and more molecule-based materials are being proposed as magnetic refrigerants. Especially since 2010, we can count over twenty new molecular coolers per year, all together summing up to more than a whopping two-hundred publications in this research field at the time of this writing (April 2017). One of the beneficial effects of such an increased popularity is that molecular coolers have found their way through the jungle of molecular magnets on the one hand and magnetocaloric materials on the other hand, reaching their own proper identity.

 

The milestones

A personal view of how this research field has evolved over time is summarized by the following milestones, chronologically listed from top to bottom.

  1. When high spins meet high magnetic anisotropy: the archetypal single-molecule magnets Mn12 and Fe8, as molecular coolers [1a,1b].
  2. When low spins meet low magnetic anisotropy: the molecular wheels Cr7Cd, as molecular coolers [2].
  3. When high spins meet low magnetic anisotropy: the molecular nanomagnets Fe14 and Mn10, as molecular coolers [3a,3b].
  4. Weak exchange coupling, as a means to enhance the field-dependence of the MCE. Experimentally reported for molecular nanomagnets, based on transition-metal ions [4a,4b].
  5. Incorporating Gd3+ ions to further enhance the MCE, as experimentally observed for 3d-4f molecular nanomagnets [5a]. Role played by the magnetic anisotropy, or its lack thereof [5b].
  6. Since ligands are nonmagnetic and thus act passively, the lighter they are, the relatively larger is the MCE, as experimentally verified for purely Gd-based molecular nanomagnets [6]. This same publication reports the first direct measurement of the MCE of a molecular cooler, though for T > 5 K only.
  7. First molecular coolers for sub-Kelvin temperatures [7].
  8. Gd-based metal-organic frameworks, as molecular coolers [8a]. The first molecular cooler with a MCE larger than the reference refrigerant Gd3Ga5O12 [8b]. Also in Ref. [8b], the first direct measurements of the MCE of a molecular cooler at sub-Kelvin temperatures, using a homemade method.
  9. Molecular coolers on silicon substrate, towards on-chip micro-refrigerators [9].
  10. Quantum signatures of magnetically frustrated molecular coolers in direct measurements of the MCE, using a homemade method [10a,10b].
  11. Cooling by rotating magnetically-anisotropic molecular nanomagnets in a static applied magnetic field [11].
  12. First magnetocaloric composite based on molecular coolers grafted onto carbon nanotubes: intermolecular heat transport in direct measurements of the MCE [12].

And more to come...

The ugly side

Unfortunately, the growth of popularity is also being accompanied by a far too appalling number of publications of doubtful quality, authored by the usual suspects. There, innovative elements are conspicuous by their absence. Shallow works often combine with embarrassingly wrong statements/analyses and at times approach closely the ethical boundaries of plagiarism. The inevitable results are (i) the discredit of this research field, especially among readers who have a solid background in magnetocalorics but are new to molecular magnetism, and (ii) the promotion of lowly lines of activity, especially among inexperienced readers.