Cool Ferric Wheels

A new form of magnetic cooling has been demonstrated on tiny ring-shaped molecules. One obvious form of cooling is for one sample of particles to give excess energy to another, surrounding, ensemble of particles. Another way of chilling atoms (used to produce Bose-Einstein condensates) is simply to allow hotter atoms to escape.

To see how "magnetic cooling" works in an ensemble of molecules consider first only the electrons spins in the molecule. The spins constitute a system all by themselves and can be "cooled" adiabatically (that is, without heat flowing in or out) by decreasing the strength of an applied magnetic field. Then some of the heat of molecular motion can be transferred to the spins; a lower molecular temperature is achieved.

This "adiabatic demagnetization" was routinely used to achieve the low temperatures (milli-kelvin) needed for studying helium-3. The principle can even be extended to the spins of nuclei, and in this way the lowest cryogenic temperature ever was reached, 50 nK in copper.

Now physicists at Erlangen-Nurnberg University in Germany (contact Oliver Waldmann, now at Ohio State, waldmann@mps.ohio-state.edu, 614-292-3705) have demonstrated, for the first time, the inverse effect: cooling molecules by increasing the strength of the applied field. This adiabatic magnetization was achieved with "ferric wheels," ring-shaped molecules featuring six iron atoms plus a few ligand hangers-on (see figure). Research like this, involving the reactions between spins and molecules, and the coherence of states over time might be beneficial to a future quantum computing scheme. (Waldmann et al., Physical Review Letters, 9 December 2002.)