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, email@example.com, 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.)