Cooling by adiabatic magnetization

Cooling by adiabatic magnetization has been seen in NaFe₆ ferric wheels. The spins in an ensemble of molecules constitute a system all by themselves and can be “cooled,” without heat flowing in or out, by decreasing the strength of an applied magnetic field. Some of the heat of molecular motion can then be transferred to the spins, which lowers the molecular temperature. Such adiabatic demagnetization was once routinely used to achieve the low temperatures needed for studying helium-3. Surprisingly, physicists at Erlangen-Nürnberg University in Germany have demonstrated the inverse effect: cooling molecules by increasing the applied field. The adiabatic magnetization was achieved with ring-shaped molecules featuring six iron atoms plus a few ligand hangers-on. The mechanisms of heat transfer are not yet fully explained. (O. Waldmann et al., Phys. Rev. Lett. 89, 246401, 2002 http://dx.doi.org/10.1103/PhysRevLett.89.246401 .)