Cooling with magnets

An international team of scientists have successfully used magnetic molecules to reach temperatures below 1K, becoming the first ever researchers to do so. This finding opens up the possibility of novel refrigeration systems. Usually, to reach low Kelvin temperatures, helium is used as the refrigerant. However, helium is becoming increasingly scarce. “The very rare helium-3 isotope with which one can get down to a few tenths of a Kelvin is now practically unaffordable,” said Professor Jürgen Schnack, co-author of the study from Bielefeld University. This new research, published in Nature Communications, demonstrates what the authors call ‘sub-Kelvin cooling’ using molecules that contain magnetic ions such as gadolinium. Professor Eric J. L. McInnes and colleagues synthesised the molecular cluster at the University of Manchester. It contains seven gadolinium ions, with six forming a hexagon and one sitting in the centre; hence it is referred to as ‘Gd7’. The weak interaction of these gadolinium ions causes a weak magnetocaloric effect (MCE) whereby when an applied external magnetic field is removed from a magnetic material, there is a reduction in temperature. “Compared to paramagnetic salts in which the temperature drops continuously as the magnetic field declines, molecules such as Gd7 behave in more complex ways. They can be used to get down to really low temperatures without switching off the magnetic field completely,” reported Dr Marco Evangelisti whose team at the Universidad de Zaragoza carried out the low temperature experiments. The computer simulations show that it starts by cooling down in a decreasing magnetic field; it then warms up again before finally cooling down once more as the magnetic field diminishes. “We were really excited when the theoretical computations were able to explain this complex behaviour in detail,” said Schnack This is the first time sub-Kelvin temperatures have been reached using magnetic molecules and is a significant step towards using these substances as a replacement for helium. Quantum signatures of a molecular nanomagnet in direct magnetocaloric measurements By Rebecca Dey