Getting to low temperature is something we take for granted today: laboratories across the world are awash with liquid nitrogen at -196 C and body scanners in hospitals use liquid helium at only 4.2 degrees above absolute zero (-269 C). Yet what underpins this low temperature technology is a subtle balancing of very weak forces between the molecules in gases - forces that are barely discernible at room temperature, but nevertheless play a crucial role in our ability to cool things down to very low temperatures.
This was first understood by the celebrated nineteenth century scientists James Joule and William Thomson who identified a special temperature below which gases can be cooled by expansion. This effect was later exploited by the technologists Carl von Linde and William Hampson as a practical means of reaching very low temperature, which is still the basic method used today.
New work by the LCN’s Dr. Laura Bovo and Prof. Steve Bramwell, in collaboration with a team of scientists from Sweden, Switzerland, Canada and Warwick, have shown that special temperatures, marking related changes in physical properties are not confined to gases, but also occur in certain magnetic materials that the workers have called “inverting magnets”.
Figure: Spheres of the magnet “spin ice” used in the study in front of a large-scale model of the atomic connections in the material. Photo taken by Dr. Laura Bovo.
In ordinary magnetic materials, such as those in fridge magnets, each atom is magnetic and the alignment of the atomic magnets gives the strong magnetism of the material as a whole. At high temperatures the permanent magnetism disappears because it is disrupted by the temperature, but there is still a tendency to strong magnetism, which can be measured by the force with which a material is attracted by another magnet.
In an inverting magnet this attraction gets larger as the temperature is lowered, but then there is a special temperature at which the trend reverses. The workers discovered that, in analogy with gases, such special temperatures reflect a subtle balance of very weak interactions, this time between the magnetic atoms comprising the material. They showed that these special temperatures – as in the case of gases – can be used as a diagnostic of very weak and competing interactions in the material. Hence, even if there is no immediate technological consequence of this, the observation of special temperatures gives a new way for researchers to model and predict magnetic properties in the low temperature limit, where various types of exotoc magnetism are expected.
“The study of special temperatures in magnets gives a surprising insight into how they eventually behave at very low temperatures, and this is all based on making an analogy with gases” says Dr. Laura Bovo of the London Centre for Nanotechnology (present address: UCL Innovation and Enterprise).
The insights found from studying special temperatures will help the researchers identify and model unusual types of magnetism that occur at very low temperature: particularly so-called spin liquid or spin ice behaviour.
Related link: UCL Mathematical & Physical Sciences