Research Fellows Directory
Dr Karl Sandeman
Imperial College London
I study solids that change their magnetic properties when they are exposed to an external stimulus (e.g. a magnetic field). At such so-called magnetic phase transitions, there is a release or take-up of heat which can be used in a cooling cycle analagous to that when a volatile liquid is alternately expanded and compressed in a conventional refrigerator. In short, the materials I study are the working substance for a future magnetic fridge.
The attraction of this research is the promise of a more efficient cooling cycle that is also free of greenhouse gas refrigerants. From a fundamental viewpoint, the reason for studying solid magnetic materials is their complexity. A magnetic solid has many different interacting "degrees of freedom" including atomic order, atomic vibrations, electrons and quantum mechanical spins. By seeking to understand the interplay of these degrees of freedom I aim not only to help in the development of magnetic refrigerants but also to discover and tailor new phenomena occuring when different degrees of freedom couple.
Most magnetic phase transitions have been observed in single-phase alloys where degrees of freedom interact within one phase on the atomic scale. Now, however, developments in nano-fabrication mean that we can examine these short-lengthscale interactions not just between atoms but also between nano-scale phases of different materials. If we move from considering atoms as building blocks to the use of material phases in this regard, the range of materials we can make is potentially very large. I am currently collaborating with researchers who are able to assemble new materials from the “bottom up” in a number of different ways. Some involve making mixtures of phases with well-controlled interfaces. Others use one “host” material to act as a molecular scaffold for another “guest” material, thus controlling inter-molecular interactions. Such developments could result in a whole new class of tailored magnetic materials.