Giant magnetocaloric effect created by strain
A European collaboration of scientists from the UK, Spain, the Department of Physics and Earth Sciences of Parma, France and the Ukraine has used interfacial strain to drive large temperature changes in thin magnetic films. Their results, published a few weeks ago in Nature Materials, pave the way for further studies and R&D, with the potential to deliver environmentally friendly, energy-efficient refrigeration in both domestic and industrial appliances.
The magnetocaloric effect, which forms the basis of magnetic refrigeration, relies on large thermal changes being driven by magnetic fields. The origin of large effects lies in a coupling of magnetism and structure so that magnetic phase transitions can drive crystal symmetry changes and vice versa. This rather rare effect can be engineered with BaTiO3 (BTO) a ferroelectric material that undergoes a structural phase transition at ~200 K, by growing magnetic La0.7Ca0.3MnO3 thin films on it. The scientists were able to create an interface that allowed the structural phase transition in the BTO to be driven by a magnetic field. This allowed a giant and extrinsic magnetocaloric effect to be generated in the LCMO/BTO system by modifying the magnetic structure of the LCMO film.
The team carried out experiments at the Diamond synchrotron science facility, with the PhotoEmission Electron Microscope (PEEM). In order to understand the microscopic origin of the magnetocaloric effect, precise magnetisation vector maps were built up using the PEEM. The team were able to clearly demonstrate that changes in the BTO substrate symmetry resulted in large areas of the LCMO thin film transforming from magnetic to non-magnetic during the structural phase transition.