Giant magneto-elastic coupling could lead to more efficient fridges

Temperature dependence of Mn-Mn nearest-neighbor distances

Temperature dependence of Mn-Mn nearest-neighbor distances in (a) CoMnSi and (b) Co0.95Ni0.05MnSi. Crystallographic positions are shown in (c), together with the closest commensurate version of the helical spin Co and Mn sublattices
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ISIS has helped scientists gather new data on how a material behaves at high temperatures and in strong magnetic fields which could lead to more efficient fridges and cooling systems.

A team of scientists from Imperial College London, Cambridge University and ISIS observed the interactions between the atoms within a material, CoMnSi, at different temperatures and magnetic fields.

Typically, the volume of a material increases modestly as it is heated. However, as this material is heated by 150˚C, the separation between the manganese (Mn) atoms changes by an extraordinary 2%, significantly more than previously observed in any metallic magnet.

“We are looking for materials which have a sharp change of magnetic state when a magnetic field is applied,” said Dr Karl Sandeman, a physicist from Imperial College who led the experiment. “Without the use of high-resolution neutron diffraction at ISIS, it would have been difficult to uncover the large and competing effects at work.”

The research group from Imperial College are ultimately looking for a magnetic change that coincides with a change with another property of the material, such as a change in the crystal structure or position of atoms. Changes such as these are known as ‘first order’ changes, and they occur very quickly, at a particular temperature, acting like a switch.

“One feature of CoMnSi was that while the material possesses a change in magnetic state, it takes place at around 200˚C above the point at which the extraordinary change in atomic separation occurs,” said Dr Sandeman. “However, we found that by applying a magnetic field, we could force both changes to happen at the same temperature. Together, these generate a ‘first order’, switch-like change. The effect is known as magneto-elastic coupling.”

In standard fridges, the contents are kept cool by condensing a gas into a liquid. This change in state absorbs heat, keeping the food in the fridge cool. A sharp change in magnetic state can produce a similar cooling effect, and could lead to the production of magnetic fridges.

Conventional refrigerants are often volatile organic compounds such as fluorocarbons (FCs) or hydrofluorocarbons (HFCs). If they escape, they can be very damaging to the environment as they are powerful greenhouse gases.

“If such volatile refrigerants escape, they have a very high greenhouse gas potential.  One recent study predicted that by 2050, between 28-45% (CO2-equivalent basis) of projected global CO2 emissions will be due to HFCs” said Dr Sandeman.

Magnetic refrigeration, on the other hand, uses solid refrigerants. Consequently, there is no potential for harmful gas leaks when using this type of cooling system.

“Using magnetic refrigeration could produce cooling systems that are more energy efficient, emit less noise, and are less harmful to the environment. Understanding the circumstances that can generate sharp, switch-like changes in the magnetic state is crucial to developing and designing magnetic refrigerants,” explained Dr Sandeman.

Beth Penrose (ISIS), Karl Sandeman (Imperial College London)

Research date: January 2010

Further Information

The team from Imperial College have received funding from a UK magnetic refrigeration company Camfridge Ltd, the European Commission (FP7 project "SSEEC"), The Royal Society, The Newton Trust, The Leverhulme Trust and the Engineering and Physical Sciences Research Council (EPSRC). This collaboration is not only looking to develop magnetic refrigeration for keeping food cool, but is also developing a magnetic heat pump which could be used for managing heat in homes.

Contact Dr K Sandeman: k.sandeman@imperial.ac.uk

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