The basis of magnetic cooling is that a metal heats up when it is magnetised and cools when it is demagnetised. Currently very expensive and rare gaolinium is used for its good magnetocalorific property. It is thought that magnetic cooling could achieve a significantly higher energy efficiency than vapour compression systems. A prototype 500 Watt system with a superconducting magnet achieved a COP of over 5, more than an equivalent vapour compression system. However, the cost of the prototype was very high and the viability of the technology depends on finding suitable cheaper materials.