In 1881, the German physicist Emil Warburg put a block of iron into a strong magnetic field and found it increased very slightly in temperature. Scientists now know the electrons pivot in the field to align at a lower energy state, releasing surplus energy. The metal warms up in what's known as the magnetocaloric effect, which is greatest near the magnetic phase transition temperature.
"If you can suddenly alter the degree of ordering of all these little spins, then you get a large response," says Sandeman. For iron at room temperature, the response is just 0.1C. Some materials cool in a magnetic field, a property that's used in low temperature research. Finding the right room temperature material is the key to a magnetic fridge, where the cooling power is derived from a positive magnetocaloric effect coupled to heat exchange.
One material works nicely: the element gadolinium (Gd). It's a silvery-white metal that's strongly attracted by a magnet, has a magnetic disordering temperature of 20C, and a giant magnetocaloric effect of several degrees. A waste product from permanent magnet manufacture, gadolinium costs around £100 per kg; a magnetic fridge would use 0.15kg. Sandeman's current research, however, is looking at other possibilities.
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