Environmentally friendly refrigerators that use magnetic refrigerants could be a step closer thanks to Ekkes Brück and colleagues at the University of Amsterdam. The researchers found that compounds containing transition metals can act as refrigerants in modest magnetic fields at room temperature. Refrigerators using such materials would also be more efficient than existing devices that rely on a vapour cycle (E Brück et al 2001 Nature 415 150).
Magnetic refrigerants heat up when they are subjected to a magnetic field because the second law of thermodynamics states that the entropy – or disorder – of a closed system must increase with time. This is because the electron spins in the atoms of the material are aligned by the magnetic field, which reduces entropy. To compensate for this, the motion of the atoms becomes more random, and the material heats up. In a magnetic refrigerator, this heat would be carried away by water or by air. When the magnetic field is turned off, the electron spins become random again and the temperature of the material falls below that of its surroundings. This allows it to absorb more unwanted heat, and the cycle begins again.
Magnetic refrigerators have two main advantages over today’s commercial devices, which extract heat from a vapour using a compressor: they do not use hazardous or environmentally damaging chemicals, such as chlorofluorocarbons, and they are up to 60% efficient. In contrast, the best gas-compression refrigerators achieve a maximum efficiency of about 40%.
The heating and cooling that takes place in magnetic refrigeration is proportional to the size of the applied magnetic field and the magnetic moments, which are generally largest in rare-earth elements. One such material, a compound based on gadolinium, has previously been shown to work as a magnetic refrigerant, but in a modest magnetic field its entropy only changes significantly at low temperatures. Brück says that in order to operate at room temperature or above this material requires large superconducting magnets, which are expensive and require extensive servicing.
In contrast, the material studied by the Amsterdam researchers, a manganese compound, performs best at room temperature. Although the magnetic moment of manganese is generally only about half that of heavy rare-earth elements, its Curie temperature of 300 kelvin means that it can undergo substantial changes in magnetic entropy using smaller permanent magnets.
Vitalij Pecharsky of the Ames Laboratory in the US thinks that the manganese compound is important scientifically, but believes that its commercial potential is still unclear. Pecharsky and colleagues showed in 1997 how to improve the cooling properties of gadolinium by adding impurities. He adds that researchers at Ames and the Astronautics Corporation of America have recently demonstrated a practical gadolinium-based magnetic refrigerator that works at room temperature using a permanent magnet.
