Magnetic refrigeration

The magnetic cooling ( cooling by adiabatic demagnetization ) is a method of low-temperature physics, with the small amounts of material at temperatures below 1 mK ( mK ) can be cooled. It is used primarily in basic research.

Theory

The magnetic refrigeration is based on the temperature dependence in the order (the entropy) of the magnetic moments of the material used. Both the magnetic moments of the electrons ( such as paramagnetic at the adiabatic demagnetization salts) as well as nuclear moments (see adiabatic Kernentmagnetisierung ) can be used.

At high temperatures, the thermal energy is larger than the interaction energy of the magnetic moments and that they are completely out of order. Does the amount of moments involved an angular momentum J, there is a constant entropy S

Per mole of, where R is the universal gas constant. Decreases at low temperatures, the thermal energy, the interaction energy of the magnetic moments, they begin to arrange themselves; entropy decreases along the dashed line of the schematic diagram. In the presence of a magnetic field a preferred direction is set and the ordering temperature is raised ( point A to point B). First, thereby per mole of the heat

Released, which must be removed by suitable measures. In general, these pre-cooling is done using the 3He - 4He Entmischungskühlung. The magnetic field is then reduced with thermal insulation ( adiabatic), due to the ordered state corresponding to a lower temperature (point C):

The attainable temperature is limited by the internal field B, which is caused by the magnetic moments themselves.

Applications

Adiabatic demagnetization of paramagnetic salts

Cooling by adiabatic demagnetization paramagnetic salts (eg Cermagnesiumnitrat / CMN) was the first method have been achieved with temperatures in the range of a few milli- Kelvin ( 10-3 K). It was already in 1926 by Debye and 1927 suggested by Giauque and uses the magnetic moments of the electrons. The method has been largely replaced since the development of the 3He - 4He Entmischungskühlung as these, in contrast to magnetic cooling operates continuously.

It is also conceivable to use the adiabatic demagnetization of substances in the vicinity of the Curie point. So it should be possible in principle, by adiabatic demagnetization of gadolinium ( Curie point: 16 ° C) to build cooling units that work without environmentally harmful CFCs and without mechanical parts. So far this method has not acquired any technical meaning.

Adiabatic Kernentmagnetisierung

The cooling by adiabatic Kernentmagnetisierung, in which the magnetic moments of nuclei are used, is still the only method by which a solid body can be cooled to well below 1 mK - it will reaches temperatures in μKelvin area. The small size of the nuclear moments (about 1/2000 of those electrons ) making a pre-cooling to a few milli- Kelvin and high magnetic fields in the range of several Tesla is required.

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