Magnetic refrigeration is a method of obtaining a temperature lower than 0.7 K. To obtain low temperatures typically liquefied gas is used. Lowering the pressure over the free surface of the liquid, you can get the temperature below the normal boiling point of the liquid. For example, the flow of nitrogen vapors can reduce temperature to the temperature of the triple point of nitrogen (63 K), by pumping hydrogen vapors (above the solid phase) you can achieve the temperature of 10 K, by helium vapor pumping (with very good conditions of the experiment) you can achieve the temperature of about 0.7 K.
In 1926, the William Francis Giauque and Peter Debye independently showed that at very low temperatures paramagnetic substances should have a great value of magnetocaloric effect and that this effect can be used to obtain low temperatures. In their first experiments in 1933, Giauque and McDougall manage to obtain the temperature of 0.25 K by demagnetizing gadolinium salts.
The method is based on the effect of heat from the paramagnetic salts at magnetizing and subsequent absorption of heat by degaussing. This allows you to get the temperature down to 0.001 K.
There is also a nuclear method of degaussing, which you can use to get the temperature up to K.
A sample of the paramagnetic salts is suspended in a tube filled with gaseous helium under slight pressure. Gaseous helium provides contact with a bath of liquid helium, refrigerated liquid evaporation under reduced pressure. While working in the bath the lowest possible pressure is maintained, usually corresponding to a temperature of ~ 1 K. Due to gas conduction paramagnetic salt is cooled to a temperature of the helium bath. Then the magnetic field is turned on.
During magnetization process the salt is heated up. The orientation of the magnetic ions along the magnetic field decreases entropy. The heat from the salt is carried away to the helium bath and salt temperature again becomes equal to 1 K.
After that, the gas that surrounds the sample and is in thermal contact with it, is pumped out and then magnetic field is turned off. In the process of magnetic refrigeration, entropy and energy of magnetic ions is partly restored by the energy of grid, and the salt temperature noticeably drops.
To obtain very low temperatures, you need the salts with a small concentration of paramagnetic ions, i.e. salts, in which neighboring paramagnetic ions are separated from each other by non-magnetic atoms. The interaction between magnetic ions in this case is very weak. For example, in chrome-potassium alum each chromium atom is surrounded by a magnetic 47 non-magnetic neighbors.
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