Question

What is principle of adiabatic demagnetisation?(please explain in detail)

Answer 1

Adiabatic demagnetisation is just another process of cooling but the range of temperature to which you can cool a substance (usually a gas) through this method is really huge. One can use demagnetisation to attain a normal refrigerating temp. of 240K and also to attain temp. as low as 0.3K! But this process is usually used to cool already cold substances i.e. reduce 5K to suppose 1K. So, what is the whole thing about adiabatic demagnetization? Well, the mechanism involves a material in which some aspect of disorder of its constituent particles exists at 4 K or below (liquid helium temperatures). Magnetic dipoles in a crystal of paramagnetic salt have this property of disorder. In them, the dipoles are randomly oriented in space. When a magnetic field is applied, these levels become separated sharply. If the magnetic field is applied while the paramagnetic salt is in contact with the liquid helium bath (an isothermal process in which a constant temperature is maintained), many more dipoles will become aligned, with a resultant transfer of thermal energy to the bath. If the magnetic field is decreased after removing it from that helium bath, no heat can flow back into the sample (an adiabatic process), and thus it will cool. Now, what exactly is the procedure? Let’s find out. First, the sample to be cooled is allowed to touch a cold He reservoir (which has a constant temperature of around 3-4 K), and a magnetic field is induced in the region of the sample. Once the sample is in thermal equilibrium with the cold reservoir, the magnetic field strength is increased. This causes the entropy of the sample to decrease, because the system becomes more ordered as the particles align with the magnetic field. The temperature of the sample is still the same as that of the cold reservoir at this point. Then the sample is isolated from the cold reservoir, and the magnetic field strength is reduced. The entropy of the sample remains the same, but its temperature drops in reaction to the reduction in the magnetic field strength. If the sample was already at a fairly low temperature, this temperature decrease can be ten-fold or greater. This process can be repeated, permitting the sample to be cooled to very low temperatures as I said. Also just for your information, though this method is reliable, safe and eco-friendly but this isn’t the only way to achieve such low temperatures. Much low temperatures can be attained by an analogous means called adiabatic nuclear demagnetization. This process relies on aligning nuclear dipoles (arising from nuclear spins), which are at least 1,000 times smaller than those of atoms. With this process, temperature of the ordered nuclei as low as 16 micro-degrees (0.000016 degree) absolute have been reached!!! Hope this was helpful. :)