ADIABATIC DEMAGNETIZATION OFPARAMAGENETIC SALTS

ADIABATIC DEMAGNETIZATION �OF�PARAMAGENETIC SALTS

1. ADIABATIC DEMAGNETIZATION OF PARAMAGENETIC SALTS ATHUL RAJ T K Mechanical Engineering

2. History Langevin (1905) pointed out that changes in the magnetization of paramagnetic substances must be accompanied by reversible temperature changes. Debye (1926) and Giaque (1927) suggested independently reversible temperature effects connected with the magnetization of certain paramagnetic salts might be used for producing temperatures well below the BP of Helium First experiments were carried out in 1933 by Giaque and MacDougall and since then magnetic cooling has become an established cryogenic technique.

3. Langevin’s Theory An assembly of non-interacting magnetic dipoles, in the absence of external magnetic field are randomly oriented. Upon application of magnetic field, B the energy of a given dipole depends on its orientation w.r.t the field direction. Magnetic moment, M depend only on the ratio B/T If pressure and volume effects are neglected, enthalpy

4. Remarks If is negative isentropic magnetization will be accompanied by heating and demagnetization by cooling. Energy of magnetization appears in the form of heat. The effect is small at ordinary temperatures where is small and the heat capacity is large.

5. Historical activities Kamerlingh Onnes (1923) discovered that some paramagnetic salts ( eg . Gadolinum sulphate) behaved according to Langevin’s theory even at the BP of He where specific heat was small and is appreciable. In the first experiment of Giaque and MacDougall, using Gadolinum sulphate reached 0.53 K starting from 3.4 K and subsequently 0.23 K demagnetizing from 1.5 K. deHaas , Wiersma and Kramers (1933) used Cerium Fluoride and reached 0.27 K . Better results were obtained by using Cerium and Dysprosium Ethyl Suplhates and Potassium Chromic Alum by the same investigators.

6. Entropy as a function of temperature

7. Demagnetization Cryostat