ADIABATIC DEMAGNETIZATION OFPARAMAGENETIC 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
)
/
( 3
Tm
Nm
volume
unit
per
moment
Dipole
M
MdB
TdS
dH
BM
U
H






7. 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.
B
T
M
)
(


B
T
M
)
(



8. 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.

9. Entropy as a function of temperature

10. Demagnetization Cryostat