This document summarizes adiabatic demagnetization, a process used to produce very low temperatures. It involves first magnetizing a paramagnetic material while it is in contact with a cold reservoir, which aligns magnetic dipoles but keeps the temperature constant. The material is then isolated and the magnetic field removed adiabatically, causing the temperature to drop significantly as magnetic entropy increases. The document provides details on the analogous gas compression/expansion process and outlines the specific steps of first magnetizing the sample at low temperature, then removing the field to demagnetize it adiabatically and lower the temperature further.

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PRESENTED BY:
J RAHUL RAO
ID NO: 172071022
M.TECH. (THERMAL ENGG.)
K L (Deemed to be) UNIVERSITY.

2. What’s Adiabatic Demagnetization.?
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MagnetoCaloric Effect
 It’s a Magneto-Thermodynamic phenomenon in which a temperature change of
a suitable material is caused by exposing the material to a changing magnetic
field
 Working Principle
 Just an another method of producing low temperatures(Around small fraction of
1K) and occurs in magneto-caloric materials.

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4.  Isothermal compression of a gas Isothermal magnetisation of a paramagne
(we apply pressure and the entropy decreases) (we apply H and the magnetic entropy decreases)
 Subsequent Adiabatic Expansion Adiabatic demagnetisation
(we lower pressure at constant S and T decreases) (we remove H, the total S remains constant and
T decreases since the magnetic entropy
increases)
Analogous to
Analogous to

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1.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.

6. Process of Lowering Temperature of a Paramagnetic Salt
1) Isothermal Magnetization
2) Adiabatic Demagnetization
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7. what exactly is the procedure
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i. 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.
ii. 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.
iii. 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.
 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.

8. Experimental Procedure
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The sample is cooled to the boiling point of helium in strong magnetic field,thermally
Isolated... and then…. removed from the field to demagnetize it.

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10. 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
 16 micro-degrees (0.000016 degree) absolute could
be reached!!
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Thank
You