What
is the process of Adiabatic Demagnetization?
How
cooling takes place by Adiabatic Demagnetization?
Cooling
by Adiabatic Demagnetization:
According to 3rd law of thermodynamics, entropy
of a substance decreases with decrease in temperature and ultimately becomes
zero at absolute zero for a perfect crystalline solid.
In 1926 William Francis Giauque and Peter Debye independently
suggested that adiabatic demagnetization would provide a practical way to reach
extremely low temperatures. Seven years later having overcome technical
problems Giauque cooled gadolinium sulfate to 0.25K a new record for
temperatures. To understand how he achieved this, the graph between entropy and
temperature for low and high values of magnetic field provides essential
information.
This graph shows entropy of paramagnetic substance as a
function of temperature in the presence and absence of applied magnetic field.
The curves x and y are S-T diagrams in
the absence and presence of applied magnetic field respectively. Adiabatic Demagnetization uses the paramagnetic nature
in some materials to cool those materials, generally in the form of gases, into
the colder range or millikelvin. To cool solid objects, this method can also be
used, but the most extreme cooling in the fractions of a Kelvin range is
normally achieved for gasses that have already been greatly cooled, which means
low-density gasses. This system consists of N paramagnetic particles together
with other particles in a crystalline solid.
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Entropy as a function of temperature in the
absence x and presence y of magnetic field |
At high temperature entropy of a system is
maximum and independent of magnetic field. But in low temperature range
entropy depends upon magnetic field.
Isothermal Magnetization:
When we apply external magnetic field on a
paramagnetic substance surrounded by helium gas at constant temperature, then
its entropy decreases along path a-b. The path a-b is an isothermal process
during which entropy of a paramagnetic substance decreases but temperature of the
system remains constant. This process is called isothermal magnetization. For
this process we have ∆T = 0 and ∆S ≠ 0
i.e.S2 > S1.
Adiabatic Demagnetization:
The path b-c is an adiabatic
demagnetization during which entropy of the system remains constant but
temperature of the system decreases from T1 to T2. This
process is called iso-entropic process. The increase in spin order is
compensated by decrease in disorderness due to lattice motion. Overall entropy
remains constant. This process can be repeated consecutively that involves
isothermal magnetization followed by adiabatic Demagnetization. In this manner,
temperatures close to 0 K can be reached. If this process is repeated an
infinite number of times, you could actually reach a temperature of absolute
zero but not less than that.
Experiment:
Figure below sketches the apparatus of
1930s. Liquid helium provides the initial temperature T1. By pumping
away helium vapor, one can cool the liquid by evaporation to 1K. The para-magnetic material is suspended
in a chamber immersed in the liquid helium. Initially the chamber contains
dilute gaseous helium also; the gas provides thermal contact between the
paramagnetic material and liquid helium and external magnetic field is raised
to the value of high magnetic field B. Thus the gas ensures isothermal
magnetization the process a to b in graph. Next, the gaseous helium is pumped
out leaving paramagnetic material thermally isolated. Slow, adiabatic demagnetization
takes the sample isoentropically from b to point c in graph. Apparatus of this
nature enabled Giauque to cool to 0.25K.
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| Apparatus for cooling by adiabatic demagnetization |
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