Yet another method of cooling objects, magnetic
, exploits the relationship between the
effects of the magnetic field strength of an applied field and the
entropy of an object.
One particular method of
magnetic cooling is Adiabatic
Demagnetization, which capitalizes on the paramagnetic
properties of some materials to cool those materials (usually in
gaseous form) down into the millikelvin -- or colder -- range.
This method can also be used to cool solid objects, but the
most drastic cooling in the fractions of a kelvin range are generally
accomplished for low-density gases that have already been greatly
cooled (around 3-4 K)
are Paramagnetic Materials?
When an outside magnetic
field is applied to these materials, a
magnetic field that is parallel to the applied field is induced.
amount of particles that align with the
applied field depends on the field strength: if there is a higher field
particles are aligned with the field.
The Process of Adiabatic
- First, the sample to be cooled (typically a gas) is allowed
to touch a cold reservoir (which has a
constant temperature of around 3-4 K, and is often liquid Helium), and
a magnetic field is induced in the region of the sample.
- Once the sample is in thermal equilibrium with the cold
magnetic field strength is increased. This causes the entropy
of the sample to decrease, because the system becomes more ordered as
the partilces align with the magnetic field. The temperature
of the sample is still the same as that of the cold reservoir at this
- Then the sample is isolated from the cold reservoir, and
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
process can be repeated, permitting the sample to be cooled to
very low temperatures.
Limitations of the Process of Adiabatic Demagnetization
- If the sample material is an electronic
temperatures that can be attained using these methods are on the order
of 1 millikelvin.
Electronic Paramagnets? These are materials with
electronic magnetic moments.
These materials are paramagnetic because electronic magnetic
moments tend to align themselves with a magnetic field.
- If the sample is a nuclear
paramagnet, the lowest achievable
much lower (because the interactions between the dipoles are much
weaker). Nuclear paramagnets have been cooled down to 250
picokelvins, and it may be possible to cool them further.
Materials with a net magnetic
moment that is
caused by the individual magnetic moments of the material's nuclei.
magnetic moments are about 1000 times smaller than electronic
magnetic moments, so the dipole interactions are weaker. As a
result, nuclear paramagnetic materials can be cooled to temperatures
that are orders of magnitude less electronic paramagnetic
materials can be cooled to.
reason for this is that
the interactions between the individual particles tend to
create their own internal magnetic field when the applied magnetic
field is very weak or absent. So, there is a limit to how
weak the applied magnetic field can be, because at some point, the
particle interactions cancel any further cooling effects of
demagnetization. Since the dipole interactions between
particles in nuclear paramagnetic materials are far weaker than those
in electronic paramagnetic materials, the temperature and
field strength limits are far lower.
An adiabatic demagnetization cooler at NASA.