Entropy has little to do with magnets loosing their magnetization. The
problems is that magnets store large amounts of energy in their magnetic
fields. This is usually described as the energy of the
and it's just another way to refer to the magnetic coupling of individual
magnetic dipoles (not the exchange coupling, just the classical
A magnetic material can lower this energy by adopting a magnetic
configuration that minimizes magnetic charges (i.e. magnetic poles). It
does so by moving it's
domain walls in a
way that lowers the total magnetic moment. A good magnet has many
crystallographic defects that pin the walls. However, if the temperature
is high enough, and if you wait long enough, the walls will eventually
“creep”. This is called “magnetic after-effect” and gives a
characteristic variation of the magnetization which is linear in
$\log(t)$. This behavior can be explained by the fact that domain walls
face a very wide distribution of energy barriers.
Good permanent magnets are supposed to show very little magnetic
after-effect. As a special case,
magnets are a type of nanomagnets that do not show this after-effect at
all, simply because they have no creepy domain walls as they would cost
too much exchange energy. An assembly of such particles can however
loose its magnetization because of individual particles switching from
one magnetic orientation to another. This is called
and is an obstacle to increasing bit density in magnetic storage (i.e.
There is some evidence that the demagnetization of a magnet is driven by
its dipolar energy rather than by entropy. First, there is the van den
Berg construction (see for example the book
Principles of Nanomagnetism,
by Alberto Passos Guimarães).
This is a geometrical way of predicting the magnetic configuration that
minimizes dipolar energy in a flat magnet. The predicted configurations
have actually been seen on real micron-sized samples, when imaged by
If the walls were driven by the urge to maximize their entropy, then one
would see them wandering randomly all around the sample. Instead, on
soft samples, they can be seen to adopt the exact configuration that was
predicted to minimize the magnetic dipolar energy.
Another evidence is numerical
This is the art of numerically predicting the magnetic configuration of
microstructures. The predictions are done by minimizing the total
magnetic energy (sum of dipolar, Zeeman, anisotropy and exchange), with
little consideration to entropy. The fact that numerical micromagnetics
can be quite successful is an evidence that energy is more important
than entropy in the behavior of magnets.
On the other hand, if one considers a diluted assembly of magnetic
nanoparticles close to their
then the demagnetization of the assembly is actually entropy-driven.
Only when the nanoparticles get quite close to one another their
interaction energy starts to play a significant role.