Weiss domains

Electron spins, which form the elementary magnetic building blocks of matter, are partially aligned parallel to each other in ferromagnetic solids and are not completely randomly distributed, even in a demagnetised state (i.e. when the material is not magnetic).
In ferromagnetic materials, there are always areas of a few tenths of a millimetre in size in which the electron spins are aligned parallel to each other. These areas are called Weiss domains (after the physicist Pierre-Ernest Weiss). An external magnetic field will cause many small domains to merge into larger domains, and the magnetisation of the solid object becomes measurable on the outside. Parallel aligned electron spins are responsible for the measurable macroscopic magnetic field.

The spins are usually not aligned in parallel as a whole, but only arranged parallel to one another within groups. And, in most cases, the alignment of the spins between the different groups is not parallel. The area in a ferromagnet in which the spins are aligned in parallel is called a Weiss domain. The domains are separated from each other by so-called Bloch walls.

Making Weiss domains visible

The phenomenon of Weiss domains can be demonstrated with a macroscopic model. For that, a set of compass needles is mounted on a board so they can freely rotate and is then observed. If the compass needles are positioned close enough together to cause the magnetic fields of neighbouring needles to influence each other, a state is achieved in which the compass needles are aligned in parallel in certain areas. The field of one compass needle aligns the other compass needles.
However, it is extremely rare that all compass needles actually align in parallel. There is usually an area boundary beyond which a differently oriented group can be found, as shown in the following illustration.

In such a model, all compass needles can be aligned by an external magnetic field. Due to the influence of temperature (movement of the compass needles) or external mechanical influence (knocks on the board), entire groups of compass needles can change their orientation. This occurs through the mutual interaction of the compass needles, as with electron spins, often suddenly for an entire group.

This abrupt behaviour can also be directly observed in electron spins and is referred to as Barkhausen jumps.

Changing the orientation of an entire Weiss domain (Barkhausen jump) leads to a change in the magnetic field. This can be made audible with the help of an amplifier and a loudspeaker. The Barkhausen jumps manifest themselves as a "crackling sound" in the loudspeaker.

The Bloch walls can also be made visible in the experiment by placing a suspension of microscopic ferromagnetic particles (e.g. iron dust) on the ferromagnetic material. This dust then arranges itself along the boundaries between different Weiss domains and can be observed under a microscope.

At first, it does not seem surprising that neighbouring electron spins interact and form Weiss domains like the compass needles, since the magnetic moments of the electron spins influence each other, and, as such, one could assume that the magnetic field of an electron spin affects the magnetic field of a neighbouring electron spin. This is what happens in the compass needle model.
However, it can be demonstrated that this magnetic force is far too small to explain the strong stabilisation of electron spins against thermal motion in ferromagnets. Instead, it is the exchange interaction that is primarily responsible for the mutual interaction of the electron spins and for the formation of Weiss domains in the ferromagnet.