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Bloch Walls
The Bloch walls were named after the Zurich-born physicist Felix Bloch. In magnetism, the term describes the transition region between the so-called Weiss domains in ferromagnetic materials, which can be seen below the Curie temperature (temperature at which ferromagnetic properties disappear).
Bloch-Walls and Weiss districts
It is important to understand that you do not want to think of a Bloch wall as a real existing wall, but rather as a boundary. At this boundary, the orientation of parallel electron spins of the neighboring Weiss districts (also called domains) differs, so that it comes to different magnetization directions.
The magnetic moments (spin directions) of the two neighboring Weiss domains do not turn suddenly in the transition layer, but gradually, since this requires the least exchange energy. This is done in a vertical, out of the layer direction. Thus, the Bloch walls differ from the so-called Néel walls, in which the change of direction takes place in the horizontal x-y plane. Thanks to the anisotropy energy, the Bloch walls cannot spread further.
When an external magnetic field is introduced, the Weiss areas merge and magnetization of the ferromagnetic material occurs. During this process, the Bloch walls shift abruptly and the alignment of the electron spins is equalized. This leads to the Barkhausen effect.
How can the Bloch walls be made visible?
If a special suspension, i.e. a liquid with ferromagnetic particles (for example iron dust), is placed on a ferromagnetic material, it can be seen that the iron dust collects at certain limits. These are the outer boundaries of the Weiss districts to each other, the Bloch walls on the material outer surface.