Magnetic domain
As white districts (also Weiss' districts) is the term for magnetism microscopic magnetized domains in the crystals of a ferromagnetic substance. They were named after the French physicist Pierre -Ernest Weiss.
Description
White recognized in 1907 that the magnetic moments of the atoms ( " elementary magnets ") of ferromagnets are aligned even without the action of an external field parallel in limited districts. The size of these districts extending from about 10-5 to 10-3 m (10 microns to 1 mm) of linear expansion. The direction of magnetization is oriented to the crystal lattice of the material. For materials whose grain size of this magnitude equal to or even less, are all crystallites single domain particles, that is, not further divided into domains.
By nature, the white areas are magnetized to saturation. The boundaries between districts are called Bloch walls. Substituting a hard magnetic material an increasing magnetic field, move at first the Bloch walls in favor of those white districts, which are aligned in the direction of the external field. A further increase in the external field eventually change more and more white districts suddenly their polarity ( Barkhausen jump ). This flipping can be made audible by inductively receives and amplifies the alternating component or the step-like increase of the magnetic field with a coil. Upon change of the external field results in a hysteresis- noise, which allows conclusions as to the magnetic properties of the material.