What exactly is the magnetic remanence? - Explanation of terms

The term magnetic remanence - or remanence flux density - refers to the magnetization of a ferromagnetic substance after switching off the external magnetic field. This means a certain residual magnetism or the residual magnetization of a material. The magnetic flux density indicates the strength of the magnetic remanence. It is measured in Gauss or Tesla unitsunitwith the following mapping:

10,000 Gauss = 1 Tesla

A high-remanence ferromagnetic material is, for example, iron. It can become magnetized when exposed to a magnetic field over a period of time. The remanence then provides information about the strength of this magnetization. A so-called hysteresis curve can be used to determine the maximum remanence: this is different for each material. By the way, remanence is especially strong in ferromagnetic materials. The magnetic field of the material is opposite to the outer one.

Remanent ferromagnetic materials

Nickel, cobalt and iron are the three elements that exhibit ferromagnetic properties at room temperature. In addition to these elements, there are also several alloys and compounds with ferromagnetic properties. Some elements become ferromagnetic only at very low temperatures - for example the so-called superconductors. Substances with ferromagnetic properties show a very strong remanence effect after switching off the external magnetic field or the magnetization (in contrast to eg paramagnets).

In everyday life you can also observe remanence: If, for example, a pair of scissors or a pin is exposed to a strong magnetic field, the objects are then attracted to objects containing iron. A remanent magnetization pin thus remains hanging on the radiator, for example. It even makes it possible to build a compass with it: simply place the magnetized pin on a piece of styrofoam and let it float in the water. It now automatically adjusts itself to the Earth's magnetic field - insofar as there are no other influencing magnetic fields - and acts as a compass in this way.

Physical explanation of remanence

It is known that a substance consists of several atoms. For metals, these combine to form a grid. Each atom has in turn:

  • Proton atomic nuclei
  • If necessary. Neutron
  • An envelope of electrons

Electrons have a so-called electron spin. This one is responsible for the magnetic properties. The remanence has something to do with this spin.

In physics lessons the magnetization is represented by small arrows in the ferromagnetic material. These align themselves and form a magnetic field. The small arrows thus represent the elementary magnets. Basically, these are nothing else than the electron spins. Without an external magnetic field, they are not subject to any order and are constantly moving. As with any body, the movement of atoms increases at higher temperatures. Normally, a ferromagnetic material is therefore not magnetic by nature - after all, the poles of the many electron spins or elementary magnets point in all directions, which are constantly changing.

In this chaos order is brought with a magnetic field: The elementary magnets or electron spins align themselves parallel to the external magnetic field. This creates a north and south pole. If the temperature is not too high, this parallel alignment will remain stable with ferromagnetic materials even when the external magnetic field is removed. The reason for this is the so-called exchange interaction - conceivable as low as possible energy level between the respective electron spins. The body remains magnetized by the stabilized alignment of each elemental magnet following removal of the external magnetic field by this remanence effect. This magnetization is called remanence. If, after removal of the magnetic field, a stronger magnetization remains behind than with the other, one speaks of a magnetically hard material (or of a magnetically soft material in the latter).

Is it possible to undo the retentivity?

The remanence can also be undone. If the magnet is exposed to the following conditions, there is the possibility that the remanence disappears:

  • Strong vibrations
  • Great heat
  • Opposite magnetic fields

For a complete demagnetization, either a so-called coercive field is necessary or the Curie temperature must be reached:

  • Nickel: 358 ° C
  • Iron: 768 ° C
  • Cobalt: 1127 ° C

However, there is no precise threshold for the complete disappearance of remanence in case of shocks.

Basically, magnets have to be supplied with energy in order to demagnetize them. Why is that? Well, one can imagine that thanks to the orientation of the individual electron spins, a certain amount of energy is stored in a magnet. This can be specified by the magnetic energy density. The amount of energy product and the maximum operating temperature are the determining factors for the quality of the magnet. This is indicated by the energy product and a subsequent letter combination for the grade - for example "N" for 80 ° C. The higher the quality, the greater the magnetic force and the greater the remanence.

The above-mentioned hysteresis is a graph showing that there is no strict proportionality between the magnetization of a ferromagnetic material and the change in the external magnetic field - the reason why remanence remains after removal of the external magnetic field.