Remanence

What exactly is magnetic remanence? – Definition

The term magnetic remanence – or remanence flux density – describes the magnetization of a ferromagnetic material after the external magnetic field has been switched off. 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 units of Gauss or Tesla, with the following assignment: 10,000 Gauss = 1 Tesla

An example of a ferromagnetic material with high remanence is iron. It can be magnetized if it is exposed to a magnetic field for a certain period. The remanence then provides information about the strength of this magnetization. The maximum remanence can be determined using a so-called hysteresis curve: this is different for each material. The remanence is particularly strong in ferromagnetic materials. The magnetic field of the material is directed opposite to the external field.

Remanent ferromagnetic materials

The three elements that exhibit ferromagnetic properties at room temperature are:

  • Nickel
  • Cobalt
  • Iron

In addition to these elements, there are also several alloys and compounds with ferromagnetic properties. Some elements only become ferromagnetic at very low temperatures - for example the so-called superconductors. Materials with ferromagnetic properties show a very strong remanence effect after the external magnetic field or magnetization is switched off (in contrast to paramagnets, for example).

Remanence in everyday life

Remanence can also be observed in everyday life: If, for example, you expose a pair of scissors or a pin to a strong magnetic field, the objects are then attracted to objects containing iron. A remanent magnetization pin can, for example, stick to a radiator or serve as the basis for a homemade compass.

DIY magnetic compass through remanence bauen

The following materials are required:
  • a magnetized pin
  • a piece of Styrofoam
  • a bowl of water

Now you simply place the magnetized pin on the piece of Styrofoam and then let it float in the water. It will now automatically align itself with the earth's magnetic field - provided there are no other influencing magnetic fields - and thus function as a compass.

Physical Basics of Remanence

We know that a substance consists of several atoms. In metals, these combine to form a lattice. Each atom in turn has:

  • Atomic nuclei made up of protons
  • Possibly neutrons
  • A shell made up of electrons

Rotation as the key to magnetic remanence: the electro spin

Electrons have what is known as an electron spin. This is responsible for the magnetic properties. The remanence is therefore directly related to this spin. In physics lessons, magnetization is represented by small arrows in the ferromagnetic material. These align and form a magnetic field. The small arrows therefore represent the elementary magnets. Basically, these are nothing other 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 the atoms increases at higher temperatures. A ferromagnetic material is therefore normally not magnetic by nature - after all, the poles of the many electron spins or elementary magnets point in all possible directions, which are also constantly changing.

From chaos to magnetic order

  • Natural state: without an external magnetic field, the electron spins are disordered.
  • Under the influence of a magnetic field: the elementary magnets align themselves parallel.
  • Remanence effect: the alignment remains even after the magnetic field is removed, if the temperature is not too high.

Role of the exchange interaction

Exchange interaction – conceivable as the lowest possible energy level between the respective electron spins. As a result, the material retains its magnetic properties even after the magnet is removed.

  • The parallel alignment of the spins is maintained
  • A permanent magnetic north and south pole is created

How can the remanence be reversed?

If the magnet is exposed to the following conditions, there is a possibility that the remanence will disappear:

  • Mechanical impact: due to strong vibrations
  • Thermal stress: due to high heat
  • Magnetic counterforce: due to opposing magnetic fields

Curie temperature as a critical point

For complete demagnetization by heat, the material-specific Curie temperature must be reached:

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

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

Energetic basics of demagnetization

Basically, magnets need to be supplied with energy to demagnetize them. Why is that? Well, you can imagine that a certain amount of energy is stored in a magnet thanks to the alignment of the individual electron spins. This can be specified by the magnetic energy density.

Magnet quality and its determination

The amount of the 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 quality - for example "N" for 80 °C. The higher the quality, the:

  • the greater the magnetic force
  • the greater the resulting remanence

Hysteresis and magnetization

The hysteresis curve shows that there is no strict proportionality between the magnetization of a ferromagnetic material and the change in the external magnetic field. This explains why the remanence remains even after the external magnetic field is removed.


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