Remanence

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What is remanence?

The remanence or remanence flux density in a ferromagnetic material (e.g. iron) manifests itself through measurable magnetic forces that remain after the material has been temporarily exposed to a magnetic field and was magnetised. The numerical value of the remanence indicates how strong the magnetisation is. The maximum remanence is a material-specific value that can be determined from the hysteresis curve.

Table of Contents

Remanence is the magnetisation that remains in a ferromagnetic material after an external magnetic field has been switched off. This is also referred to as remanence flux density.

When an object is exposed to an external magnetic field, magnetisation occurs. The magnetic field created by the magnetisation itself is particularly strong in ferromagnetic materials and is aligned with the external magnetic field.

Remanence in ferromagnetic materials

At room temperature, only the elements iron, nickel and cobalt are ferromagnetic. In addition, there are also ferromagnetic alloys and compounds as well as elements that become ferromagnetic at low temperatures.

Ferromagnetic materials, in turn, show strong remanence when the external magnetic field is switched off.

Remanence can be observed in everyday life when you expose an iron-containing object, such as a pair of scissors, to a strong magnetic field. Afterwards, you will observe that the scissors can attract ferrous pins, for example, even though the magnet has already been removed from the scissors.

Physical explanation of remanence

To explain remanence, one can conceive that every substance consists of atoms with atomic nuclei and electrons. The electrons have a so-called 'spin', which has magnetic properties.

The physics of remanence has to do with electron spin. Electron spins behave like tiny elementary magnets. Without an external magnetic field, they are not evenly aligned and are also constantly in motion. This movement increases at higher temperatures. If you imagine the electron spins as rod magnets, the poles of these many tiny rod magnets point in different and constantly changing directions. The object as a whole is, therefore, not magnetic.

When a ferromagnetic body is magnetised in an external magnetic field, the elementary magnets are all aligned in parallel. The north pole of all microscopic magnets points in one direction, the south pole in the other.

If the temperature is not too high, the electron spins in a ferromagnet remain aligned even if the external magnetic field is removed. This is due to the mutual interaction of the electron spins, the so-called exchange interaction, which is particularly strong in ferromagnets. Each elementary magnet is stabilised in its alignment. The object as a whole remains noticeably magnetic. This permanent magnetisation is called remanence.

With the same external magnetic field, a greater remanence remains in magnetically hard materials than in magnetically soft materials.

Remanence disappears if the magnetised object is exposed to high heat or strong vibrations, as this changes the alignment of the electron spins again. An oppositely polarised magnetic field can also cause the remanence to disappear. This requires a very specific magnetic field strength, the so-called coercive field, so that complete demagnetisation occurs on the one hand, but an oppositely polarised remanence does not build up on the other.

The effect that the magnetisation of ferromagnetic objects is not strictly proportional to the change in the external magnetic field, i.e. in particular that remanence remains when the external magnetic field is switched off, is also referred to as hysteresis.

The strength of the remanence is indicated by the so-called magnetic flux density,

which is measured in the units tesla or gauss. The conversion is 1 T = 1 tesla = 10 000 gauss = 10 kG.

Ill. Hysteresis curve for a magnetically soft material (left) and a magnetically hard material (right). For the still non-magnetised material, the red 'initial magnetisation curve' shows the course of the magnetisation over the external field. The respective curve applies to the course shown by the arrows.

Typical points on the hysteresis curve are the coercive field H_c, which is necessary to compensate for the remanence of the material by the external field, and the remanence B_R itself, which denotes the remaining flux density when the external magnetic field disappears. Finally, there is the saturation flux density B_S, at which all electron spins are aligned.

Due to the alignment of the electron spins, a certain amount of energy is stored in each magnet, which is indicated by the energy product. The alignment of the electron spins is destroyed at high temperatures. The stored magnetic energy (amount of the energy product as a number) and the maximum operating temperature (indicated by a combination of letters, e.g. 'N' for 80 °C) determine the grade of the magnet. A magnet with a high grade also has a high remanence and therefore strong magnetic forces.

Author:
Dr Franz-Josef Schmitt

Dr Franz-Josef Schmitt is a physicist and academic director of the advanced practicum in physics at Martin Luther University Halle-Wittenberg. He worked at the Technical University from 2011-2019, heading various teaching projects and the chemistry project laboratory. His research focus is time-resolved fluorescence spectroscopy in biologically active macromolecules. He is also the Managing Director of Sensoik Technologies GmbH.

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