Magnetic polarisation
What is magnetic polarisation?
Magnetic polarisation of a ferromagnetic material refers to the alignment of the elementary magnets in the material, i.e. the process of magnetisation. Often, the direction and strength of the magnetic forces created by the aligned elementary magnets is also called magnetic polarisation. Magnets with reversed polarisation are those where the north pole of one magnet is at the location of the south pole of the other magnet and vice versa.Table of Contents
Magnetic polarisation is the process of aligning magnetic moments.
Once alignment has been completed, polarisation refers to the direction along which the elementary magnets
have been aligned.
Magnetic polarisation manifests itself as magnetisation
and also indicates the direction of magnetisation.
Two different magnets are said to have reverse or opposite polarisation if both magnets have their north
and south poles
at opposite ends.
Origin of magnetic polarisation
Physically, magnetic polarisation occurs when the magnetic moments present in matter are aligned by an external magnetic field. This causes the magnetic forces of all magnetic moments to add up, resulting in an amplification of the external magnetic field by factor μ, the magnetic permeability. This is precisely the case when magnetic moments are present in the matter that can also be aligned. These can be individual electron spins, for example. Materials with this property are paramagnetic or even ferromagnetic. The process of polarisation of matter can be observed when a ferromagnetic material is placed in an external magnetic field.When an external magnetic field H0 acts on a material with magnetic permeability μ, a magnetic flux density B is created which is greater by the factor μ of the material than in a vacuum: B=μμ0H0.
This apparent change in magnetic flux density, which is caused by the influence of matter as opposed to a vacuum, is known as magnetic polarisation and is denoted by the letter J. A ferromagnetic material, for example, increases the magnetic flux density. The magnetic polarisation in an external field H0 is determined by the magnetic susceptibility χ of the material: J=χμ0H0.
Because B=μ0H0+J,
μ=(1+χ)
represents the relationship between magnetic susceptibility χ
and magnetic permeability μ.
Experimental determination
In an experiment, magnetic polarisation can be determined by measuring the magnetic flux density at the end of a wire coil when a current flows through the coil. This can be done using a Hall probe, for example. If a ferromagnetic material is inserted into the coil, for example a cylinder made of iron, a much greater magnetic flux density will be measured than without the iron core.
When a voltage U is applied to the ends of a coil, a current I flows.
This current in turn causes a magnetic flux density B.
The magnitude of the flux density depends crucially on the magnetic permeability of the volume enclosed by the coil.
If the coil is filled with air (μ=1, left side), a magnetic flux B is formed, which is represented by field lines in the illustration.
However, when a ferromagnetic material with a permeability greater than 1 is introduced into the coil (for iron, μ can reach values of up to 100000), the atomic spins of the material align themselves parallel to the generated magnetic field (the atomic spins are shown in red, right side).
The magnetic flux B is significantly greater due to this process of so-called magnetic polarisation.
A significantly stronger magnetic field can then be measured on the surface of the magnet and the magnetic forces of such a coil with a magnetically polarised iron core are also significantly greater.
Relationship between magnetic polarisation and magnetic flux density
In a vacuum (μ=1), the magnetic field H0 corresponds to a magnetic flux density B=μ0H0.The matter also introduces the magnetic polarisation J: B=μ0H0+J.
This corresponds to an increase in the magnetic field H
compared to the external magnetic field H0
by the magnetisation M:
H=H0+M.
Magnetic polarisation is therefore fundamentally equivalent to magnetisation. Magnetic polarisation is the magnetic flux density associated with the magnetic field of a certain magnetisation. The magnetisation is a magnetic field, whereas magnetic polarisation is a magnetic flux density.
Because B=μ0H, it follows that B=μ0H0+μ0M.
Because B=μ0H0+J, the magnetic polarisation J is, in turn, equal to the product of the magnetisation M and the permeability constant of the vacuum μ0: J=μ0M
Magnetisation is measured in amperes per metre (A/m), whereas the unit of magnetic polarisation is the same as the unit of magnetic flux density tesla (T).
In ferromagnets, part of the magnetic polarisation is retained even when the external magnetic field is switched off. This remaining magnetic polarisation is called remanence. The reason for the residual magnetic polarisation in ferromagnetic materials is the permanent alignment of the magnetic moments of the electron spins. The electron spins remain in the aligned state due to the exchange interaction. The alignment of the magnetic moments can only be destroyed by applying heat, by heavy impacts or by a suitable opposing field of coercive field strength.
In diamagnets, no permanent magnetic moments exist. However, if a diamagnetic material is placed in an external magnetic field, an effect nevertheless occurs, namely the induction of circular currents in the material. The induction of circular currents in the material is an effect that is also present in paramagnetic and ferromagnetic substances, but is masked by the permanent magnetic moments, i.e. the para- and ferromagnetism. The induced circular currents themselves have magnetic moments. However, they do not strengthen the external field, but instead weaken it (according to Lenz's law) because they are oriented in the opposite direction to the external magnetic field.
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.
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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