Permanent magnet

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What is a permanent magnet?

Permanent magnets are materials that exhibit lasting magnetic forces. They can attract ferromagnetic materials (e. g. iron) or repel each other at like poles (north pole to north pole, south pole to south pole). A demagnetisation of a permanent magnet is possible through heat, strong mechanical shock or strong external magnetic fields. In addition to permanent magnets, there are also electromagnets.

Table of Contents

The north pole of a permanent magnet attracts the south pole of another permanent magnet and vice versa. In contrast, repulsive magnetic forces act between like poles (north pole to north pole, south pole to south pole). Ferromagnetic materials (iron, cobalt, nickel and some alloys) are always attracted to permanent magnets.

Permanent magnets are magnetic materials which, unlike electromagnets, do not require an electrical current for their magnetic field. Permanent magnets always consist of ferromagnetic materials whose elementary magnets, the atomic spins, have been aligned in parallel through a process of magnetisation.

This can happen when molten ferromagnetic rocks cool. Such stones were historically found by the ancient Greeks near the town of Magnesia. Thus, the city of Magnesia is the historical eponym of magnetism.

However, permanent magnets can also be produced artificially. To do so, strongly ferromagnetic metals, usually alloys such as samarium cobalt, are magnetised by strong external magnetic fields. This process of magnetisation exhibits a so-called hysteresis, i.e. a non-symmetrical material behaviour when the external magnetic field is increased and then decreased. Hysteresis occurs because the alignment of the elementary magnets in the ferromagnet is stabilised by exchange interaction and, consequently, an already magnetised material has different properties than a ferromagnet that has not yet been magnetised.

Because of hysteresis, a magnetic field remains in the ferromagnet even when the external magnetic field is switched off. The magnetised material thus becomes a permanent magnet. The remaining magnetic flux density is called remanence.

Demagnetisation of permanent magnets

While an electromagnet can be switched off simply by turning off the electrical current and have its polarity reversed by reversing the direction of the current, it is not easily possible to "switch off" a permanent magnet. Hence the term "permanent". A permanent magnet remains magnetic until the alignment of the magnetic spins is disturbed again through external influences (heat, strong impacts, magnetic fields). The magnetic forces will vanish, and the material will have to be magnetised again. In extreme cases, the material could even be damaged. Consequently, each permanent magnet has a maximum operating temperature. Above this temperature, damage can occur. Above the material-specific Curie temperature, the magnet will always be completely demagnetised.

Strength of permanent magnets

The strength of a permanent magnet depends not only on the material used but also on the precision with which the material is magnetised. Magnetisation only leads to high remanence if a complete orientation of all atomic spins is achieved. This requires suitable material and technical expertise.

As described by Maxwell’s equations, magnetic fields always arise from moving charges. There are only magnetic fields caused by the movement of charge, and it always creates a magnetic field with a north pole and a south pole.

The magnetic forces of permanent magnets are explained by microscopic charge movement in matter. The electrons in the atoms move at high speed and with a characteristic electron spin. The overall state of motion of the electrons creates a magnetic moment and thus a magnetic force.
Magnetic forces always act along the magnetic field. This can be demonstrated with field lines. The field lines will indicate the direction and magnitude of the magnetic forces.

A current-carrying conductor loop (pictured left) creates a magnetic field. The strength of this magnetic field is measured by the magnetic moment. There are numerous magnetic moments in a ferromagnetic material (centre). When all of them are aligned in parallel, a permanent magnet is created. The permanent magnet has a magnetic field that is identical to the magnetic field of a coil. Only a few field lines are schematically indicated in the picture shown. Permanent magnets can be produced in a wide variety of shapes. A horseshoe magnet is shown on the right. In a horseshoe magnet, the north and south poles face each other. Since magnetic field lines are always closed as a whole, they run from the north to the south pole and then back to the north pole inside the material. In the air gap of the horseshoe magnet, a homogenous magnetic field is created with parallel field lines between the poles.

The magnetic forces of a permanent magnet depend primarily on the magnitude of the atomic magnetic moments and the completeness of the alignment, as well as the magnitude of the exchange interaction. These factors also influence the total magnetic energy stored in a permanent magnet. Magnetic energy is measured by the energy product. The energy product determines the grade of a magnet. The greater the energy product, and therefore the magnetic energy of the permanent magnet, the higher the grade.

Buying permanent magnets

supermagnete has specialised in the sale of permanent magnets for many years. With a huge stock of neodymium magnets and other permanent magnets, we can usually deliver immediately and in large quantities. Don’t hesitate to contact us if you have any questions. The following magnets have become top sellers in our assortment:

Other neodymium magnets

Other ferrite magnets

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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