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Permanent magnet
A permanent magnet or permanent magnet is a material from which a magnetic force always emanates. Such a permanent magnet can attract ferromagnetic substances (for example iron or cobalt). Furthermore, the poles of the same name of two permanent magnets repel each other. A permanent magnet can be demagnetized by a strong external magnetic field, which counteracts the magnetic field of the permanent magnet. A magnetization is also possible by a strong mechanical force or by reaching a temperature above the so-called Curie temperature
Which permanent magnets are there?
There are basically three types of permanent magnets: neodymium magnets, ferrite magnets and AlNiCo magnets. Between two poles of the same name of two magnets, for example between two north poles or two south poles, the magnetic forces are repulsive. If the poles are opposite, ie south pole to north pole or vice versa, the two magnets attract each other. Various alloys, nickel, cobalt and iron are ferromagnetic substances. They can be magnetized. They are attracted by a permanent magnet as well.
Permanent magnet and electromagnet
Unlike a solenoid, a permanent magnet does not require electrical energy to maintain a magnetic field. The electron spins in the permanent magnet were aligned in parallel and remain in this orientation thanks to the exchange interaction. The parallel alignment itself can be achieved, for example, by means of a magnetic field. Magnets can also arise naturally, for example by cooling molten ferromagnetic rock. Therefore, the name comes from the way: The ancient Greeks found magnetic stones at that time near the city Magnesia.
An electromagnet can also be effectively shut off by turning off the power. Furthermore, it is also possible to reposition him, in which simply the current direction is changed. However, a permanent magnet can not simply be switched off - which is why the designation also touches on this. Thus, only the supply of mechanical, thermal or magnetic energy leads to a demagnetization. In particular, in the first two cases, it is possible that the ground substance of the permanent magnet is damaged. Furthermore, the permanent magnet must be magnetized again after the magnetization. As already mentioned, a permanent magnet above the specific Curie temperature is completely demagnetized. It is therefore only logical that a permanent magnet has a maximum operating temperature.
How a ferromagnetic material becomes a permanent magnet
The process of magnetization itself follows a so-called hysteresis - this is called an asymmetric behavior of the material or the magnetization during the rise of an external magnetic field and the subsequent reduction of the magnetic field. The formation of the hysteresis is based on the exchange interaction, which stabilizes the orientation of the elementary magnets in the ferromagnetic material. A non-magnetized ferromagnet therefore has different magnetic properties than a magnetized ferromagnet. The magnetic field of the ferromagnet remaining after alignment of the electron spins after switching off the external magnetic field makes the ferromagnetic material a permanent magnet. Furthermore, this remaining magnetization is called remanence.
Often the strength of a magnetic field of a permanent magnet depends on the materials used. The magnetic field strength is also dependent on how and especially how exactly the material was magnetized. A large remanence can only be achieved if all atomic spins are fully aligned. This requires corresponding machines and technical know-how. Magnetic fields themselves only arise through a charge motion, as described by Maxwell's equations. They also show that a magnetic field always arises with a south pole and a north pole. The moving charges in the permanent magnet are the electrons of the individual atoms with their characteristic electron spin. This microscopic charge motion and the resulting motion state of the electrons results in a magnetic moment and in a magnetic force. These forces follow a magnetic field, which can be represented by field lines. Depending on the distance of the field lines to each other, the magnetic field is stronger or weaker. In addition, the field lines indicate the direction of the magnetic field: The field lines always show outside the magnetic field from the north to the south pole. However, they do not stop in the magnet itself, but go on there: in the magnet itself, field lines point from the south pole to the north pole.
The magnetic force that results from a permanent magnet depends mainly on the size, the energy of the exchange interaction, the completeness of the orientation and the size of the magnetic moments of the individual atoms. The energy product measures the magnetic energy of a magnet. Such magnetic energy is influenced by the just mentioned quantities and is also stored in a permanent magnet. The energy product, in turn, is an indicator of the quality of a magnet: the larger the energy product, the greater the quality, and the greater the magnetic energy of a permanent magnet.