Operating temperature

Permanent magnets have a maximum operating temperature - so they can not withstand any temperature. This is because high temperatures re-mix the elementary magnets that are aligned in parallel to the magnetic field. The magnetic field disappears at high temperatures therefore. For each magnet, this results in a maximum operating temperature specified by the manufacturer - usually in the form of a letter in the quality specification.

Materials with ferromagnetic properties are magnetized by an external magnetic field. The persistence of a permanent after removal of the external magnetic field keeps the material magnetic. It can act as a magnet by itself. At high temperatures, this remanence disappears, causing the magnetization to disappear. To avoid demagnetization, a certain operating temperature must not be exceeded. If this temperature is exceeded, it can very likely lead to demagnetization of the material or the magnet. This must then be magnetized again after it has cooled down.

Remanence physical background
To understand the effect of the magnetization of ferromagnetic matter, it is best to consider the physical background of remanence. Easily understood, remanence can be explained by the microscopic observation of an atom and its magnetization:

At each atom, there is a magnetic moment caused by the electron spin from an unplanned electron. This moment acts like a magnet with a small magnetic field. It has a north and a south pole. The spins of the unpaired electrons of the many atoms are rotated by the external magnetic field and its acting force on the magnetic moment. They align parallel to the external magnetic field. The so-called exchange interaction between the individual electron spins leads to a stabilization of this orientation after the orientation of the magnetic moments. This happens only in ferromagnetic materials or matter with ferromagnetic properties. The background of the exchange interaction is the low energy level in the parallel position of all electron spins. The interaction however has a limited strength, after all the electrons are mobile. The electron spins can be realigned by an external influence. It stands to reason that the aligned system of electron spins can be mixed again by a strong disturbance. All that needs to be done is to overcome the exchange interaction between the individual spins.

This can be done in three different ways:

  • An external magnetic field: When a magnetic field is applied that is opposite to the spins of the electrons, they can change their orientation. But the magnetic field has to be strong enough for that.
  • Mechanical shock: A sudden, strong force also makes it possible to demagnetize a magnet.
  • Thermal Energy: As already explained, the third way to remove the magnetization is to heat the ferromagnetic material. With the warming of the magnet, the temperature and thus the kinetic energy of each atom rise. This also increases the movement of the electron spins. Despite exchange interaction, one electron spin can leave the parallel alignment with the others. The probability of this effect increases with the temperature. Once the thermal energy exceeds the exchange interaction, all actually aligned electron spins will quickly and randomly rearrange. The temperature threshold for rapid conversion is called the Curie temperature - it indicates when a ferromagnet suddenly becomes a paramagnet. The remanence drops above this temperature to 0. Like the exchange interaction, the Curie temperature is material dependent: for iron it is 769 ° C, for nickel at 358 ° C and for cobalt at 1127 ° C.

Basically, the maximum operating temperature must not exceed the Curie temperature. However, it has also been stated that there is a likelihood that individual electron spins will rearrange even before reaching the Curie temperature. To avoid this, the specified operating temperature is usually slightly below the Curie temperature. However, the reason for this is not only the probability of demagnetization: with increasing temperatures, the probability of material distortions or instabilities in the material also increases. Therefore, the maximum operating temperature is chosen so that on the one hand no demagnetization of the magnet occurs, on the other hand, no cracks or other lattice defects occur.

The maximum operating temperature of a magnet is characterized in quality by a letter.

"50M" stands, for example, for an energy product with 50 MegaGaussOersted (for the 50) at a maximum operating temperature of 100 ° C (for the M).
"N" again stands for 80 ° C,
"H" for 120,
"SH" for 150 and
"UH" for 180 ° C.
"EH" even stands for 200 ° C.