Maximum operating temperature

Ferromagnetic materials are magnetised in an external magnetic field. When the external field is switched off, a remanence remains. Thus, the material stays magnetic and can itself be used as a magnet. However, the remanence decreases at high temperatures. The magnetisation disappears. Each magnet has a maximum operating temperature that is set to prevent demagnetisation. Above this temperature, which depends on the material of the magnet, demagnetisation can occur. The material must then be cooled and re-magnetised.

Physics background

To understand this effect, it is necessary to consider the physical background of remanence. Remanence can be easily understood by looking at magnetisation on a microscopic level of individual atoms.

In ferromagnetic materials, there is a magnetic moment on each atom, which is caused by the electron spin of an unpaired electron. This magnetic moment itself acts like a small magnet with a north and a south pole. An external magnetic field now exerts a force on the magnetic moments of all electron spins, which rotates the spins and aligns them parallel to the external magnetic field. Once the magnetic moments are aligned, this alignment is stabilised in ferromagnetic materials by the exchange interaction between the electron spins.

The exchange interaction means that it is energetically more favourable if all electron spins are parallel. However, this interaction only has a limited strength. The electrons themselves are mobile, and the alignment of the electron spins can be changed by external influence.

Demagnetisation of magnets

Just as all electron spins were aligned by an external magnetic field, it is conceivable to mix the electron spins again if the system is disturbed to such an extent that the exchange interaction between the electron spins is overcome. This can be achieved by an external magnetic field that is directed in the opposite direction to the magnetic field of the electron spins. The material can also be demagnetised by hard mechanical impacts because the alignment of the electron spins is then mechanically disturbed.

A third way to eliminate remanence is to heat the magnetised material. The temperature of a solid is determined by the kinetic energy of the individual atoms. Heating a ferromagnet also increases the movement of the electron spins. As the kinetic energy (thermal energy) increases, the probability of an electron spin twisting out of its parallel alignment increases despite the exchange interaction. If the thermal energy is greater than the exchange interaction, a rapid disorganisation of the originally aligned electron spins occurs.

Maximum operating temperature of magnets: Not to exceed the Curie temperature

The temperature at which a ferromagnet transforms into a paramagnet is the so-called Curie temperature. Above the Curie temperature, a magnetised material is completely demagnetised. The remanence therefore drops to zero. As the exchange interaction is characteristic of each material, different materials also have different Curie temperatures. It is 769 °C for iron, 1127 °C for cobalt and 358 °C for nickel.

The maximum operating temperature, therefore, cannot exceed the Curie temperature of the material. To avoid even partial demagnetisation, the maximum operating temperature is usually well below the Curie temperature. The disorder of the electron spins increases steadily with temperature. Material distortions or general material instabilities can also occur at temperatures far below the Curie temperature. The maximum operating temperature is therefore set in such a way that demagnetisation of the magnet due to heat will definitely not occur below the maximum operating temperature. The manufacturer designates the maximum operating temperature of its magnets with a letter in the grade. A grade of 50M, for example, identifies a magnet with an energy product of 50 megagauss-oersted and a maximum operating temperature of 100 °C (M). Other identification codes are 'N' for 80 °C, 'H' for 120 °C, 'SH' for 150 °C, 'UH' for 180 °C and 'EH' for 200 °C.

High temperature magnets

Our assortment includes the following neodymium magnets that are suitable for higher temperatures:

We also carry various shapes of ferrite raw magnets, all of which can be used at temperatures of up to 250 °C:

Other ferrite magnets