Curie temperature

The Curie temperature is the temperature above which ferromagnetic materials no longer possess a permanent magnetic field. This happens because the atoms vibrate so intensely that there is no longer any global orientation. Above the Curie temperature the material behaves paramagnetically.

As the temperature rises, the molecular excitement gradually disrupts the spin alignment. When the Curie temperature is reached, the alignment collapses because the thermal energy exceeds the energy of the magnetic interaction.

It is difficult to measure the Curie temperature exactly. For one thing, the permanent magnetic field around the material only gradually disappears. Secondly, the Curie temperature varies greatly based on even small quantities of contaminants in the material.

For example, if we heat an AlNiCo magnet above its Curie temperature of 850°C, it will no longer be ferromagnetic. It then becomes paramagnetic. Even after the magnet cools down, the permanent magnetic field does not return. There will, however, be new magnetic fields present in small areas within the material, the so-called Weiss domains (Weiss 1865-1904), but these fields are aligned in random directions so their vector sum does not result in an external magnetic field. Nevertheless, it is possible to remagnetize the magnet.

The ferromagnetic elements and alloys with their Curie temperatures:

Material   Curie temp.

Fe            770°C
Co           1115°C
Ni             354°C
Gd            19°C
AlNiCo      850°C
Ferrite       450°C
Sm Cobalt 750-825°C
Nd-Fe-B    310-340°C

Pierre Curie (1859-1906)