# Ferromagnetisme

##### Article

August 13, 2022

Ferromagnetism occurs in materials that contain unpaired spins between which there is an interaction that causes the atomic magnetic moments to align parallel to each other. This leads to spontaneous and permanent magnetic fields around an object made of a ferromagnetic material. Although there are usually interactions in a material that want to set the spins in the same direction and interactions that set the spins in the opposite direction, so the first forces in a ferromagnet dominate (otherwise antiferromagnetism arises). In principle, all spins in a ferromagnet can move in the same direction - in that case the object reaches its magnetic saturation and has a large spontaneous magnetic field. However, it is also possible that the ordering of the spins takes place in smaller domains, the so-called Weiss areas. If the magnetization direction of the domains is arbitrary, the object's total field is zero, although there is magnetic ordering. By exposure to a strong external field, all domains can be pulled in the same direction (magnetized). As the temperature increases, the temperature movement gradually breaks the spin order. At a certain temperature, the Curie temperature ( t c ) {\textstyle (T_{c})} , the order collapses because the thermal energy has become greater than the energy of the magnetic interaction. above t c {\textstyle T_{c}} the material behaves paramagnetically, the reciprocal susceptibility plotted against the absolute temperature then forms the characteristic straight line of a paramagnet. However, the line continues t t c {\textstyle TT_{c}} instead of by t {\textstyle T} 0 K because the interaction between the spins continues to exist, even though the thermal energy prevents the ordering.

## Scientific description

The magnetic permeability 0 ( 1 + ) {\textstyle \mu \mu _{0}(1+\chi )} and thus also the magnetic susceptibility {\textstyle \chi } is not constant in ferromagnets, but a nonlinear function of applied field strength huh {\textstyle H} and the history of magnetization. Therefore, usually the (differential) magnetic susceptibility {\textstyle \chi } considered as a derivative of the magnetization from the field strength. The magnetization becomes zero in the saturation region. The relationship between magnetization m → {\displaystyle {\vec {M}}} and magnetic flux density B → {\displaystyle {\vec {B}}} is: B → 0 ( huh → + m →