Fe, Co and Ni are materials associated with ferromagnetism, together with their alloys. However, there are also many other materials showing a ferromagnetic behaviour. A ferromagnetic substance shows a permanent (i.e. also without outer fields) spontaneous magnetizing, being greatest at 0K. It is reduced by temperature, being zero at its curie-temperature. The material is showing a paramagnetic behaviour above this temperature. It is a secondary phase transition, which can be seen by the discontinuity of the heat capacity, i.e. at Fe. An attempt to explain why these materials are ferromagnetic will now follow.

Band magnetism can be considered qualitatively. If taking Ni as an example, it has a partly filled d-band. This band is divided in two sub-bands which are spin-up and spin-down occupated respectively (see fig. to the left) These bands have different energy, i.e. one band is filled while the other is only partly filled. Measurements show that Ni has 0.6 bohr magnetons per atom. As the orbital moment is locked as above, it means there are 0.6 non-compensated electrospin on average, which contribute to the total magnetic moment. The valence electrons form an electronic gas which is distributed on the different bands and this explains why it is possible to have magnetical moments even if they are not integral numbers. The result will be a decimal value of non-compensated electronic spin per atom.

The reason why these sub-bands would be separated in energy, is that the reduction of the coulumb repulsion is greater than the increase of the kinetic energy the atom receives from the change of electrons with spin-down to spin-up. This spin energy alteration is small if the band is narrow, i.e. has high state density at the fermi level. This is called band magnetism or collective electron ferromagnetism and arise at reciprocal action between the atoms within a crystal. The entire crystal becomes magnetized. The magnetizing disappears at temperatures above the Curie-temperature and a paramagnetic behaviour is obtained. An assumption can be made that the band structure is destroyed.

The rare earths have a complicated behaviour, though Gd for instance is ferromagnetic. The magnetic moment is for Gd emanates from the 4f-shell, which is strongly localized. Reciprocal action between the atom intermediates through the conduction band which is polarized magnetically, resulting in an extra magnetization per atom. The reciprocal action is called RKKY(Rudeman, Kittel, Kasuya, Yosida).

Regions in the ferromagnetic material, with the same direction of the magnetization, are called domains. All the domains have different directions, and in total, the magnetization become zero, or close to zero. Only a weak magnetic field is needed to direct the domains, in order for the material to show its whole spontanious magnetization.

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