Stoner model of ferromagnetism
Ferromagnetism ultimately stems from electron-electron repulsion. The simplified model of a solid which is nowadays usually called the Stoner model, can be formulated in terms of dispersion relations for spin up and spin down electrons,
where the second term accounts for the exchange energy, is the Stoner parameter, () is the dimensionless density of spin up (down) electrons and is the dispersion relation of spinless electrons where the electron-electron interaction is disregarded. If is fixed, can be used to calculate the total energy of the system as a function of its polarization . If the lowest total energy is found for , the system prefers to remain paramagnetic but for larger values of , polarized ground states occur. It can be shown that for
A non-zero state may be favoured over even before the Stoner criterion is fulfilled.
Relationship to the Hubbard model
The Stoner model can be obtained from the Hubbard model by applying the mean-field approximation. The particle density operators are written as their mean value plus fluctuation and the product of spin-up and spin-down fluctuations is neglected. We obtain
With the third term included, which was omitted in the definition above, we arrive at the better-known form of the Stoner criterion
- Having a lattice model in mind, is the number of lattice sites and is the number of spin-up electrons in the whole system. The density of states has the units of inverse energy. On a finite lattice, is replaced by discrete levels and then .
- Stephen Blundell, Magnetism in Condensed Matter (Oxford Master Series in Physics).
- Teodorescu, C. M.; Lungu, G. A. (November 2008). "Band ferromagnetism in systems of variable dimensionality". Journal of Optoelectronics and Advanced Materials. 10 (11): 3058–3068. Retrieved 24 May 2014.