The Stoner Parameter I and Spin-Polarization

If we allow for these terms, the energy of the system is to be written in a more complex form E = E(n, ), where we have chosen as a parameter = (n+ - n )/n, i.e. the relative magnetization. We make two observations  

I) is the effect of spin-polarization. Wohlfarth had already shown very early that AE(n, ) may be expressed as an expansion in 

The Stoner model retains only the first term, proportional to (molecular field approximation which leads to terms such as (26) in the Hamiltonian). 

The splitting of the band energy E° into two sub-bands, for spin-up and spin-down 

A permanent magnetic moment and, possibly, collective magnetic phenomena. 

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