Itinerant ferromagnetism
= Itinerant ferromagnetism
{title2=$\epsilon_{k\sigma}=\epsilon_k-\sigma M$}
Ferromagnetic spin polarization carried by mobile electrons rather than a separate fixed-spin lattice. A homogeneous spin field splits the electron bands and competes with a positive field cost. Mean-field instability is governed by the <Stoner criterion>; the ordered metal supports transverse spin waves as well as fermionic particle-hole excitations.