Quantum spin fluctuation theory of the magnetic equation of state of weak itinerant-electron ferromagnets

Abstract

On the basis of the spin fluctuation mechanism, magnetic properties of itinerant-electron weak ferromagnets are discussed, for the wide temperature range from the ground state to the paramagnetic state, explicitly taking into account the effects of zero-point quantum spin fluctuations. Particular attention is focused on properties of the ordered phase. The temperature dependence of the spontaneous magnetic moment, for instance, is quantitatively analysed in close comparison with experiments. It is also shown that the fourth-order expansion coefficient of the free energy in powers of the static magnetic moment is temperature dependent, and therefore magnetic isotherms are not so simple as was originally anticipated in the Stoner–Wohlfarth theory.