Zero-point spin fluctuations and the magnetovolume effect in itinerant-electron magnetism

Quantum dynamical effects of zero-point spin fluctuations (SF) are shown to give rise to the strong spin anharmonicity effects neglected in the conventional SF theory of weak itinerant-electron magnetism, which is based on the weak SF coupling constraint. A theory of weak itinerant magnets is presented generalizing the quantum Ginzburg-Landau (GL) approach to account for large zero-point SF and spin anharmonicity effects in a wide temperature range both below and far above the Curie temperature. The theory is based on a variational procedure for the free energy treated as a functional of the magnetic susceptibilities which are defined self-consistently via the free energy. The magnetic equation of state and magnetovolume effect are analyzed in terms of thermal and zero-point SF. The theory presents the microscopic grounds for the phenomenological GL approach where the zero-point SF effects are incorporated in the model parameters and establishes a new link between the SF theory and the first-principles band-structure calculations.