Spin Wave Instability of Itinerant Ferromagnet

TL;DR

This paper investigates the conditions under which the ferromagnetic state becomes unstable in the Hubbard and double exchange models, linking spin-wave softening to metal-insulator transitions.

Contribution

It provides a variational analysis showing spin-wave instability as a key factor in ferromagnetic breakdown, highlighting differences across doping and coupling regimes.

Findings

Spin-wave softening signals ferromagnetic instability in the Hubbard model.

In over-doped strong-coupling regimes, particle excitations become unstable first.

Spin-wave instability is identified as a precursor to metal-insulator transition.

Abstract

We show variationally that instability of the ferromagnetic state in the Hubbard model is largely controlled by softening of a long-wavelength spin-wave excitation, except in the over-doped strong-coupling region where the individual-particle excitation becomes unstable first. A similar conclusion is drawn also for the double exchange ferromagnet. Generally the spin-wave instability may be regarded as a precursor of the metal-insulator transition.