Metallic ferromagnetism without exchange splitting

TL;DR

This paper explores a novel mechanism for metallic ferromagnetism where spin polarization leads to band broadening rather than energy band shifts, driven by kinetic energy gain, with implications for optical properties and real materials.

Contribution

It introduces a new class of models where ferromagnetism arises without exchange splitting, emphasizing band broadening and kinetic energy effects, derived from generalized tight binding models with Coulomb interactions.

Findings

Ferromagnetism driven by band broadening instead of energy shifts.

Spectral weight transfer in optical absorption occurs upon spin polarization.

Reentrant ferromagnetism observed in certain parameter regimes.

Abstract

In the band theory of ferromagnetism there is a relative shift in the position of majority and minority spin bands due to the self-consistent field due to opposite spin electrons. In the simplest realization, the Stoner model, the majority and minority spin bands are rigidly shifted with respect to each other. Here we consider models at the opposite extreme, where there is no overall shift of the energy bands. Instead, upon spin polarization one of the bands broadens relative to the other. Ferromagnetism is driven by the resulting gain in kinetic energy. A signature of this class of mechanisms is that a transfer of spectral weight in optical absorption from high to low frequencies occurs upon spin polarization. We show that such models arise from generalized tight binding models that include off-diagonal matrix elements of the Coulomb interaction. For certain parameter ranges it is also found that reentrant ferromagnetism occurs. We examine properties of these models at zero and finite temperatures, and discuss their possible relevance to real materials.