Ferromagnetism, spiral magnetic structures and phase separation in the two-dimensional Hubbard model

TL;DR

This paper investigates how electronic dispersion and correlations influence ferromagnetism in the two-dimensional Hubbard model, highlighting the importance of next-nearest-neighbor hopping t' in shaping magnetic phases.

Contribution

It introduces a detailed analysis of ferromagnetism formation considering next-nearest-neighbor hopping and electron correlations in 2D Hubbard models, emphasizing the role of Fermi surface curvature.

Findings

Next-nearest-neighbor hopping t' critically affects ferromagnetism.

Ferromagnetic phase is asymmetric around half-filling.

Fermi surface curvature near van Hove points stabilizes ferromagnetism.

Abstract

The quasistatic approximation and equation-of-motion decoupling for the electron Green's functions are applied to trace the effect of electronic dispersion and electron correlations on the ferromagnetism of two-dimensional itinerant-electron systems. It is found that next-nearest-neighbor hopping t' is of crucial importance for ferromagnetism formation yielding the magnetic phase diagram which is strongly asymmetric with respect to half-filling. At small t' in the vicinity of half-filling the ferromagnetic phase region is restricted by the spin-density wave instability, and far from half-filling by one-particle (spin-polaron) instability. At t' close to t/2 ferromagnetism is stabilized at moderate Hubbard U due to substantial curvature of the Fermi surface which passes in the vicinity of the van Hove singularity points. The results obtained are of possible importance for high-T_c compounds and layered ruthenates.