Magnetic Order Beyond RKKY in the Classical Kondo Lattice

TL;DR

This paper investigates the classical Kondo lattice model in three dimensions, revealing how intermediate coupling regimes lead to magnetic orders that differ from traditional RKKY predictions, emphasizing the importance of electronic structure and coupling effects.

Contribution

It demonstrates the limitations of RKKY theory in the intermediate coupling regime and highlights the role of electronic structure and coupling effects in determining magnetic order.

Findings

Phase boundaries deviate from RKKY predictions at moderate coupling.

Non-interacting Fermi surface no longer determines magnetic order.

Weak coupling spiral phases are replaced by collinear order.

Abstract

We study the Kondo lattice model of band electrons coupled to classical spins, in three dimensions, using a combination of variational calculation and Monte Carlo. We use the weak coupling `RKKY' window and the strong coupling regime as benchmarks, but focus on the physically relevant intermediate coupling regime. Even for modest electron-spin coupling the phase boundaries move away from the RKKY results, the non interacting Fermi surface no longer dictates magnetic order, and weak coupling `spiral' phases give way to collinear order. We use these results to revisit the classic problem of 4f magnetism and demonstrate how both electronic structure and coupling effects beyond RKKY control the magnetism in these materials.