Insulating Ferromagnetism in La$_{4}$Ba$_{2}$Cu$_{2}$O$_{10}$: an \textit{Ab Initio} Wannier Function Analysis

TL;DR

This study uses  ab initio Wannier function analysis to reveal that intersite direct exchange, not superexchange, drives the insulating ferromagnetism in LaBaCuO
0, predicting a pressure-induced magnetic transition.

Contribution

It uncovers the dominant intersite direct exchange mechanism behind ferromagnetism in LaBaCuO
0 using energy-resolved Wannier states, a novel insight in this context.

Findings

Intersite direct exchange dominates magnetic coupling.

Pressure can induce a transition from ferromagnetic to antiferromagnetic.

In-plane magnetic order is not determined by nearest-neighbor interactions.

Abstract

Microscopic mechanisms of the puzzling insulating ferromagnetism of half-filled La$_{4}$Ba$_{2}$Cu$_{2}$O$_{10}$ are elucidated with energy-resolved Wannier states. The dominant magnetic coupling, revealed through evaluated parameters ($t$, $U$, and $J$), turns out to be the intersite direct exchange, a currently ignored mechanism that overwhelms the antiferromagnetic superexchange. By contrast, the isostructural Nd$_{4}$Ba$_{2}$Cu$_{2}$O$_{10}$ develops the observed antiferromagnetic order via its characteristics of a 1D chain. Surprisingly, the in-plane order of both cases is \textit{not} controlled by coupling between nearest neighbors. An intriguing pressure-induced ferromagnetic to antiferromagnetic transition is predicted.