Effects of Crystal Structure and the On-Site Coulomb Interactions on the Electronic and Magnetic Structure of Pyrochlores $A_2$Mo$_2$O$_7$ (A= Y, Gd, and Nd)

TL;DR

This study investigates how crystal structure and Coulomb interactions influence the electronic and magnetic properties of Mo pyrochlores, revealing their roles in metal-insulator transitions, magnetic ordering, and Hall effects.

Contribution

It introduces a comprehensive mean-field approach combining electronic structure, spin-orbit coupling, and Coulomb interactions to explain experimental phenomena in Mo pyrochlores.

Findings

Coulomb U controls metal-insulator transition.

Nearly collinear ferromagnetism observed in metallic phase.

Large anomalous Hall effect predicted near transition.

Abstract

Being motivated by recent experimental studies, we investigate magnetic structures of the Mo pyrochlores $A_2$Mo$_2$O$_7$ ($A$= Y, Nd, and Gd) and their impact on the electronic properties. The latter are closely related with the behavior of twelve Mo($t_{2g}$) bands, located near the Fermi level and well separated from the rest of the spectrum. We use a mean-field Hartree-Fock approach, which combines fine details of the electronic structure for these bands, extracted from the conventional calculations in the local-density approximation, the spin-orbit interaction, and the on-site Coulomb interactions amongst the Mo(4d) electrons, treated in the most general rotationally invariant form. The Coulomb repulsion U plays a very important role in the problem, and the semi-empirical value U$\sim$1.5-2.5 eV accounts simultaneously for the metal-insulator (M-I) transition, the ferromagnetic (FM) - spin-glass (SG) transition, and for the observed enhancement of the anomalous Hall effect (AHE). The M-I transition is mainly controlled by $U$. The magnetic structure at the metallic side is nearly collinear FM, due to the double exchange mechanism. The transition into the insulating state is accompanied by the large canting of spin and orbital magnetic moments. The sign of exchange interactions in the insulating state is controlled by the Mo-Mo distances. Smaller distances favor the antiferromagnetic coupling, which preludes the SG behavior in the frustrated pyrochlore lattice. Large AHE is expected in the nearly collinear FM state, near the point of M-I transition, and is related with the unquenched orbital magnetization at the Mo sites. We also predict large magneto-optical effect in the same FM compounds.