Interplay of orbital and spin ordering in the iron pnictides

TL;DR

This paper investigates how orbital ordering influences electronic anisotropy and magnetic properties in iron pnictides, combining theoretical approaches to explain experimental observations and predict new scaling behaviors.

Contribution

It demonstrates that orbital ordering alone can account for key experimental features and predicts a quadratic relationship between orbital polarization and magnetic moment.

Findings

Orbital ordering explains lattice distortion and magnetic moment correlation.

Orbital polarization scales as the square of the magnetic moment near T_N.

Orbital polarization induces in-plane anisotropic exchange interactions.

Abstract

A number of recent experiments exhibit electronic anisotropy in the iron pnictides, and there is a growing body of experimental evidence that its origin is related to orbital ordering in Fe d_{xz} and d_{yz} orbitals. We examine this problem in the parent compounds of the iron pnictides by a combination of ab initio band theory calculations, phenomenological Ginzburg-Landau theory of coupled orbital and magnetic order parameters, and a microscopic mean-field study of the Kugel-Khomskii model. We find that orbital ordering is sufficient to explain a number of key experimental observations, in particular a linear correlation between the orthorhombic lattice distortion and the magnetic ordered moment. We predict that orbital polarization should scale as a square of magnetic moment close to T_N. Mediated by orbital polarization, the effective spin-spin exchange interactions develop anisotropy in the ab-plane, in accord with recent neutron scattering measurements.