Ferromagnetic redshift of the optical gap in GdN

TL;DR

This study measures the optical gap in GdN across temperatures, revealing a ferromagnetic redshift and refining theoretical band structure models to match experimental data.

Contribution

It provides new experimental measurements of the GdN optical gap in both phases and refines theoretical models to accurately describe the ferromagnetic redshift.

Findings

Optical gap is 1.31 eV in paramagnetic phase.

Gap red-shifts to 0.9 eV below Curie temperature.

Refined band structure matches experimental gaps in both phases.

Abstract

We report measurements of the optical gap in a GdN film at temperatures from 300 to 6K, covering both the paramagnetic and ferromagnetic phases. The gap is 1.31eV in the paramagnetic phase and red-shifts to 0.9eV in the spin-split bands below the Curie temperature. The paramagnetic gap is larger than was suggested by very early experiments, and has permitted us to refine a (LSDA+U)-computed band structure. The band structure was computed in the full translation symmetry of the ferromagnetic ground state, assigning the paramagnetic-state gap as the average of the majority- and minority-spin gaps in the ferromagnetic state. That procedure has been further tested by a band structure in a 32-atom supercell with randomly-oriented spins. After fitting only the paramagnetic gap the refined band structure then reproduces our measured gaps in both phases by direct transitions at the X point.