  Rare-earth nitrides, such as gadolinium nitride (GdN), have great potential
for spintronic devices due to their unique magnetic and electronic properties.
GdN has a large magnetic moment, low coercitivity and strong spin polarization
suitable for spin transistors, magnetic memories and spin-based quantum
computing devices. Its large spin splitting of the optical bandgap functions as
a spin-filter that offers the means for spin-polarized current injection into
metals, superconductors, topological insulators, 2D layers and other novel
materials. As spintronics devices require thin films, a successful
implementation of GdN demands a detailed investigation of the optical and
magnetic properties in very thin films. With this objective, we investigate the
dependence of the direct and indirect optical bandgaps (Eg) of half-metallic
GdN, using the trilayer structure AlN(10 nm)/GdN(t)/AlN(10 nm) for GdN film
thickness t in the ranging from 6 nm to 350 nm, in both paramagnetic (PM) and
ferromagnetic (FM) phases. Our results show a bandgap of 1.6 eV in the PM
state, while in the FM state the bandgap splits for the majority (0.8 eV) and
minority (1.2 eV) spin states. As the GdN film becomes thinner the spin split
magnitude increases by 60%, going from 0.290 eV to 0.460 eV. Our results point
to methods for engineering GdN films for spintronic devices.
