We characterize optical transitions of neodymium ions $\left({\mathrm{Nd}}^{3+}\right)$ in antiferromagnetic neodymium gallate $\left({\mathrm{NdGaO}}_3\right)$ with applied fields up to 3 T. The magnetic phase of this material has not previously been studied with the field along its magnetization axis. The measured optical spectra indicate three magnetic phases—antiferromagnetic, intermediate, and saturated—where the intermediate phase likely forms a different magnetic structure from typical spin-flop phases. The observed absorptions were classified into two distinct families of optical transitions: single-Nd and two-Nd absorptions. We demonstrate that the optical transitions in the antiferromagnetic and saturated phases can be modeled using a standard single-ion crystal-field Hamiltonian that interacts with a mean magnetization from the rest of the lattice, and we expand that model to encompass pairs of ions, explaining the origins of the two-Nd transitions. This study offers a deeper understanding of the optical transitions in rare-earth antiferromagnetic crystals, which have been recently attracting significant interest for microwave-to-optical quantum transduction, despite being relatively unexplored to date.

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• Electronic excitation & ionization
• Electronic structure of atoms & molecules
• Lineshapes & shifts
• Magneto-optical spectra
• Zeeman effect