Structural, Optical, and Magnetic Studies of Nickel-Doped β-Ga2O3 Monoclinic and Spinel Polycrystalline Powders

β-Gallium oxide has well-studied electrical characteristics but relatively less explored optical as well as magnetic properties. In this work, pure and Ni-doped β-Ga₂O₃ polycrystalline powders were prepared using a hydrothermal method to study the structural, optical, and magnetic properties at various concentrations of Ni at 1 M%, 3 M%, 5 M%, and 7 M%. XRD analysis confirmed the formation of monoclinic β-Ga₂O₃ up to Ni 1 M% doping. The formation of additional peaks was observed exclusively for the samples doped with Ni from 3 M% to 7 M%. These additional peaks belong to NiGa₂O₄ that has an inverse spinel structure. The reflectance studies using UV–Vis diffuse reflectance spectroscopy shows a reduction in bandgap from approximately 4.7 eV to 4.1 eV with the addition of the dopant. The emission peaks observed from photoluminescence studies shows UV, blue, and green emissions with varying intensity. Room-temperature magnetic studies performed using a vibrating sample magnetometer showed a transition from the diamagnetic state of the pure sample to the antiferromagnetic state with increasing Ni concentration in the doped samples. The diamagnetic properties of β-Ga₂O₃ makes it ineffective in spintronic applications. From the present work, the improved magnetism due to Ni doping coupled with the optical properties suggests that nickel-doped gallium oxide can be used as an optical magnetic bifunctional material.