Oxygen vacancies mediated ferromagnetism with enhanced optical and dielectric properties in Gd³⁺ and Co²⁺/Mn²⁺ co-doped ZnO nanocrystals

Herein, diluted magnetic semiconductor (DMS) nanocrystals (NCs) of Gd_xTM_0.07Zn_0.93−xO (TM=Co, Mn) with Gd concentrations of 3 at% and 6 at% and a fixed TM concentration of 7 at% were synthesized using sol-gel technique, and their multifunctional properties were studied. X-ray diffraction (XRD) analysis confirmed a wurtzite hexagonal structure for all synthesized NCs with no detectable impurity peaks. Williamson–Hall (W–H) plots revealed a trend of decreasing crystallite size and increasing lattice strain with higher doping levels. Transmission electron microscopy (TEM) based morphological analysis showed irregularly shaped NCs with an average crystallite size of 32–40 nm. X-ray photoelectron spectroscopy (XPS) confirmed the 3 + oxidation state of Gd and 2 + states for Co/Mn, as well as the presence of oxygen defects. Ultraviolet–visible (UV–Vis) absorption spectroscopy indicated a reduction in the band gap for the doped samples. Photoluminescence (PL) spectra revealed the formation of defects and oxygen vacancies in the samples. Magnetic characterization demonstrated room-temperature ferromagnetic (RTFM) behavior in all doped NCs, which was further verified by electron paramagnetic resonance (EPR) spectroscopy. Combining with PL and XPS results, the observed ferromagnetism is attributed to interactions between dopant ions and trapped electrons, mediated by magnetic polarons. Dielectric measurements revealed an increase in both the dielectric constant and dielectric loss, suggesting enhanced dielectric polarization. The AC conductivity increased with frequency due to hopping charge carriers. These results show that oxygen vacancies created by Gd³⁺ and Co²⁺/Mn²⁺ co-doping in the host ZnO lattice have a significant impact on the multifunctional properties of ZnO nanocrystals.