Ferromagnetism and dielectric properties in Zn_0.95−xNd_xTM_0.05O (TM=Co, Fe) nanocrystals: Collective role of grain boundaries and oxygen vacancies

Herein, Zn_0.95−xNd_xTM_0.05O (TM = Co, Fe) (x = 2%, 6%) nanocrystals with spherical shape, wurtzite-like hexagonal structure, and average crystallite size of 35 nm were prepared by sol-gel method, those exhibit ferromagnetic (FM) behavior at room temperature. X-ray photoelectron spectroscopy (XPS) results confirmed the successful incorporation of dopants ions such as Nd³⁺ and Co²⁺/Fe²⁺ into the host ZnO lattice. Size-strain analysis based on the Williamson-Hall plot was performed to evaluate the crystallite size and lattice strain. High-resolution transmission electron microscope characterization shows that the fine nanocrystals are separated by grain boundaries (GBs). The calculated specific grain boundary area (S_GB) of 60 × 10⁶m²m^-3 in the co-doped samples is above the threshold value of 5.3 × 10⁷ m²m^-3. The UV–Visible analysis shows that the energy band gap decreases with increasing Nd³⁺ concentration. The photoluminescence spectra (PL) show a broad emission band in the visible region corresponding to the various structural defects and oxygen vacancies in the prepared nanocrystals responsible for the activation of bound magnetic polarons (BMPs). Fitting the experimental magnetization data with the BMP model based on the Langevin function shows that the density of magnetic polarons (10²⁰cm⁻³) in the doped samples is equal to the typical percolation threshold. The dielectric constant and dielectric loss was observed to increase at lower frequency due to the presence of GBs and oxygen vacancies, and this behavior was explained based on Koop's phenomenological theory. Based on these results, it was demonstrated that the FM response and the dielectric properties in studied samples are strongly associated with GBs and oxygen vacancies.