Impact of Cr doping on the structure, optical and magnetic properties of nanocrystalline ZnO particles

The doping of Cr ions on the structure, optical and magnetic properties of ZnO have been investigated. Zn_1−xCr_xO (0 ≤ x ≤ 0.05) samples were prepared by the solution combustion method. Rietveld refined x-ray diffraction results reveal that all the samples are in a hexagonal structure. Transmission electron microscopy (TEM) revealed that the particle size was 48.2 ±2.1 nm, 29.4 ± 2.2 nm and 25.3 ± 2.1 nm for the Zn_1−xCr_xO with x = 0, 0.03 and 0.05, respectively. The band-gap decreases slightly from 3.308 ± 0.003 eV to 3.291 ± 0.003 eV with increase of Cr content from x = 0 to 0.05, respectively. The field-dependent magnetization M(μ_οH) measurements of ZnO and Cr-doped ZnO exhibit room temperature ferromagnetism (RTFM). Photoluminescence and electron paramagnetic resonance exhibits negatively charged zinc vacancies (V_Zn⁻) and singly ionized oxygen vacancies (V_o⁺) in ZnO and these defects are responsible for RTFM. And the V_Zn⁻ accountable for RTFM in Cr-doped ZnO. EPR results confirmed that Cr³⁺ ions replaces the Zn²⁺ sites in the hexagonal ZnO. The observed long-range ferromagnetism ordering is explained based on the bound magnetic polaron (BMP) mechanism. This work elucidates the origin of RTFM in synthesized ZnO and Cr-doped ZnO.