Defect structures and (ferro)magnetism in Zn1-xFexO nanoparticles with the iron concentration level in the dilute regime (x = 0.001 – 0.01) prepared from acetate precursors

• Published in: Results in physics 2023-08, Vol.51, p.106644, Article 106644
• Main Authors: Mihalache, V., Negrila, C., Secu, M., Mercioniu, I., Iacob, N., Kuncser, V.
• Format: Article
• Language: English
• Subjects: Coupling between Fe ions in defective sites; Dilute Fe-doping: x = 0.001–––0.01; Dilute magnetic semiconductors; RTFM; SPM inverse spinel ZnFe2O4; Zn1-xFexO nanostructures
• ISSN: 2211-3797; 2211-3797
• DOI: 10.1016/j.rinp.2023.106644

Superexchange coupling (resembling that in the inverse spinel ZnFe2O4) between Fe3+ in defective sites – a hypothesised mechanism for RTFM in 0.1 – 1 at.% Fe-doped ZnO. [Display omitted] •Dilute Fe-doped Zn1-xFexO (x = 0.001–0.01) nanoparticles prepared from acetate precursors.•RTFM for x ≤ 0.01. AFM behavior of M vs T and increasing FM contribution with Fe doping.•XPS, 57Fe Mossbauer, Photoluminescence – Fe3+ in distorted surface sites; intrinsic defects.•SPM (ultra)fine ZnFe2O4 on the surface of ZnO - the only impurity phase for x ≥ 0.03.•Fe coupled with intrinsic defects. Superexchange coupling (resembling that in inverse spinel ZnFe2O4) between Fe3+ at the defective sites - a suggested mechanism of RTFM for x ≤ 0.01. Zn1-xFexO nanoparticles with the iron concentrations level in the dilute regime (x = 0.001–––0.01) were produced by a sol–gel route from acetate precursors along with an un-doped and 3 at.% Fe-doped reference. The X-ray diffraction of the un-doped and 0.1–1 at.% Fe-doped samples reveal the reflections for only the ZnO wurtzite structure. Fe doping enhances the a-axis lattice constant, the unit cell volume and the microstrain. Iron doping reduces the average crystallite/particle size (confirmed by Scanning Electron Microscopy), improving the surface-to-volume ratio or the concentration of defective surface sites. XPS identifies the iron in both Fe3+ and Fe2+ states. XPS and 57Fe Mossbauer spectroscopy indicate a broad distribution (distortion) of Fe3+ sites on the surface of ZnO nanoparticles. The blue shift and broadening of the UV emission, and quenching of defect-related photoluminescence in the Fe-doped samples verify the presence of iron in the ZnO lattice and surface intrinsic defects. 0.1–1 at.% Fe-doped ZnO show room temperature ferromagnetism, RTFM, characteristic of dilute magnetic semiconductors, DMS. The magnetization measurements with temperature evidence an antiferromagnetic alignment and an increase of ferromagnetic contribution with Fe doping up to 1 at.%. Zn0.97Fe0.3O reference is a superparamagnetic ZnO/ZnFe2O4 nanocomposite with a blocking temperature of 20 K; HRTEM shows (ultra)fine ZnFe2O4 particles at the surface of ZnO nanoparticles. The analysis of experimental data of 0.1–1 at.% Fe-doped ZnO was done in terms of iron coupling with intrinsic defects, which can generate surface Fe3+ states with geometries similar to the Fe3+ in inverse spinel ZnFe2O4. The superexchange interaction (resembling that in the inverse spine