Insights into the structural and microscopic origin of magnetic and optical properties of monodispersed γ-Fe2O3, ZnO, γ-Fe2−2xZn2xO3 and γ-Fe2O3@ZnO nanostructures†

Abstract

Monodispersed γ-Fe₂O₃, ZnO, γ-Fe_2−2xZn_2xO₃ (substitution, SUB) and γ-Fe₂O₃@ZnO (core@shell, CS) nanoparticles were synthesized via a thermolysis method, and their changes in optical and magnetic behaviour were investigated. γ-Fe₂O₃, γ-Fe_2−2xZn_2xO₃ and γ-Fe₂O₃@ZnO nanoparticles were crystallized in cubic structure, whereas ZnO was found to be amorphous. γ-Fe₂O₃ nanoparticles were spherical, whereas SUB and CS nanoparticles were nanorods. In the magnetization versus applied magnetic field (M–H curve) study, γ-Fe₂O₃ nanoparticles were superparamagnetic at room temperature and ferromagnetic at 5 K. At room temperature, SUB nanoparticles showed paramagnetic behaviour at an applied magnetic field below 2500 Oe, and above 2500 Oe, these nanoparticles exhibited diamagnetic behaviour, which can be attributed to the contribution of ZnFe₂O₄ and ZnO. At 5 K, SUB nanoparticles exhibited paramagnetic behaviour. CS nanoparticles showed paramagnetic/diamagnetic behaviour at room temperature but showed weak ferromagnetic behaviour at 5 K. This can be represented as small size core γ-Fe₂O₃@interface ZnFe₂O₄@shell ZnO. ZnO nanoparticles were diamagnetic at room temperature, but it became paramagnetic at 5 K. At zero-field cooled and field-cooled curves, γ-Fe₂O₃ nanoparticles showed a blocking temperature (T_B) at 210 K. ZnO, SUB and CS nanoparticles showed a peak at 50–60 K in the zero-field cooled curve, which suggested the presence of antiferromagnetic interactions in these materials. In the photoluminescence study, luminescence intensity was highest in ZnO, followed by CS, SUB and γ-Fe₂O₃. In terms of the proximity effect, SUB could be considered as ZnFe₂O₄/ZnO/FeO (mixture), whereas CS could be considered as γ-Fe₂O₃@ZnFe₂O₄@ZnO (core@interface@shell). The decrease in photoluminescence intensity was owing to the presence of magnetic impurities, which was supported by the decay lifetime study. The relations between crystal structure, microstructures, magnetic and optical properties are studied in this work.