Zn–Fe–oxide nanostructures of different iron concentrations for multifunctional applications: properties and precursor influence

Abstract

Zn–Fe–O nanoparticle systems (Z3F, Z20F and Z60F) were produced by changing the Zn:Fe ratio (0.97 : 0.03, 0.8 : 0.2 and 0.4 : 0.6 in at%, respectively) in Zn(II)–Fe(III)–carboxylate precursors. According to X-ray diffraction, Z60F is nearly single-phase ZnFe₂O₄ (5.9 nm crystallite size), Z20F is a ZnO/ZnFe₂O₄ nanocomposite consisting of 48.8% ZnFe₂O₄ (4.7 nm crystallite size), and Z3F is apparently pure ZnO (9.5 nm). We found evidence for a ZnFe₂O₄ spinel of high inversion degree (80–100%) and with superparamagnetic (SPM) behaviour at room temperature in all three samples by a remarkable correlation between HRTEM, FTIR, XPS, Mössbauer and magnetization analyses. Iron modifies the decomposition process of the precursor and enhances its viscosity, which appears to favour the separation of Zn- and Fe-rich phases. As a consequence, two-phase systems of individual nanocrystals/nanoparticles (ZnO and ZnFe₂O₄) are formed. The large anisotropy constant, 10⁶–10⁷ erg cm⁻³, of the ZnFe₂O₄ nanoparticles and the concentration dependence of their magnetic energy barrier are explained in terms of interparticle interactions interlinked with finite size effects and high inversion degree; these factors also control the other parameters of importance for applications, including the blocking temperature (13–111 K), saturation magnetization (1.08–17.7 emu g⁻¹ at 300 K, 4.6–44.8 emu g⁻¹ at 5 K) and coercivity (85.4–491 Oe at 5 K). Magnetic dynamic results, particularly modelled by the Néel-Brown and Vogel–Fulcher laws, yield fitting parameters which validate the presence of concentration-dependent dipole-like interactions between ZnFe₂O₄ nanoparticles. A fraction of iron was found in the Fe²⁺ state, presumably substituting for Zn²⁺ in zinc oxide; however, the samples behave like ZnFe₂O₄ SPM nanoclusters/nanoparticles dispersed in a nonmagnetic ZnO particle assembly, rather than Zn(Fe)O dilute magnetic semiconductors. The relevance of the properties of the investigated material for specific applications is highlighted throughout the manuscript.