Abstract
In this work, we present the role of vanadium ions (V+5 and V+3), oxygen vacancies (VO), and interstitial zinc (Zni) to the contribution of specific magnetization for a mixture of ZnO-V2O5 nanoparticles (NPs). Samples were obtained by mechanical milling of dry powders and ethanol-assisted milling for 1 h with a fixed atomic ratio V/Zn?=?5% at. For comparison, pure ZnO samples were also prepared. All samples exhibit a room temperature magnetization ranging from 1.18?×?10−3 to 3.5?×?10−3 emu/gr. Pure ZnO powders (1.34?×?10−3 emu/gr) milled with ethanol exhibit slight increase in magnetization attributed to formation of Zni, while dry milled ZnO powders exhibit a decrease of magnetization due to a reduction of VO concentration. For the ZnO-V2O5 system, dry milled and thermally treated samples under reducing atmosphere exhibit a large paramagnetic component associated to the formation of V2O3 and secondary phases containing V+3 ions; at the same time, an increase of VO is observed with an abrupt fall of magnetization to σ?~?0.7?×?10−3 emu/gr due to segregation of V oxides and formation of secondary phases. As mechanical milling is an aggressive synthesis method, high disorder is induced at the surface of the ZnO NPs, including VO and Zni depending on the chemical environment. Thermal treatment restores partially structural order at the surface of the NPs, thus reducing the amount of Zni at the same time that V2O5 NPs segregate reducing the direct contact with the surface of ZnO NPs. Additional samples were milled for longer time up to 24 h to study the effect of milling on the magnetization; 1-h milled samples have the highest magnetizations. Structural characterization was carried out using X-ray diffraction and transmission electron microscopy. Identification of VO and Zni was carried out with Raman spectra, and energy-dispersive X-ray spectroscopy was used to verify that V did not diffuse into ZnO NPs as well to quantify O/Zn ratios.
Highlights
The complex mechanisms that allow ferromagnetic order at room temperature in diluted magnetic oxides (DMO) are a controversial subject as magnetic behavior is strongly dependent on the synthesis method and it is very difficult to obtain reproducible homogeneity on samples
We report results concerning the structural and magnetic behavior of pure ZnO NPs milled under different conditions, and on the second part, we present a complete analysis of ZnO-V2O5 NPs, getting a clear conclusion about the role of each structural defect
It has been proposed that complexes involving N impurities could be stable at room temperature [17]
Summary
The complex mechanisms that allow ferromagnetic order at room temperature in diluted magnetic oxides (DMO) are a controversial subject as magnetic behavior is strongly dependent on the synthesis method and it is very difficult to obtain reproducible homogeneity on samples. The most popular mechanism for ferromagnetic order in DMO is the bound magnetic polaron (BMP) where a trapped electron at the site of the VO, with a hydrogenic radius (0.4 to 0.6 nm), intercepts and polarizes the magnetic moment from 3d ions creating ferromagnetic order. Percolation of such BMPs creates a spin-polarized impurity band. It was proposed that substitutional Zn induced the formation of Sni defects to which is attributed the magnetic moment This model is reinforced by theoretical calculations carried out by several groups [13,14]. Structural defects do not mean partially amorphous material, but a concentration of punctual defects in monocrystalline structures
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