F. H. Aragón

Long-range ferromagnetic order induced by a donor impurity band exchange in SnO2:Er3+ nanoparticles

In this work, the structural and magnetic properties of Er-doped SnO2 (SnO2:Er) nanoparticles are reported. The SnO2:Er nanoparticles have been synthesized by a polymer precursor method with Er content from 1.0% to 10.0%. X-ray diffraction results indicate the formation of only the rutile-type structure in all samples. The estimated mean crystallite size shows a decrease from ∼10 to ∼4 nm when the Er content is increased from 1.0% to 10.0%. The particle size values have been corroborated by transmission electron microscopy technique. The thermal dependence of the magnetization is consistent with the 3+ oxidation state of erbium ions for all samples. A strong paramagnetic-like behavior coexisting with a ferromagnetic phase has been determined for samples with Er content below 5.0%. Above this concentration, only a paramagnetic behavior has been determined. Isothermal magnetization curves are consistent with the occurrence of long-range ferromagnetic order mediated by donor electrons forming bound magnetic ...

Fe doping effect on the structural, magnetic and surface properties of SnO2 nanoparticles prepared by a polymer precursor method

In this study the structural, magnetic and surface characterization of Fe-doped SnO2 nanopowders synthesized by a polymer precursor method is presented. The x-ray diffraction (XRD) data analysis shows the formation of rutile-type structure for all samples. For Fe-content up to 5.0 mol% lattice constants and unit cell volume values suggest substitutional solution of Fe3+- and Sn4+-ions in the SnO2 matrix and the likely generation of oxygen vacancies to account for charge compensation. Above 5.0 mol% Fe-content the entrance of Fe3+-ions into interstitial sites seems to be the dominant regime. Magnetic measurements confirm the ferric valence state and suggest the coexistence of weak ferromagnetic (FM) with strong paramagnetic (PM) phases. Using the bound magnetic polaron (BMP) model the FM contribution has been associated to electrons trapped within oxygen vacancies (donor electrons) that form BMPs which overlap to create a spin-split impurity band. Despite the small size of the particles no evidence of thermal relaxation effects has been observed, which was assigned to the formation of aggregates of strongly interacting naked particles. Above ≈1.0 mol% Fe-content, the antiferromagnetic (AFM) interaction associated to Fe-clusters seems to be dominant and only a PM phase is observed. These results are consistent with XPS data analysis which indicates that the magnetic properties are strongly correlated with the surface properties of the particles.

Fe-doping effects on the structural, vibrational, magnetic, and electronic properties of ceria nanoparticles

In this work, we report on a single-pot synthesis route based on a polymeric precursor method used for successfully producing undoped and iron-doped CeO2 nanoparticles with iron contents up to 10.0 mol. %. The formation of high-crystalline nanoparticles with a cubic fluorite structure is determined for all the studied samples. Meanwhile, the magnetic measurements of the undoped ceria nanoparticles revealed the occurrence of ferromagnetism of bound magnetic polarons of a fraction of Ce3+ at room temperature, and only a paramagnetic behavior of Fe3+ ions was determined for Fe-doped ceria nanoparticles. A monotonous reduction of the effective magnetic moment of the Fe3+ ions was determined. It suggests a change from a high-spin to low-spin state of Fe ions as the Fe content is increased. The 3+ valence state of the iron ions has been confirmed by the Fe K-edge X-ray absorption near-edge structure (XANES) and Mossbauer spectroscopy measurements. X-ray photoelectron spectroscopy data analysis evidenced a coexi...

Evolution of the doping regimes in the Al-doped SnO2 nanoparticles prepared by a polymer precursor method.

In this study, we report on the structural and hyperfine properties of Al-doped SnO2 nanoparticles synthesized by a polymer precursor method. The x-ray diffraction data analysis carried out using the Rietveld refinement method shows the formation of only rutile-type structures in all samples, with decreasing of the mean crystallite size as the Al content. A systematic study of the unit cell, as well as the vicinity of the interstitial position show strong evidence of two doping regimes in the rutile-type structure of SnO2. Below 7.5 mol% doping a dominant substitutional solution of Al(+3) and Sn(4+)-ions is determined. However, the occupation of both substitutional and interstitial sites is determined above 7.5 mol% doping. These findings are in good agreement with theoretical ab initio calculations.

Magnetic nanohydrogel obtained by miniemulsion polymerization of poly(acrylic acid) grafted onto derivatized dextran

This study describes the synthesis of magnetic nanohydrogels by miniemulsion polymerization technique. Dextran was derivatized by the glycidyl methacrylate to anchor vinyl groups on polysaccharides backbone, allowing its use as a macromonomer for miniemulsion polymerization, as confirmed by proton nuclear magnetic resonance spectroscopy (13C NMR). Magnetite nanoparticles were synthesized by coprecipitation, followed by air oxidation to maghemite. The results of X-ray diffractometry (XRD), Raman and transmission electron microscopy (TEM) analysis showed that maghemite nanoparticles were obtained with a diameter of 5.27nm. The entrapment of iron oxide nanoparticles in a dextran nanohydrogel matrix was confirmed by thermogravimetric analysis (TGA), scanning transmission electron microscopy (STEM) and Zeta potential data. The magnetic nanohydrogels presented superparamagnetic behavior and were colloidal stable in physiological during 30days. Our findings suggest that the synthesized magnetic nanohydrogel are potential candidates for use in drug delivery systems due to its physicochemical and magnetic properties.