J. A. H. Coaquira

Synthesis, characterization and magnetic properties of polymer-Fe3O4 nanocomposite.

The chemical stability of magnetic particles is of great importance for their applications in medicine and biotechnology. The most challenging problem in physics of disordered systems of magnetic nanoparticles is the investigation of their dynamic properties. The chemical coprecipitation process was used to synthesize spherical magnetite nanoparticles of 14 nm. The as-prepared magnetite nanoparticles have been aged in the matrix. Magnetic properties and aging effect were studied by Mössbauer spectroscopy at temperatures ranging from 77 to 300 K, and X-ray diffraction. At room temperature, the Mössbauer spectrum showed superparamagnetic behavior of the particles, while well-defined sextets were observed at 77K, indicating a blocked regime. The superparamagnetic magnetite nanoparticles can be used as microbead biosensors.

Evidence of a cluster glass-like behavior in Fe-doped ZnO nanoparticles

We report on the study of the structural and magnetic properties of crystalline Fe-doped ZnO nanoparticles with Fe content up to 10% synthesized by a co-precipitation method. The Rietveld analysis indicates that the Fe-doped ZnO nanoparticles are formed in a single phase wurtzite structure. DC magnetization (M) vs. applied magnetic field (H) curves obtained at 5 K show the occurrence of a ferromagnetic behavior. The coercive field and saturation magnetization depend on the Fe content. At room temperature, M vs. H curves show features consistent with a superparamagnetic state of nanoscale system. The temperature dependence of the AC and DC magnetic susceptibilities show features related to the thermal relaxation of the nano-sized particles. From the AC data analysis, a magnetic transition from the superparamagnetic to cluster-glass state is determined.

Structural and magnetic properties of core-shell Au/Fe 3 O 4 nanoparticles

We present a systematic study of core-shell Au/Fe3O4 nanoparticles produced by thermal decomposition under mild conditions. The morphology and crystal structure of the nanoparticles revealed the presence of Au core of d = (6.9 ± 1.0) nm surrounded by Fe3O4 shell with a thickness of ~3.5 nm, epitaxially grown onto the Au core surface. The Au/Fe3O4 core-shell structure was demonstrated by high angle annular dark field scanning transmission electron microscopy analysis. The magnetite shell grown on top of the Au nanoparticle displayed a thermal blocking state at temperatures below TB = 59 K and a relaxed state well above TB. Remarkably, an exchange bias effect was observed when cooling down the samples below room temperature under an external magnetic field. Moreover, the exchange bias field (HEX) started to appear at T~40 K and its value increased by decreasing the temperature. This effect has been assigned to the interaction of spins located in the magnetically disordered regions (in the inner and outer surface of the Fe3O4 shell) and spins located in the ordered region of the Fe3O4 shell.

Facile approach to suppress γ-Fe2O3 to α-Fe2O3 phase transition beyond 600 °C in Fe3O4 nanoparticles

Magnetic iron oxide nanoparticles on a zeolite template have been synthesized using wet chemical approach. The average particle size initially decreases from 8.5 to 6 nm (increasing zeolite concentration from 0 to 75 mg) but increases to 11 nm for higher zeolite concentration (100 mg). Room temperature magnetization curves show an initial decrease in saturation magnetization from 62 to 42 emu per gram due to decrease in particle size as well as increase in contribution from nonmagnetic zeolite template. Further increase in zeolite concentration to 100 mg results in a significant increase in saturation magnetization from 42 to 51 emu per gram. Calorimetric studies show a continuous enhancement in γ-Fe2O3 to α-Fe2O3 phase transition temperature from 590 to 715 °C by increasing the zeolite concentration from 0 to 75 gm. The exothermic peak corresponding to the γ-Fe2O3 to α-Fe2O3 phase transition has been completely suppressed for nanoparticles prepared in presence of 100 mg of zeolite. Mossbauer spectra of as-synthesized nanoparticles show an increase of superparamagnetic components from 7 to 36% corresponding to increase in zeolite concentration from 0 to 100 mg. Mossbauer spectra of pure Fe3O4 nanoparticles annealed at 500 °C shows formation of pure α-Fe2O3 phase and Mossbauer spectra of particles prepared in presence of 25 mg shows only 18% of α-Fe2O3 phase after annealing at 550 °C. Further increase in zeolite concentration to 50 and 75 mg (annealed at 550 °C) leads to pure γ-Fe2O3. Annealing of Fe3O4 nanoparticles prepared in the presence of 100 mg of zeolite at 650 °C shows formation of only 8% α-Fe2O3 phase. Our results show an easy and effective method to enhance the thermal stability of magnetic iron oxide nanoparticles making it suitable for high temperature applications.

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 ...

Optical and magnetic properties of Co-doped ZnO nanoparticles and the onset of ferromagnetic order

In this study, we report on the optical and magnetic properties of Co-doped ZnO nanoparticles with increasing Co-content (CoxZn1−xO; x = 0.000, 0.005, 0.010, 0.030, 0.050, 0.070, and 0.100) synthesized by the combustion reaction method. The X-ray diffraction patterns and the Raman spectra of all samples indicated the formation of the ZnO hexagonal wurtzite phase (space group C46V). The Raman data also show the formation of a secondary Co3O4 phase, which is barely seen in the X-ray spectra. Photoacoustic spectroscopy and electron paramagnetic resonance confirm the presence of the two phases (CoxZn1−xO and Co3O4). Vibrating sample magnetometer measurements performed at room temperature exhibited hysteresis loops, indicating the presence of long-range magnetic ordering in the samples. Analysis of the magnetization as a function of magnetic field and temperature shows that the ferromagnetism in the as-synthesized samples comes from small Co-metallic inclusions, with an estimated radius of about 4.8 nm and blo...

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...

Characterization of copper microelectrodes, following a homemade lithography, technique, and gold electroless deposition

We report the fabrication and characterization of copper microelectrodes obtained by a homemade lithography technique and after gold electroless deposition. For the fabrication, planes consisting of arrays of electrodes (black in color) with bow tie shape were designed and printed on a transparent paper (Canson ltd.). Using an embroidery frame with a silk fabric, a photographic emulsion was spread on the silk and simultaneously pressing the Canson paper on it. The system was introduced into a closed box and exposed with a UV light. The designed electrode templates prevented direct exposition of the UV light over copper films and indelible ink was spread over it. After the ink was dried, the copper film is immersed into ferric acid to attack the uncovered copper parts (where there is no ink). In this way, we obtained copper electrodes with initial gap separation of ~142μm and subsequently, they followed electroless deposition of gold to make the copper electrodes to contact. For the characterization, electrical measurements were performed. They present ohmic resistance values in the order of 106 Ω produced by surface scattering of the electrons within the gold microwire and enhanced by oxidation of the copper electrodes.

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.