Magnetic properties of Fe-doped NiO nanoparticles.

Undoped and Fe-doped NiO nanoparticles were successfully synthesized using a lyophilization method and systematically characterized through magnetization techniques over a wide temperature range, with varying intensity and frequency of the applied magnetic fields. The Ni1-xFexO nanoparticles can be described by a core-shell model, which reveals that Fe doping enhances exchange interactions in correlation with nanoparticle size reduction. The nanoparticles exhibit a superparamagnetic blocking transition, primarily attributed to their cores, at temperatures ranging from above room temperature to low temperatures, depending on the Fe-doping level and sample synthesis temperature. The nanoparticle shells also exhibit a transition at low temperatures, in this case to a cluster-glass-like state, caused by the dipolar magnetic interactions between the net magnetic moments of the clusters. Their freezing temperature shifts to higher temperatures as the Fe-doping level increases. The existence of an exchange bias interaction was observed, thus validating the core-shell model proposed.

Did Salts in Seawater Play an Important Role in the Adsorption of Molecules on Minerals in the Prebiotic Earth? The Case of the Adsorption of Thiocyanate onto Forsterite-91.

Thiocyanate may have played as important a role as cyanide in the synthesis of several molecules. However, its concentration in the seas of the prebiotic Earth could have been very low. Thiocyanate was dissolved in two different seawaters: a) a composition that comes close to the seawater of the prebiotic Earth (seawater-B, Ca2+ and Cl-) and b) a seawater (seawater-A, Mg2+ and SO42-) that could be related to the seas of Mars and other moons in the solar system. In addition, forsterite-91 was a very common mineral on the prebiotic Earth and Mars. Two important results are reported in this work: 1) thiocyanate adsorbed onto forsterite-91 and 2) the amount of thiocyanate adsorbed, adsorption thermodynamic, and adsorption kinetic depend on the composition of the artificial seawater. For all experiments, the adsorption was thermodynamically favorable (ΔG < 0). The adsorption data fitted well in the Freundlich and Langmuir-Freundlich models. When dissolving thiocyanate in seawater 4.0-A-Gy and seawater 4.0-B-Gy, the adsorption of thiocyanate onto forsterite-91 was ruled by enthalpy and entropy, respectively. As shown by n values, the thiocyanate/foraterite-91 system is heterogeneous. For all kinetic data, the pseudo-first-order model presented the best fit. The constant rate for thiocyanate dissolved in seawater 4.0-A-Gy was twice that compared to thiocyanate dissolved in seawater 4.0-B-Gy or ultrapure-water. The interaction between thiocyanate and Fe2+ of forsterite-91 was with the nitrogen atom of thiocyanate. In the presence of thiocyanate, sulfate interacts with forsterite-91 as an inner-sphere surface complex, and without thiocyanate as an outer-sphere surface complex.

Spin-Glass Transitions in Zn1-xFexO Nanoparticles.

Monophasic Zn1-xFexO nanoparticles with wurtzite structure were synthesized in the 0 ≤ x ≤ 0.05 concentration range using a freeze-drying process followed by heat treatment. The samples were characterized regarding their optical, structural, and magnetic properties. The analyses revealed that iron doping of the ZnO matrix induces morphological changes in the crystallites. Iron is substitutional for zinc, trivalent and distributed in the wurtzite lattice in two groups: isolated iron atoms and iron atoms with one or more neighboring iron atoms. It was also shown that the energy band gap decreases with a higher doping level. The samples are paramagnetic at room temperature, but they undergo a spin-glass transition when the temperature drops below 75 K. The magnetic frustration is attributed to the competition of magnetic interactions among the iron moments. There are a superexchange interaction and an indirect exchange interaction that is provided by the spin (and charge) itinerant carriers in a spin-polarized band situated in the vicinity of the Fermi level of the Fe-doped ZnO semiconductor. The former interaction actuates for an antiferromagnetic coupling among iron ions, whereas the latter constitutes a driving force for a ferromagnetic coupling that weakens, decreasing the temperature. Our results strongly contribute to the literature because they elucidate the controversies reported in the literature for the magnetic state of the Fe-doped ZnO system.