Tailoring the physical and chemical properties of Sn1-xCoxO2 nanoparticles: an experimental and theoretical approach.

In this work, we present a coupled experimental and theoretical first-principles investigation on one of the more promising oxide-diluted magnetic semiconductors, the Sn1-xCoxO2 nanoparticle system, in order to see the effect of cobalt doping on the physical and chemical properties. Our findings suggest that progressive surface enrichment with dopant ions plays an essential role in the monotonous quenching of the surface disorder modes. That weakening is associated with the passivation of the oxygen vacancies as the Co excess at the surface becomes larger. Room-temperature 119Sn Mössbauer spectroscopy data analysis revealed the occurrence of a distribution of isomer shifts, related to the different non-equivalent surroundings of Sn4+ ions and the coexistence of Sn2+/Sn4+ at the particle surfaces provoked by the inhomogeneous distribution of Co ions, in agreement with the X-ray photoelectron spectroscopy measurements. Magnetic measurements revealed a paramagnetic behavior of the Co ions dispersed in the rutile-type matrix with antiferromagnetic correlations, which become stronger as the Co content is increased. Theoretical calculations show that a defect with two Co mediated by a nearby oxygen vacancy is the most likely defect. The predicted effects of this defect complex are in accordance with the experimental results.

Misaligned anisotropies in spin-valve films studied through magnetoresistance and magnetization measurements.

Magnetization and magnetoresistance properties of Py/Cu/Py/IrMn spin valve (SV) films are studied in the framework of the modified antiferromagnetic domain-wall model applied to a granular multidomain system. In the simulations, a misalignment between the in-plane easy magnetization axes of the Py and IrMn is considered. Magnetization and magnetoresistance data are simulated for a number of field orientations and a fairly good agreement with the experiment is found. The same holds for the respective distributions of the coercive and exchange-bias field values determined from magnetoresistance first-order reversal curves (MR-FORC) obtained for magnetic field parallel to the direction of that applied during the sample deposition. Both experimental and theoretical data of the angular variations of the magnetoresistance at constant fields are successfully used to obtain the misalignment angles. For some samples, mostly those with thinner Py layer coupled to IrMn one, our results indicate that the misalignment is due to interfacial magnetic frustration. Moreover, it is shown that MR-FORC diagrams are useful to extract information about the alignment of the grains along the field-cooling directions in SVs, as well as that these can be used to determine the threshold of the continuity of the pinned magnetic layer.

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.

Properties of nanoparticles prepared from NdFeB-based compound for magnetic hyperthermia application.

Nanoparticles were prepared from a NdFeB-based alloy using the hydrogen decrepitation process together with high-energy ball milling and tested as heating agent for magnetic hyperthermia. In the milling time range evaluated (up to 10 h), the magnetic moment per mass at H = 1.59 MA m(-1) is superior than 70 A m(2) kg(-1); however, the intrinsic coercivity might be inferior than 20 kA m(-1). The material presents both ferromagnetic and superparamagnetic particles constituted by a mixture of phases due to the incomplete disproportionation reaction of Nd(2)Fe(14)BH(x) during milling. Solutions prepared with deionized water and magnetic particles exposed to an AC magnetic field (H(max) ~ 3.7 kA m(-1) and f = 228 kHz) exhibited 26 K ≤ ΔT(max) ≤ 44 K with a maximum estimated specific absorption rate (SAR) of 225 W kg(-1). For the pure magnetic material milled for the longest period of time (10 h), the SAR was estimated as ~2500 W kg(-1). In vitro tests indicated that the powders have acceptable cytotoxicity over a wide range of concentration (0.1-100 µg ml(-1)) due to the coating applied during milling.