Sabrina Nicolodi

Controlled rotation of the exchange-bias direction in IrMn∕Cu∕Co via ion irradiation

Co∕Cu∕IrMn films were irradiated with 40keV He+ ions varying the fluence and the current, with magnetic field applied at 120° with respect to the original exchange-bias direction. The angular variations of the exchange-bias field of the irradiated samples were compared with those of the as-made and the thermally annealed films. Gradual deviation of the exchange-bias direction with the fluence increase was observed. Complete reorientation of the easy axes of both ferromagnet and antiferromagnet toward that of the field applied during irradiation was achieved for fluences higher than 1×1015ions∕cm2, accompanied with a significant enhancement of the exchange-bias field.

Quantifying magnetic anisotropy dispersion : theoretical and experimental study of the magnetic properties of anisotropic FeCuNbSiB ferromagnetic films

The Stoner-Wohlfarth model is a traditional and efficient tool to calculate magnetization curves and it can provides further insights on the fundamental physics associated to the magnetic properties and magnetization dynamics. Here, we perform a theoretical and experimental investigation of the quasi-static magnetic properties of anisotropic systems. We consider a theoretical approach which corresponds to a modified version of the Stoner-Wohlfarth model to describe anisotropic systems and a distribution function to express the magnetic anistropy dispersion. We propose a procedure to calculate the magnetic properties for the anisotropic case of the SW model from experimental results of the quadrature of magnetization curves, thus quantifying the magnetic anisotropy dispersion. To test the robustness of the approach, we apply the theoretical model to describe the quasi-static magnetic properties of amorphous FeCuNbSiB ferromagnetic films. We perform calculations and directly compare theoretical results with longitudinal and transverse magnetization curves measured for the films. Thus, our results provide experimental evidence to confirm the validity of the theoretical approach to describe the magnetic properties of anisotropic amorphous ferromagnetic films, revealed by the excellent agreement between numerical calculation and experimental results.

Abrupt suppression of the exchange bias across a non-magnetic insulator spacer

This paper reports results obtained on exchange-biased IrMn/Al2O3/Co films deposited by magnetron sputtering, where the thickness of the non-magnetic insulator layer, tAl2O3, was varied. Ferromagnetic resonance and static magnetization measurements were used to study the exchange interaction between the antiferromagnet (IrMn) and ferromagnet (Co) layers. X-ray diffractometry and x-ray reflectometry as well as high-resolution transmission electron microscopy were employed for structural characterization of the films. It was found that the IrMn/Co exchange coupling decreases very abruptly with tAl2O3 being the exponential decay length at least four times smaller than the values previously reported for exchange-bias systems. Such a rapid suppression of the coupling is explained in terms of the prompt loss of the direct contact between the antiferromagnet and the ferromagnet with tAl2O3.

Discrimination between coupling and anisotropy fields in exchange-biased bilayers

In the framework of models that assume planar domain wall formed at the antiferromagnetic part of the interface of exchange-biased bilayers, one cannot distinguish between the cases of high or low ratios between the coupling and the antiferromagnet’s anisotropy fields by using hysteresis loop measurement, ferromagnetic resonance, anisotropic magnetoresistance, or ac susceptibility techniques applied on one and the same sample. The analysis of the experimental data obtained on a series of FeMn/Co films indicated that once the biasing is established the variation in the coercivity with the FeMn layer thickness could be essential for solving this problem. If the coercivity decreases with the thickness then the interlayer exchange coupling is the parameter that varies while the domain-wall energy of the antiferromagnet remains practically constant.

Magnesium surface enrichment of CoFe2O4 magnetic nanoparticles immobilized with gold: reusable catalysts for green oxidation of benzyl alcohol

Gold nanoparticles have shown excellent activity for selective oxidation of alcohols; such catalytic systems are highly dependent on the initial activation of the substrates, which must occur on the catalyst surface in heterogeneous catalysts. In many cases, an extra base addition is required, although the basicity of the support may also be of significant importance. Here, we explored the intrinsic basicity of magnesium-based enrichments on CoFe2O4 magnetic nanoparticles for the oxidation of benzyl alcohol using molecular oxygen as oxidant. The MgO and Mg(OH)2 enrichments enabled gold impregnation, which was not possible on the bare CoFe2O4 nanoparticles. The Au/MgO/CoFe2O4 and Au/Mg(OH)2/CoFe2O4 catalysts reached 42% and 18% conversion, respectively without base promotion, in 2.5 hour and 2 bar of O2. When the catalysts were tested with sub-stoichiometric amounts of base, they became more active (>70% of conversion) and stable in successive recycling experiments without metal leaching, under the same reaction conditions. We also showed the oxide phases of the enrichments performed using Rietveld refinements and how the Mg(OH)2 phase interferes with the activity of MgO-based materials.

Magnetic biocatalysts of pectinase and cellulase: Synthesis and characterization of two preparations for application in grape juice clarification

In the present study, we prepared two different magnetic biocatalysts of pectinase and cellulase: carrier-free magnetic CLEAs (CLEA-MP*) and immobilization on glutaraldehyde-activated magnetite (Enz-Glu-MP*). The biocatalysts were compared to their magnetic properties, immobilization parameters, stability and grape juice clarification. Enz-Glu-MP* presented higher magnetic properties than CLEA-MP*, whereas this presented higher surface area and pore volume. The KM of the enzyme immobilized on Enz-Glu-MP* was 25.65mM, lower in comparison to the CLEA-MP* (33.83mM). On the other hand, CLEA-MP* was the most active and stable biocatalyst, presenting higher recovered activity (33.4% of cellulase), higher thermal stability (2.39 stabilization factor) and improved reusability (8cycles). The integration of magnetic technology with enzymatic immobilization emerges as a possibility to increase the recover and reuse of biocatalysts for application in juice technology.

Chitosan-stabilized gold nanoparticles supported on silica/titania magnetic xerogel applied as antibacterial system

AbstractThe sol-gel method is an excellent choice to produce composite materials with enhanced performance by efficiently combining the individual features of their components. In this work, chitosan-stabilized gold nanoparticles (ChAuNPs) were immobilized onto a SiO2/TiO2 magnetic xerogel, which was synthesized through hetero-condensation of silica and titania precursors in the presence of magnetite particles covered with a silica shell. This system allies the antimicrobial capacity of ChAuNP, the surface reactivity of titania, porous structure of silica, and magnetic response of the magnetite particles. The magnetite phase was characterized by X-ray diffraction and the shape and size of the particles were observed by scanning and transmission electron microscopy. ChAuNPs were obtained in spherical shape with size below 10 nm, as characterized by UV–Vis spectroscopy and transmission electron microscopy. SiO2/TiO2 magnetic xerogel containing the ChAuNP was also characterized by thermogravimetric and textural analysis, transmission electron microscopy, and magnetism. The ChAuNP-SiO2/TiO2 magnetic xerogel is mesoporous with facile magnetic recovering and its performance as antimicrobial agent was assessed against the pathogen E. coli. The ChAuNP-SiO2/TiO2 magnetic xerogel presented inhibitory effect against the tested bacteria, even with such low gold content. After the magnetic recovering, the material was reused and maintained its antibacterial activity. HighlightsMagnetic composite embedding magnetite particles in silica/titania network.Adhesion of chitosan-stabilized gold nanoparticles to silica/titania surface.Porous and high surface area material containing gold nanoparticles as antimicrobial agent.Efficient and reusable antimicrobial system against E. coli bacteria.

Synthesis, Characterization and Catalytic Evaluation of Magnetically Recoverable SrO/CoFe2O4 Nanocatalyst for Biodiesel Production from Babassu Oil Transesterification

Biodiesel production has gained a lot of attention due to its environmental benefits and production from renewable sources. Here, it is reported a magnetically recoverable catalyst of SrO immobilized on a CoFe2O4 support (SrO/CoFe2O4), which was efficiently applied for babassu oil transesterification and used up to four cycles without a significant activity loss. The stability and performance of the catalyst were analyzed considering the solvent used for its synthesis. Characterizations such as vibrating sample magnetometer (VSM), thermogravimetry (TG), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) were performed. The present study demonstrated that the catalyst has a strong magnetic response, which reflects the nature of the nanoparticles. For the first run, the material presented a yield of 96% when synthesized in acetone in a molar proportion of SrO/CoFe2O4 of 5:1.