Showing papers by "Marcelo Knobel published in 2011"

An effective method to probe local magnetostatic properties in a nanometric FePd antidot array

Abstract: A simple method to quantitatively characterize the local magnetic behaviour of a patterned nanostructure, like a ferromagnetic thin film of antidot arrays, is proposed. The first-order reversal curve (FORC) analysis, coupled with simulations using physically meaningful hysterons, allows us to obtain a quantitative and physically related description of the interaction field and each magnetization reversal process. The hysterons system is built from previously known hypotheses on the magnetic behaviour of the sample. This method was successfully applied to a highly hexagonal ordered FePd antidot array with nanometric dimensions. We achieved a complete characterization of the two different magnetization reversal mechanisms in function of the in-plane applied field angle. For a narrow range of high fields, the magnetization initiates rotating reversibly around the pores, while at lower fields, domain walls are nucleated and propagated. This in-plane magnetization reversal mechanism, partly reversible and partly irreversible, is the only angularly dependent one. While going away from the easy axis, its reversible proportion increases, as well as its switching field distribution. Finally, the results indicate that the high surface roughness between adjacent holes of the antidot thin film induces a parallel interaction field. The proposed method demonstrates its ability also to be applied to characterizing patterned nanostructures with rather complex magnetization reversal processes.

Magnetic and structural properties of fcc/hcp bi-crystalline multilayer Co nanowire arrays prepared by controlled electroplating

Abstract: We report on the structural and magnetic properties of crystalline bi-phase Co nanowires, electrodeposited into the pores of anodized alumina membranes, as a function of their length. Co nanowires present two different coexistent crystalline structures (fcc and hcp) that can be controlled by the time of pulsed electrodeposition. The fcc crystalline phase grows at the early stage and is present at the bottom of all the nanowires, strongly influencing their magnetic behavior. Both structural and magnetic characterizations indicate that the length of the fcc phase is constant at around 260–270 nm. X-ray diffraction measurements revealed a strong preferential orientation (texture) in the (1 0–1 0) direction for the hcp phase, which increases the nanowire length as well as crystalline grain size, degree of orientation, and volume fraction of oriented material. The first-order reversal curve (FORC) method was used to infer both qualitatively and quantitatively the complex magnetization reversal of the nanowires. Under the application of a magnetic field parallel to the wires, the magnetization reversal of each region is clearly distinguishable; the fcc phase creates a high coercive contribution without an interaction field, while the hcp phase presents a smaller coercivity and undergoes a strong antiparallel interaction field from neighboring wires.

Electronic depiction of magnetic origin in undoped and Fe doped TiO2−d epitaxial thin films

Abstract: We have investigated the electronic and magnetic properties of the pulsed laser deposited epitaxial thin films of undoped and Fe doped (4 at. %) anatase TiO2−d by photoemission, magnetization measurements, and ab-initio band structure calculations. These films show room temperature magnetic ordering. It is observed that Fe ions hybridize with the oxygen vacancy induced Ti3+ defect states. Our study reveals the formation of local magnetic moment at Ti and Fe sites to be responsible for magnetic ordering. A finite density of states at the Fermi level in both undoped and Fe doped films is also observed, suggesting their degenerate semiconducting nature.

ROOM TEMPERATURE FERROMAGNETISM IN PURE AND Cu DOPED ZnO NANORODS: ROLE OF COPPER OR DEFECTS

Abstract: We report structural, magnetic and electronic structure studies of pure and Cu doped ZnO nanorods with the aim to understand the origin of ferromagnetism. A structural study indicates that all the samples exhibit single phase nature and rules out the formation of secondary phase. NEXAFS measurements reveal that Cu ions exist in Cu2+ state. Magnetic hysteresis loop measurements reflect that the pure and Cu doped ZnO nanorods exhibit room temperature ferromagnetism. The increase in the intensity of green emission in photoluminescence study indicates that defects density increases with Cu doping.

Structural and magnetic study of a diluted magnetic semiconductor: Fe-doped CeO2 nanoparticles.

Abstract: This paper reports the effect of Fe doping on the structure and room temperature ferromagnetism of CeO2 nanoparticles. X-ray diffraction and the selective area electron diffraction measurements performed on the Ce(1-x)Fe(x)O2 (0 < or = x < or = 0.07) nanoparticles showed a single-phase nature with a cubic structure, and none of the samples showed the presence of any secondary phase. The mean particle size, which was calculated using transmission electron microscopy, increased with the increase in the Fe content. The DC magnetization measurements that were performed at room temperature showed that all the samples exhibited ferromagnetism. The saturation magnetic moment increased with the increase in the Fe content.

Investigations of lanthanum doping on magnetic properties of nano cobalt ferrites

Abstract: The magnetic properties of nano-crystallite cobalt lanthanum ferrite (CoLaxFe2-x O4) with varied quantities of lanthanum (x = 0, 0.1, 0.15, 0.2, 0.25, 0.3) prepared by co-precipitation method have been studied by vibrating sample magnetometer (VSM) and LCR meter. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the size, structure, and morphology of the ferrite samples. The average crystallite size varied from 17.83 nm to 49.99 nm. All the samples, although, in nano range, show significant hysteresis. The saturation magnetization (Ms) values decreased from 60.57 emu/g to 30.15 emu/g. The remanence (MR) fell from 10.85 emu/g to 6.39 emu/g. Doping with lanthanum La3+ ions modulates significantly the magnetic properties of cobalt spinel ferrites without sacrificing the ferromagnetic character.

Synthesis of Ag–CoFe2O4 dimer colloidal nanoparticles and enhancement of their magnetic response

Abstract: This paper reports the structural and magnetic properties of Ag–CoFe2O4 colloidal dimer nanoparticles (NPs) synthesized using a two-step solution-phase route. Ag NPs were used as seeds to grow Ag–CoFe2O4 dimer NPs using thermal decomposition of metallic precursor. By means of temperature and field dependent dc magnetization measurements, it is found that the silver due to its interface with CoFe2O4 particles leads to thermal stabilization of the dimer NPs superior as compared to CoFe2O4 alone. Our results show enhancement of the magnetic anisotropy and a large coercivity at 2 K for dimer NPs, which could be ascribed to interface effect between Ag and CoFe2O4 components and the related structural defects.

INTERNATIONALIZING BRAZIL’S UNIVERSITIES: Creating Coherent National Policies Must Be a Priority

Abstract: It is estimated that approximately 3 million students are enrolled as international students, and it is possible to project that this number may reach more than 7 million by 2025 As global demand exceeds the supply, competition is building for the best of these students Some countries (or regions) clearly envisage the opportunity this represents and have been strongly stimulating student mobility There is a race for “brains”, be it for professors at the end of their careers looking for new professional opportunities and/or the opportunity to return to their native countries, or for researchers at the beginning of their careers, looking for a place that might offer them a better future, or even for students, who seek more appealing alternatives How will Brazil fare in this competition for talent? If it is to internationalize its higher education study programs, Brazil must deal with a number of practical problems, including a lack of specific policies and guidelines Bureaucracy, for instance, is one major problem A foreigner who comes to live in Brazil faces many obstacles, mainly due to the bureaucracy involved in everything from getting a Visa through the Federal Police office, to opening up a bank account, renting an apartment, registering at school, amongst many other processes and regulations that make it difficult for anyone to come and live in Brazil One rarely finds a course offered in English or Spanish in a Brazilian university and the selection of faculty are normally held in Portuguese Currently, there are no plans or projects at either the federal or state level, to address these obstacles This should be a major concern to all who hold positions of responsibility in the educational process, as Brazil is not keeping pace with higher education reforms found globally The internationalization movement is growing, and Brazil must actively seek reforms to keep pace with economic competitors

Probing the interdependence between irreversible magnetization reversal processes by first-order reversal curves

Abstract: A procedure to probe the interdependence between irreversible magnetic processes is presented. It consists of measuring the first-order reversal curves (FORCs) without saturating the system. Depending on the variation of the reversal fields during the curves’ acquirement (increasing or decreasing), it fixes the hardest or softest hysterons into their negative saturation level throughout the measurement. Differences between these FORC diagrams and the classical one, as well as variation of the end magnetization as a function of the reversal field, indicate and characterize the requirement that some irreversible processes arise from others. The procedure is described to investigate magnetic systems, but can be directly used to study any hysteretic system.

INTERNATIONALIZING BRAZIL’S UNIVERSITIES: Creating Coherent National Policies Must Be a Priority

Abstract: It is estimated that approximately 3 million students are enrolled as international students, and it is possible to project that this number may reach more than 7 million by 2025. As global demand exceeds the supply, competition is building for the best of these students. Some countries (or regions) clearly envisage the opportunity this represents and have been strongly stimulating student mobility. There is a race for “brains†, be it for professors at the end of their careers looking for new professional opportunities and/or the opportunity to return to their native countries, or for researchers at the beginning of their careers, looking for a place that might offer them a better future, or even for students, who seek more appealing alternatives. How will Brazil fare in this competition for talent? If it is to internationalize its higher education study programs, Brazil must deal with a number of practical problems, including a lack of specific policies and guidelines. Bureaucracy, for instance, is one major problem. A foreigner who comes to live in Brazil faces many obstacles, mainly due to the bureaucracy involved in everything from getting a Visa through the Federal Police office, to opening up a bank account, renting an apartment, registering at school, amongst many other processes and regulations that make it difficult for anyone to come and live in Brazil. One rarely finds a course offered in English or Spanish in a Brazilian university and the selection of faculty are normally held in Portuguese. Currently, there are no plans or projects at either the federal or state level, to address these obstacles. This should be a major concern to all who hold positions of responsibility in the educational process, as Brazil is not keeping pace with higher education reforms found globally. The internationalization movement is growing, and Brazil must actively seek reforms to keep pace with economic competitors.