Wallace C. Nunes

Chemical Synthesis and Structural Characterization of Highly Disordered Ni Colloidal Nanoparticles

This work focuses on synthetic methods to produce monodisperse Ni colloidal nanoparticles (NPs), in the 4-16 nm size range, and their structural characterization. Narrow size distribution nanoparticles were obtained by high-temperature reduction of a nickel salt and the production of tunable sizes of the Ni NPs was improved compared to other methods previously described. The as-synthesized nanoparticles exhibited spherical shape and highly disordered structure, as it could be assigned by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Annealing at high temperature in organic solvent resulted in an increase of nanoparticle atomic ordering; in this case, the XRD pattern showed an fcc-like structure. Complementary data obtained by X-ray absorption spectroscopy confirmed the complex structure of these nanoparticles. Temperature dependence of the magnetic susceptibility of these highly disordered Ni NPs showed the magnetic behavior cannot be described by the conventional superparamagnetic theory, claiming the importance of the internal structure in the magnetic behavior of such nanomaterials.

Effects of dipolar interactions on the magnetic properties of γ-Fe2O3 nanoparticles in the blocked state

The role played by dipolar interactions in the coercive field and remanence of nanoparticle systems is studied using different concentrations of nearly monodisperse nanoparticles of γ-Fe2O3. The 9.0nm particles are isolated by organic encapsulation and diluted in paraffin. Magnetic properties were investigated by means of hysteresis obtained at different temperatures, zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves, and the so-called ΔM technique. The analysis of the experimental data points to the existence of a demagnetizing role played by the dipolar interaction in the coercive field.

γ-Fe2O3 nanoparticles dispersed in porous Vycor glass : A magnetically diluted integrated system

An investigation of the effect of interparticle interaction and particle size distribution has been carried out on iron oxide nanoparticles dispersed into porous Vycor glass. γ-Fe2O3 nanoparticles dispersed into monoliths of Vycor glass were obtained using impregnation-decomposition cycles through the single-source metallo-organic decomposition process. Magnetic properties were investigated by ac magnetic susceptibility measurements, as a function of temperature at different frequencies, by measuring zero-field-cooled and field-cooled magnetization curves and by constructing hysteresis loops at different temperatures. A log-normal size distribution of monodomain nanoparticles has been deduced from the analysis of the magnetization curves. F57e Mossbauer spectroscopy was also employed for investigating the magnetic behavior as a function of nanoparticle size. The systems exhibit typical superparamagnetic behaviors with a wide particle size distribution that can be changed without significantly affecting th...

Thermal behavior of hard-axis magnetization in noninteracting particles with uniaxial anisotropy

Experimental evidence is presented to support predictions made by an analytical model describing the temperature-dependent behavior of an assembly of noninteracting magnetic nanoparticles with uniaxial anisotropy under an external field. When the applied field is smaller than the anisotropy field of the particles and is oriented perpendicular to the easy axis, a maximum of the magnetization occurs at finite temperature. The theory shows good agreement with measurements of an array of CoCrPt nanoislands with uniaxial anisotropy. Deviations are discussed taking into account the thermal dependencies of the saturation magnetization and the anisotropy of the magnetic material.

Effects of postdeposition heat treatment on the structural and magnetic properties of CoFe2O4 nanoparticles produced by pulsed laser deposition

In this work, we investigated the effects of postdeposition heat treatment on structural and magnetic properties of CoFe2O4 nanoparticles produced by pulsed laser deposition. Structural analysis by X-ray diffraction, transmission electron microscopy (TEM) and Mossbauer spectroscopy indicate the formation of a single phase cobalt ferrite nanoparticles with the size ranging from 4.3 to 33.3 nm depending on the annealing temperature. The magnetic properties of the samples were investigated in a wide temperature range (50–400 K). Noticeable effects of the cubic magnetocrystalline anisotropy on the magnetization process of nanoparticles were observed for samples annealed at 450 °C and 600 °C, while for samples as-deposited and annealed at 300 °C the magnetization properties were dominated by a uniaxial effective anisotropy. ΔM technique was used to investigate the magnetic interaction among the nanoparticles. Only demagnetizing interactions were observed for the sample annealed up to 300 °C, while for the samp...

Influence of substrate on the magnetic properties of Ni and permalloy sub-micrometric patterned stripes

The magnetic properties of sub-micrometric magnetic structures of Ni and permalloy (Ni80Fe20) patterned stripes are studied as functions of temperature. The samples are produced by combining an interferometric lithographic technique with sputtering. At room temperature, ferromagnetic resonance and hysteresis data indicate the existence of an in-plane uniaxial anisotropy in both samples. The NiFe sample has an easy axis along the stripes direction in the entire studied temperature range (2 < T < 300 K). On the other hand, an interesting change in the easy magnetization direction is observed in Ni samples, from parallel to perpendicular to the axis of the stripes, when the temperature is decreased. The results are interpreted in terms of a competition between shape and magnetoelastic anisotropies.

Role of magnetic anisotropy on the magnetic properties of Ni nanoclusters embedded in a ZnO matrix

We have investigated the magnetic properties of Ni nanoaggregates produced by ion implantation in ZnO single crystals. Several deviations from classical models usually adopted to describe the magnetic properties of nanoparticle systems were found. The strain between host and Ni nanoaggregates induces a magnetic anisotropy with a preferred direction. We show that these anisotropy effects can be misinterpreted as a ferromagnetic or antiferromagnetic coupling among the nanoaggregates similar to that of an oriented, interacting nanocrystal ensemble.

SÍNTESE, CARACTERIZAÇÃO E ESTUDO DAS PROPRIEDADES MAGNÉTICAS DE UM POLÍMERO DE COORDENAÇÃO CONTENDO COBALTO(II) E COBRE(II)

This work describes the synthesis and characterization of two new compounds with ligand opy (N-(2-pyridyl)oxamate): the copper(II) precursor [Me4N]2[Cu(opy)2].5H2O and CoIICuII coordination polymer {[CoCu(opy)2]}n×4nH2O. This latter compound was obtained by reaction of [Me4N]2[Cu(opy)2].5H2O and CoCl2.6H2O in water. The heterobimetallic CoIICuII chain was characterized by elemental analysis, IR spectroscopy, thermogravimetry and magnetic measurements. Magnetic characterization revealed typical behavior of one-dimensional (1D) ferrimagnetic chain as shown in the curves of temperature (T) dependence of magnetic susceptibility (χM), in the form of χMT versus T, and dependence of magnetization (M) with applied field (H).

Positive exchange bias in Fe on antiferromagnetic semiconductor EuTe

In this paper, the exchange bias effect in Fe/EuTe(111) bilayers prepared by molecular beam epitaxy (MBE) is investigated by SQUID. For cooling fields from 2 to 70 kOe, only positive shifts was found of the magnetic hysteresis loops. This result indicates antiferromagnetic coupling between the ferromagnetic (F) and the antiferromagnetic (AF) layer at the Fe/EuTe(111) interface independently of the applied cooling field. Fe/EuTe bilayers prepared on EuTe(111) surfaces with stoichiometry (Eu:Te = 2.1) and, consequently, magnetic order altered by Ar-sputtering do not present exchange bias.