M. B. Fernández van Raap

Structural and magnetic study of zinc-doped magnetite nanoparticles and ferrofluids for hyperthermia applications

Cubic-like shaped ZnxFe3−xO4 particles with crystallite mean sizes D between 15 and 117 nm were obtained by co-precipitation. Particle size effects and preferential occupation of spinel tetrahedral site by Zn2+ ions led to noticeable changes of physical properties. D ≥ 30 nm particles displayed nearly bulk properties, which were dominated by Zn concentration. For D ≤ 30 nm, dominant magnetic relaxation effects were observed by Mossbauer spectroscopy, with the mean blocking size DB ~ 13 to 15 nm. Saturation magnetization increased with x up to x ~ 0.1–0.3 and decreased for larger x. Power absorbed by water and chitosan-based ferrofluids from a 260 kHz radio frequency field was measured as a function of x, field amplitude H0 and ferrofluid concentration. For H0 = 41 kA m−1 the maximum specific absorption rate was 367 W g−1 for D = 16 nm and x = 0.1. Absorption results are interpreted within the framework of the linear response theory for H0 ≤ 41 kA m−1. A departure towards a saturation regime was observed for higher fields. Simulations based on a two-level description of nanoparticle magnetic moment relaxation qualitatively agree with these observations. The frequency factor of the susceptibility dissipative component, derived from experimental results, showed a sharp maximum at D ~ 16 nm. This behaviour was satisfactorily described by simulations based on moment relaxation processes, which furthermore indicated a crossover from Neel to Brown mechanisms at D ~ 18 nm. Hints for further improvement of magnetite particles as nanocalefactors for magnetic hyperthermia are discussed.

Analysis of the structure, configuration, and sizing of Cu and Cu oxide nanoparticles generated by fs laser ablation of solid target in liquids

We report on the analysis of structure, configuration, and sizing of Cu and Cu oxide nanoparticles (Nps) produced by femtosecond (fs) laser ablation of solid copper target in liquids. Laser pulse energy ranged between 500 μJ and 50 μJ. Water and acetone were used to produce the colloidal suspensions. The study was performed through optical extinction spectroscopy using Mie theory to fit the full experimental spectra, considering free and bound electrons size dependent contributions to the metal dielectric function. Raman spectroscopy and AFM technique were also used to characterize the sample. Considering the possible oxidation of copper during the fabrication process, two species (Cu and Cu2O) arranged in two structures (bare core or core-shell) and in two configuration types (Cu-Cu2O or Cu2O-Cu) were considered for the fitting depending on the laser pulse energy and the surrounding media. For water at high energy, it can be observed that a Cu-Cu2O configuration fits the experimental spectra of the collo...

Quasi-static magnetic measurements to predict specific absorption rates in magnetic fluid hyperthermia experiments

In this work, the issue on whether dynamic magnetic properties of polydispersed magnetic colloids modeled using physical magnitudes derived from quasi-static magnetic measurement can be extrapolated to analyze specific absorption rate data acquired at high amplitudes and frequencies of excitation fields is addressed. To this end, we have analyzed two colloids of magnetite nanoparticles coated with oleic acid and chitosan in water displaying, under a radiofrequency field, high and low specific heat power release. Both colloids are alike in terms of liquid carrier, surfactant and magnetic phase composition but differ on the nanoparticle structuring. The colloid displaying low specific dissipation consists of spaced magnetic nanoparticles of mean size around 4.8 nm inside a large chitosan particle of 52.5 nm. The one displaying high specific dissipation consists of clusters of magnetic nanoparticles of mean size around 9.7 nm inside a chitosan particle of 48.6 nm. The experimental evaluation of Neel and Brown relaxation times (∼10−10 s and 10−4 s, respectively) indicate that the nanoparticles in both colloids magnetically relax by Neel mechanism. The isothermal magnetization curves analysis for this mechanism show that the magnetic nanoparticles behave in the interacting superparamagnetic regime. The specific absorption rates were determined calorimetrically at 260 kHz and up to 52 kA/m and were well modeled within linear response theory using the anisotropy density energy retrieved from quasi-static magnetic measurement, validating their use to predict heating ability of a given polydispersed particle suspension. Our findings provide new insight in the validity of quasi-static magnetic characterization to analyze the high frequency behavior of polydispersed colloids within the framework of the linear response and Wohlfarth theories and indicate that dipolar interactions play a key role being their strength larger for the colloid displaying higher dissipation, i.e., improving the heating efficiency of the nanoparticles for magnetic fluid hyperthermia.

Influence of size-corrected bound-electron contribution on nanometric silver dielectric function. Sizing through optical extinction spectroscopy

The study of metal nanoparticles (NPs) is of great interest due to their ability to enhance optical fields on the nanometric scale, which makes them interesting for various applications in several fields of science and technology. In particular, their optical properties depend on the dielectric function of the metal, its size, shape and surrounding environment.This work analyses the contributions of free and bound electrons to the complex dielectric function of spherical silver NPs and their influence on the optical extinction spectra. The contribution of free electrons is usually corrected for particle size under 10?nm, introducing a modification of the damping constant to account for the extra collisions with the particles boundary.For the contribution of bound electrons, we considered the interband transitions from the d-band to the conduction band including the size dependence of the electronic density states for radii below 2?nm. Bearing in mind these specific modifications, it was possible to determine optical and band energy parameters by fitting the bulk complex dielectric function. The results obtained from the optimum fit are: Kbulk?=?2???1024 (coefficient for bound-electron contribution), Eg?=?1.91?eV (gap energy), EF?=?4.12?eV (Fermi energy), and ?b?=?1.5???1014?Hz (damping constant for bound electrons).Based on this size-dependent dielectric function, extinction spectra of silver particles in the nanometric?subnanometric radius range can be calculated using Mies theory, and its size behaviour analysed. These studies are applied to fit experimental extinction spectrum of very small spherical particles fabricated by fs laser ablation of a solid target in water. From the fitting, the structure and size distribution of core radius and shell thickness of the colloidal suspension could be determined. The spectroscopic results suggest that the colloidal suspension is composed by two types of structures: bare core and core?shell. The former is composed by Ag, while the latter is composed by two species: silver?silver oxide (Ag?Ag2O) and hollow silver (air?Ag) particles. High-resolution transmission microscopy and atomic force microscopy analysis performed on the dried suspension agree with the sizing obtained by optical extinction spectroscopy, showing that the latter is a very good complementary technique to standard microscopy methods.

Size dependent Cu dielectric function for plasmon spectroscopy: Characterization of colloidal suspension generated by fs laser ablation

Copper metal nanoparticles (Nps) have received increasing interest during the last years due to their potential applications in several fields of science and technology. Their optical properties depend on the characteristics of the dielectric function of the metal, their size, and the type of environment. The contribution of free and bound electrons on the dielectric function of copper Nps is analyzed as well as their influence on its plasmonic properties. The contribution of free electrons is corrected for particle size under 10 nm, introducing a term inversely proportional to the particles radius in the damping constant. For bound electron contribution, interband transitions from the d-band to the conduction band are considered. For particles with sizes below 2 nm, the larger spacing between electronic energy levels must be taken into account by making the electronic density of states in the conduction band size-dependent. Considering these specific modifications, optical parameters and band energy val...

Tool induced contamination of elemental powders during mechanical milling

Several metallic and semimetallic elements have been submitted to mechanical milling to investigate their contamination with chrome‐steel milling tools. Contamination was followed with 57Fe Mössbauer spectroscopy and X‐ray diffraction. The contamination yield, defined as the number of incorporated Fe moles per gram of sample, was found to be more simply related to the Poisson ratio than to the Young, shear or bulk moduli, or to the enthalpy of mixing of the Fe‐element couple.

Dipolar interaction and demagnetizing effects in magnetic nanoparticle dispersions: Introducing the mean-field interacting superparamagnet model

Fil: Sanchez, Francisco Homero. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Fisica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Fisica La Plata; Argentina

Small-angle x-ray scattering study of nanocrystalline FeyCu1-y alloys produced by ball milling

Small-angle x-ray scattering measurements on ball-milled FeyCu1-y giant-magnetoresistive alloys (0.141≤y≤0.45) have been performed using synchrotron radiation. For samples with different nominal compositions, prepared under exactly the same conditions, the scattered intensity recorded as a function of the wavevector varied systematically. For this reason, the strong scattering signal was attributed mainly to composition fluctuations in the crystalline grains. The system was treated as a two-phase model consisting of Fe-rich regions in a homogeneous Cu matrix, with composition-dependent relative volume fractions. Real-space analysis of the scattering was performed in terms of a volumetric distribution of spherical particles. Results indicate that the main contribution corresponds to 2 nm scattering objects of well-defined density contrast. The invariant Q was calculated to assess the variations in electron-density contrast as a function of the Fe content in the samples. Calculations based on these results allowed the determination of 0.35 at.% iron concentration in the iron-rich phase for all the nominal compositions studied.

Thermal Evolution of Fe65Ni20Nb6B9 Nanocrystalline Metastable Alloy

The alloy Fe65Ni20Nb6B9 was obtained from the elemental constituents in a high-energy planetary ball mill and was studied by 57 Fe Mössbauer effect spectroscopy, Mössbauer thermal scans and X-ray diffraction. The as prepared nanocrystalline alloy consisted primarily of metastable bcc α-Fe(Ni) nanocrystals (57 nm average size) and small amounts of γ-(Fe,Ni) with Ni concentration of about 58%. Up to about 693 K only defect recovery is inferred. Between 693 and 873 K the α-phase transformed gradually into the fcc γ-phase, whose starting Ni concentration decreased continuously with increasing temperature, reaching a final value which was below 32 at. %. Introduction Magnetic nanocrystals embedded in magnetic and non-magnetic amorphous matrices are of great interest nowadays because of different reasons. On the one hand the behavior of an ensemble of magnetic nanoparticles with interactions of increasing intensity (from free particles to collective systems) is a subject of great interest, which presents basic questions to be answered [1]. On the other hand, one of the most promising routes for developing softer and better magnetic materials for technological use is the manufacturing of dense dispersions of nanocrystals with high saturation magnetization and high permeability embeded in magnetic amorphous matrices [2]. One criterion used to attain this goal is the achievement of systems in which the exchange length lK ≈ (A/K) 1/2 >δ, where A is the exchange constant, K the anisotropy energy density and δ the characteristic size of magnetic entities which have a defined easy direction (as nanocrystals in the above referred dispersions). Such criterion is based on the anisotropy random model, which predicts that under the stated condition the effective anisotropy will vary as δ [3,4] and the system will therefore have softer magnetic properties as δ becomes smaller. In this work we study the alloy Fe65Ni20Nb6B9 obtained from the elemental powders in a highenergy planetary ball mill. Fe and Ni bear magnetic moments and their high global concentration (85 at.%) should impart a high saturation magnetization to the system, whereas Ni, especially within an fcc (γ) phase, would contribute to the softening of the magnetic response. Nb was added to reduce atomic diffusion and hence to prevent grain growth above certain critical size within the nanometric scale. B was used to promote amorphization or disorder in the space left between nanocrystals. Ball milling provides a non-equilibrium route which is proven suitable for the preparation of this sort of nanocomposites. The aim of this article is to present a characterization of the system and of its evolution with temperature from the point of view of composition, structure and microstructure. The system’s, magnetic properties will be published elsewhere. To this end, we applied local and non-local experimental techniques such as Mössbauer effect spectroscopy (MS), Mössbauer thermal scans (MTS)[5] and X-ray diffraction (XRD). Journal of Metastable and Nanocrystalline Materials Online: 2004-07-07 ISSN: 2297-6620, Vols. 20-21, pp 571-575 doi:10.4028/www.scientific.net/JMNM.20-21.571

Mössbauer study of the thermally induced transformation of the Fe0.91B0.09 rapidly quenched crystalline alloy

The transformation of the Fe0.91B0.09 rapidly quenched crystalline alloy from its original metastable state to its equilibrium state has been studied by Mossbauer effect spectroscopy. It was found that the alloy transforms in a single step from a fine dispersion of orthorhombic‐Fe3B‐like complexes embedded in metallic bcc iron to a system consisting of Fe2B precipitates in the α‐Fe matrix. The kinetics and temperature dependence of the process have been measured. It was determined that the relative fraction of Fe2B increases as Ω=1−exp(−kt3/2) with k=k0 exp(−Ea/kBT), indicating a diffusion‐controlled, Arrhenius‐type transformation, where k0=(1.92±0.26)×1014 s3/2 and Ea=(3.56±0.07) eV/atoms. Magnetization versus temperature scans have been simulated and compared to previously measured scans. A simple picture of the transformation process is given.