G. A. Pasquevich

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.

A new application of Mössbauer effect thermal scans: determination of the magnetic hyperfine field temperature dependence

Mossbauer thermal scans proved to be suitable to determining the magnetic hyperfine field temperature dependence at the Fe site of the antiferromagnet FeSn2, if the Doppler energy is fixed at a value such that some of the nuclear transition energies cross that of the incident gamma ray when temperature is varied.

A constant-velocity Mössbauer spectrometer with controlled temperature sweep

A constant-velocity Mossbauer spectrometer with controlled temperature sweep of the sample is presented. The equipment was developed for Mossbauer thermal scanning experiments, allowing the determination of the evolution of the magnetic hyperfine field with temperature. It is based in a three level architecture implemented in a medium sized personal computer (PC). The instrument runs the widely used Linux operating system, supporting several custom microprocessor-based boards, extending the hardware capabilities of the PC. This scheme optimizes the required time for the experiment, exploiting in the background the full capabilities of modern multitasking operating systems in terms of networking, graphical user interfaces, and data storage. The prototype was tested applying the method to the simplest case, i.e., a symmetric sextet collapsing into a singlet. It was found that required times are an order of magnitude smaller than those demanded by the conventional methodology. The results obtained for the 57...

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

Study of Magnetic Materials by Mössbauer Thermal Scans. Application to Nanocrystalline Systems

Mossbauer thermal scans (2 K/min) have been applied to the classical amorphous finemet-type precursor Fe 73.5 Si 13.5 Nb 6Cu 1B9 alloy. They allowed the determination of the ordering temperatures of the amorphous phase before (T C = 624 K) and after partial (T C = 674 K) and complete (T C = 595 K) crystallization. The existence of three well differentiated temperature regions is observed, corresponding to the ferro to paramagnetic transition of the amorphous, nanocrystallization of bcc Fe(Si) and the ferro to superpara or paramagnetic transition of the crystalline phase. The crystallization kinetics at 763 K and 805 K was studied by two different approaches. At 763 K spectra were recorded every two hours up to a total time of 117 h. At 805 K scans were used due to the much higher transformation rate. Both results were analized with theoretical contributions representing the amorphous, very small crystals or embryos and grown up crystals. The evolution of their relative abundances obtained with the two approaches was consistent. The embryo nucleation was almost completed after two and eight hours at 763 and 805 K, respectively.

Mössbauer thermal scan study of a spin crossover system

Programmable Velocity equipment was used to perform a Mossbauer Thermal Scans to allow a quasi-continuous temperature study of the magnetic transition between the low-spin and a high-spin configurations in (Fe(Htrz)2(trz))(BF4) system. The material was studied both in bulk as in nanoparticles sample forms.

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

Optimal configuration for programmable Mössbauer experiments

Based on channel independency of recently developed M?ssbauer instrumentation an approximation to optimal configuration of experiments is presented. The analysis relies on the presumption that all the available channels of the spectrum are not equally efficient for a given experimental application. A quantification of this concept is presented and a method for different channel layout comparison is proposed. The optimization of recorded spectra is important in dynamic experiments where efficiency in data taking imposes feasibility limits as well as in static applications as a way of reducing experimental time.

Design and testing of a pilot scale magnetic separator for the treatment of textile dyeing wastewater

Iron nanoparticles can be incorporated on the structure of natural clays to obtain magnetic clays, an adsorbent that be easily removed from a wastewater by magnetic means. Magnetic clays have high adsorption capacities of different contaminants such as heavy metals, fungicides, aromatic compounds and colorants and show rapid adsorption kinetics, but crucial data for achieving its full or pilot scale application is still lacking. In this work, magnetic bentonites with different amounts of magnetite (iron fractions on the clay of 0.55, 0.6 and 0.6) were used to remove color from a real textile wastewater. On a first stage the optimal conditions for the adsorption of the dye, including pH, temperature and clay dosage were determined. Also design parameters for the separation process such as residence time, distance from magnet to magnetic clay and magnet strength were obtained. Finally a pilot scale magnetic drum separator was constructed and tested. A removal of 60% of the dye from a wastewater that contained more than 250 ppm of azo dye was achieved with only 10 min of residence time inside the separator.