Silvia E. Jacobo

Synthesis of ultrafine particles of barium ferrite by chemical coprecipitation

A new method to synthesize barium hexaferrite by chemical coprecipitation is described. A mixed precursor was precipitated by addition of a barium salt to a strongly alkaline ferrate (VI) solution. The precursor yielded barium hexaferrite on heating for 6 h at 800 °C; partial transformation was evident even at lower temperatures, from X-ray patterns and Mossbauer spectra. Scanning electron microscopy of the powders fired at 800 °C, showed that the particles were less than 0.5 μm in diameter.

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.

Mössbauer analysis of Nd-Co M-type strontium hexaferrite powders with different iron content

The structural analysis of strontium hexaferrites SrFexO19 (for x = 12, 11 and 10) and substituted samples Sr0.7Nd0.3Fe12 − yCo0.3O19 (for y = 0.3,1.3 and 2.3) prepared through the citrate precursor method is shown. Nd and Co substitution modifies saturation magnetization (MS) and increases coercivity (Hc) in samples heat-treated at 1100°C for two hours. Mössbauer analyses show different iron occupancy and the influence of the Fe3 +  content is particularly emphasized. Hematite segregation is observed for some compositions. Samples with low Fe3 +  content show the best magnetic properties with no secondary phase segregation.

Microwave Resonant and Zero-Field Absorption Study of Doped Magnetite Prepared by a Co-Precipitation Method

Fe3O4 and ZnxFe3−xO4 pure and doped magnetite magnetic nanoparticles (NPs) were prepared in aqueous solution (Series A) or in a water-ethyl alcohol mixture (Series B) by the co-precipitation method. Only one ferromagnetic resonance line was observed in all cases under consideration indicating that the materials are magnetically uniform. The shortfall in the resonance fields from 3.27 kOe (for the frequency of 9.5 GHz) expected for spheres can be understood taking into account the dipolar forces, magnetoelasticity, or magnetocrystalline anisotropy. All samples show non-zero low field absorption. For Series A samples the grain size decreases with an increase of the Zn content. In this case zero field absorption does not correlate with the changes of the grain size. For Series B samples the grain size and zero field absorption behavior correlate with each other. The highest zero-field absorption corresponded to 0.2 zinc concentration in both A and B series. High zero-field absorption of Fe3O4 ferrite magnetic NPs can be interesting for biomedical applications.

Coercivity Enhancement of Hexagonal Ferrites

Hexagonal ferrites have been widely used as permanent magnets since their discovery in the 1950s. In spite of their relatively modest magnetic properties, ferrite magnets still show the best performance-to-cost ratio and different investigators are trying to improve their magnetic capabilities by using different synthesis methods and compositions. Different scientific investigations and techniques (Mössbauer spectrometry, X-ray diffraction, and magnetic measurements) have allowed to optimize the permanent magnet properties of rare earth substituted hexagonal ferrite magnets such as La-Co and Nd-Co Sr and Ba ferrites. However, the solubility of rare earth ions in M-type hexaferrite is very low and their introduction leads to the formation of secondary phases, which must be avoided in order to obtain permanent magnets with optimal properties. We report results on enhanced coercivity of hexagonal Sr ferrites with Nd-Co substitution synthesized by the self-combustion method and calcination at 1100°C for two hours. The synthesis of this kind of ferrite is performed with a deficient, non-stoichiometric iron content (ratio Fe/ Sr1xRxof 10 and 11 instead of 12) in order to explore the presence of secondary phases. Comparison with samples of the same composition and stoichiometric formulation is made. Samples with lower iron content show the highest saturation magnetization, remanence and/or coercivity, indicating that the best results for applications of this ferrite will be obtained with an iron deficiency in the stoichiometric formulation. Nd substitution enhances the ferrite anisotropy and coercivity with respect to the unsubstituted sample.

Structural and Magnetic Properties of Nanoparticles of NiCuZn Ferrite Prepared by the Self-Combustion Method

NiCuZn ferrites were prepared by the sol-gel self-combustion method. Nanosized, homogeneous and highly reactive powders were obtained at relatively low temperatures. In present work the variations of structural, magnetic, and microwave properties of NiCuZn ferrite nanoparticles were studied as a function of the annealing temperature. The analysis of XRD patterns showed that only the spinel phase is present. Cell parameters slightly vary with thermal treatment while a crystalline size increases. Magnetic nanoparticles were mixed with an epoxy resin for reflectivity studies with a microwave vector network analyzer using the microwave-guide method in the range of 7.5 to 13.5GHz. Static saturation magnetization value (measured by SQUID) and microwave absorption show clear dependence on the annealing temperature/particle size and the absorption maximum moves towards the higher frequencies with an increase in the average size of the particles.

Mössbauer Study of Ni-Zn Ferrites Obtained by the Self-Propagated Method

Ni1−xZnxFe2O4 ferrites (x = 0, 0.5, 0.6 and 0.7) were prepared from self-propagated powders. Their structural and magnetic properties were characterised with x-ray diffraction, Mossbauer spectroscopy and ac susceptibility. Mossbauer analysis was performed at room temperature and at 15K, which was expected to yield useful information on the microstructure. The sintered samples present high initial permeability that increases as the concentration of Zn increases. The x= 0.7 Mossbauer spectrum exhibits superparamagnetism at room temperature and magnetic order at low temperature. The roles of cation-site occupation and magnetic coupling are discussed.

Interfacial chemistry of powdered barium ferrites immersed in aqueous solutions: leaching in acidic chloride media

The kinetics of leaching and dissolution of barium mono- and hexa-ferrite in acidic chloride media are presented and discussed. The monoferrite is appreciably more reactive, and dissolves in a nearly congruent way in low pH media. In the case of barium hexaferrite, iron and barium dissolution are independent processes. Initially, fast barium leaching is observed, which is followed by a slower process controlled by diffusion through a growing iron oxide layer. Iron dissolves at nearly constant rates that are a function of pH; the apparent kinetic order on H+ is 0.66, typical of acid dissolution mechanisms. The differing reactivities of the mono- and the hexa-ferrite relates to their crystal chemistry.

Synthesis and Characterization of Carbon-Coated Magnetite for Functionalized Ferrofluids

Carbon-coated magnetite nanoparticles (NPs) were synthetized by the mechanochemical method in a high-energy ball mill using hematite and amorphous carbon as precursors. The milling was performed at 700 r/min, with a ball/powder mass ratio of 35 in stainless steel vials with WC balls and Ar atmosphere. The precursor powders were milled from 1 to 18 hours and they were annealed for 2 h in Ar at 500 °C. Structural and magnetic properties of the NPs were investigated by X-ray diffraction (XRD), vibrating sample magnetometry and high resolution transmission electron microscopy (HRTEM). XRD patterns, refined with the Rietveld method, show that magnetite is present in samples milled from 6 hours onward and that after milling for 18 hours and annealing, the sample contains a single crystalline phase. Magnetization curves for samples with different milling times show saturation magnetizations ranging from 34.1 emu/g after 1 h to 78.0 emu/g after 18 h. Coercive fields are about 500 Oe for all samples. HRTEM studies reveal that the samples are made of amorphous carbon clusters with magnetite NPs less than or equal to 20 nm. This system seems appropriate for biomedical applications.

Magnetic and Dielectric Properties of Nanophase Lithium-Substituted Manganese-Zinc Ferrite

Nanocrystalline lithium-substituted manganese-zinc ferrites Li0.5xMn0.4Zn0.6-xFe2+0.5xO4 were prepared by the sol-gel autocombustion method. X-ray diffraction analysis (XRD) confirmed that samples are single-phase and that only a spinel phase is present. The saturation magnetization increases while the cell parameter of the cubic phase decreases with Li concentration. Magnetic permeability and dielectric permittivity of all samples were measured at room temperature as a function of frequency. Reflection loss calculations show that the prepared samples are good electromagnetic wave absorbers in microwave range. Li substitution plays an important role in changing the structural and magnetic properties of these MnZn ferrites.