Ioan Dumitru

Study of magnetic interactions in metallic nanowire networks

Nanowire arrays are characterized using ferromagnetic resonance (FMR) experiments. The dipolar interactions between nanowires are studied by investigating the angular variation of the FMR resonant field. Two qualitatively different angular variations in the resonant field are experimentally observed and explained using the interaction field. The experimental data are explained using a theoretical model that considers the interaction field in nanowire networks as arising from magnetic charges at the wires surface (wires extremities and lateral cylindrical surface) and takes into account the finite wire length and porosity of the sample.

Annealing effects on the crystallite size and dielectric properties of ultrafine Ba1−xSrxTiO3 (0 < x < 1) powders synthesized through an oxalate-complex precursor

Nanocrystalline Ba1−xSrxTiO3 (0 < x < 1) nanopowders were prepared through the oxalate route and the influence of the thermal history on their dielectric properties was investigated. For pelletized samples obtained by decomposing the corresponding oxalate intermediates at 600 °C and then annealed at different temperatures ranging from 600 to 1000 °C the dielectric constant decreases upon increasing the strontium content of the samples. Similar behavior was observed with decreasing the temperature of the thermal treatment of the pellets, a trend which is presumably ascribed to a synergic effect of decreasing the porosity of the samples, the increase of the crystallite size as well as the growth of nanograins. Additionally, a two-fold increase of the dielectric constant is also achievable by using a different annealing strategy, which consists of the direct heat treatment of the intermediates at different temperatures ranging between 600 and 1000 °C. In this latter case, the experimental data unambiguously show a direct correlation between the variation of the dielectric constant and that of the crystallite size/microstrain of the BSTO nanopowders.

Structural and Magnetic Characterizations of Coprecipitated Ni&#8211;Zn and Mn&#8211;Zn Ferrite Nanoparticles

Two mixed ferrite systems, namely Ni0.65Zn0.35 Fe2O4 (Ni-Zn) and Mn0.75 Zn0.18 Fe2.07 O4 (Mn-Zn) have been prepared by coprecipitation method, and then the resulting ultrafine powders were heat treated at different temperatures from 200 to 800degC for improved crystallinity and magnetic properties. The samples were characterized by X-ray diffraction, vibrating sample magnetometry, and ferromagnetic resonance spectrometry. As a result of the heat treatment, the average particle size has been found to increase from 9.9 to 15.7 nm for Ni-Zn ferrites and from 2.4 to 10.2 nm for Mn-Zn ferrites, and the corresponding magnetization values have increased from 9.1 to 23 emu/g for Ni-Zn ferrites and from 7.9 to 11.7 emu/g for Mn-Zn ferrites, respectively. The results are discussed in the light of changes in particle size and inversion degree parameter for cationic distribution at nanoscales

Probing two-dimensional magnetic switching in Co∕SiO2 multilayers using reversible susceptibility experiments

The magnetic switching behavior of Co∕SiO2 multilayers has been studied using reversible susceptibility experiments performed along different orientations in the sample’s plane. A sensitive method for critical curve determination of two-dimensional magnetic systems was proposed. It was shown that this method, based on reversible susceptibility’s singularities detection, is general and can be applied independent of the expression of free energy describing the magnetic system under study. It is found that as the Co∕SiO2 ratio increases in the samples, the switching mechanism is governed by a noncoherent rotation mechanism.

Controlling temperature in magnetic hyperthermia with low Curie temperature particles

Hyperthermia induced by the heating of magnetic particles (MPs) in alternating magnetic field receives a considerable attention in cancer therapy. An interesting development in the studies dedicated to magnetically based hyperthermia is the possibility to control the temperature using MPs with selective magnetic absorption properties. This paper analyzes the temperature field determined by the heating of MPs having low Curie temperature (a FeCrNbB particulate system) injected within a malignant tissue, subjected to an ac magnetic field. The temperature evolution within healthy and tumor tissues was analyzed by finite element method simulations in a thermo-fluid model. The cooling effect produced by blood flowing in blood vessels was considered. This effect is intensified at the increase of blood velocity. The FeCrNbB particles, having the Curie temperature close to the therapeutic range, transfer the heat more homogeneous in the tumor keeping the temperature within the therapeutic range in whole tumor vol...

High Frequency Absorption of PVC/Iron Oxides and PVC/CoFe

In this work, we present a study on the microwave frequency absorption of PVC/γ-Fe₂O₃, PVC/Fe₃O₄, and PVC/CoFe₂O₄/CoO nanofibers. The composite nanofibers with size ranging between 100 nm and 1 μm where obtained by electrospinning technique. The absorption properties of synthesized nanofibers were measured between two open-end rectangular waveguides in the 8-12 GHz frequency domain. The value of the transmission loss was found to be below -10 dB for PVC/Fe₃O₄ composite nanofibers demonstrating that this material can be used as absorber of electromagnetic radiation in the mentioned frequency range.

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Magnetic hyperthermia uses the targeted therapeutic heat from magnetic particles (MPs) in an alternating magnetic field to kill cancer cells. MPs with low Curie temperature (Tc) (in the range 42°C-45°C), high magnetization, and magnetic permeability/susceptibility are good candidates for their use in hyperthermia therapy. This paper analyzes the hyperthermic effects determined by the MPs with low Tc within a tumoral configuration from healthy tissue when an alternating magnetic field is applied. The temperature field was determined as a solution of the Pennes bioheat transfer equation. The spatial distribution of the particles after their injection within the tissues was computed by solving the convection-diffusion equation in porous tissues. Results show that the MP injection rate significantly influences the spatial distribution of the particles and the temperature field of the tumor. Higher values of the ferrofluid flow rate push more particles to the tumor center, thus also elevate its central temperature. The temperature field becomes uniform in a percentage of 90% of tumor volume if a blood vessel is localized at the tumor center.

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In this paper, the temperature dependence of the hysteretic processes of Co nanowires, squarelly ordered in an array prepared by electrodeposition in nanopores of alumina membranes was analyzed. Both the magnetostatic interactions induced in the nanowires arrays and the thermal stresses radial, azimuthal and axial stresses, which appear during the cooling of the system nanowire and alumina template from room temperature to 3 K was evaluated. The analysis of thermal induced stresses provides useful informations concerning the magnetic anisotropy in the Co nanowires. The temperature dependence of the remanent magnetization and coercitive field as an effect of the induced thermal stresses and magnetostatic interactions between nanowires was studied.

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The ferroelectric-magnetic composite materials are very suitable for designing and producing microwave devices and components, due to their dielectric and magnetic properties. For microwave antennas, sensors, resonators, oscillators, filters, phase shifters, attenuators, and amplifiers, the output and input impedances, the resonance frequencies, the quality factor (the losses), and the attenuation or the amplification can be very easily adjusted by modifying the dielectric permittivity or/and the magnetic permeability. This is possible by a right choice of the ratio between the ferroelectric and magnetic phases in the composite material. Therefore, it is highly important to accurately determine these characteristics of the composite material in microwave range.

/CoO Nanofibers Produced by Electrospinning Technique

The effect of interparticle interactions on the precessional switching process of coupled ellipsoidally shaped particles subject to pulsed magnetic fields is analyzed by using the Landau–Lifshitz–Gilbert equation. The analysis of switching process is made using two different representations previously proposed in the case of isolated single domain particle. In one switching diagram the final configurational state of the magnetization vector is displayed as a function of the pulse strength and direction whereas in the other representation the final state of magnetization is correlated to the initial position of magnetization vector. It is found that the dynamics of the coupled system, reflected in both switching diagrams is strongly dependent of the interparticle distance and their bond angle.