Solid State Phenomena Vol. 202

Abstract: The Ferrite term is used to refer to all magnetic oxides containing iron as major metallic component which has great to technological applications because of their ferromagnetic and insulating properties at room temperature. Among such ferrites, the hexagonal ones (hexaferrites) have long been used for permanent magnets and are of interest for microwave applications. The hexaferrite M-type has a structure built up from the S blocks interposed by the R block and are symbolically described as RSR*S*. In the last decades there has been great interest in the hexaferrites M-Type for applications as electronic components for mobile and wireless communications at microwave/GHz frequencies, electromagnetic wave absorbers for electromagnetic compatibility (EMC), radar absorting material (RAM) and stealth technologies and as composite materials. This review aimed study the structure, magnetic and dielectric properties of the hexaferrite Ba_xSr_1-xFe₁₂O₁₉, which is a promising material for electronic devices and for small dielectric resonator antennas (MRA).The outline of this Review Paper is as follows:

Abstract: Due to a fast progress in the development of communication systems, the dielectric and magnetic ceramics (ferrites) have become attractive to be used in devices. Although the ferrites of the spinel type were the first material to be used in the microwave range, garnets have smaller dielectric losses and, therefore, are chosen for many applications. High demands for modern electric applications in magnetic materials results in new techniques and products being permanently studied and researched, with a consequent appearance of new solutions for a wide applications series. This work presents the study of the ferrimagnetic composite, constituted by Y₃Fe₅O₁₂ (YIG) and Gd₃Fe₅O₁₂ (GdIG) phases, through solid state synthetic route and submitted to high-energy mechanical milling. Additionally, experiments were made in order to evaluate the electric and magnetic behavior of the composites at radio frequency and microwave range and then later suggest an adequate technological application. The composites were efficient as ferrite resonator antennas (FRAs) and microstrip antennas (thick films deposited on metalized surface alumina substrate by screen-printing technique), in the microwave frequency range. The experiments with FRAs showed satisfactory results due to the control of the antennas radiation characteristics and their tuning by the use of an external magnetic field. The composite resonators studied in this work can be important to the development of a third generation (3G) wideband antennas to cell phones and other wireless products.

Abstract: Materials with both electrical and magnetic properties are required for various applications ranging from electric and magnetic shields, molecular electronics, and sensors to microwave absorbers. Conducting polymers with the addition of magnetic oxides composites are suitable for such uses. Such is the case of the polyaniline (PANI), an organic polymer, mixed with magnetite. We have prepared through a new chemical method a PANI/ Fe₃O₄ composite with different amount of magnetic oxide from 0.3% m/m on the composite up to 42% m/m. The final product is a powder with good solubility in some organic solvents as chloroform. Structural morphological studies, transport properties (the electrical conductivity as a function of temperature) and magnetic characterization were performed on the oxides, on PANI and on the composites. We have explored the answer of these composites as absorbers in the microwave region to determine its potential technological application. An interesting magnetoresistance (MR) behavior was observed. The results are related to the particle size and to the influence of the percentage of oxide in the composite.

Abstract: 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/ Sr_1xR_x of 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.

Abstract: The algorithm for fast evaluation of the hysteresis loops of uniaxial or textured ferroelectric microcrystal or grains with long-range interactions is developed. Two types of ceramic microstructures are considered: 1. Random ceramics with complete isotropic distribution function of the crystallographic orientations of grains; and 2. Textured ceramics with anisotropic distribution function of the crystallographic orientations of grains. The qualitative analysis of the hysteretic behavior in terms of the grain distribution function is successfully demonstrated. Comparison of the calculated results with experimental data for Pb (Mg_1/3Nb_2/3)O₃ PbTiO₃ ceramics is presented.

Abstract: We report the Swift Heavy Ion (SHI) irradiation effect on structural and magnetic properties of Mn_0.5Zn_0.5Fe₂O₄ ferrite nanoparticles. The irradiation experiment has been carried out on three different sized, i.e. 63, 82 and 126 Å, Mn_0.5Zn_0.5Fe₂O₄ particles with 100 MeV Si⁺⁸ ions to a fluence 5 × 10¹³ ions/cm² using the 15UD Tandem accelerator. The XRD measurement reveals that the basic single phase spinel structure remains unaltered after SHI irradiation; however, the lattice parameters and particle size decrease on SHI irradiation. The magnetization measurement carried out at 300 K indicates that all the particles are super-paramagnetic at room temperature, but the applied magnetic field of 70 kOe is insufficient to saturate the system. DC and AC magnetization measurements indicate that on irradiation, the sample with an intermediate size is affected more than the other two.

Abstract: The spinel ferrite system CoZn_zGe_zCr_x-zFe_2-x-zO₄ with z= x^2; x = 0.60, 0.65 and 0.70 was studied by Mà T (ZFC-FC) DC magnetization at low and high fields in the temperature range 5K to 300K, MàH measurements with high field up to 55 kOe at 15K temperature and low field AC susceptibility in temperature range 77K to 300K. We observe thermo-magnetic irreversibility (TMI) in MàT study, high field slope with high coercivity in MàH study. The composition x = 0.70 shows constricted hysterisis loop at 15K. We observed a cusp at transition temperature in AC susceptibility measurements. We have shown that the features observed in all these measurements are adequately explained using domain wall dynamics and inherent magneto-crystalline anisotropy without invoking the concept of cluster spin-glass type ordering in x = 0.60 and x = 0.65 in temperature range 5K to 300K. The sample x= 0.70, however showed signatures of cluster spin-glass type of ordering at low temperature as revealed in non-Brillouin type shape of low field FC curves in MàT DC magnetization and constricted hysterisis loop at 15K temperature.

Abstract: MFe₂O₄ (Where M = Cu²⁺, Co²⁺ and Ni²⁺) has been synthesized using a citrate sol-gel self-ignition process in order to investigate their catalytic performance for reduction of p-nitrophenol. The ignited precursors were annealed at 400, 600, 800 and 1000 ^°C for 2 hrs. to obtain nanoferrite particles. The prepared samples were characterized using various instrumental techniques like FT-IR, XRD, and UV-VIS. X-ray analysis confirms the formation of single phase. Powder X-Ray diffraction patterns showed the formation of body centered tetragonal structure for CuFe₂O₄ and cubic structure for CoFe₂O₄ and NiFe₂O₄. It was observed that as the annealing temperature increases from 400 to 1000 ^°C, the XRD peaks grow sharper attributing to an increase in particle size with increasing temperature; which is consistent with TEM. Transmission Electron micrographs analysis revealed an enhancement in grain size with the annealing temperature. The disappearance of yellow colour indicated the conversion of p-nitrophenol to p-aminophenol and the occurrence of colour disappearance has been observed in one minute using CuFe₂O₄ (8mol %). This suggested the efficiency of CuFe₂O₄ in catalyzing p-nitrophenol reduction. FT-IR and UV-Visible spectroscopy has been used to confirm this reduction process. FT-IR spectra showed N-H stretching vibrations at ~3365 cm^-1 and 3350 cm^-1 attributing to the formation of primary amine using CuFe₂O₄ for catalyzing reduction of p-nitrophenol. The disappearance of the absorption band at 400 nm in the UV-Visible spectrum also confirms the conversion of p-nitrophenol to p-aminophenol.

Abstract: Nano-crystalline Co_0.6Zn_0.4Cu_0.2Cr_xFe_1.8-xO₄ (x = 0.2, 0.4, 0.6 and 0.8), have been synthesized using a citrate sol-gel auto combustion method and annealed at different temperature 400 ^°C, 600 ^°C, 800 ^°C and 1000 ^°C. The effect of chromium substitution on the structural, magnetic and dielectric properties of cobalt-zinc-copper ferrite has been studied. The structural and magnetic characteristics have been studied by X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) and Vibrating Sample Magnetometer (VSM) techniques. The X-ray diffractogram of all the annealed samples confirm the formation of single phase with Fd-3m space group. The crystalline sizes have been found to increase (from ~15 nm – to 60 nm) with the annealing temperature. The DC electrical resistivity of all the ferrites has been measured using a two-probe method between temperature range of 30 ^°C to 100 ^°C. The DC electrical resistivity of all the samples decrease with increase in temperature indicates semiconducting nature. However, the DC resistivity increase as the Cr³⁺ concentration increases because the Cr³⁺ ions enter the octahedral sites and reduce the electron exchange between Fe²⁺ and Fe³⁺ causing a decrease in polaron hopping of Fe²⁺-Fe³⁺ions. The value of the Seebeck coefficient (S) for all the ferrites is found to be positive indicating that all the ferrite samples behave as p-type semiconductors. The effect of copper chromium cation distribution among the tetrahedral (A) and octahedral (B) sites of Co-Zn substituted ferrite on magnetization and coercivity field have been investigated using VSM (vibrating sample magnetometer) technique. The decrease in the saturation magnetization (M_s) with increasing chromium content may be attributed to the copper and chromium enters into the octahedral site of the Co-Zn ferrite.