Solid State Phenomena Vol. 241

Abstract: Hexagonal ferrites, or hexaferrites, are hugely important materials commercially and technologically, with common applications as permanent magnets, magnetic recording and data storage media, components in electrical devices operating at wireless frequencies, and as GHz electromagnetic wave absorbers for EMC, RAM and stealth technologies. Hexaferrites are all ferrimagnetic materials, and their magnetic properties are intrinsically linked to their crystalline structures, all having a strong magnetocrystalline anisotropy; that is the induced magnetisation has a preferred orientation within the crystal structure. They can be divided into two main groups: those with an easy axis of magnetisation (known as uniaxial), the hard hexaferrites, and those with an easy plane (or cone) of magnetisation (known as ferroxplana or hexaplana), soft ferrites. The common hexaferrite members are:M-type ferrites, such as BaFe₁₂O₁₉ and SrFe₁₂O₁₉Z-type ferrites (Ba₃Me₂Fe₂₄O₄₁)Y-type ferrites (Ba₂Me₂Fe₁₂O₂₂)W-type ferrites (BaMe₂Fe₁₆O₂₇)X-type ferrites (Ba₂Me₂Fe₂₈O₄₆)U-type ferrites (Ba₄Me₂Fe₃₆O₆₀)where Me = a small 2⁺ ion such as cobalt, nickel or zinc, and Ba can be fully substituted by Sr. Generally, the M ferrites are hard, the Y, Z and U ferrites are soft, and the W and X ferrites can very between these two extremes, but all have large magnetisation (M) values.There is currently increasing interest in composite materials containing hexaferrite fibres. It had been predicted that properties such as thermal and electrical conductivity, and magnetic, electrical and optical behaviour will be enhanced in material in fibrous form. This is because a continuous fine fibre can be considered as effectively one-dimensional, and it does not behave as a homogeneously distributed solid. Although the intrinsic magnetisation of the material is unaffected, the effective magnetisation of an aligned fibre sample should be greater when a field is applied parallel with fibre alignment compared to when applied perpendicularly to fibre alignment. This feature was first demonstrated by the author for aligned hexaferrite fibres in 2006. This chapter will deal with progress in the manufacture and properties of hexaferrite fibres, from the first syntheses of BaM, SrM, Co₂Y, Co₂Z, Co₂W, Co₂X and Co₂U micron-scale fibres by the author 12-15 years ago, to recent developments in M ferrite hollow fibres and nanofibres, and hexaferrite-coated CNTs (carbon nanotubes).The relative properties of all reported hexaferrite fibres are compared and summarised at the end of this chapter.

Abstract: The work deals with the study of electrical and magnetic properties of Magnesium-Chromium and Cobalt-Chromium nanoFerrites for their potential applications synthesized by Citrate-Gel auto-combustion method. Structural Characterization of prepared nanoferrites was performed using XRD, SEM, EDS and TEM. XRD patterns confirmed the formation of homogeneous single phased cubic spinel belonging to the space group Fd3m (in agreement with ICSD Ref. data). It is found that crystallite size of Mg-Cr ferrites was in the range of 7-23nm and that of Co-Cr nanoferrites in the range of 6-12 nm. Structural morphology of both the ferrites was studied by SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy). Elemental compositional analysis was carried out by EDS.Electrical properties such as D.C. resistivity of prepared Mg-Cr and Co-Cr nanoferrites were studied by two probe method. The dielectric measurements as a function of frequency at room temperature were performed in the frequency range of 20Hz to 2MHz using LCR meter. Various dielectric parameters like dielectric constant (ε′), dielectric loss (ε′′) and dielectric loss tangent (tan δ) were measured as a function of frequency. The magnetization measurements of the prepared nanoferrite samples were carried out at room temperature in the applied field of ±15KOe using Vibrating Sample Magnetometer (VSM). From the obtained M-H loops, various magnetic parameters such as Saturation Magnetization (M_s), Coercivity (H_c) and Remanence Magnetization (M_r) were measured. Magnetization as a function of field (±10T) at 5K, 25K, 300K temperatures was measured using VSM. Super-paramagnetic nature of some specified samples in Mg-Cr nanoferrites system was investigated from the temperature dependence of both the field cooled (FC) and the zero-field cooled (ZFC) magnetization measurements under a field of 100 Oe in the temperature range 5K to 350K.From the results of D.C. resistivity measurements it is observed that the resistivity decreases with increase in temperature for both Mg-Cr and Co-Cr nanoferrites suggesting the semiconducting behavior of the samples. Dielectric measurements suggest that the conduction in the ferrite systems may be due to the polaron hopping mechanism. The low loss tangent values at high frequency show the potential applications of these materials in high frequency microwave devices. Mg-Cr nanoferrites were synthesized yielding narrow hysteresis loops which are the characteristic of very soft magnetic materials that are desirable for their utility in Transformers, Inductor cores, Microwave devices and Magnetic shielding. Hysteresis loops of Co-Cr nanoferrites show the medium hard magnetic behavior of the materials. MgFe₂O₄ shows superparamagnetic behavior above room temperature and MgCr_0.9Fe_1.1O₄ shows superparamagnetic behavior with the blocking temperature 94.5K. Hence these nanoferrites find applications in targeted drug delivery and in Magnetic Resonance Imaging (MRI) in biomedical field. The observed results can be explained in detail on the basis of composition and crystallite size.Contents of Paper

Abstract: The present study is concerned with the fabrication and characterization of Me₂Y substituted hexaferrites, Ba₂Me₂Fe_12-xT_xO₂₂ (Me = Co²⁺, Mg²⁺, and Cr²⁺, and T = Fe³⁺, and Ga³⁺). The samples were prepared by the conventional ball milling technique and sintering at 1200° C. The effect of the choices of Me and T ions on the structural and magnetic properties of the hexaferrites were investigated. XRD patterns, magnetic parameters, and Mössbauer spectra of the Co₂Y were consistent with a single phase Y-type hexaferrite. However, the CoCr-Y sample was found to be dominated by the Y-type hexaferrite, and M-type and BaCrO₄ minority phases were observed in the XRD pattern of the sample. The small increase in saturation magnetization from about 34 emu/g up to 37.5 emu/g was therefore attributed to the development of the M-type phase. On the other hand, XRD pattern of the Cr₂Y sample indicated the dominance of the M-type phase in this sample. The high coercivity (1445 Oe) of this sample is evidence of the transformation of the material from a typically soft magnetic material (Y-type) to a hard magnet (M-type). The Ga-substitution for Fe in Co₂Y did not affect the saturation magnetization significantly, but the coercivity was reduced. However, the sample Ba₂CoMgFe₁₁GaO₂₂ exhibited a significant reduction of the saturation magnetization down to a value 26.6 emu/g, which could be due to the attenuation of the super-exchange interactions induced by the Mg²⁺ substitution.

Abstract: Recent applications of transition metal nanoferrites as catalyst in thermal decomposition of ammonium perchlorate (AP) and combustion of composite solid propellant (CSP), have been reviewed. Catalytic applications include the use of mainly cobalt, nickel, copper, zinc, manganese, cadmium nanoferrites, as well as their mixed-metal combinations. The nanoferrites are obtained mainly by wet-chemical, sol-gel, solvo-thermal, auto-combustion and co-precipitation methods. Addition of nanoferrites to AP led to shifting of the high temperature decomposition peak toward lower temperatures which shows their catalytic activity. The burning rates of CSPs have also been enhanced by these nanoferrites. Contents of Paper

Abstract: The study of ceramic materials has attracted the attention of many researchers due to the possibility of their use in nanotechnology. The spinel ferrites form a large group of materials with a broad range of applications. Some examples include electronic devices such as high-frequency transformer cores, antenna rods, induction-tuners, among many others. However, when the ferritic materials display superparamagnetic behavior, their potential for biological applications like drug delivery, hyperthermia, resonance magnetic imaging and magnetic separation, become amazingly high. Therefore, the superparamagnetism is a characteristic strongly desired for spinel ferrites. Since this phenomenon is size-dependent, the methodologies to synthesize these materials has emerged as a crucial step in order to obtain the desired properties. In this regarding, several synthetic processes have been developed. For example, co-precipitation is a fast and cheap method to synthesize superparamagnetic spinel ferrites. However, methodologies involving microwave, ultrasound or polymers frequently result in these kind of materials. Therefore, this review brings a brief historic introduction about spinel ferrites as well as essential concepts to understand their structure and magnetic properties. In addition to this, recent advances in synthesis and applications of the superparamagnetic spinel ferrites are mentioned. Contents of Paper

Abstract: The discovery of novel materials, processes, and phenomena at the nanoscale and the development of new experimental and theoretical techniques for research provide fresh opportunities for the development of innovative nanosystems and nanostructured materials. Nanomaterials with tailored unique properties have limitless possibilities in materials science. The most widely used synthesis routes for iron oxide nanoparticles are based on precipitation from solution. Most of the nanoparticles available to date have been prepared using chemical route. Physical processes have also been recently developed to produce high quality monodisperse and monocrystalline iron oxide nanoparticles. Magnetite has recently attracted attention because bulk Fe₃O₄ has a high Curie temperature of 850 K and nearly full spin polarization at room temperature, and due to its wide range of applications in almost all branches of science and technology. Clearly, nanoscale magnetite offers potential for creation of novel technology in multiple fields of study. Opportunities for magnetite nanoparticles to be effectively incorporated into environmental contaminant removal and cell separation magnetically guided drug delivery, imaging of tissue and organs, magnetocytolysis, sealing agents (liquid O-rings), dampening and cooling mechanisms in loudspeakers, high gradient magnetic separation (HGMS) techniques and contrasting agents for magnetic resonance imaging (MRI). Advancement of synthesis and stabilization procedures towards production of uniformly sized, dispersed (potentially embedded) magnetite nanoparticles has clearly inspired creative imagination and application in various fields.

Abstract: Three series of NiMgCuZn ferrites were prepared by conventional double sintering ceramic process. The formation of single phase in these ferrites was confirmed by X-ray diffraction. A brief review of the important investigations carried out on the internal friction behaviour of NiMgCuZnFe₂O₄, in the temperature range 40oC to 360oC, has been reported. In the present investigation, the composite piezoelectric resonator method has been used. The effect of compositional changes of ferrites on Curie temperature and internal friction are reported. Results and discussions on the temperature variation of internal friction of the three series of NiMgCuZnFe₂O₄ samples are discussed. In all the series studied, only single stress induced relaxation peaks are observed. These studies were carried out to develop a ferrite composition for their use as core materials for microinductor applications. The results are explained in the light of structural phase transitions.

Abstract: Influence of Ca substitution on structural, magnetic and dielectric properties of Ba₃Co_2-xCa_xFe₂₄O₄₁ (where x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0), prepared by Sol-Gel auto-combustion method, has been investigated in present studies. The obtained powder was sintered at 950 oC for 4 hrs. in the static air atmosphere. Structural analysis of Ca-doped Ba₃Co_2-xCa_xFe₂₄O₄₁ powders revealed pure Z-type hexaferrite phase at low temperature. The frequency dependent dielectric constant (Єʹ) and magnetic properties such as remanent magnetization (Mr), saturation magnetization (Ms) and coercivity (Hc) were studied. It is observed that coercivity increased gradually with increase in calcium content. The real dielectric constant (Єʹ) and dielectric loss tangent (tan δ) were studied in the frequency range of 20Hz to 2MHz. The dielectric parameters for all samples show normal dielectric behavior as observed in hexaferrites. Contents of Paper