Solid State Phenomena Vol. 232

Abstract: Oxide nanomaterials are in great demand due to their unique physical, chemical and structural properties. The nanostructured materials with desired magnetic properties are the future of power electronics. Unique magnetic properties and excellent biocompatibility of these materials found applications in pharmaceutical field also. For these applications, the synthesis of magnetic oxide nanomaterials with required properties is highly desirable. Till now, various techniques have been evolved for the synthesis of oxide nanomaterials with full control over their shape, size, morphology and magnetic properties. In nanoscale, the magnetic properties are totally different from their bulk counterparts. In this range, each nanoparticle acts as a single magnetic domain and shows fast response to applied magnetic field. This review article discusses the synthesis techniques, properties and the applications of magnetic oxide nanomaterials. Various characterization techniques for magnetic materials have been discussed along with the literature of iron oxide, nickel oxide, and cobalt oxide nanomaterials. The challenges for further development of these materials have also been presented to broaden their rapidly emerging applications.

Abstract: Nanocrystalline Mn_1-xZn_xFe₂O₄ (x=0, 0.2, 0.4, 0.6, 0.8 and 1.0) ferrites have been successfully synthesised using microwave–hydrothermal method for high frequency applications. The nanopowders were characterised using X-ray diffraction (XRD) and sintered using microwave furnace at 900°C and the total time taken for sintering is 30 min. The frequency dependence of real and imaginary part of permeability were measured in the range 1 MHz to 1.8 GHz. The saturation magnetisation and coercive force were obtained using a vibration sample magnetometer (VSM) in the field of 1.5 T. The temperature dependence of initial permeability (μ_i) was measured in the temperature range of 300K to 600K at 10 kHz. The high values of permeability and saturation magnetization enables these materials to be the potential candidates for a number of applications, for example, in transformers, choke coils, noise filters and recording heads.

Abstract: In this article we report on the structural and magnetic properties of BaFe_12-4xMo_xZn_3xO₁₉ hexaferrites with Mo-Zn substitution for Fe ions. The starting materials were commensurate with the BaM stoichiometry, and the Mo:Zn ratio was 1:3. The powder precursors were prepared by high energy ball milling, and subsequently sintered at temperatures from 1100 to 1300° C. The structural analyses indicated that all samples sintered at 1100° C were dominated by a major M-type hexaferrite phase. The relative abundance of the BaMoO₄ and Zn-spinel secondary phases increased with increasing the concentration of the substituents, resulting in a decrease of the saturation magnetization from about 67 emu/g (for x = 0.0) to 55 emu/g (for x = 0.3). The coercivity also decreased from 3275 Oe (for x = 0.0) to 900 Oe (for x = 0.3), demonstrating the ability to tune the coercivity to the range useful for magnetic recording by the substitution process. The saturation magnetization improved significantly with sintering at T > 1100° C, and the coercivity decreased significantly, signaling the transformation of the samples to soft magnetic materials. These magnetic changes were due to the high-temperature reaction of the spinel phase with the BaM phase to produce the W-type hexaferrite phase on the one hand, and to the growth of the particles on the other hand. The magnetic phases were further investigated using Mössbauer spectroscopy and thermomagnetic measurements. Our study indicated that the sample with x = 0.2 has the highest saturation magnetization (74 emu/g at sintering temperature of 1300° C) and a tunable coercivity between 2100 Oe and 450 Oe.

Abstract: Strontium copper hexaferrite powder with composition Sr₂Cu₂Fe₁₂O₂₂ was synthesized in presence of a non-ionic surfactant Tween-80 using chemical co-precipitation route. The prepared samples were calcinated at 950 oC for 4 hrs in a furnace and then slowly cooled to room temperature to obtain Sr2Cu2Fe12O22 hexaferrite powder. The effect of surfactant concentration on phase formation, microstructure, magnetic and dielectric properties of Sr₂Cu₂Fe₁₂O₂₂ were investigated using XRD, SEM, TEM, VSM, dielectric and low field AC susceptibility measurement techniques. The XRD analysis reveals the formation of mixed phases of Y and M type hexaferrites. The synthesized samples exhibited magnetic properties typical for soft magnetic materials, with saturation magnetization typical for Y-type hexaferrites. The dielectric properties were studied within the frequency range 100 H_Z to 2 MHz. SEM images show formation of non-uniform, spongy and porous structure. The low field AC susceptibility measurements indicate that formed Sr-Cu hexaferrite powder possesses ferrimagnetic to paramagnetic transition at Curie temperature.

Abstract: Superparamagnetic Iron oxide nanoparticles (SPIONs) have fascinated researchers due to their vast applications in biomedical fields such as magnetic resonance imaging, cell sorting, hyperthermia, drug delivery etc. The special properties of SPIONs depend on the method of synthesis and surface modification. Among various synthetic protocols, hydrothermal method has attracted much attention due to simplicity, uniformity and excellent magnetic properties of iron oxide nanoparticles. Magnetic properties of SPIONs could be tuned by controlling the size and shape of the particles as well as by the surface modification. Low colloidal stability and high hydrophobic nature of SPIONs result in aggregation of the particles which could be avoided by surface modification of the SPIONs using various capping agents. The size, shape and surface environment of SPIONs can also be controlled by the surface coating. SPIONs are promising contrast agents due to their non-poisonous nature, biocompatibility and large surface area. The biocompatibility of SPIONs is enhanced by the surface coating/modification. The present review focuses on the hydrothermal synthesis of SPIONs and their characterization using various techniques and the applications of SPIONs in the MRI.Table of Contents

Abstract: We present a brief review of spin torque nanooscillator, which has triggered extensive research interests in the field of nanomagnetism and applied spintronics in recent years. The underlying physical mechanism governing the spin torque nanooscillator is the spin momentum transfer effect, where the angular momentum of itinerant electrons can be passed to localized magnetic moments. The typical device architectures and design of spin torque nanooscillator have been reviewed in this paper, with a particular focus on potential applications of spin torque nanooscillator in the fields of nanotechnology, computing, and biotechnology.Contents of Paper

Abstract: Ferroelectric superlattices with polarization perpendicular to the surface or interface are studied within the framework of the Landau-Ginzburg theory. An interface energy is introduced in the free energy to describe the effect of mixing and local polarization coupling at interface. Internal electric field is considered in the model. For superlattices grown on substrate, the influence of substrate on the properties of ferroelectric superlattices is required. This brief review is a sequel to the previous review article [1], which summarizes the recent development in Landau-Ginzburg theory developed for studying ferroelectric superlattices over approximately the last three years.

Abstract: Nickel substituted cobalt zinc nanoferrites (Co_0.6Zn_0.4Ni_xFe_2-xO₄, x=0.2, 0.4, 0.6, 0.8 and 1.0) were successfully synthesised by sol gel method. FT-IR studies showed two absorption bands in the range of 400-600 cm^-1 corresponding to the M-O bond in the tetrahedral and octahedral clusters, respectively. Powder X-ray diffraction patterns revealed that all the samples had cubic structure with Fd-3m space group.The lattice constant was observed to increase with increase in nickel substitution, thus altering the unit cell volume. An examination of the magnetic properties revealed an increase in saturation magnetization with increasing Ni concentration upto x=0.4, and a decrease there after.These results could be explained using Neel's collinear two-sub-lattice model and three sub-lattice non-collinear model suggested by Yafet and Kittel. DC resistivity was found to decrease with increase in temperature due to semiconductor nature of nanoferrites. The catalytic activity was found to be maximum at x = 0.2 and further found to decrease with increase in nickel concentration

Abstract: Piezoelectricity (PE) is defined as the polarization under homogeneous application of stress on polar/non-centrosymmetry/no-inversion symmetry dielectrics, whereas it has been commonly accepted that flexoelectricity (FLX) is the induced polarization due to strain gradient in any polar/nonpolar dielectrics, the latter effect is universal and can be generated in any materials under inhomogeneous stress. Flexoelectricity is inversely proportional to the size of materials and devices which further suggests that giant FLX effects may develop in nanoscale materials. Flexoelectricity represents the polarization due to strain gradient and have significant effects on the functional properties of nanoscale materials, epitaxial thin films, one-dimensional structure with various shape and size, liquid crystals, polymers, nanobio-hybrid materials, etc. Till late sixties, very few works on flexoelectricity have been reported due to very weak magnitude compared to piezoelectricity. Advancement in nanoscale materials and device fabrication process and highly sophisticated electronics with detection of data with high signal to noise ratio lead the scientists/researchers to get several orders of higher flexoelectric coefficients compared to the proposed theoretical limits. Recently, giant FLX have been observed in nanoscale materials and their magnitudes are six to seven orders larger than the theoretical limits. In this review article, we describe the basic mechanism of flexoelectricity, brief history of discovery, theoretical modeling, experimental procedures, and results reported by several authors for bulk and nanoscale ferroelectric and dielectric materials.