Structural, electrical and magnetic characterizations of Ni-Cu-Zn ferrite synthesized by citrate precursor method

Fine powders of NiCuZn ferrite with composition Ni_(0.7−x)Cu_xZn_0.3Fe₂O₄ (where x=0, 0.2, 0.4 and 0.6) were prepared by the citrate precursor method. X-ray diffraction measurements confirm the formation of single-phase cubic spinel structure. The grain size was estimated by SEM micrograph which increases with Cu content. Dielectric constant (?) and loss tangent (tan δ) were measured as a function of frequency. The ? and tan δ show a decreasing trend with increase of frequency for all the samples. The DC resistivity was measured as a function of temperature. The temperature-dependent DC resistivity measurements show that the room-temperature DC resistivity of NiCuZn ferrite with x=0.2 is of the order of 10⁹ Ω cm. The AC conductivity (σ_AC) was studied as a function of frequency. The hysteresis data indicate that the maximum saturation magnetization of 38.66 emu/g is obtained for the composition with x=0.2.     

Novel conducting lithium ferrite/chitosan nanocomposite: Synthesis,characterization, magnetic and dielectric properties

A study on Lithium ferrite/chitosan nanocomposite (LFCN), easily moldable into arbitrary shapes, as the conducting polymer and ferromagnetic characteristics is presented. The composite material is produced in the presence of Li_0.5Cr_0.1Fe_2.4O₄ and Li_0.5Co_0.1Fe_2.4O₄ nanoparticle by ex-situ polymerizations process. Various characterizations techniques have been used to explore the characteristic of the synthesized products. The frequency dependent dielectric properties and electrical conductivity of all the samples have been measured through complex impedance plot in the frequency range of 1 kHz–6 MHz at room temperature. It was observed that in case of (LFCN), fluctuation in value of (ε′) and (ε″) is ceased over the frequency range of 4 Mz which can be attributed to the steady storage and dissipation of energy in the nanocomposite system. Moreover, it is also observed that electrical conductivity of (LFCN) increases with frequency and its value was found to be (0.032–0.048) (ohm-cm)⁻¹ in frequency range of 1 kHz–6 MHz. Due to its low cost, a simple synthesis process and high flexibility, the proposed LFCN may find applications in various types of electronic components.     

Dielectric,electrical transport and magnetic properties of Er3+substituted nanocrystalline cobalt ferrite

Erbium substituted cobalt ferrite (CoFe_2−xEr_xO₄; x=0.0–0.2, referred to CFEO) materials were synthesized by sol-gel auto-combustion method. The effect of erbium (Er³⁺) substitution on the crystal structure, dielectric, electrical transport and magnetic properties of cobalt ferrite is evaluated. CoFe_2−xEr_xO₄ ceramics exhibit the spinel cubic structure without any impurity phase for x≤0.10 whereas formation of the ErFeO₃ orthoferrite secondary phase was observed for x≥0.15. All the CFEO samples demonstrate the typical hysteresis (M–H) behavior with a decrease in magnetization as a function of Er content due to weak superexchange interaction. The frequency (f) dependent dielectric constant (ε′) revealed the usual dielectric dispersion. The ε′–f dispersion (f=20 Hz to 1 MHz) fits to the modified Debye's function with more than one ion contributing to the relaxation. The relaxation time and spread factor derived are ∼10⁻⁴ s and ∼0.61(±0.04), respectively. Electrical and dielectric studies indicate that ε′ increases and the dc electrical resistivity decreases as a function of Er content (x≤0.15). Complex impedance analyses confirm only the grain interior contribution to the conduction process. Temperature dependent electrical transport and room temperature ac conductivity (σ_ac) analyses indicate the semiconducting nature and small polaron hopping.     

Synthesis and magnetic properties of the size-controlled Mn-Zn ferrite nanoparticles by oxidation method

Size-controlled Mn_0.67Zn_0.33Fe₂O₄ nanoparticles in the wide range from 80 to 20 nm have been synthesized, for the first time, using the oxidation method. It has been demonstrated that the particle size can be tailor-made by varying the concentration of the oxidant. The magnetization of the 80 nm particles was 49 A m² kg⁻¹ compared to 34 A m² kg⁻¹ for the 20 nm particles. The Curie temperatures for all the samples are found to be within 630±5 K suggesting that there is no size-dependent cation distribution. The critical particle size for the superparamagnetic limit is found to be about 25 nm. The effective magnetic anisotropy constant is experimentally determined to be 7.78 kJ m⁻³ for the 25 nm particles, which is about an order of magnitude higher than that of the bulk ferrite.     

Influence of annealing temperature on the formation, microstructure and magnetic properties of spinel nanocrystalline cobalt ferrites

Nanocrystalline cobalt ferrites were synthesized by a simple, general, one-step sol–gel auto-combustion method. An interpretation based on the measurement of the adiabatic flame temperature and the amounts of gas evolved during reaction had been proposed for the nature of combustion. The influence of annealing temperatures on the magnetic properties was investigated. The microstructure was characterized by means of X-ray diffractometer (XRD) and transmission electron microscopy (TEM). It was found that the particle size and magnetic properties of the as-prepared ferrite samples showed strong dependence on the annealing temperature. The coercivity initially increased and then decreased with increasing annealing temperature whereas the particle size and saturation magnetization continuously increased.     

Effect of Cd on the structural, magnetic and electrical properties of nanostructured Mn-Zn ferrite

Nanoparticles of Mn_0.5Zn_0.5−xCd_xFe₂O₄ (x=0.0, 0.1, 0.2 and 0.3) have been synthesized by a chemical co-precipitation method. The lattice constant increases with increasing Cd content. X-ray calculations indicate that there is deviation in the cation distribution in the nanostructured spinel ferrite. The dielectric constant and dielectric loss decrease for the samples with Cd content up to x=0.2. However the dielectric constant rises for x=0.3. This is due to an increase in the hopping process at the octahedral (B sites). The dielectric constant increases with increase in temperature, indicating a thermally activated hopping process. The DC resistivity increases with Cd content up to x=0.2 and decreases for Cd content x=0.3. The maximum magnetization of all the samples decreases with increase in Cd content.     

Synthesis of cobalt ferrite nano-particles and their magnetic characterization

Cobalt ferrite nano-particles (CoFe₂O₄) were synthesized by the co-precipitation method with ammonium hydroxide as an alkaline solution. The reactions were carried out at different temperatures between 20 and 80 °C. The nano-particles have been investigated by magnetic measurements, X-ray powder diffraction and transmission electron microscopy. The average crystallite size of the synthesized samples was between 11 and 45 nm, which was found to be dependent on both pH value of the reaction and annealing temperatures. However, lattice parameters, interplane spacing and grain size were controlled by varying the annealing temperature. Magnetic characterization of the nano-samples were carried out using a vibrating sample magnetometer at room temperature. The saturation magnetization was computed and found to lie between 5 and 67 emu/g depending on the particle size of the studied sample. The coercivity was found to exhibit non-monotonic behavior with the particle size. Such behavior can be accounted for by the combination between surface anisotropy and thermal energies. The ratio of remanence magnetization to saturation magnetization was found to exhibit almost linear dependence on the particle size.     

Synthesis and characterization of a POM-based nanocomposite as a novel magnetic photocatalyst

A novel magnetic photocatalyst, prepared by grafting polyoxometalates (POM) anions PW₁₂O₄₀³⁻ onto Fe₃O₄ nanoparticles via a layer of Ag, was synthesized and characterized. The coated Ag layer was used as an intermediate bond for anchoring POM anions onto the magnetite cores. Resulting materials have been characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption-desorption isotherm, magnetization, and inductively coupled plasma (ICP). The activity of the photocatalyst was tested by the photocatalytic degradation of Rhodamine B. It was found that, compared to pure POM, the decolorization fraction of Rhodamine B in 2 h operation was 2.8-3.4 times higher by using the POM-based nanocomposite. ICP analysis of the concentration of Fe, W and P in treated water showed that photodissolution was minimal. In addition, as the synthesized composite possesses a magnetite core, it is possible to retrieve the photocatalyst by exerting an external magnetic field, which is easier than the recovery of conventional TiO₂ fine particles and homogeneous POM photocatalysts. The exhibited photocatalytic activity and magnetization of the novel photocatalyst provide a promising solution for the degradation of water contaminants and photocatalyst recovery.     

Synthesis and characterization of Co3O4 nanorods by thermal decomposition of cobalt oxalate

Cobalt oxalate was used as a precursor to prepare Co₃O₄ nanorods by thermal decomposition. The combinations of triphenylphosphine and oleylamine were added as surfactants to control the morphology of the particles. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The diameters of Co₃O₄ nanorods are 20 nm and the average lengths are around 500 nm. The hysteresis loops of the obtained samples reveal the ferromagnetic behaviors, the enhanced coercivity (H_c) and decreased saturation magnetization (M_s) in contrast to their respective bulk materials. The study provides a simple and efficient route to synthesize Co₃O₄ nanorods at low temperature.     

Fabrication and characterization of magnetic Fe3O4-CNT composites

Carbon nanotubes (CNTs) decorated with magnetite nanoparticles on their external surface have been fabricated by in situ solvothermal method, which was conducted in benzene at 500 °C with ferrocene and CNTs as starting reagents. The as-prepared composites were characterized using XRD, FTIR, SEM and TEM. It has been found that the amount of magnetite nanoparticles deposited on the CNTs can be controlled by adjusting the initial mass ratio of ferrocene to CNTs. The Fe₃O₄-CNT composites display good ferromagnetic property at room temperature, with a saturation magnetization value (M_s) of 32.5 emu g⁻¹ and a coercivity (H_c) of 110 Oe.     

Synthesis modified structural and dielectric properties of semiconducting zinc ferrospinels

The influence of preparation techniques on structural and dielectric properties of ZnCr_xFe_1−xO₄ (x=0, 0.1 abbreviated as Z and ZC) ferrite nano-particles synthesized using chemical co-precipitation (CCP), sol-gel (SG) and solid state reaction (SS) techniques is discussed. XRD profiles are used to confirm the single phase spinel ferrite formation. TEM images indicate the change in size and shape of particles on changing either the composition or the synthesis methodology. The TEM micrograph of samples obtained through CCP shows uniform particle size formation compared to those obtained through SG and SS. Sample prepared through CCP possess porosity >70% making these materials suitable for sensing applications. The dielectric loss, dielectric constant and ac conductivity are analyzed as a function of frequency, temperature and composition using impedance spectroscopy. A universal dielectric behavior has been predicted through temperature and frequency variations of different parameters. Dielectric constant is found to possess highest value for sample synthesized through SG which marks the possibility of using the SG derived ferrospinels as microwave device components.     

Improved multiferroic properties and a novel magnetic behavior of Bi0.8La0.2Fe1−xCoxO3 nanoparticles

Bi_0.8La_0.2Fe_1−xCo_xO₃ nanoparticles of single phase (BLFCO_x, x=0, 0.005, 0.01, 0.02) were prepared by a sol-gel method using polyvinyl alcohol as a surfactant. Co substitution at Fe site improved further dielectric properties of Bi_0.8La_0.2FeO₃ nanoparticles in the frequency range below 25 MHz at room temperature. Magnetization at 10 kOe, coercivities, and remanence of BLFCO_x nanoparticles increased with increasing Co content. It is interesting that the hysteresis loop of all the BLFCO_x nanoparticles presented a wasp-waisted shape. The property can open an important way to design new multiferroic applications of low hysteresis loss in low magnetic fields.     

Studies on the activation energy from the ac conductivity measurements of rubber ferrite composites containing manganese zinc ferrite

Manganese zinc ferrites (MZF) have resistivities between 0.01 and 10 Ω m. Making composite materials of ferrites with either natural rubber or plastics will modify the electrical properties of ferrites. The moldability and flexibility of these composites find wide use in industrial and other scientific applications. Mixed ferrites belonging to the series Mn_(1−x)Zn_xFe₂O₄ were synthesized for different ‘x’ values in steps of 0.2, and incorporated in natural rubber matrix (RFC). From the dielectric measurements of the ceramic manganese zinc ferrite and rubber ferrite composites, ac conductivity and activation energy were evaluated. A program was developed with the aid of the LabVIEW package to automate the measurements. The ac conductivity of RFC was then correlated with that of the magnetic filler and matrix by a mixture equation which helps to tailor properties of these composites.     

Preparation of immunomagnetic nanoparticles and their application in the separation of mouse CD34 hematopoietic stem cells

The magnetic nanoparticles with a diameter of about 60 nm were synthesized by coprecipitation from ferrous and ferric iron solutions and coated with silica. Then the nanoparticles were modified with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPS) in order to immobilize anti-CD34⁺ monoclonal antibodies to the surface of modified magnetic particles. The results of transmission electron microscope (TEM) and Fourier transformed infrared (FT-IR) indicated that the nanoparticles were successfully prepared. Scanning electron microscope (SEM) photo confirmed that the mouse CD34⁺ cells (cells expressing CD34) were separated by the immunomagnetic nanoparticles. The viability of the separated cells was studied by hematopoietic colony-forming assay, the result of which showed that the target cells still had an ability of proliferation and differentiation. The application of the separated CD34⁺ cells was in testing the pharmacological effect of three samples isolated from enzyme-digested traditional Chinese medicine Colla corii asini.     

Magnetic and dielectric behaviors of Ba0.2Sr0.8Co0.8Fe0.2O3-δ oxide

Ba_0.2Sr_0.8Co_0.8Fe_0.2O_3-δ (BSCFO) ceramic oxide has been synthesized by combined citrate-EDTA complexing method and studied with regard to their structural, magnetic and dielectric properties. It is shown that the compound exhibits perovskite-type cubic structure. It depicts hysteresis loop in presence of magnetic field—indicating its magnetic nature. The dielectric properties of sintered oxide were investigated in temperature range (373-873 K) and frequency (100 kHz-1 MHz).The ferroelectric and ferrimagnetic transition temperatures were found to be around 700 K.     

Chemical synthesis of self-assembled Ni nanoparticles and understanding of nonmagnetic layer inside their surface

To chemically synthesize mono-dispersed and self-assembled Ni nanoparticles, it was important to find the best combination of a Ni precursor and a ligand. Our Ni nanoparticles exhibited a face-centered cubic structure and superparamagnetism at room temperature. The value of saturation magnetization for our Ni nanoparticles was largely different from that of bulk Ni. Because of the relationship between the diameter and saturation magnetization per volume, the number of atoms composing the Ni nanoparticle was correlated with magnetization. This result indicated that a magnetic core/shell structure inside a Ni nanoparticle was produced. The nonmagnetic layer, as a magnetic shell of the core/shell structure, was created due to the low crystallinity of Ni nanoparticles and was composed of amorphous Ni‒O states. As a result, antiferromagnetic spins arrayed in the Ni‒O states were broken. Disordered spins were generated, which eventually decreased the total magnetization of the Ni nanoparticles.     

Synthesis and magnetic properties of three-dimensional metal (II) organophosphates

A new series of metal (II) organophosphates with the formula M(II) 2(H₂O)₂[O₃PCH₂(C₆H₄)CH₂PO₃] (M=Mn, Fe and Ni) have been prepared by hydrothermal synthesis. The structure consisted of two-dimensional metal–oxygen inorganic layers is pillared by p-xylylenediphosphonate to form a three dimensional framework. The layers are constructed by corner-sharing metal oxygen polyhedron. A study on the magnetism of the materials indicates the presence of spin canted antiferromagnetc interactions. The manganese and iron compounds represent the interesting 3D metal organophosphate molecular metamagnet due to spin canted antiferromagnetic with high critical temperature (40 K for Mn; 16 K for Fe). The infinite M–O–M layers are believed to be responsible for this high performance.     

Hopping conductivity of Ni-doped p-CdSb in strong magnetic fields

Magnetoresistance (MR) of oriented single crystals of the anisotropic semiconductor p-CdSb doped with 2 at% of Ni is investigated between T=1.5 and 300 K in transversal pulsed magnetic fields up to B=30 T. In fields B∼4-15 T at T below 4.2 K, the resistivity obeys the law ln ρ∼η[B?(B)]^1/2 with ?(B)=a(0)/a(B), where a is the carrier localization radius and parameter η depends on a(0), on the acceptor concentration N_A and on the direction of the magnetic field with respect to the crystallographic axes, but does not depend on T. Such behavior gives evidence for MR realized by hopping charge transfer over the nearest-neighbor sites in strong magnetic field. The analysis of the experimental data yields the values of η, agreeing with calculated ones within an error of 10%, taking into account the effects of the anisotropy of the acceptor states and of the explicit dependence of a(B) due to the increase in the activation energy of shallow acceptors in magnetic field and the sensitivity of the metal-insulator transition to B.     

Structural and magnetic properties of Cu1−xMnxO nanocrystal prepared by combustion synthesis

Nanoscale Cu_1−xMn_xO powder is prepared by using the combustion synthesis technique with two different fuels. The structural properties of the powder are determined using Rietveld refinement of X-ray diffraction data, high-resolution transmission electron microscopy, and Fourier transform infrared spectroscopy, while its magnetic properties are analyzed by means of hysteresis loop and temperature dependence of magnetization. The results show that (1) the Cu_1−xMn_xO nanocrystal is of monoclinic CuO structure, with grain size of 10-30 nm varying with the type of fuel, the nitrate/fuel ratio (N/F), and the Mn concentration, the doping of Mn has a little influence on the lattice parameters; (2) when the Mn concentration is higher than 7%, a small amount of impurity phase of CuMn₂O₄ appears and annihilates the potential cation vacancies; (3) all of the samples with x≥5% exhibit low-temperature ferromagnetism with the Curie temperature of ∼90 K, which increases slightly by raising the Mn concentration; (4) the paramagnetic moment per Mn ion is around 2-4 bohr magneton above the Curie temperature, which decreases with increasing Mn concentration, implying that the nearest Mn ions are antiferromagnetically coupled and the ferromagnetic order could originate from the super-exchange of next nearest Mn ions along the [1 0 1?] direction.     

Experimental study on the preparation,characterization and conductivity improvement of reduced graphene-oxide papers

In the present study, reduced graphene-oxide (r-GO) papers were prepared by vacuum filtration method using chemically obtained graphene oxide as raw materials. Different reduction methods, chemical, thermal or the combination were designed to investigate the influence of reduction process on the structure and conductivity of r-GO papers. The reducibility of the obtained papers was investigated by XPS and Raman. The structure, morphology and electrical conductivity were examined by XRD, SEM and four point resistivity test system, respectively. Results showed that chemical reduction using hydrazine or annealing in reducing ambinent alone was not sufficient to achieve maximum reduction, the highest C/O ratio and highest conductivity was obtained in paper reduced via a combination of hydrazine and thermal annealing treatment. In order to further improve the conductivity of the paper, Ag nanoparticles have been decorated into the paper.