Photoluminescence of nanoparticles of (Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>)<Subscript>0.95</Subscript>(Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.05</Subscript> solid solution

We have ground bulk samples to obtain nanoparticles of (Ga₂S₃)_1–x (Eu₂O₃) _x solid solutions, the sizes of which were determined using an atomic force microscope. The photoluminescence spectra of the nanoparticles were studied in the temperature interval 77–300 K. We have established the mechanisms for emission and transfer of energy from the matrix to the rare-earth ion, and we determined the Stokes shift (ΔS = 0.7 eV), the Huang–Rhys parameter (S = 16), and the optical phonon energy (ħ−ω = 23 meV).     

Magnetic and dielectric properties of Mn<Subscript>0.2</Subscript>Ni<Subscript>0.8</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles synthesized by PEG-assisted hydrothermal method

We present a systematic investigation on the structural and magnetic properties of Mn_0.2Ni_0.8Fe₂O₄ nanoparticles synthesized by a polyethylene glycol (PEG)-assisted hydrothermal route. XRD, FT-IR, TEM and VSM were used for the structural, morphological, dielectric properties and magnetic investigation of the products, respectively. Average crystallite size of product was estimated using Line profile fitting as 6 ± 1 nm and particle size as 6.5 ± 1.0 nm from TEM micrographs. Magnetization measurements have shown that the particles have a blocking temperature of 134 K. Magnetization and the coercive field of the sample increase by decreasing the temperature. The conductivity measurements reveal the semiconducting behaviour for the sample. Temperature-dependent dielectric properties: dielectric permittivity (ε) and ac conductivity (σ_ac) for the sample were studied as a function of applied frequency in the range from 1 Hz to 3 MHz. These studies indicated that the dielectric dispersion curve for the sample showed usual dielectric dispersion which can be explained on the basis of Koop’s theory, which is based on the Maxwell–Wagner model for the interfacial polarization of homogeneous double structure.     

Frustrated exchange interactions formation at low temperatures and high hydrostatic pressures in La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>Mn<Subscript>O2.85</Subscript>

The magnetic and thermal properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite are investigated in wide temperature (4–350 K) range, including under hydrostatic pressure (0–1.1 GPa). Throughout the pressure range investigated, the sample is spin glass with diffused phase transition into paramagnetic state. It is established, that spin glass state is a consequence of exchange interaction frustration of the ferromagnetic clusters embeded into antiferromagnetic clusters. The magnetic moment freezing temperature T _f of ferromagnetic clusters increases under pressure, freezing temperature dependence on pressure is characterized by derivative value ∼4.5 K/GPa, while the magnetic ordering T _MO temperature dependence is characterized by derivative value ∼13 K/GPa. The volume fraction of sample having ferromagnetic state is V _fer ∼ 13% and it increases under a pressure of 1.1 GPa by ΔV _fer ≈ 6%. Intensification of ferromagnetic properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite under hydrostatic pressure is a consequence of oxygen vacancies redistribution and unit cell parameters decrease. The most likely mechanism of frustrated exchange interactions formation is discussed.     

Temperature dependence of the magnetic properties and magnetoimpedance of nanocrystalline Fe<Subscript>73.5</Subscript>Si<Subscript>16.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> ribbons

The temperature dependences of the magnetic properties and the magnetoimpedance effect of soft magnetic nanocrystalline Fe_73.5Si_16.5B₆Nb₃Cu₁ alloy ribbons are studied in the temperature range 24–160°C. A high temperature sensitivity of the impedance and the magnetoimpedance effect of the ribbons are detected in the ac frequency range 0.1–50 MHz. At an ac frequency of 50 MHz, the change in the impedance reaches 0.2 Ω/°C (0.5%/°C) in the temperature range 85–160°C. When the temperature increases, a monotonically decreasing character of the dependence of the magnetoimpedance effect on the applied magnetic field changes into a dependence having an increasing initial segment. The effect of temperature on the magnetoimpedance properties of the soft magnetic nanocrystalline ribbons is shown to result from temperature-induced changes in their electrical conductivity, magnetization, and effective magnetic anisotropy.     

Synthesis and characterization of mono-dispersed Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Er<Superscript>3+</Superscript>-coated SiO<Subscript>2</Subscript> nanoparticles by co-precipitation process

In this paper, a facile co-precipitation process for preparing mono-dispersed core–shell structure nanoparticles is reported. The 110 nm SiO₂ cores coated with an yttrium aluminum garnet (Y₃Al₅O₁₂) layer doped with Er³⁺ were synthesized and the influence of the concentration ratio of [urea]/[metal ions] on the final product was investigated. The structure and morphology of samples were characterized by the X-ray powder diffraction, Fourier transform IR spectroscopy and transmission electron microscopy, respectively. The results indicate that a layer of well-crystallized garnet Y₃Al₅O₁₂:Er³⁺ were successfully coated on the silica particles with the thickness of 20 nm. The near infrared and upconversion luminescent spectra of the SiO₂@Y₃Al₅O₁₂:Er³⁺ powders further confirm that a Y₃Al₅O₁₂:Er³⁺ coating layer has formed on the surface of silica spherical particles.     

Microemulsion-mediated in-situ synthesis and magnetic characterization of polyaniline/Zn<Subscript>0.5</Subscript>Cu<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocomposite

Polyaniline/Zn_0.5Cu_0.5Fe₂O₄ nanocomposite was synthesized by a simple, general and inexpensive in-situ polymerization method in w/o microemulsion. The effects of polyaniline coating on the magnetic properties of Zn_0.5Cu_0.5Fe₂O₄ nanoparticles were investigated. The structural, morphological and magnetic properties of as-prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, scanning electron microscopy (SEM) and magnetic measurements. The morphology analysis confirmed that polyaniline was deposited on the porous surface of magnetic Zn_0.5Cu_0.5Fe₂O₄. It was shown that the saturation magnetization and coercivity of Zn_0.5Cu_0.5Fe₂O₄ decreased after polyaniline coating, which can be interpreted by the interparticle dipole–dipole interactions that contributed to magnetic anisotropy and changed the magnetic properties of the nanoparticles. PACS  74.25.Ha; 81.05.-t; 81.05.Lg     

Ultrasound-assisted dispersion of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanoparticles in organic solvents and the use of the dispersion as magnetic cosmetics

A new method of dispersing the aggregated strontium hexaferrite (SrFe₁₂O₁₉) magnetic nanoparticles in organic solvents such as propylene glycol monomethyl ether acetate (PGMEA), propylene glycol (PG), and glycerol, by an ultrasonic bath is reported herein. The particles size of SrFe₁₂O₁₉ after treatment with the PGMEA is in the range 70–100 nm. The structure of dispersed SrFe₁₂O₁₉ was characterized using transmission electron microscopy (TEM), high resolution scanning electron microscopy (HR SEM) and thermo gravimetric analysis (TGA). This dispersed material was used for the preparation of a topical magnetic cosmetic product as follows: The dispersion of SrFe₁₂O₁₉ in PG was mixed with “Dermud-Ahava Body Cream”, an ‘oil in water’ emulsion of a Dead Sea mineral cosmetic, “AHAVA”, and the magnetic properties of the created composite were determined. The ferrimagnetic behavior of the composite has been demonstrated as being very similar to the behavior of strontium hexaferrite itself.     

Synthesis of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel nanoparticles using a mixture of bayerite and magnesium sulfate

This article reports a novel method to prepare MgAl₂O₄ spinel nanoparticles. By calcining a powder mixture of bayerite and magnesium sulfate at 800 °C and washing with water, single-phase MgAl₂O₄ spinel nanoparticles were prepared. The powder mixture and the calcined products were characterized by differential thermal and thermogravimetric analysis (DSC-TG), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Brunauer–Emmett–Teller (BET) nitrogen-gas adsorption method. The obtained MgAl₂O₄ spinel nanoparticles have an average particle size of 12 nm, a narrow size distribution, and weak agglomeration. The specific surface area of the MgAl₂O₄ spinel powder is 110 m²/g. The formation of MgAl₂O₄ spinel is attributed to a solid-state reaction between γ-Al₂O₃ and MgSO₄.     

Optical absorption spectra and magnetic phase transitions in TbFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Optical absorption spectra of the trigonal crystal of TbFe₃(BO₃)₄ in the vicinity of the ⁷F₆ → ⁵D₄ transition in a Tb³⁺ ion were studied as a function of temperature (2–70 K) and magnetic field strength (0–60 kOe) at 2 K. The splitting of the excited states of Tb³⁺ due to both the magnetic ordering of iron and an external magnetic field was determined. Abrupt splitting of the absorption lines of Tb³⁺ at temperature T_N of the magnetic ordering of the subsystem of iron was revealed, suggesting that the nature of such splitting is not entirely magnetic.     

Magnetic properties of the Nd<Subscript>0.5</Subscript>Gd<Subscript>0.5</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystal

The magnetic properties of the Nd_0.5Gd_0.5Fe₃(BO₃)₄ single crystal have been studied in principal crystallographic directions in magnetic fields to 90 kG in the temperature range 2–300 K; in addition, the heat capacity has been measured in the range 2–300 K. It has been found that, below the Néel temperature T _N = 32 K down to 2 K, the single crystal exhibits an easy-plane antiferromagnetic structure. A hysteresis has been detected during magnetization of the crystal in the easy plane in fields of 1.0–3.5 kG, and a singularity has been found in the temperature dependence of the magnetic susceptibility in the easy plane at a temperature of 11 K in fields B < 1 kG. It has been shown that the singularity is due to appearance of the hysteresis. The origin of the magnetic properties of the crystal near the hysteresis has been discussed.     

Synthesis and characterization of a polyaniline-modified SnO<Subscript>2</Subscript> nanocomposite

Polyaniline-modified tin oxide and tin oxide nanoparticles were synthesized using a solution route technique. The obtained pristine products were characterized with X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and optical absorption spectroscopy. Thermogravimetric analysis results showed that the polyaniline-modified SnO₂ nanoparticles exhibit higher thermal stability than the SnO₂ nanoparticles. Scanning electron microscopy analysis on the as-synthesized powders showed spherical particle in the range of 50–100 nm.     

Magnetic properties of LiNi<Subscript>0.5</Subscript>Mn<Subscript>0.47</Subscript>Al<Subscript>0.03</Subscript>O<Subscript>2</Subscript> as positive electrode for Li-ion batteries

The layered LiNi_0.5Mn_0.47Al_0.03O₂ was synthesized by wet chemical method and characterized by X-ray diffraction and analysis of magnetic measurements. The powders adopted the α-NaFeO₂ structure. This substitution of Al for Mn promotes the formation of Li(Ni_0.47²⁺Ni_0.03³⁺Mn_0.47⁴⁺Al_0.03³⁺)O₂ structures and induces an increase in the average oxidation state of Ni, thereby leading to the shrinkage of the lattice unit cell. The concentration of antisite defects in which Ni²⁺ occupies the (3a) Li lattice sites in the Wyckoff notation has been estimated from the ferromagnetic Ni²⁺(3a)–Mn⁴⁺(3b) pairing observed below 140 K. The substitution of 3% Al for Mn reduces the amount of antisite defects from 7% to 6.4–6.5%. The analysis of the magnetic properties in the paramagnetic phase in the framework of the Curie–Weiss law agrees well with the combination of Ni²⁺ (S = 1), Ni³⁺ (S = 1/2) and Mn⁴⁺ (S = 3/2) spin-only values. Delithiation has been made by the use of K₂S₂O₈. According to this process, known to be softer than the electrochemical one, the nickel ions in the (3b) sites are converted into Ni⁴⁺ in the high spin configuration, while Ni²⁺(3a)–Mn⁴⁺(3b) ferromagnetic pairs remain, as the Li⁺(3b) ions linked to the Ni²⁺(3a) ions in the antisite defects are not removed. The results show that the antisite defect is surrounded by Mn⁴⁺ ions, implying the nonuniform distribution of the cations in agreement with previous NMR and neutron experiments.     

Antiferromagnetic interactions in Er-doped SnO<Subscript>2</Subscript> DMS nanoparticles

Diluted magnetic semiconductor (DMS) nanoparticles of Sn_1−x Er _x O₂ (x = 0.0, 0.02, 0.04, and 0.1) were prepared by sol–gel method. The X-ray diffraction patterns showed SnO₂ rutile structure for all samples with no impurity peaks. The decrease in crystallite size with Er concentration was confirmed from TEM measurements (from 12 to 4 nm). The UV–Visible absorption spectra of Er-doped SnO₂ nanoparticles showed blue shift in band gap compared to undoped SnO₂. The electron spin resonance analysis of Er-doped SnO₂ nanoparticles indicate Er³⁺ in a rutile lattice and also decrease in intensity with Er concentration above x = 0.02. Temperature-dependent magnetization studies and the inverse susceptibility curves indicated increased antiferromagnetic interaction with Er concentration.     

One-step synthesis of colloidal Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> and γ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles at room temperature

A facile room-temperature synthesis has been developed to prepare colloidal Mn₃O₄ and γ-Fe₂O₃ nanoparticles (5 to 25 nm) by an ultrasonic-assisted method in the absence of any additional nucleation and surfactant. The morphology of the as-prepared samples was observed by transmission electron microscopy. High-resolution transmission electron microscopy observations revealed that the as-synthesized nanoparticles were single crystals. The magnetic properties of the samples were investigated with a superconducting quantum interference device magnetometer. The possible formation process has been proposed.     

Preparation of TiO<Subscript>2</Subscript>-coated LiCo<Subscript>1/3</Subscript>Ni<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> by electrostatic self-assembly method and its properties

A precursor of TiO₂–LiCo_1/3Ni_1/3Mn_1/3O₂ was prepared by electrostatic self-assembly method. The final product was obtained by heating the precursor at 400–450 °C for 4–6 h in air. X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical tests were used to examine the structural, morphology, elementary valence, and electrochemical characteristics. XRD indicated that the TiO₂-coated material can be indexed by α-NaFeO₂ layered structure, which belongs to hexagonal-type space group R3m. XPS results confirmed the existence of TiO₂ compound on the surface of the coated sample. The SEM image showed that the material had spherically porous morphology with the uniform size about 6 μm. The initial charge–discharge capacity of the TiO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ material was 168.8/160.0 mAh/g. After 60 cycles, the discharge capacity of the TiO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ sample was 147.0 mAh/g, and the coulombic efficiency was 94.0%. Compared with the uncoated sample, the electrochemical performance of TiO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ was improved.     

Mechanochemical reactions in Co<Subscript>80</Subscript>B<Subscript>20</Subscript> and Co<Subscript>60</Subscript>B<Subscript>40</Subscript> mixtures during ball milling and postannealing

The solid-phase interaction of cobalt with boron during high-energy ball milling of Co₈₀B₂₀ and Co₆₀B₄₀ mixtures was studied. It was found that Co₃B boride forms during the mechanical alloying of components in the presence of both modifications of Co with a reduction in the size of blocks of up to 7–15 nm. The transformation of the metastable Co₃B phase into stable Co₂B and CoB borides is observed in the temperature range of 590–700°C.     

Magnetotransport properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> with different grain sizes

The magnetotransport and magnetoresistive (MR) properties of manganese-based La_0.67Ca_0.33MnO₃ perovskite with different grain sizes are reported. The electrical resistivity was measured as a function of temperature in magnetic fields of 0.5 and 1 T. The insulator–metal transition temperature, T _IM, shifted to a higher temperature with the application of the magnetic field. In zero field, T _IM is almost constant (∼271 K) for all samples except for the sample with the largest grain size, where T _IM=265 K. The temperature dependence of resistivity was fitted with several equations in the metallic (ferromagnetic) region and the insulating (paramagnetic) region. The density of states at the Fermi level, N(E _F), and the activation energy of electron hopping were estimated by fitting the resistivity versus temperature curves. The ρ–T ² curves are nearly linear in the metallic regime, but the ρ–T ^2.5 curves exhibit a deviation from linearity. The variable range hopping model and small polaron hopping model fit the data well in the high-temperature region, indicating the existence of the Jahn–Teller distortion that localizes the charge carriers. MR was found to increase with an increase in the magnetic field, an effect which is attributed to the intergrain spin tunneling effect.     

Electrophysical properties and the structure of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> compound thermally restored after low-temperature decomposition

The electrophysical properties and structure of the nonstoichiometric high-temperature superconductor YBa₂Cu₃O _y restored at T = 930–950°C after low-temperature decomposition (T = 200°C) into phases different in the oxygen content have been studied. It has been shown that, unlike heat treatments at T ≤ 900°C, the superconducting properties are almost completely restored for 3–5 h during grain recrystallization, which is impossible at lower temperatures. After short-term annealing at T = 930–950°C (for 1–2 h), the ceramic material still contains a significant number of structural defects, most likely, in cation sublattices. These defects can contribute to the pinning of magnetic vortices, which substantially increases the critical current density in magnetic fields up to 2 T as compared to ceramic materials produced by the conventional technology.     

Polarization modes in the Ba<Subscript>2</Subscript>Mg<Subscript>2</Subscript>Fe1<Subscript>2</Subscript>O<Subscript>22</Subscript> multiferroic

The spectra of complex permittivity of a Ba₂Mg₂Fe₁₂O₂₂ single crystal belonging to the family of Y-type hexaferrites have been measured over a wide temperature range (10–300 K) with the aim of determining the dynamic parameters of the phonon and magnetic subsystems in the terahertz and infrared frequency ranges (3–4500 cm⁻¹). A factor-group analysis of the vibrational modes has been performed, and the results obtained have been compared with the experimentally observed resonances. The oscillator parameters of all nineteen phonon modes of E _u symmetry, which are allowed by the symmetry of the Ba₂Mg₂Fe₁₂O₂₂ crystal lattice, have been calculated. It has been found that, at temperatures below 195 and 50 K, the spectral response exhibits new absorption lines due to magnetic excitations.     

Magnetic state of the structural separated anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

The results of neutron diffraction studies of the La_0.70Sr_0.30MnO_2.85 compound and its behavior in an external magnetic field are stated. It is established that in the 4–300 K temperature range, two structural perovskite phases coexist in the sample, which differ in symmetry (groups R[`3]cR\bar 3c and I4/mcm). The reason for the phase separation is the clustering of oxygen vacancies. The temperature (4–300 K) and field (0–140 kOe) dependences of the specific magnetic moment are measured. It is found that in zero external field, the magnetic state of La_0.70Sr_0.30MnO_2.85 is a cluster spin glass, which is the result of frustration of Mn³⁺-O-Mn³⁺ exchange interactions. An increase in external magnetic field up to 10 kOe leads to fragmentation of ferromagnetic clusters and then to an increase in the degree of polarization of local spins of manganese and the emergence of long-range ferromagnetic order. With increasing magnetic field up to 140 kOe, the magnetic ordering temperature reaches 160 K. The causes of the structural and magnetic phase separation of this composition and formation mechanism of its spin-glass magnetic state are analyzed.