Synthesis, characterization and magnetic properties on nanocrystalline BaFe12O19 ferrite

Single phase BaM (BaFe₁₂O₁₉) ferrites are prepared by using sol–gel method. The preparing conditions of samples are investigated in detail, such as acid/nitrate ratio, the value of pH and annealing temperature. The best conditions on preparing BaFe₁₂O₁₉, which can be obtained on a Fe/Ba ratio of 12, the citric acid contents R = 3, the starting pH of solution is 9, and annealing temperature 950 °C. The thermal decomposition behavior of the dried gel was examined by TG–DSC, the structure and properties of powders were measured respectively by XRD techniques. The magnetic properties of barium ferrites are emphatically researched about the changing crystallite size and annealing temperature by the vibrating sample magnetometer (VSM). Magnetic measurement shows that the barium ferrite samples annealed at 1000 °C has the maximal coercive field of 5691.91 Oe corresponding to the maximal remnant magnetization of 35.60 emu/g and the sample synthesized at 1000 °C has the maximal saturation magnetization of 60.75 emu/g.     

Synthesis and optical properties of Ce-doped Y3Mg2AlSi2O12 phosphors

Y_3−xMg₂AlSi₂O₁₂:Ce_x³⁺ (x=0.015, 0.03 and 0.06) phosphors possessing garnet crystal structure were synthesized by the sol–gel combustion technique. The samples were characterized by application of powder X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, thermal quenching (TQ) and scanning electron microscopy (SEM). Moreover, luminous efficacies (LE), color points and quantum efficiencies (QE) were calculated. Optical properties were studied as a function of Ce³⁺ concentration and annealing temperature. XRD analysis revealed that sintering of polycrystalline Y₃Mg₂AlSi₂O₁₂:Ce³⁺ powders at 1550 °C results in nearly single-phase garnet materials. Phosphors showed broad emission band in the range of 500–750 nm and had the maximum intensity at 600 nm, which results in strongly red-shifted phosphors compared with conventional YAG:Ce phosphors emitting at 560 nm. However, strong concentration quenching has also been observed, probably due to increased Stokes shift.     

High temperature transport properties of Ag-added (Ca0.975La0.025)3Co4O9 ceramics

Ag-added (Ca_0.975La_0.025)₃Co₄O₉ ceramics were fabricated using spark plasma sintering from the precursor powder synthesized by a polyacrylamide gel method. The results indicated that Ag precipitated as a second phase in Ca₃Co₄O₉ matrix. The addition of Ag was effective in enhancing the electrical conductivity and had a slight effect on Seebeck coefficient. In addition, the temperature dependence of electrical conductivity showed that the hole hopping conduction mechanism was dominant for the Ag-added (Ca_0.975La_0.025)₃Co₄O₉ ceramics. The activation energy remained unchanged with the increasing Ag content. The thermoelectric power factor of Ag-added (Ca_0.975La_0.025)₃Co₄O₉ ceramics reached about 5×10⁻⁴ Wm⁻¹ K⁻² at 700 °C, suggesting a promising thermoelectric oxide candidate at high temperatures.     

Microfabrication of gold dots in SiO2/TiO2 glass films by two-photon absorption

Photogeneration of Au nanoparticles in SiO₂/TiO₂ glass films was carried out by two-photon absorption with a femtosecond pulse laser. Exquisite microdot-arrays of Au with micrometer spatial resolution were achieved by scanning of the focused laser beam. These structures were constructed in SiO₂/TiO₂ glass films by a sol–gel method. The sol–gel method demonstrated that Au dots microarray are fabricated at any position by two-photon absorption in the glass. The results show the utility of a two-photon absorption technique in the fabrication of complicated patterns with metal particles.     

Effect of C doping on the structural and optical properties of sol–gel TiO2 thin films

Undoped and C-doped TiO₂ thin films have been prepared by sol–gel process. Their structure and optical properties have been investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–vis spectroscopy. It has been observed that C dopants retard the transformation from anatase-to-rutile phase. Namely, C doping effect is attributed to the anatase phase stabilization. The optical analyses show that the optical band gap of anatase C-doped TiO₂ decreases with increasing amount of C. Also, it is founded that C dopants have been shown to make TiO₂ have a visible light photoresponse.     

Crystal structure, electronic and transport properties of AgSbSe2 and AgSbTe2

Undoped and p- and n-doped AgSbX₂ (X=Se and Te) materials were synthesized by direct fusion technique. The structural properties were investigated by X-ray diffraction and SEM microscopy. The electrical conductivity, thermal conductivity and Seebeck coefficient have been measured as a function of temperature in the range from 300 to 600 K.To enlighten electron transport behaviours observed in AgSbSe₂ and AgSbTe₂ compounds, electronic structure calculations have been performed by the Korringa-Kohn-Rostoker method as well as KKR with coherent potential approximation (KKR-CPA) for ordered (hypothetical AgX and SbX as well as AgSbX₂ approximates) and disordered systems (Ag_1−xSb_xX), respectively. The calculated density of states in the considered structural cases shows apparent tendencies to opening the energy gap near the Fermi level for the stoichiometric AgSbX₂ compositions, but a small overlap between valence and conduction bands is still present. Such electronic structure behaviour well agrees with the semimetallic properties of the analyzed samples.     

Magnetic and related properties of U4Rh13Si9 and U4Ir13Si9

The compounds U₄Rh₁₃Si₉ and U₄Ir₁₃Si₉ crystallize with the orthorhombic Er₄Ir₁₃Si₉-type structure that contains three non-equivalent positions of uranium atoms. Their magnetic, electrical transport and thermal properties were studied down to liquid helium temperature in magnetic fields up to 9 T. Both compounds have been found to order antiferromagnetically at low temperatures and to exhibit complex magnetic behavior in the ordered state. Some features characteristic of spin fluctuators (U₄Rh₁₃Si₉) and Kondo lattices (U₄Ir₁₃Si₉) indicate that the two ternaries studied are novel strongly correlated electron systems.     

Orbital-decomposed electronic and magnetic properties of the double perovskite Sr2FeReO6

The detailed orbital-decomposed electronic structures and magnetic properties of the double perovskite Sr₂FeReO₆ have been studied using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA). Both occupied and unoccupied s and three p states of Fe³⁺ ion are located far away from the Fermi level, while all up-spin states and most down-spin states are completely filled for the s and three p states of Re⁵⁺ ion. The octahedral crystal field of the oxygen atoms around transition-metal (TM) sites splits the five-fold degenerate d states of the free TM atoms into triply degenerate t_2g states with smaller bonding-antibonding splitting and doubly degenerate e_g states with larger bonding-antibonding splitting. The Fe³⁺ and Re⁵⁺ ions are in the states (3d⁵, S=5/2) and (5d², S=1) with magnetic moments 3.70 and −0.86μ_B, respectively and thus antiferromagnetic coupling via oxygen between them. There are no direct interactions between two nearest Fe-Fe or Re-Re pairs, whereas along each Fe-O-Re-O-Fe or Re-O-Fe-O-Re chains, the hybridizations between Fe 3d and 4s, O 2s and 2p, as well as Re 5p, 5d and 6s orbitals are fairly significant.     

Transport and magnetic properties of La0.8Ce0.2MnO3 thin films grown by pulsed laser deposition

We report on the growth of La_0.8Ce_0.2MnO₃ thin films on (0 0 1) LaAlO₃ substrates by pulsed laser deposition. CeO₂ as an impurity is present in both the bulk and film samples. The electrical transport and magnetic properties of the films are similar to that of the divalent cation-doped or La deficient LaMnO₃, which show colossal magnetoresistance. Thermopower measurements indicate that the carriers are holes. The results are explained in terms of a La site deficiency due to the existence of CeO₂.     

Mn2 CoSb compound: Structural, electronic, transport and magnetic properties

High-ordered Mn₂CoSb compound has been synthesized successfully by a melt-spinning technique. The band structure calculation shows that Mn₂CoSb is a true half-metallic ferromagnet characterized by an indirect Γ–X band gap of about 0.4007 eV around the Fermi level for minority-spin electrons. The calculated magnetic moment is per formula unit, which is close to the experimental value of . The electronic resistivity shows a power-law T^1.33 temperature dependence at low temperature. The T^1.5 dependence of the magnetization was observed at low temperature, which is expected from Bloch’s law.     

Effect of the synthesis route on the structural properties and shape of the indium oxide (In2O3) nano-particles

Nano-crystalline indium oxide (In₂O₃) particles have been synthesized by sol–gel and hydro-thermal techniques. A simple hydro-alcoholic solution consisting indium nitrate hydrate and citric acid (in sol–gel method) and 1, 4-butandiol (in hydro-thermal method) have been utilized. The structural properties of indium oxide nano-powders annealed at 450 °C (for both methods) have been characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and specific surface area (SSA) analysis. Structural analysis of the samples shows cubic phase in sol–gel and cubic-hexagonal phase mixture in hydro-thermally prepared particles. The nano-particles prepared by sol–gel method have nearly spherical shape, whereas hydro-thermally-made ones display wire- and needle-like shape in addition to the spherical shape. The obtained In₂O₃ nano-particles surface areas were 23.2 and 55.3 in sol–gel and hydro-thermal methods, respectively. The optical direct band gap of In₂O₃ nano-particles were determined to be 4.32 and 4.24 eV for sol–gel and hydro-thermal methods, respectively. These values exhibit 0.5 eV blue shift from that the bulk In₂O₃ (3.75 eV), which is related to the particle size reduction and approaching the quantum confinement limit of nano-particles.     

Structural,electronic and magnetic properties of the double perovskite Pb2FeReO6

The structural, electronic and magnetic properties of the double perovskite Pb₂FeReO₆ have been studied by using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA) as well as taking into account the on-site Coulomb repulsive and exchange coupling interactions (GGA+U). The optimized crystal structure of the Pb₂FeReO₆ is a body-centered tetragonal (BCT) with a space group of I4/m and the lattice constants of a=b=5.59 Å and c=7.93 Å, consistent with the experimental results. The two axial transition metal and oxygen (TM–O) distances are slightly larger than the four equatorial TM–O distances and shows the existence of the Jahn–Teller structural distortion in FeO₆ and ReO₆ octahedra. The Fe³⁺ and Re⁵⁺ ions are in the states (3d⁵, S=5/2) and (5d², S=1) with magnetic moments 3.929 and −0.831μ_B respectively and thus antiferromagnetic (AFM) coupling via oxygen between them. The half-metallic (HM) ferromagnetic (FM) nature implies a potential application of this new compound in magnetoelectronic and spintronics devices.     

Synthesis of self-assembly BaTiO3 nanowire by sol–gel and microwave method

Self-assembly ferroelectric BaTiO₃ nanowires were fabricated using sol–gel and microwave method. The X-ray diffraction patterns show that BaTiO₃ nanowires belong to the tetragonal perovskite structure. An increase in the intensity of (1 1 0) peak was observed as the annealing time increased. The shape of BaTiO₃ nanowires microwave-annealed for different minutes was investigated using atomic force microscopy. It is found that nanowires of BaTiO₃ annealed for 2.5 min are very clear-cut, orderly and almost uninterrupted. The height of nanowire is near to the film thickness. However, nanowires of BaTiO₃ annealed for 5 min are lesser, shorter and lower, and the distances among these nanowires are wider and well-proportioned. The origin of the distinct differences due to the remotion of atoms obtained enough energy was discussed.     

Magnetic properties for the Cu2MnSnSe4 and Cu2FeSnSe4 compounds

Magnetic susceptibility χ measurements in the range from 2 to 300 K were carried out on samples of the Cu₂FeSnSe₄ and Cu₂MnSnSe₄ compounds. It was found that Cu₂FeSnSe₄ was antiferromagnetic showing ideal Curie-Weiss behavior with a Néel temperature T_N of about 19 K and Curie-Weiss temperature θ=−200 K, while for Cu₂MnSnSe₄ the behavior was spin-glass with a freezing temperature T_f of about 22 K and Curie-Weiss temperature θ=−25 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory.     

Magnetic and transport properties of Nd1.67Sr0.33NiO4

Nd_1.67Sr_0.33NiO₄ polycrystals have been prepared by modified sol–gel method and subsequent annealing. X-ray diffraction analysis, electrical resistivity, magnetic susceptibility and thermal magnetisation have been measured. Rietveld analyses show a tetragonal or pseudo-tetragonal K₂NiF₄-type structure. The resistivity measurements present a change in conduction mode close to 230 K, which corresponds to the charge ordering temperature. Below this temperature, the material adopts a variable range hopping conduction mode; and above, the conduction follows adiabatic thermal activated mode. The magnetic measurements show paramagnetic behaviour in the range of 80–300 K. Moreover, the magnetic susceptibility data show a sign of the charge ordering transition about 230 K.     

First-principles calculations of electronic, optical and elastic properties of ZnAl2S4 and ZnGa2O4

The optimized crystal structures, band structures, partial and total densities of states (DOS), dielectric functions, refractive indexes and elastic constants for ZnAl₂S₄ and ZnGa₂O₄ were calculated using the CASTEP module of Materials Studio package. Pressure effects were modeled by performing these calculations for different values of external hydrostatic pressure up to 50 GPa. Obtained dependencies of the unit cell volume on pressure were fitted by the Murnaghan equation of state, and the relative changes of different chemical bond lengths were approximated by quadratic functions of pressure. Variations of applied pressure were shown to produce considerable re-distribution of the electron densities around ions in both crystals, which is evidenced in different trends for the effective Mulliken charges of the constituting ions and changes of contour plots of the charge densities. The longitudinal and transverse sound velocities and Debye temperatures for both compounds were also estimated using the calculated elastic constants.     

Synthesis and magnetic properties of NiFe2−xAlxO4 nanoparticles

Nanocrystalline Al-doped nickel ferrite powders have been synthesized by sol–gel auto-ignition method and the effect of non-magnetic aluminum content on the structural and magnetic properties has been studied. The X-ray diffraction (XRD) revealed that the powders obtained are single phase with inverse spinel structure. The calculated grain sizes from XRD data have been verified using transmission electron microscopy (TEM). TEM photographs show that the powders consist of nanometer-sized grains. It was observed that the characteristic grain size decreases from 29 to 6 nm as the non-magnetic Al content increases, which was attributed to the influence of non-magnetic Al concentration on the grain size. Magnetic hysteresis loops were measured at room temperature with a maximum applied magnetic field of ≈1 T. As aluminum content increases, the measured magnetic hysteresis curves become more and more narrow and the saturation magnetization and remanent magnetization both decreased. The reduction of magnetization compared to bulk is a consequence of spin non-collinearity. Further reduction of magnetization with increase of aluminum content is caused by non-magnetic Al³⁺ ions and weakened interaction between sublattices. This, as well as the decrease in hysteresis was understood in terms of the decrease in particle size.     

Study of magnetotransport in double-layered La1.4Ca1.6Mn2O7 manganite: Presence of nano-ferromagnetic domains in paramagnetic matrix

Double-layered manganite La_1.4Ca_1.6Mn₂O₇ has been synthesized using the solid-state reaction method. It had a metal-to-insulator transition at temperature T_M1≈127 K. The temperature dependence of ac susceptibility showed a broad ferromagnetic transition. The two-dimensional (2D)-ferromagnetic ordering temperature (T_C2) was observed as ≈245 K. The temperature dependence of its low-field magnetoresistance has been studied. The low-field magnetoresistance of double-layered manganite, in the temperature regions between T_M1 and T_C2, has been found to follow 1/T⁵. The observed behaviour of temperature dependence of resistivity and low-field magnetoresistance has been explained in terms of two-phase model where ferromagnetic domains exist in the matrix of paramagnetic regions in which spin-dependent tunneling of charge carriers occurs between the ferromagnetic correlated regions. Based on the two-phase model, the dimension of these ferromagnetic domains inside the paramagnetic matrix has been estimated as ∼12 Å.     

The effects of elements doping on transport and thermoelectric properties of Sr3Ti2O7

The Ruddlesden–Popper (RP) phase compounds (Sr_0.95R_0.05)₃Ti₂O₇ (R=Er, Y, Dy, Gd, Eu, Sm, Nd and La) were prepared, and their transport and thermoelectric properties were investigated. The results indicate that high-T electrical resistivity ρ (300 K<T<1000 K) increases monotonically with temperature and basically has a relation ρ∝T^M, with M varying from 0.91 to 1.92 at temperatures T>~650 K, suggesting acoustic phonon scattering is dominant. At low temperatures (5 K<T<300 K), ρ for (Sr_0.95R_0.05)₃Ti₂O₇ (R=Nd and La) decreases monotonously with decreasing temperature, whereas ρ for (Sr_0.95R_0.05)₃Ti₂O₇ (R=Er, Y, Dy, Gd, Eu and Sm) decreases first, and then increases instead as T decreases to a critical temperature T_c. Moreover, electrical conductivity σ∝T^1/2 holds at lower temperatures, indicating that the electron–electron interaction caused by the presence of disorder dominates the transport process at the low temperatures. Besides, experiments show that at T<~400 K the lattice thermal conductivity of the doped compounds basically decreases with increase of the atomic mass of dopants. Generally, the figure of merit (ZT) at 1000 K increases first, and then decreases with the increase of the dopants' ionic radius, and the largest ZT is achieved in (Sr_0.95Gd_0.05)₃Ti₂O₇ mainly owing to its lower lattice thermal conductivity.