W.F. Liu

Structure and magnetoresistance of the double perovskite Sr2FeMoO6 doped at the Fe site

The structural, magnetic and transport properties of the compounds Sr2(Fe1−xMnx)MoO6 (0 ≤ x ≤ 0.45) have been studied. The saturated magnetization, Curie temperature and low-field magnetoresistance of the compounds decrease with x, while the resistivity increases by several orders of magnitude when x exceeds a critical value. A positive magnetoresistance has been observed for x = 0.45. A possible percolation mechanism is proposed to elucidate the observations; it also suggests a coexistence of (Mn3+, Mo5+) and (Mn2+, Mo6+) valence pairs and a saturated substitution of Mn3+ for Fe3+.

Mixing effects of light and heavy rare earths in the Laves phase compounds Nd1−xTbxCo2

The lattice constant and magnetic properties of compounds Nd1-xTbxCo2 were investigated by x-ray power diffraction and magnetic measurements. The lattice constant and Curie temperature show linear variations with the Tb concentration, while the saturation moment exhibits an anomaly at a critical concentration x(c)approximate to0.33. The compounds exhibit a field-induced metamagnetic transition from a weak ferrimagnetism to a strong ferrimagnetism in the vicinity of x(c)

The abnormal electrical and optical properties in Na and Ni codoped BiFeO3 nanoparticles

Bi0.97Na0.03Fe1−xNixO3 (x = 0, 0.005, 0.01, 0.015) nanoparticles are prepared via a sol-gel method. Weak ferromagnetism and exchange bias phenomenon without field cooling are observed in the samples. The oxygen vacancy concentration and leakage current density are increased with increasing the Ni content. However, with the increase of Ni content, the band gap of Bi0.97Na0.03Fe1−xNixO3 nanoparticles first decreases and then increases. To explain the abnormal phenomenon, the interplay of oxygen vacancy donor and hole acceptor is analyzed and a phenomenological qualitative model based on the electronic energy band is proposed. Additionally, the threshold switching behavior appears in Bi0.97Na0.03Fe1−xNixO3 samples with x = 0.01, 0.015 and the effect is qualitatively explained by introducing a conducting channel model based on the high-density mobile charges.

Crystal structure and magnetic properties of Pr5Si4−xGex compounds

The magnetic properties of Pr5Si4-xGex (x = 0.0, 1.0, 2.0, 2.5, 3.0, 3.5, 4.0) were investigated by magnetic measurements. The compounds Pr5Si4-xGex crystallize in the tetragonal Zr5Si4-type structure for x = 0.0 and 1.0, in the monoclinic Gd5Si2Ge2-type structure for x = 2.0 and 2.5; and in the orthorhombic Sm5Ge4-type structure for x = 3.0-4.0. Unlike the heavy rare earth RE5Si4-xGex (RE = Gd, Tb, Dy) system, the Curie temperature of this system exhibits the following features: the Curie temperature of the compound increases with the increasing of the Ge content in the tetragonal phase region, whereas it shows a weak dependence on the Ge content in the monoclinic phase region and decreases in the orthorhombic phase region. All of the compounds in this system may have a canted magnetic structure

Structure dependence of magnetic properties of Nd5Si4−xGex (x=1.2 and 2)

The crystal structure and magnetic properties of ternary compounds Nd5Si4-xGex (x = 1.2 and 2) were investigated by means of X-ray powder diffraction and magnetic measurements. The as-cast samples alpha-Nd5Si2.8Ge1.2 and beta-Nd5Si2Ge2 crystallize in the orthorhombic Gd5Si4-type structure with space group Puma and the monoclinic Gd5Si2Ge2-type structure with space group P112(1)/a, respectively. The samples beta-Nd5Si2.8Ge1.2 and beta-Nd5Si2Ge2 annealed at 1273 K all crystallize in the tetragonal Zr5Si4-type structure with space group P4(1)2(1)2. All phases of alpha-Nd5Si2.8Ge1.2, beta-Nd5Si2.8Ge1.2, alpha-Nd5Si2Ge2 and beta-Nd5Si2Ge2 exhibit a spin glasslike behavior and may have a canted magnetic structure. The Curie temperature shows a strong dependence on the crystal structure of the compounds. The average interatomic distances of magnetic Nd atoms and the amount of the covalent (Si,Ge)-(Si,Ge) bond pairs in these phases are different, which may be responsible for their different magnetic properties

Abnormal variation of band gap in Zn doped Bi0.9La0.1FeO3 nanoparticles: Role of Fe-O-Fe bond angle and Fe-O bond anisotropy

Bi0.9La0.1FeO3 (BLFO) and Bi0.9La0.1Fe0.99Zn0.01O3 (BLFZO) nanoparticles were prepared via a sol-gel method. The oxygen vacancies and holes increase with Zn doping analyzed through X-ray photoelectron spectroscopy, which could contribute to the increase of leakage current density. However, with the increase of the defects (oxygen vacancies and holes), the band gap of BLFZO also is increased. To explain the abnormal phenomenon, the bandwidth of occupied and unoccupied bands was analyzed based on the structural symmetry driven by the Fe-O-Fe bond angle and Fe-O bond anisotropy.

Structure dependence of magnetic properties of Pr5Si2Ge2

The crystal structure and magnetic and electronic transport properties of the ternary intermediate compound Pr5Si2Ge2 were investigated by means of x-ray powder diffraction, magnetic and electrical resistance measurements. The compound has two polymorphs: alpha-Pr5Si2Ge2 crystallizing in a monoclinic structure with space group P112(1)/a, for the as-cast sample, and alpha-Pr5Si2Ge2 crystallizing in a tetragonal structure with space group P4(1)2(1)2, for. the sample annealed at 1273 K. The average interatomic distances of magnetic Pr atoms and the amounts of covalent (Si, Ge)-(Si, Ge) bond pairs in these two phases are different, which may be, responsible for their different magnetic properties. The Curie temperatures T-C of alpha-Pr5Si2Ge2 and beta-Pr5Si2Ge2 are 38 and 50 K, respectively. An irreversibility between zero-field-cooling and field-cooling magnetization curves is observed. The zero-field resistance of these two phases exhibits an anomaly near T-C. On the basis of the results of magnetic and electrical resistance measurements, it is inferred that both of these phases exhibit spin-glass-like behaviour. The magnetic properties and structure dependence of the Pr5Si2Ge2 can be elucidated well using the Ruderman-Kittel-Kasuya-Yosida model.

Room temperature threshold switching behaviors of Bi0.9Nd0.1Fe1−xCoxO3 nanoparticles

Many investigations have reported threshold switching (TS) effects in amorphous semiconductors, but it is rarely observed in BiFeO3 nanoparticles. Here, the effects of cobalt doping on the electrical conductivity of Bi0.9Nd0.1Fe1−xCoxO3 (x = 0, 0.01, 0.03, 0.05) nanoparticles were investigated. Leakage current density (J) versus applied voltage (V) curves were measured and it was found that the leakage current increases with increases of the cobalt dopant concentration. With the increase of the maximal applied voltage, the samples with cobalt doping exhibit nearly symmetric TS behaviors at room temperature. With the increase of cobalt dopant concentration, the optical band gap is decreased, and the smallest optical band gap value (∼2.01 eV) is observed for the 3% Co doped BiFeO3 sample. X-ray photoelectron spectroscopy of the samples shows that the cobalt dopant induces an enhancement of the oxygen vacancy concentration. Possible mechanisms for the threshold switching effects are discussed on the basis of a conductive filament model. The mobile charge carriers align to form conducting filaments when the threshold voltage (VTH) is achieved, corresponding to the transition from a high-resistance state to a low-resistance state, and the rupture of the conductive path takes place when the applied voltage is decreased. The leakage current decreases dramatically and eventually returns to the initial high-resistance state.

Dielectric, multiferroic properties and resistance modulation of Ta-doped Bi0.97Ba0.03FeO3 ceramics

Single phase Bi0.97Ba0.03Fe1-xTaxO3 ceramics with x = 0, 0.01, 0.03, 0.05 were synthesized by modified rapid sintering process method. The formation of rhombohedral perovskite-like structure was confirmed by X-ray diffraction investigation for all the samples. Dielectric and leakage current measurements indicated that the content of the oxygen vacancy in the samples decreased as a function of the substitution of Ta5+ ions. A distinct threshold switching behavior was observed in the leakage current density. The impedance measurements suggested that the grain effect made a major contribution to the resistance. The changes in dielectric, multiferroic properties and resistance modulation of the Ta5+ and Ba2+ co-doped BiFeO3 ceramics could have a huge potential for material application.

Crystal structure and spin-reorientation transition in TbFe10−xNixSi2 compounds

The crystal structure and magnetic properties of TbFe10-xNixSi2 compounds have been investigated by means of X-ray diffraction and magnetic measurements. It is revealed that Ni substitution for Fe results in a slight variation of the Curie temperature, but a salient decrease of the spin-reorientation temperature of the compounds. A spin-phase diagram of TbFe10-xNixSi2 is constructed. At room temperature the compounds show planar magnetocrystalline anisotropy for x less than or equal to 1.5, but easy-axis anisotropy for x > 1.5. Temperature dependent X-ray diffraction indicates a noticeable magnetostriction around the spin-reorientation temperature