Yongquan Guo

Interplay of structure and transport properties of sodium-doped lanthanum manganite

The crystal structure, magnetic and electrical transport properties of the sodium-doped lanthanum manganites La1-xNaxMnO3 (0.07≤x≤0.40) have been studied in detail using x-ray powder diffraction, atomic absorption spectroscopy, a SQUID (superconducting quantum interference device) magnetometer and the four-probe resistivity measurement technique. A rhombohedrally distorted perovskite structure has been observed in the range 0.07≤x≤0.20. Both the lattice parameter and unit-cell volume decrease with increase in the Na content. A ferromagnetic-to-paramagnetic phase transition associated with a metal-insulator transition is observed for all the La1-xNaxMnO3 compounds. There is a systematic change in both the Mn-O-Mn bond angle and the tolerance factor with Na content. The compositional variation of the magnetic and metal-insulator transition temperatures is explained as due to the distortion of the MnO6 octahedron and increase in the tolerance factor that controls the hopping interaction. In the metallic region a ρ~AT2 behaviour is observed due to the magnon excitation effect. The resistivity shows a field-dependent minimum at low temperature that has been explained as due to the intergrain transport phenomenon.

Structure and magnetic properties of RFe6−xGa6+x (R=rare earth)

The crystal structures and magnetic properties of RFe6−xGa6+x (R=Ce, Pr, Nd, Sm, and Gd) alloys have been investigated by x-ray powder diffraction and magnetic measurement. A ternary intermetallic compound with an orthorhombic ScFe6Ga6 type structure is found in these alloys. The lattice parameter contraction is observed when the higher atomic number lanthanides are substituted into the intermetallic. In each unit cell, there are six nonequivalent crystal positions, i.e., 2a, 4e, 4f, 4g, 4h, and 8k, which are occupied by 2R, 4GaI, 4(FeI,Ga), 4GaII, 4GaIII, and 8FeII, respectively. The RFe6−xGa6+x alloys have ferromagnetic ordering but the magnetic transitions have been found in Nd based alloy. The RFe6−xGa6+x alloys show a soft magnetic behavior but the magnetization hysteresis loops have been observed in the Nd and Sm alloys at 5 K. The saturated magnetic moments per molecule tend to increase from Ce to Nd. However, a drastic decrease of magnetic moment in GdFe6Ga6 alloy is observed and is likely due to ...

Magnetic and electronic transport properties of YbxCa1−xMnO3 compounds

The polycrystalline YbxCa1−xMnO3 (x=0.1,0.2) were studied by x-ray powder diffraction, magnetic, and electrical resistivity measurements. The YbxCa1−xMnO3 crystallizes in an orthogonally distorted perovskite structure, and shows the ferromagnetic ordering with TC more than 110 K. However, the field dependence of magnetization and electrical resistivity exhibits very complicated behavior. A magnetic field induced insulator–metal transition has been found in the Yb0.2Ca0.8MnO3 compound. In addition, the large asymmetry in magnetization and resistivity hysteresis loops has been observed in this compound at 10 K, which might be due to the charge ordering and magnetocrystalline anisotropy.

The effect of element doping and heat treatment on the impact resistance of Nd–Fe–B sintered magnets

The effects of rare-earth contents, substitutions, and heat treatment parameters on the impact resistance of the Nd–Fe–B sintered magnets were investigated. Analysis shows that the impact resistance increases linearly with increasing plastic rare-earth-rich phase in the investigated magnets. Heavy rare-earth Dy substitutions for Nd help us to improve the impact resistance. The bending strength and impact resistance during annealing process was investigated. It is found that the annealing process affects the bending strength and impact resistance remarkably. Both the bending strength and the intrinsic coercivity are poor in the as-sintered specimen. In the postsintering annealing process, both of them get improved and the change of intrinsic coercivity is similar to that of the bending strength. Study showed that the optimized heat treatment parameters help us to improve the bending strength and impact resistance of the sintered Nd–Fe–B magnets. Study shows that the sintered Nd–Fe–B magnets with improved i...

Effect of nitrogen incorporation in La2/3Ca1/3MnO3 manganite

We have studied the crystal structure, the magnetic and electron transport properties and the magnetoresistance effect of nitrogenation of La2/3Ca1/3MnO3 by x-ray powder diffraction, magnetic measurement and standard four-probe resistance-temperature measurement. Nitrogenation results in an increase in the lattice parameter and unit cell volume, and induces the formation of a superlattice. The N atom prefers to occupy the 4c crystal position in the superlattice structure. Nitrogenation weakens the ferromagnetic ordering and results in the decrease of Curie temperature and the saturation magnetic moment but increases the electrical resistivity significantly. The electrical resistivity exhibits a metal-insulator (MI) transition, but the MI transition temperature decreases after nitrogenation. The behaviour of the electrical resistivity follows the equation ρ = ρ0(1-aT + bT2 + cT4) in the metal regime. However, it follows the formula ρ = ρ0exp (E/kBT) in the insulator regime. The magnetoresistance ratio measurement shows an increase of 15% after nitrogenation.

Unusual electronic transport properties in La0.96-xNdxK0.04MnO3

Polycrystalline La0.96−xNdxK0.04MnO3 has been studied by means of x-ray powder diffraction, dc magnetization, and electrical resistance measurement. La0.96−xNdxK0.04MnO3 initially crystallizes in a rhombohedral distorted perovskite structure. However, an orthorhombic structure is obtained with increasing Nd content more than 20%. Both the rhombohedral and the orthorhombic phases show ferromagnetic order. However, the compound shows a magnetic inhomogeneity behavior with increasing Nd content. With increasing Nd doping, the ferromagnetic order is significantly weakened, and the second magnetic transition occurs at a low temperature and its peak intensity enhances. The resistivity of these compounds also exhibits an imhomogeneity behavior. The rhombohedral phase shows a metal–insulator (MI) transition but the MI transition peak intensity is obviously weakened and broadens with increasing Nd content and a low temperature insulator state is observed in the orthorhombic phase. It is interesting that the second...