J Du

Effect of Ag substitution on the transport property and magnetoresistance of LaMnO3

Abstract The structural, magnetic and electronic properties of the polycrystalline La1−xAgxMnO3 are systematically investigated as a function of Ag-doping level. The result of the Rietveld refinement of X-ray powder diffraction shows that the samples are single rhombohedral ( R 3 C ) structure phase for x R 3 C perovskite phase and a nonmagnetic metal Ag phase. The temperature dependence of resistivity shows that all samples undergo a sharp insulator–metal (I–M) transition accompanying a paramagnetic to ferromagnetic transition with the decrease of temperature. However, for x

Improvement of the phase formation and superconductivity of the (Bi, Pb) 2Sr2Ca2Cu3Ox silver-sheathed tapes with B2O3 addition

In (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed (Bi2223/Ag) tapes, melted liquid phase plays an important role to form the Bi2223 phase. We have added a small amount of B2O3 into Bi2223/Ag tapes to assist in inducing melted phase because B2O3 has a melting point (460°C) much lower than the general sintering temperature of the tapes, and studied the influence of B2O3 doping on the microstructure and critical current density (Jc) of Bi2223/Ag tapes. The results show that B2O3 doping is really effective to result in the faster growth and better alignment of the Bi2223 grains in the superconducting core and improve the magnetic field dependence of the Jc value.

Preparation and magnetic properties of the double-perovskite A2FeMoO6 (A=Ca, Sr, Ba) polycrystals with nanometer-scale particles

Polycrystalline double perovskites A2FeMoO6u200a(A=Ca,u200aSr,u200aBa) with nanometer-scale grain size have been synthesized using a sol–gel method. The grain size of the samples is controlled within nanometer scale by sintering at different temperatures. The phase purity and the crystal structure of the samples are analyzed by x-ray powder diffraction measurements. The electrotransport and magnetic properties are also measured in this article. In comparison with the large grain samples with micrometer scale, the nanometer-scale grain samples have different magnetic properties, such as lower magnetic transition temperatures and larger magnetoresistance, which can be explained in terms of size effect.

Magnetic properties and enhanced magnetic refrigeration in (Mn1−xFex)5Ge3 compounds

Magnetic and magnetocaloric effects of (Mn1-xFex)(5)Ge-3 compounds are studied systematically. The maximum of magnetic entropy changes of 8.01 J/kg K under an external field change of 5 T is obtained for (Mn0.9Fe0.1)(5)Ge-3, which is the largest value in Mn5Ge3-based solid solutions. Moreover, the Fe substitution increases the refrigeration capacity (RC) value greatly. The largest RC value of 237 J/kg in (Mn0.8Fe0.2)(5)Ge-3 even compares favorably to that of many well-known magnetic refrigeration materials. Thus the Fe-containing (Mn1-xFex)(5)Ge-3 compounds are much-improved magnetic refrigerants for the application of room-temperature magnetic refrigeration. The increase of the RC value is probably resulted from the formation of magnetic nanostructure. (c) 2007 American Institute of Physics.

Synthesis and magnetic properties of compounds

The synthesis and magnetic properties of have been investigated. The formation of is found to depend strongly on annealing temperature. Pure single-phase compound can be synthesized at ambient pressure when . The main phase of the Laves structure can still be observed up to x = 0.8. The study of crystalline parameters, magnetic moments and Mossbauer spectra for single-phase is performed. Curie temperature and local Fe magnetic moment increase with the increasing Pr concentration. Extrapolating the trends the spontaneous magnetization and anisotropy constant of are 4.98 and at 1.5 K, respectively. Magnetostriction increases with increasing x and the largest saturation magnetostriction, 200 ppm, is observed for x = 0.5. With the increase of Pr concentration, lattice parameter and spontaneous magnetization anomalies exist in a specific range 0.2 < x < 0.5 that can be attributed to the change of Ce ion valence toward tetravalent.

Large low-field inverse magnetocaloric effect in Ni 50−x Mn 38+x Sb 12 alloys

The magnetic properties and magnetocaloric effects of Ni(50-x)Mn(38+x)Sb(12) ferromagnetic shape-memory alloys with x = -1, 0, 1 and 2 that undergo a martensitic transformation were investigated. The magnetic-entropy changes Delta S of nominal Ni(49)Mn(39)Sb(12), or Ni(49.5)Mn(38.6)Sb(11.9), at 279K is 6.15 J kg(-1) K(-1) for a magnetic-field change Delta B = 1 T, with negligible hysteresis loss, as it transforms from a low-temperature martensitic phase to a high-temperature austenitic one. The large inverse Delta S in a small field change and the negligible hysteresis loss, along with the low cost of Sb, indicate that Ni(49)Mn(39)Sb(12) is a promising candidate for room-temperature magnetic refrigeration.

Visible-light-induced persistent photoconductivity in perovskite manganite films of (La0.3Nd0.7)2/3Ca1/3MnO3

The persistent photoconductivity (PPC) induced by visible light of a He–Ne laser with a wavelength of 632.8 nm in perovskite manganite films of (La0.3Nd0.7)2/3Ca1/3MnO3 at low temperatures, below ∼50 K, was observed. Based on the measurements of transport and magnetic properties, it is suggested that the samples undergo a transition of cluster-glass state below ∼50 K. The PPC phenomenon can be qualitatively explained in terms of a photoinduced ferromagnetic cluster growth model.

Ordered double-perovskite Ca2FeMoO6 compounds with nanometer-scale grains: structure, magnetism, and intergrain tunneling magnetoresistance

Abstract Polycrystalline ordered double-perovskite Ca2FeMoO6 bulk samples with nanometer-scale grain size have been synthesized using a sol–gel method. A large magnetoresistance (MR) of 7.6% at a low magnetic field of 0.5 T and room temperature is obtained. The result of ac susceptibility shows that the sample has two magnetic transitions at TC1=374.6 K and TC2=336.4 K, which may correspond to the orthorhombic structure and the monoclinic structure phase, respectively. Due to the large MR at room temperature and well ferromagnetic conductivity of Ca2FeMoO6, it may be a desirable material for practical applications in future.

EFFECT OF SILICON SUBSTITUTION ON THE MAGNETIC PROPERTIES OF TB2FE17 SINGLE CRYSTALS

The growth of single crystals Tb2Fe17−xSix (x=0, 1, 2, 3, and 3.3) by the Czochralski method is performed. The crystal structure and magnetic properties are investigated. The substitution of silicon for iron leads to the reduction of the lattice parameters, and the significant increase of the Curie temperature from 413 to 526 K. The mean field analysis shows that the Fe–Fe exchange interaction is much strengthened while the Tb–Fe one is slightly weakened by the introduction of silicon. The anisotropy constants up to the sixth order are calculated by fitting the magnetization process of the samples at 1.5 K under the field up to 65 kOe. In addition, a narrow domain wall pinning-dominated process caused by the substitution of Si for Fe is confirmed and analyzed.

Visible-light induced persistent photoconductivity in trilayered films of perovskite manganites

The persistent photoconductivity (PPC) effect has been observed in trilayered films made of perovskite manganites La2/3Ca1/3MnO3/(La0.3Nd0.7)2.3Ca1/3MnO3La2/3Ca1/3MnO3(LNL) induced by He–Ne laser with wavelength of 632.8 nm. According to the result obtained in the thin film of (La0.3Nd0.7)2/3Ca1/3MnO3(LNCMO), which the PPC effect is also observed below ∼50 K, the PPC effect observed in trilayered film LNL should originate from the middle layer LNCMO. Compared with the thin film of LNCMO, the PPC effect of the trilayered film LNL appears at ∼86 K, which is higher than that of LNCMO at ∼50 K. The PPC effect of LNL can be quenched on thermal cycling in the vicinity of 98 K, which is also higher than that of LNCMO at ∼77 K. The difference of PPC behavior between the thin films of LNCMO and LNL can be attributed to the variation of cluster-glass state in the trilayered films of LNL caused by the strong coupling of interlayer between the middle layer LNCMO and the top/bottom layers LCMO.