S. L. Ye

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

Effect of Li substitution on the crystal structure and magnetoresistance of LaMnO3

The crystal structure and magnetoresistance of the polycrystalline La1−xLixMnO3 (x=0.10, 0.15, 0.20, 0.30) are investigated. The result of the Rietveld refinement of x-ray powder diffraction shows that the room temperature structural transition from rhombohedral (R3C) to orthorhombic (Pbnm) symmetry occurs at the Li-doped level x⩾0.2. Accompanying the occurrence of the structural transition, the lattice distortion and the bending of the Mn–O–Mn bond increase and the ferromagnetic transition temperature TC decreases. For x=0.10 and 0.15 samples, double metal–insulator (M–I) transitions accompanying a single ferromagnetic transition and a negative magnetoresistance as high as 26% in a magnetic field of 0.8 T are observed. For x=0.20 and 0.30, the samples manifest nonmetallic behavior throughout the measured temperature range. We suggest that the double M–I transitions phenomena of low Li-doped samples originate from the magnetic inhomogeneity due to the formations of the Mn3+ and Mn4+-rich regions induced ...

Mn2+ concentration manipulated red emission in BaMg2Si2O7 : Eu2+,Mn2+

The luminescent properties of concentration dependence are reported in BaMg2Si2O7:Eu2+,Mn2+ red phosphor. It is observed that the broad red emission of Mn2+ consists of two bands, located at 620 and 675 nm, respectively, which are attributed to two different Mn2+ centers [Mn2+(I) and Mn2+(II)] substituting for two nonidentical Mg2+ sites [Mg2+(I) and Mg2+(II)] in the host. It is also found that the relative emission intensity of the Mn2+(II) to the Mn2+(I) increases with increasing Mn2+ concentration, leading to a red-shift of the overall emission. A detail analysis on the energy transfer from Eu2+ to the two Mn2+ centers is presented, which indicates that the number ratio of Mn2+(II) to Mn2+(I) increases with increasing Mn2+ concentration. This result is interpreted by the preferential formation of Mn2+(I) substituting for Mg2+(I) site. Based on energy transfer, the emission intensity ratios of Mn2+(I) to Eu2+ and Mn2+(II) to Eu2+, which is Mn2+ concentration dependent, are calculated using related fluor...

Mn2+ activated red phosphorescence in BaMg2Si2O7: Mn2+, Eu2+, Dy3+ through persistent energy transfer

Based on persistent energy transfer, Mn2+ activated red phosphorescence is achieved in BaMg2Si2O7. Two types of Mn2+ centers with emitting peaks at 620 and 675nm, respectively, are observed and found to govern both the red luminescence and red phosphorescence spectral shapes. The spectral distribution of the red phosphorescence as a function of irradiation wavelengths and Mn2+ concentrations is systematically studied in Mn2+ and Dy3+ doubly doped and Mn2+, Eu2+, and Dy3+ tridoped samples, indicating the dominant role of persistent energy transfer on generation of the red phosphorescence. Moreover, it is found that the incorporation of Mn2+ in BaMg2Si2O7: Eu2+, Dy3+ may prolong the phosphorescent persistence time (>1h) compared with the Mn2+ free materials.

Luminescence and Energy Transfer in Eu2 + and Mn2 + Co-doped Ca2P2O7 for White Light-Emitting Diodes

Eu 2+ and Mn 2+ co-doped α- and β-Ca 2 P 2 C 7 are prepared by solid-state reaction. An orange emission band originating from the 4 T 1 ( 4 G)- 6 A 1 ( 6 S) transition of Mn 2+ is observed in both phases upon UV excitation through energy transfer from Eu 2+ to Mn 2+ . Photoluminescence and energy transfer in α-Ca 2 P 2 C 7 :Eu 2+ , Mn 2+ are investigated as a function of Mn 2+ concentrations. The energy transfer rates are calculated based on the analysis of the luminescence dynamical process as well as fluorescence lifetime measurements. The intensity ratios of the orange to blue band as a function of Mn 2+ concentrations are also calculated and the results are in good agreement with that obtained directly from emission spectra. A white light emitting diode (LED) fabricated through the integration of a GaN near-UV chip and two phosphor blends (α-Ca 2 P 2 O 7 :Eu 2+ , Mn 2+ blue-orange phosphor and Ba 2 SiO 4 :Eu 2+ green phosphor) into a single package shows a warm white light of 3797 K correlated color temperature, a good color rendering index (CRI) of 84, and a luminescent efficiency of 11 1m/W with color coordinates of x = 0.39, y = 0.40. Moreover, the CRI can be improved to 94 by using blue Ca 2 (PO 4 ) 3 Cl:Eu 2+ phosphor added to the two phosphor blends.

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

Polycrystalline double perovskites A2FeMoO6 (A=Ca, Sr, Ba) 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.

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.

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.

Effect of Li doping on the charge ordering state of La0.5Ca0.5MnO3

The effect of Li doping on the charge ordering (CO) state in perovskite-type manganates La0.5Ca0.5−xLixMnO3 with x=0, 0.05, 0.1, 0.15, 0.2 is investigated by transport and magnetic property measurements. The resistivity of the samples decreases with the increase of the Li-doped level. As x⩾0.1, the insulator–metal (IM) transition has been induced. Accompanying the variation of transport properties, the sample undergoes the variation from antiferromagnetic order for x<0.05 to ferromagnetic (FM) order for x⩾0.1. The experimental results indicate that the Li doping can destroy the CO state and induce the FM metallic (FMM) phase. These phenomena can be interpreted in terms of the strong competition between the CO state and FM phase induced by the Li+ ions substitution for the Ca2+ ions.