S.-J. Feng

Competition between ferromagnetic metallic and paramagnetic insulating phases in manganites

La0.67Ca0.33Mn1−xCuxO3 (x=0 and 0.15) epitaxial thin films were grown on the (100) LaAlO3 substrates, and the temperature dependence of their resistivity was measured in magnetic fields up to 12 T by a four-probe technique. We found that the competition between the ferromagnetic metallic (FM) and paramagnetic insulating (PI) phases plays an important role in the observed colossal magnetoresistance (CMR) effect. Based on a scenario that the doped manganites approximately consist of phase-separated FM and PI regions, a simple phenomenological model was proposed to describe the CMR effect. Using this model, we calculated the resistivity as functions of temperature and magnetic field. The model not only qualitatively accounts for some main features related to the CMR effect, but also quantitatively agrees with the experimental observations.

The effect of phase separation on charge ordering state in La1-xCaxMnO3 (x = 1/2, 2/3, and 3/4)

The magnetic phase separation characteristics are found in manganese perovskite La1/3Ca2/3MnO3 by electron spin resonance (ESR) and magnetization measurements. An extra resonance signal observed in ESR spectra just above the charge ordering (CO) temperature TCO provides strong evidence for the existence of ferromagnetic (FM) clusters near the CO state. The investigation of the resistivity of La12xCaxMnO3 (x ¼ 1/2, 2/3, and 3/4) in different magnetic fields up to 14 T shows that the effect of magnetic fields on CO state decreases with increasing x. Our results indicate that the percolative characteristics of the phase separation between FM clusters and CO state for x ¼ 1=2 and 2/3 samples are related to the magnetic field dependence of CO state. However, for x ¼ 3=4 it is assumed that there are no obvious FM clusters in the CO phase. q 2002 Published by Elsevier Science Ltd.

Structural, transport and magnetic properties of RuSr2Sm1.4Ce0.6Cu2O10−δ

yThe structural, transport and magnetic properties of RuSr2Sm1.4Ce0.6Cu2O10-delta have been studied. The increase of oxygen content results in a decrease of lattice parameters and an increase of superconducting transition temperature. For Raman spectra, the hardening of the 650 cm(-1) mode with decreasing temperature is due to the thermal contraction of lattice. Magnetic measurements exhibited a ferromagnetic transition around 150 K, indicating the coexistence of superconductivity and ferromagnetism in this compound

Energy dissipation in Bi2Sr2CaCu2O8+δ single crystal

Abstract The in-plane resistivity of Bi 2 Sr 2 CaCu 2 O 8+ δ single crystal in different magnetic fields was measured for magnetic field H parallel and perpendicular to the CuO 2 plane. The irreversibility line H irr ⊥ c ( T ) for H parallel to the CuO 2 plane locates in higher field and higher temperature region in the H – T phase diagram than H irr ∥ c ( T ) for H perpendicular to the CuO 2 plane. H irr ∥ c ( T ) is dominated by surface barrier pinning, while H irr ⊥ c ( T ) shows different temperature dependencies above and below 2 T, which may indicate different energy dissipation above and below the field. The field dependence of activation energy above 2 T in H ⊥ c configuration could be explained by vortex–antivortex expansion in CuO 2 plane.

Effect of Pb doping on the superconducting and magnetic resonance properties of Ru-1222

Abstract The resistivity, electron paramagnetic resonance (EPR) and microstructure of polycrystallines Ru 1− x Pb x Sr 2 Gd 1.4 Ce 0.6 Cu 2 O 10− δ ( x =0, 0.03, 0.2, 0.4, 0.5) have been studied systematically. Pb doping reduces the conductivity and suppresses the superconductivity, which may reflect the increase in hole localization due to disorder effects resulting from Pb doping. The temperature dependencies of magnetic resonance for samples Ru 1− x Pb x Sr 2 Gd 1.4 Ce 0.6 Cu 2 O 10− δ show that there are two absorption peaks. The first one around the magnetic field 3300 Oe corresponds to the electron paramagnetic resonance (EPR) of the Gd ions, and the second that appears below the ferromagnetic transition temperature is a ferromagnetic resonance (FMR) due to the ordered Ru ions.

Transport and magnetic resonance properties of Ru1−xSbxSr2Gd1.4Ce0.6Cu2O10−δ

Abstract The resistivity, electron paramagnetic resonance (EPR) and microstructure of polycrystallines Ru 1− x Sb x Sr 2 Gd 1.4 Ce 0.6 Cu 2 O 10− δ ( x =0, 0.01, 0.02, 0.05, 0.07) have been studied systematically. Sb doping reduces the conductivity of the system and the onset superconducting transition temperature decreases from 42 K for x =0 to 19 K for x =0.07. This may reflect the increase in hole localization due to a distortion of RuO 6 octahedra. The temperature dependencies of magnetic resonance for samples Ru 1− x Sb x Sr 2 Gd 1.4 Ce 0.6 Cu 2 O 10− δ show that there are two absorption peaks. The first one around the magnetic field 3300 Oe corresponds to the EPR of the Gd ions, and the second that appears below the ferromagnetic transition temperature is a ferromagnetic resonance due to the ordered Ru ions.

Transport and magnetic properties in La0.7Ca0.3Mn1−xCuxO3

Abstract The resistivity, ac susceptibility, electron spin resonance (ESR), and X-ray photoemission spectroscopy (XPS) were investigated in La0.7Ca0.3Mn1−xCuxO3 (x=0, 0.1, and 0.15). It was found that the resistivity becomes larger and the paramagnetic to ferromagnetic transition temperature TC shifts to lower temperatures with increasing Cu-doping level. However, the insulator–metal and magnetic transition temperatures of the system are still high even when the content of Cu is equal to 0.15, which is also confirmed by the characterizations of magnetization and resistivity of La0.7Ca0.3Mn0.85Cu0.15O3 epitaxial film. Our analysis shows that the main effect arises from the proportion variation of Mn4+ ions. Taking Cu 2p3/2 core level spectra into consideration, we pointed out that there are both Cu3+ and Cu2+ ions in the system.

Structural modifications and phonon softening in Bi2Sr2Ca1−xRxCu2O8+δ (R = Pr and Gd) single crystals

The phonon Raman spectra of Bi2Sr2Ca1-xRxCu2O8+delta (R = Pr and Gd) single crystals are systematically investigated. The experimental results show that the O(2)(Sr)A(1g) mode softens with Pr and Gd doping, while the O(1)(Cu) B-1g mode softens with Pr doping but hardens with Gd doping. The changes of average ionic radius on the Ca site in Bi-based cuprates can account well for the Raman frequency shifts of the O(I)(Cu) mode, but have little influence on the O(2)(Sr) mode. The frequency softening of the O(2)(Sr) mode in Pr- and Gd-doped Bi2212 crystals mainly results from contraction of the BiO bilayers with doping content. The correlation between the O(2)(Sr) mode frequency and the c-axis parameter as well as the incommensurate modulation wavelength is discussed.

Insulator-metal transition shift related to magnetic polarons in La0.67-xYxCa0.33MnO3

Abstract:The magnetic transport properties have been measured for La0.67-xYxCa0.33MnO3(0 ≤ x ≥ 0.14) system. It was found that the transition temperature Tp almost linearly moves to higher temperature as H increases. Electron spin resonance confirms that above Tp, there exist ferromagnetic clusters. From the magnetic polaron point of view, the shift of Tpvs.H was understood, and it was estimated that the size of the magnetic polaron is of 9.7 ˜ 15.4 Å which is consistent with the magnetic correlation length revealed by the small-angle neutron-scattering technique. The transport properties at temperatures higher than Tp conform to the variable-range hopping mechanism.

Irreversibility line in the superconductor RuSr2Gd1.4Ce0.6Cu2O10−δ

Abstract The resistive transition of the superconductor RuSr2Gd1.4Ce0.6Cu2O10−δ is measured under magnetic fields from 0 to 10 T. The results show that the irreversibility line determined from the different reduced resistivity criterion can be fitted well by the expression: Hi=α(1−T/Tc)γ. The activation energy (U) is obtained from the low dissipation parts of the ρ(T) curves using the Arrhenius activation law ρ(T,H)=ρ0(H)exp{−U(H)[1−T/Tc]/kBT}. The U value obtained is smaller than that of other high temperature cuprate superconductors, which may be due to the coexistence of superconductivity and ferromagnetism in RuSr2Gd1.4Ce0.6Cu2O10−δ. It is also found that the activation energy can be fitted by U=157H−0.59.