Xiaojuan Fan

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.

Field-induced crossover from cluster-glass to ferromagnetic state in La0.7Sr0.3Mn0.7Co0.3O3

The thermodynamics and kinetics of the magnetic states of the semiconductor-like compound La0.7Sr0.3Mn0.7Co0.3O3 were studied by detailed magnetic measurements. The spontaneous magnetic state was found to be a cluster-glass phase which can cross over to a ferromagnetic state under an external magnetic field H. The defreezing temperature Tf for the cluster-glass phase and the merging temperature Tm for zero field cooled and field cooled magnetizations are related to the field H by Hj=H0j(1−Tj/Tc)nj, where H0j and nj are constants and j=f,m. Codoping tends to destroy the double exchange in Mn4+–O–Mn3+ and broadens the coexistence region of the cluster-glass and ferromagnetic states. The magnetic relaxation was found to deviate from the usual logarithmic time dependence and follow a power law.

Transport and magnetic properties in perovskite compound

Magnetic and transport properties of the series (x = 0, 0.01, 0.03, 0.05, 0.1, 0.3, 0.5, 1.0) have been investigated. It was found that with increasing Co content, the Curie temperature decreases, while the spin glass transition temperature decreases first then increases. As for the sample with x = 0.3, the relation of and the merging point (for the zero-field-cooled and field-cooled magnetization curves starting to merge) with the applied magnetic field H follows a relation , ), where and are constants. The resistivity increases drastically with Co doping and a semiconductor-like behaviour is displayed in the sample x = 0.3. These facts strongly suggest that the double-exchange interaction in the perovskite materials is very fragile since a small Co content can change the magnetic and relevant transport properties of the system.

Pr-doping effect on the structure and superconductivity of Bi2Sr2Ca1-xPrxCu2Oy single crystals

Abstract Effect of Pr-doping on the structure and superconductivity of Bi2212 system was studied for Bi 2 Sr 2 Ca 1− x Pr x Cu 2 O y single crystals with x =0–0.78. The a - and b -axis lengths increase with increasing Pr concentration, while the c -axis length and the period of modulation decrease with Pr-doping. These can be explained by the higher valence of Pr ion substituting at Ca site, the extra oxygen in BiO bilayers and the lattice mismatch between perovskite layer and BiO bilayer. In addition, the changes in lattice parameters are more dramatic in the Pr-rich region than those in the Ca-rich region, which can be attributed to the difficulty in the intercalation of extra oxygen in the BiO bilayer at high-doping level. The superconducting transition temperature increases slightly at first and then drops gradually upon Pr doping and the complete suppression of superconductivity occurs when the Pr content reaches 0.60. The superconducting volume faction also decreases with Pr content and is proportional to 1− ax 2 .

Growth of Bi2Sr2Ca1−xPrxCu2Oy single crystals from Bi-rich melts

Abstract Bi 2 Sr 2 Ca 1− x Pr x Cu 2 O y single crystals were successfully grown by a self-flux method from Bi-rich melts for the first time. For starting composition x s =0–0.6, all the crystals possess the pure Bi2212 phase with the actual Pr content varying from x =0 to 0.78 correspondingly. However, the grown crystals are of the Bi2201 and the Bi2222 phase for starting composition x s =0.7–0.9 and x s =1.0, respectively, with evident difference between the nominal and actual composition of crystals. The superconducting transition temperature of Bi2212 crystals increases slightly at first and then drops gradually upon Pr doping and the complete suppression of superconductivity occurs when the actual Pr content reaches 0.60.

Absorption of microwaves in La1−xSrxMnO3 manganese powders over a wide bandwidth

We present the frequency dependence of microwave-absorbing properties of La1−xSrxMnO3 (x=0.4, 0.5, 0.6, and 0.7) powders at room temperature. The absorbing properties change gradually with x in the frequency range of 8–12 GHz. The optimal absorption can be achieved for a x=0.4 sample and its microwave loss peak value is about 25 dB. Further experimental results show that the absorption can be attributed to magnetic and dielectric losses and the microwave loss peak corresponds to the maximum dielectric loss tangent tan δe near 10.5 GHz. Furthermore, the absorbing properties of the oxides mixed with carbonic fiber and Y-type planar hexagonal ferrite have been rudimentarily studied. Results show that these additives greatly enhance the microwave-absorbing properties of the oxides.

Correlation between Tc and ns/m* in Bi2Sr2CaCu2O8+δ single crystals

Abstract The reversible magnetization measurements with applied field parallel to the c -axis were carried out for a series of Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi2212) single crystals with different oxygen content varying from underdoped to overdoped states, from which the doping dependence of penetration depth and n s / m * can be obtained. It was firstly found in this system that with increasing oxygen content, the value of n s / m * increases in the underdoped region and reaches a maximum at the optimal doping, then decreases in the overdoped region. The observed deviation from the linear relationship of T c ∝ n s / m * in the underdoped region can be regarded to be due to the structural distortion and the disorder scattering, while the decrease of n s / m * in the overdoped region is suggested to be due to the phase-separation related pair breaking.

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.

The Jahn-Teller effect and electron-phonon interaction in La0.25Ca0.75Mn1-xCrxO3

The ultrasonic (longitudinal and transverse) velocities and the transport and magnetic properties of polycrystalline La0.25Ca0.75Mn1-xCrxO3 (x = 0, 0.03, 0.05, and 0.07) have been studied systematically. It was found that with increasing Cr content, the resistivity increases, the charge-ordering transition temperature TCO shifts to low temperature, and the magnetic moment of the system is strengthened. From the temperature dependence of the ultrasonic velocities, one can establish that the Jahn-Teller energy and phonon exchange constant decrease with increasing Cr content.

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