X.Q. Xiang

Exchange bias tuned by cooling field in phase separated Y0.2Ca0.8MnO3

The effect of cooling field HFC on the exchange bias field HEB in a spontaneous lamellar ferromagnetic/antiferromagnetic phase separated Y0.2Ca0.8MnO3 has been studied. It is found that with increasing HFC from 1to6T the value of HEB decreases by 37% at 2K and is inversely proportional to the ferromagnetic layer thickness tFM. This suggests that the tuning of HEB by HFC in Y0.2Ca0.8MnO3 arises from the variation of tFM with HFC. This phenomenon is essentially different from other types of magnetic tunings.

Effect of particle size on the structure and magnetic properties of La0.6Pb0.4MnO3 nanoparticles

We studied the effect of particle size on the structure and magnetic properties of La0.6Pb0.4MnO3 compounds with particle diameters varying from 5 to 100 nm. With decreasing particle size, the increase of the rhombohedral distortion narrows the bandwidth W-O2p and hence results in the decrease of ferromagnetic transition temperature. At the same time, the magnetic domain structure evolves from multidomain to single domain, and finally a superparamagnetic behavior is detected. The critical single domain size determined by experiment is about 25 nm, and the critical superparamagnetic behavior size obtained by calculation is 5.8 nm. It is found that the variation of coercivity is related to the evolution of magnetic domain states with particle size, and the saturation magnetization decreases significantly due to the increasing contribution of the surface effect with decreasing particle size. (c) 2006 American Institute of Physics.

Effect of stripe order on the vortex phase transition in La1.44Nd0.4Sr0.16CuO4 films

The vortex phase transition in La1.44Nd0.4Sr0.16CuO4 epitaxial films with different thicknesses under a compressive strain were studied systematically by measuring the current?voltage (IV) characteristics in magnetic fields perpendicular to the CuO2 planes. For a strongly compressed thin film, in which the static stripe order is destroyed, the IV curves show a good three-dimensional (3D) vortex glass (VG) scaling collapse in various magnetic fields, indicating a 3D VG phase transition. By applying the standard scaling procedure to the data of a strain-released thick film with static stripe order, neither a 3D nor a quasi-2D VG model works well. Further investigation shows that, in order to achieve a good scaling collapse, the Kosterlitz?Thouless correlation length should be used for the isothermals above Tg. The scaling of isothermals below Tg can also be optimized if the VG correlation length and adjusted static critical exponent ? are used with D = 2 only, indicating that a quasi-2D VG phase might exist. Our findings reveal that the nature of the vortex phase transition is completely altered in the presence of the stripe order.

I‐V characteristics of La1.84Sr0.16CuO4∕Nb-doped SrTiO3 heterojunction

Current-voltage (I‐V) characteristics of the La1.84Sr0.16CuO4∕1.0wt% Nb-doped SrTiO3 heterojunction were measured in the temperature range from 5to290K under magnetic fields up to 14T. All I‐V curves show a fine rectifying property. A visible reduction of the diffusion potential (Vd) can be observed at the superconducting transition temperature due to the opening of the superconducting gap in La1.84Sr0.16CuO4. The reduction of Vd from its original track, ΔVd, is found to be of similar temperature and field dependencies as those of the superconducting gap. This suggests an alternative technique for detecting the superconductivity related features in superconducting films.

Ultrasonic study on magnetic-field-induced stripe order in La1.88Sr0.12-xBaxCuO4

The ultrasonic sound velocities and attenuations of La1.88Sr0.12-xBaxCuO4 (0 <= x <= 0.12) under magnetic fields of up to 14 T show two important features of charge stripe order in the system. The first is that the ultrasonic characteristics indicate the existence of local stripe order associated with low temperature tetragonal structure domains for samples with x <= 0.04. The second is that the local stripe order can be enhanced by magnetic fields. The results indicate a strong interaction between charge stripes and crystal lattice in the system.

The crossover from parallel shift to fan-shape broadening of the superconducting transition of La1.44Nd0.4Sr0.16CuO4 films in magnetic fields

In this paper we report the transport properties of c-axis-oriented superconducting La(1.44)Nd(0.4)Sr(0.16)CuO(4) films on LaSrAlO(4) substrates with different thicknesses in various magnetic fields parallel and perpendicular to the c axis of the films. It is found that with decreasing film thickness there is a crossover from the parallel shift to fan-shape broadening in the resistive transitions. Moreover, for both field configurations, the flux pinning potential U(0)(H) of the film shifts upward with an order of magnitude with increasing in-plane compressive strain. These results indicate that the compressive epitaxial strain and strain-induced destabilization of the static charge stripes are important aspects for the promotion of superconducting transition temperature as well as the flux pinning potential.

Effect of Temperature on La1.85Sr0.15CuO4 Single Crystal Growth by Floating Zone Method

Superconducting single crystals of La1.85Sr0.15CuO4 have been grown at various temperatures without single crystal seeds by the traveling-solvent floating-zone method. In order to avoid the formation of bubbles during the crystal growth process, a flowing atmosphere of 2 atm oxygen or 1 atm air was applied in different temperature range. It was found that the crystal quality could be improved by raising the growth temperature in a certain range, and the orientation changed from (110) to (100) at higher temperature. X-ray diffraction results showed that the full-width at half-maximum of the best as-prepared crystal was 0.086°. The crystals grown in 2 atm oxygen showed a superconducting transition temperature (Tc) of 37.3 K, while the crystals grown in floating air showed a Tc of about 35 K and it could be improved to 36.5 K by annealing in flowing oxygen.

In-plane substitution effect on the local structure of La1.88Sr0.12CuO4

The effect of in-plane substitution on the local structure has been studied by ultrasonic method in a series sample of La1.88Sr0.12Cu1−yZnyO4 (y=0.00,0.01,0.02,0.03). It is found that a small amount of Zn doping greatly promotes the formation of the local low-temperature tetragonal phase, whereas further doping is harmful to it, the tendency of which is to show a marked resemblance to the Zn-concentration dependence of the magnetic transition temperature TN reported by muon-spin-relaxation (μSR) study. Our results also provide additional evidence that the slight in-plane nonmagnetic doping introduces local pinning centers to stabilize the charge stripes in La1.88Sr0.12CuO4.