T. F. Zhou

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.

Crossover of magnetoresistance from negative to positive in the heterojunction composed of La0.82Ca0.18MnO3 and 0.5 wt % Nb-doped SrTiO3

We report the magnetotransport properties of the heterojunction composed of La0.82Ca0.18MnO3 (LCMO) and 0.5 wt % Nb-doped SrTiO3 (SNTO). At temperature below 120 K, the heterojunction only exhibits a negative magnetoresistance (MR) independent of bias current. At temperatures above 120 K, with increasing bias current a crossover of MR from negative to positive is observed. This intriguing crossover of MR can be ascribed to the dominant spin character changing from the majority spin carriers to the minority spin carriers at the LCMO-SNTO interface due to their occupation in the complicated bands of La0.82Ca0.18MnO3. (c) 2006 American Institute of Physics.

Forward and backward diodelike rectifying properties of the heterojunctions composed of La1−xSrxCoO3−δ and 0.7wt% Nb-doped SrTiO3

Heterojunctions composed of La1-xSrxCoO3-delta (x= 0.4 and 0.5) and 0.7 wt % Nb- doped SrTiO3 were fabricated and exhibited good rectifying properties. The asymmetric current- voltage relations for the x=0.4 and x=0.5 junctions display opposite shapes: the former has a forward shape, while the latter shows a backward one. The x=0.4 junction can be viewed as a Schottky diode. For the x=0.5 junction, the forward bias currents show much less temperature dependence than the reverse ones and the reverse voltage V-char, at which the bias current is 10 mu A, displays a distinct change at the ferromagnetic transition temperature of the cobalt oxide of the junction. These observations in the x=0.5 junction can be understood by a nearly degenerate model together with its temperature- dependent magnetism

Self-generated in-plane superlattice in relaxed epitaxial La0.67Sr0.33MnO3 films

The strain relaxation and its effect on the in-plane superlattice formation of epitaxial La0.67Sr0.33MnO3 (LSMO) thin films on the (LaAlO3)0.3(Sr2AlTaO6)0.7 (001) [LSAT (001)] substrates were investigated as a function of film thickness by x-ray diffraction. Rocking curves and reciprocal space mappings around (002) and (103) reflections reveal that the film with a thickness above 27nm is almost fully relaxed and has a modulated superlattice structure. The recovery of the pseudocubic angle of LSMO resulting from the relaxation of the shear strain introduced by the growth of rhombohedral LSMO on cubic LSAT was suggested to be the origin of the superlattice structure.

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.

Magnetic field dependent evolution of band structure in La1−xCaxMnO3 (x=0.18 and 0.30)

Probing of the evolution of electronic structure in magnetic field is a key issue for the design and realization of manganite-based devices. Here, the shift of the maximum (or minimum) position in unusual magnetoresistance (MR) versus V curves of the heterojunctions composed of La1-xCaxMnO3 (LCMO, x=0.18 and 0.3) and 0.5 wt % Nb-doped SrTiO3 (SNTO) was used to investigate the change of band gaps of LCMO with magnetic field. Negative and positive MRs which depend on temperature, bias current (or voltage), and Ca concentration observed in these heterojunctions are directly determined by the band structure at the LCMO-SNTO interface. (c) 2006 American Institute of Physics.

Modulation of rectifying behaviour by oxygen contents in La0.5Sr0.5CoO3−δ/0.7 wt% Nb-doped SrTiO3 heterojunctions

Heterojunctions composed of La0.5Sr0.5CoO3−δ and 0.7 wt% Nb-doped SrTiO3 were deposited at different oxygen partial pressures and display opposite rectifying shapes: the junctions fabricated at a lower oxygen pressure show a backward shape and they gradually display a forward shape by tuning up the oxygen partial pressures. The former tends to form degenerate p–n junctions due to oxygen deficiency, which is responsible for the backward rectifying shape. The latter, however, can be viewed as a Schottky-type diode. The oxygen contents for the junctions can be estimated from their x-ray diffraction patterns, and are also confirmed from their resistance and magnetization measurements.

Effects of Neutron Irradiation on Superconducting Properties of GdBa2Cu3O7- Single Domain Superconductor

The effects of neutron irradiation with a fluence of 1015 n/cm2 on the superconducting properties of GdBa2Cu3O7−δ single domain sample were studied. The point and cascade defects produced by the neutron irradiation were observed by high-resolution transmission electron microscopy. The cascade defects were found to have the sizes of about 4-7 nm which is comparable to the coherence lengths of high temperature superconductors. The point defects disappear while the cascades still exist through thermal annealing. The temperature dependence of magnetization for the magnetic field parallel to the crystallographic c axis shows that the neutron irradiation leads to a dramatic degradation of superconductivity for the as-irradiated sample, a decrease of critical current density (Jc), and the weakening of the fishtail effect in the Jc versus magnetic induction B curve. However, for the as-irradiated sample annealed in the flowing oxygen atmosphere, it shows that Jc under high magnetic fields is greatly enhanced, the fishtail shifts towards higher magnetic fields, and its superconductivity is partially recovered as well due to the remaining effective pinning centers of the cascades. These results suggest a prospective application for such a treated GdBa2Cu3O7−δ superconductor.