Zhang Shi-Yuan

A study of pseudo-elasticity in the superconducting oxide YBa2Cu3O7-δ

The pseudo-elasticity in YBa2Cu3O7- delta has been investigated by a mechanical method from room temperature to 79 K. Several ferroelastic-type loops were observed near room temperature, 240, 220 and 170 K. These phenomena are discussed in conjunction with the results of X-ray diffraction.

Effects of Ag Doping on Magnetoresistance of La0.833K0.167MnO3Polycrystalline Perovskite Manganites

The microstructure, magnetic and transport properties of the heterogeneous system with a nominal composition (1/m)Ag2O-La0.833K0.167MnO3 (1/m is molar ratio with m = 32, 16, 8, 4, and 2) have been studied. The x-ray diffraction patterns show that all the samples are two-phase composite and consist of a magnetic La0.833K0.167MnO3 (LKMO) perovskite phase and a nonmagnetic metal Ag phase. The room-temperature magnetoresistance (MR) effect is enhanced significantly due to the addition of Ag. For the m = 4 sample, we obtain MR values up to 32% under a lower field of 0.5 T and 67 % under a higher field of 5.5 T at the temperature of 300 K, which are much larger than the corresponding MR values (10% and 35%) of the LKMO sample without Ag. In the low-temperature range from 4.2 K to 250 K, the MR values of the Ag-added samples however decrease with the increasing Ag content in the samples. This effect is discussed qualitatively by using the relative contribution from the intrinsic MR effect and the extrinsic MR effect including the spin-polarized-tunnelling and spin-dependent scattering effects in different temperature regions.

Magnetic-Field-Induced Semimetal-Insulator-like Transition in Highly Oriented Pyrolitic Graphite

We report the extraordinarily large positive magnetoresistances (MR, 69400% at 4.5 K under a magnetic field of 8.15 T), de Hass–van Alphen oscillations effect at 10 K and the semimetal-insulator-like transition in a wide range of temperature in highly oriented pyrolitic graphite (HOPG). Besides a dominating ordinary MR (OMR) mechanism in the free-electron mode, it is realized from qualitative analysis that the Coulomb interacting quasiparticles within graphite layers play some roles. However it is difficult to associate the transition with the simple OMR theory. In order to investigate the possible origins of the transition, further analysis is carried out. It is revealed that the magnetic-field-induced behaviour is responsible for the semimetal-insulator-like transitions in HOPG.

Magnetic Properties and Low-Field Magnetoresistance of Pr0.7Pb0.3MnO3 Single Crystals

The magnetic properties and magnetoresistance of Pr0.7Pb0.3MnO3 single crystals have been studied. At low temperature, the temperature dependence of magnetization can be represented as M(T) = M(0)(1-BT3/2). A large B value was observed. The spontaneous magnetic moment is slightly larger than the spin-only value for Mn3+ and Mn4+ ions. Above the Curie temperature, the susceptibility obeys the Curie-Weiss Law and the resistivity R can fit well with R0exp (T0/T)1/4. It is found that the effective paramagnetic moment obtained from the Curie constant is larger than the theoretical value. The large effective paramagnetic moment may originate from the magnetic clusters. A large low-field magnetoresistance of ΔR/R(0) = -50% is observed at 230 K in 1 T.

Injection Efficiency of Spin-Polarized Quasi-particles in Y-Ba-Cu-O Thin Film

The injection efficiency of spin-polarized quasi-particles in a superconducting Y-Ba-Cu-O (YBCO) film is investigated by means of current injection into the Nd0.7Sr0.3MnO3/SrTiO3/YBa2Cu3O7-δ heterostructures. We have successfully prepared six injection junction windows in the YBCO thin film strip, which are all 56 µm wide, but 80 µm, 40 µm, 20 µm, 10 µm, 5 µm, and 2 µm long, respectively. At 16 K, our YBCO thin film strip (80 nm thick) has Jc = 2×105A/cm2. Under the spin-polarized injection current of Iinj = 0.5 mA, with decreasing injection junction length L, injection efficiency η (η≡ΔJc/ΔJinj; ΔJc: the suppression of critical current density due to finite ΔJinj) increases gradually, until L is not more than 20 µm. In that case, η hardly varies with the change of L, reaching up to about 6. We think that these results are related to the spin diffusion length of spin-polarized quasi-particles in the superconducting thin film. In our heterostructures, the YBCO thin film keeps good superconductivity and the injection junction windows are of proper size, therefore, we have obtained relatively high injection efficiency of spin-polarized quasi-particles.

稀土掺杂La-Sr-Mn-O的室温磁致伸缩效应

A series of polycrystaline bulk (La 1-x R x ) 2/3 Sr 1/3 MnO 3 (R=Sm,Tb) over a wide composition range were prepared by two different techniques, and their structure nd magnetostriction effect were studied carefully at room temperature. The X-ray diffraction pattern indicated that the structure of samples transformed from rhombohedral type to orthorhombic phase with decreasing R contents. The sample, composition near the transformation point, had the maximum value of magnetostriction. The magnetostriction may come from the exchange magnetostriction in the samples at room temperature.A series of polycrystaline bulk (La 1-x R x ) 2/3 Sr 1/3 MnO 3 (R=Sm,Tb) over a wide composition range were prepared by two different techniques, and their structure nd magnetostriction effect were studied carefully at room temperature. The X-ray diffraction pattern indicated that the structure of samples transformed from rhombohedral type to orthorhombic phase with decreasing R contents. The sample, composition near the transformation point, had the maximum value of magnetostriction. The magnetostriction may come from the exchange magnetostriction in the samples at room temperature.

Spin-Polarized Tunnelling Magnetoresistance Effects in La0.833K0.167MnO3/SrTiO3 Polycrystalline Perovskite Manganites

A sample of La0.833K0.167MnO3–SrTiO3 (LKMO/STO) is fabricated by the sol-gel method. The microstructure, magnetic and transportation properties have been studied. X-ray diffraction patterns indicate that the structure of LKMO/STO is a homogeneous solid solution phase. The resistivity of LKMO/STO shows the insulator behaviour, which is different from La0.833K0.167MnO3 (LKMO) whose resistivity shows metal–insulator transition with decreasing temperature. The low-field (m0H = 0.02 T) magnetoresistance decreases from 11% to 0.2% with the increasing temperature from 4 K to 220 K for the LKMO/STO sample. The magnitude of magnetoresistance in a strong field (μ0H = 5.5 T) almost increases linearly with decreasing temperature and reaches the maximum of 65% at the low temperature of 4.2 K, which is much higher than that of LKMO (40%). The enhanced low-field magnetoresistance effects are quantitatively explained by the spin-polarized tunnelling at grain boundaries.

Anomalous magnetostrictive effects in (La1-xTbx)2/3Sr1/3MnO3

A series of (La1-xTbx)2/3Sr1/3MnO3 polycrystalline samples has been studied by means of x-ray diffraction, magnetostriction and thermal expansion measurements. It has been found that this series undergoes a phase transition from a rhombohedral to an orthorhombic form at the doping level x≈0.20 at room temperature accompanied by an anisotropic magnetostriction up to -50×10-6 under a magnetic field of 1T. The linear and volume magnetostrictions vary with chemical composition, even change sign. At T=80K, the magnetostrictions for the samples of x=0.20 and 0.40 exhibit different behaviours. The sample of x=0.20 has positive volume and linear magnetostrictions and a negative anisotropic magnetostriction, while the sample of x=0.40 has an opposite behaviour. The magnitude of volume magnetostriction for both the samples is essential (~10-4) at T=80K under a magnetic field of 4T. We conclude that these anomalous effects are due to the charge delocalization and the structural phase transition between orthorhombic and rhombohedral forms induced by the applied magnetic field.

Correlation Between Magnetovolume and Colossal Magnetoresistance Effects in Terbium Doped La-Sr-Mn-O Perovskite

(La0.67Tb0.33)2/3Sr1/3MnO3 has been studied in order to probe mechanisms responsible for the giant magnetoresistance ratios and the lattice effect in this kind of compound. The experiment has shown a strong connection between the magnetotransport and magnetovolume properties. An applied magnetic field not only gives rise to a large negative magnetoresistance (-900%) but also produces two different magnetovolume effects which reflect two different magnetostriction mechanisms in the compound.