Wei-ng Ji

PREPARATION OF PIEZOELECTRIC-COEFFICIENT MODULATED MULTILAYER FILM ZNO/AL2O3 AND ITS ULTRAHIGH FREQUENCY RESONANCE

Multilayer films composed of piezoelectrically active ZnO and inactive Al2O3 layers were prepared on silicon by a pulsed laser deposition technique. The ZnO layers were completely (001) textured to generate a single piezoelectric coefficient d33 perpendicular to the substrate surface and the Al2O3 layers were amorphous at 375 °C. The interfacial sharpness and the film orientation were analyzed by low and high angle x-ray diffraction θ–2θ scanning. High frequency resonance of 10.6 GHz was measured and higher values up to 100 GHz are expected in the multilayer films with periods 320 nm or smaller.

Multiferroic properties of (1 − x)[0.94Bi0.5Na0.5TiO3–0.06BaTiO3]–xCoFe2O4 ceramics

Multiferroic composite ceramics of (1 − x)[0.94Bi0.5Na0.5TiO3–0.06BaTiO3]–xCoFe2O4 (BBNTO–CFOx), x = 0.1, 0.2, 0.3, 0.4, 0.5, have been prepared and their structure, magnetoelectric (ME) effect, magnetic and ferroelectric properties have been investigated. x-ray diffraction and scanning electron microscopy show that the ceramics consist of separated perovskite BBNTO and spinel CFO. The ME effect is revealed by measuring the magnetic hysteresis loops of the same sample before and after electric poling. The magnetization increases gradually with increasing CFO content. Further electric measurements reveal that BBNTO–CFO0.30 has the largest polarization with a remanent polarization of 60.9 µC cm−2, which is even larger than that of pure BBNTO. The enhanced ferroelectricity might be ascribed to the ME effect. Our results may be helpful for developing new lead-free multiferroic composites with improved properties.

Giant positive magnetoresistance in half-metallic double-perovskite Sr2CrWO6 thin films

Ferrimagnetic half-metallic Sr2CrWO6 thin films show giant positive magnetoresistance up to 17,200%. Magnetoresistance (MR) is the magnetic field–induced change of electrical resistance. The MR effect not only has wide applications in hard drivers and sensors but also is a long-standing scientific issue for complex interactions. Ferromagnetic/ferrimagnetic oxides generally show negative MR due to the magnetic field–induced spin order. We report the unusually giant positive MR up to 17,200% (at 2 K and 7 T) in 12-nm Sr2CrWO6 thin films, which show metallic behavior with high carrier density of up to 2.26 × 1028 m−3 and high mobility of 5.66 × 104 cm2 V−1 s−1. The possible mechanism is that the external magnetic field suppresses the long-range antiferromagnetic order to form short-range antiferromagnetic fluctuations, which enhance electronic scattering and lead to the giant positive MR. The high mobility may also have contributions to the positive MR. These results not only experimentally confirm that the giant positive MR can be realized in oxides but also open up new opportunities for developing and understanding the giant positive MR in oxides.

Quantitative control of Fe/Mo anti-site defect and its effects on the properties of Sr2FeMoO6

Using a facile method, Fe/Mo anti-site defect (ASD) concentration in double perovskite Sr2FeMoO6 is quantitatively controlled within a wide range of 2%–21%. Our systematic investigation demonstrates the significant effects of ASD on the physical properties, such as magnetization, magnetoresistance and the Curie temperature.

Microstructure and ferromagnetic property in CaRuO3 thin films with pseudoheterostructure

CaRuO3 thin films were synthesized on SrTiO3 substrates by pulsed laser deposition. Detailed microstructure analysis by transmission electron microscopy revealed the pseudoheterostructure in CaRuO3 films. It consists of a coherently strained cubic CaRuO3 layer contacted with substrate, as well as a strained orthorhombic CaRuO3 layer. The orthorhombic CaRuO3 layer is composed of two types of domains. The ferromagnetic property of the pseudoheterostructure CaRuO3 was revealed by superconducting quantum interference device measurement. This is due to the cubic CaRuO3 layer, which is supported by first-principle calculations. The formation mechanism of pseudoheterostructure in ultrathin CaRuO3 thin films was proposed.

Magnetic and electrical transport properties of Pb1-xLaxTi1-xMnxO3 ceramics

Pb1-xLaxTi1-xMnxO3 (PLTM100x, x = 0.20, 0.40, 0.50, 0.60, and 0.80) ceramics have been prepared and investigated. X-ray diffractions show all ceramics are crystallized into perovskite structures with tetragonal symmetry for PLTMO20 and orthorhombic symmetry for the other compositions. X-ray photoelectron spectroscopy results confirm that the Mn cations in PLTMO20 and PLTMO40 have the valence state of +3, whereas in other compositions, the Mn cations have mixed valences states of +3 and +4. With increasing x from 0.20 to 0.80, the materials change from superparamagnetic to coexisting ferromagnetic and antiferromagnetic magnetic behaviour whereas all compositions persist insulating behaviour, as confirmed by the magnetization-magnetic field (M-H), the temperature dependent magnetization (M-T) and transport measurements. Especially, the PLTMO80 has a large negative magnetoresistance of −41.5% at 10 K with the applied filed of 2 T. By considering the chemical states of Mn cations, these results are attributed...