Liya Lv

Structural, optical, and ferromagnetic properties of Co-doped TiO2 films annealed in vacuum

Polycrystalline anatase Ti1−xCoxO2 (x=0–0.06) films have been fabricated by sol-gel spin coating. The effects of Co doping on the structural, optical, and magnetic properties are investigated. It is shown that oxygen vacancies and/or defects in the films are generated during thermal treatment in vacuum. Co doping reduces crystal quality and inhibits crystalline grain growth. Due to the introduction of Co, photoluminescence (PL) spectra become weak and the band gap emission has a blueshift. PL spectra reveal that the solubility of Co is lower than 0.02. At 300 K, the saturated magnetization is around 1.8u2002μB/Co, which is independent of the concentration of Co. This value is approximately equivalent to the average magnetic moment of bulk metallic cobalt (1.75u2002μB/Co). Zero-field-cooling/field-cooling measurements indicate that room temperature ferromagnetism in Co-doped TiO2 films is not an intrinsic property of the material. The presence of Co metal is identified by x-ray photoelectron spectroscopy and scann...

Metal nanotubes prepared by a sol?gel method followed by a hydrogen reduction procedure

A sol?gel method followed by a hydrogen reduction procedure was proposed in this work for the preparation of metal nanotubes. The tube length, diameter, wall thickness and, most importantly, the chemical components can be adjusted conveniently. Several examples of metal nanotubes (Fe, Ni, Pb and CoFe) were prepared by using anodic aluminium oxide (AAO) templates. The fabricated nanotubes are uniform in diameter and wall thickness through the entire tubes. The length of the nanotubes is about 100??m, with an aspect ratio of about 1000.

Observation and manipulation of paramagnetic oxygen vacancies in Co-doped TiO2 nanocrystals

Electron paramagnetic resonance measurements were presented to investigate paramagnetic oxygen vacancies (F+ centers) in Co-doped TiO2 nanocrystals. Surface and interior F+ centers were manipulated by washing or/and annealing. Anisotropic surface F+ centers are so instable as to be destroyed at high temperature, while isotropic interior F+ centers are rather stable even at 430°C. The correlation of ferromagnetism with F+ centers was observed, which provided direct experimental evidence for F+-center-mediated coupling in diluted magnetic oxides.

Enhancement of the spin entropy in NaxCo2O4 by Ni doping

The effects of Ni doping on the spin entropy in NaxCo2O4 (x=1.2) has been carefully studied. A strong magnetic-field suppression of the thermopower indicates the emergence of large spin-entropy effect. The magnetothermopower increases with increasing Ni doping level, suggesting that Ni doping improves the spin entropy of NaxCo2O4. Based on analysis of x-ray absorption spectra, we propose a spin-entropy competition mechanism to explain well the spin-entropy enhancement.

Effects of Zr substitution on structures and ferroelectric properties of Bi3.25La0.75Ti3O12 thin films

Thin films of Bi3.25La0.75Ti3O12 (BLT) and B-site substituted BLT by Zr, i.e. Bi3.25La0.75Ti3−xZrxO12 (BLTZx, x = 0.20, 0.50, 0.75, 1.00 and 1.50) were fabricated on Pt/Ti/SiO2/Si substrates by pulsed laser deposition. Structures and electrical characteristics of the polycrystalline films were studied as functions of Zr composition. Structures were investigated by x-ray diffraction, scanning electron microscopy and Raman spectroscopy. It is revealed that the Zr substitution does not destroy the layered structures of BLT but the Zr substitution tolerance is limited. With increasing Zr composition, generally, the octahedra-related vibration modes show a significant low-frequency shift. Compared with the well-known BLT films, the BLTZ0.20 films have larger remnant polarization (Pr), smaller coercive field (Ec) and better fatigue resistance. However, further increasing Zr composition leads to weakened ferroelectricity of the BLTZx films with decreased Pr and reduced fatigue resistance. The effects of Zr substitution on structures and ferroelectric properties of BLT and the correlation between structures and properties are discussed in detail.

Anomalous Hall effect and magnetoresistance of (FexSn1?x)1?y(SiO2)y films

Structural, magnetic and transport properties were investigated for heterogeneous (FexSn1?x)1?y(SiO2)y films deposited on oxidized silicon substrates at room temperature with RF magnetron sputtering. X-ray diffraction indicated that the films consist of a nanocrystalline phase of FeSn2 embedded into an amorphous background. For (FexSn1?x)92.33(SiO2)7.67, it was demonstrated that with the increase in the Fe component an evolution from negative isotropic magnetoresistance (MR) behaviour to one with a mixture of anisotropic magnetoresistance and isotropic negative MR occurs. On fixing the SiO2 percentage and decreasing the Fe?Sn ratio, the negative transverse MR first increases and then decreases. For samples having a fixed Fe?Sn ratio, the negative transverse MR increases with the increase in SiO2. Compared with the Fe?Sn system, the addition of 7.67?at.% SiO2 enhances the saturated Hall resistivity. Further increase in SiO2 introduces a complicated saturated Hall resistivity dependence on the concentration of SiO2.

High-temperature ferromagnetism of helical carbon nanotubes

We report the experimental results on the magnetism of curvature-induced helical carbon nanotubes (HCNTs). It is demonstrated that without any magnetic impurities in the sample, the as-prepared HCNTs show clear ferromagnetism with a Curie point as high as 970 K.