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Nanocrystalline ZnO?:?Co samples (having 5?at.% Co) exhibiting room temperature ferromagnetism (RTFM) have been synthesized from acetate precursors. The effect of sequential sintering of the samples at higher temperatures for prolonged duration in air ambient has been investigated for their phase-purity and magnetic characteristics. The nanocrystalline ZnO?:?Co obtained after calcinations at 600??C exhibits RTFM. The study further indicates that the Co3O4 phase progressively forms with increasing sintering temperature, and partially transforms into CoO at and above 800??C. Irrespective of the sintering temperature and duration, the samples sintered at and above 800??C exhibited identical magnetization behaviour with regard to the saturation moment and field required for technical saturation. The ferromagnetism persisting even at higher temperatures (?800??C) arises intrinsically in the insulating Zn1?xCoxO phase. The recently proposed bound magnetic polaron model appears to account for the observed ferromagnetic ordering in this carrier deficient oxide system. A paramagnetic contribution to the overall magnetization is also observed, which is attributed to the paramagnetic Co3O4 and CoO and possibly to isolated polarons and/or non-interacting Co2+ ions.

Very thin fluorine-doped tin oxide films (60–70 nm) were prepared on glass substrates at low deposition temperatures (275–300°C), these films showed an electrical resistivity of 3.2 ×10-4Ω-cm and a transparency of 88% at 625 nm with the application of a dc electric field on the film surface during growth using a spray pyrolysis deposition technique. In this first study of its kind, the applied electric field during growth by spray pyrolysis resulted in the reduction in the critical thickness and the increases in both the electrical conductivity and transparency of continuous films. The obtained X-ray diffraction (XRD) patterns showed that the films prepared with an electric field were polycrystalline, whereas those prepared without an electric field were amorphous. This method shows potential for producing very thin oxide films at a low deposition temperature with a high growth rate, an enhanced optical quality and an improved electrical conductivity.

A detailed study of the effect of ambient and humid atmosphere on the stability and physical properties of single and multiphasic samples of a BSCCO superconductor has been carried out. It has been observed that, on exposure to atmosphere for a period of one year, the single-phase samples 2223 or 2212 did not show any phase decomposition. However, a decrease in Tc was observed in these samples. Mixed-phase samples showed the partial conversion of the 2223 phase into the 2212 phase on the surface with a corresponding decrease in Tc, depending upon the initial amount of added Pb. X-ray photoelectron spectroscopy analysis of the multiphase, leaded samples showed the decrease in the ratio of Cu2+/Cu1+ as well as the formation of various surface carbonates responsible for the degradation in the superconducting parameters. In a humid atmosphere, a single-phase 2223 sample did not show any decomposition whereas single-phase 2212 or mixed-phase samples showed partial degradation to Bi2CuO4, SrCO3, CaCO3 and CuO as evidenced by X-ray diffraction and EDX analysis. Based on these observations a possible degradation mechanism has been proposed and the role of Pb has been discussed.

We report here the room temperature ferromagnetic ordering in spray-pyrolized polycrystalline thin films of Ti1−xCoxO2 (x = 0, 0.025, 0.05 and 0.10). The saturation magnetic moment in these films with x = 0.05 and 0.10 is estimated to be of 0.28 and 0.38µB/Co, respectively. The ambient air employed in the present case of the spray pyrolysis process resulted in ferromagnetic films possessing very high resistivity values (∼10 7 � cm). This is in contrast to Ti1−xCoxO2 films deposited by vacuum based techniques, which are relatively conducting. While pristine TiO2 film exhibited the anatase phase, the rutile phase of TiO2 also evolved with an increase in Co concentration in these films. X-ray diffractograms could not reveal segregation of Co atoms in the TiO2 matrix; also, no formation of any oxide of cobalt was detected. X-ray photoelectron spectroscopic investigation established the Co cations to be bivalent, and suggests that Co +2 is substituting Ti +4 in the TiO2 matrix. Due to the highly resistive nature of these films, the observed room temperature ferromagnetism could be attributed either to the formation of bound magnetic polarons or to defect mediated ordering.

Magnetic ordering in Co-substituted spray pyrolyzed SnO2 thin films has been investigated by magneto-transport studies of Sn1−xCoxO2−δ and F co-doped films. The free carrier concentration and room temperature ferromagnetism induced in Sn1−xCoxO2−δ thin films with cobalt concentrations up to x = 0.10 are found to be closely related to each other. Both the carrier concentration and the saturation magnetic moment increase monotonically with the concentration of Co, and attain maximum values of 2.2 × 1019 cm−3 and 0.26 μB/Co-ion, respectively, at x = 0.10. The substitution of O2− with F− (nearly 10 atomic percent (a/o)) has resulted in much higher values of both carrier concentration (2.07 × 1020 cm−3) and magnetic moment (0.80 μB/Co-ion). Also, anomalous behavior of the Hall effect has been observed. The carrier mediated interaction is proposed to be the most probable mechanism responsible for ferromagnetism in these films. The temperature dependence of the magnetization of these films predicts a Curie temperature exceeding 573 K. All the films are optically transparent (>75%) in the visible spectral region.

Optical properties of spray pyrolysed Sn1?xCoxO2?? (0?x?0.15) thin films exhibiting room temperature ferromagnetism have been investigated in the energy range 1?6?eV. The onset of optical absorption is blueshifted by as much as 220?meV for a cobalt concentration of 10a/o (x = 0.10). Our detailed analysis indicates that the most probable cause of the blueshift is the Burstein?Moss effect. The optical conductivity calculations reveal that Co does not introduce any mid-gap states. Also the small change in the slope of ?2 versus h? indicates that the O and Sn band edges are affected on Co addition. This band-edge shift also implies the presence of strong Coulomb interaction between Co, Sn and O sites, as a plausible cause of the observed ferromagnetism in these samples. The Burstein?Moss effect seems to be masking the effect of band structure modification introduced by Co addition. The high (~75%) optical transparency of these films in the visible region remains practically unaffected on Co addition.