and 600 1C for 90 min each. The hysteresis loops establish that all the samples have spontaneous magnetization (or ferromagnetism) at room temperature after subtracting the diamagnetic effect. The M–H curves indicate that saturation in magnetization is attained within an applied field of 10 kOe. It is clearly seen that with increasing TA, saturation magnetization (Ms) decreased. Maximum Ms of 0.039 emu/g was obtained for the composite nanofibers heat treated at 500 1C, while the sample heat treated at 600 1C exhibited the lowest Ms of 0.007 emu/g. These values are compar- able to the values of 0.047 emu/g and 0.0054 emu/g reported for mechanically milled [21] and co-pecipitated [12] undoped ZnO. Inset in Fig. 4 shows variation of Ms with TA in the range of study. Though RTFM has been reported by other research groups in
4.In summary, ZnO nanofibers were prepared via electrospinning and annealing in air or vacuum. XRD data show that there is no impurity phase in the wurtzite ZnO nanofibers. Raman data indicates the presence of defects in the wurtzite ZnO nanofibers. Room temperature ferromagnetism is observed in all the samples with maximum saturation magnetization value in samples annealed at 550 1C. Higher magnetization exhibited by vacuum annealed ZnO nanofibers establishes the role of O vacancies in the RTFM observed in this wurtzite ZnO nanostructure. The present studies not only provide evidence for the role of O vacancies in the observed RTFM in ZnO nanofibers but also provides a means to tailor the O vacancies in ZnO nanostructures by controlling the ambient during the heat treatment of electrospun ZnO/PVA composites.
semiconducting oxides, there is doubt whether this is an intrinsic or extrinsic property of these materials. As pointed out earlier, though there seems to be a general consensus about the role of defects in this phenomenon, it is not clear whether one can attribute it to the metal ion vacancies [14] or O [5,25].
XRD and Raman data presented in the earlier part of the paper
clearly establish that ZnO exists in single (wurtzite) phase in 1-d
form without any other impurity phases. Our Raman studies show
that defects are present in the single phase wurtzite ZnO nanofi-
bers which confirm the role of defects in the observed RTFM in
ZnO nanofibers.
n order to understand the influence of O
vacancies on RTFM, we performed heat treatment of the as-spun
nanofiber composite in air and under vacuum ( $ 10
550 1C for 90 min and recorded their M–H loops. Fig. 5 shows the M–H curves of air and vacuum annealed samples which show that both samples are ferromagnetic at room temperature. It is also clearly noticeable that the vacuum annealed sample exhibits higher magnetization (Ms1⁄40.056emu/g) than the air annealed sample (Ms 1⁄4 0.022 emu/g). Since the oxygen available in the two annealing procedures are different and the vacuum annealed samples have more O vacancies, the influence of O vacancies is obvious in the higher magnetization exhibited by the vacuum annealed ZnO nanofibers.