An unexpected presence of ferromagnetic (FM) ordering in nanostructured nonmagnetic metal oxides has been reported previously. Though this property was attributed to the presence of defects, systematic experimental and theoretical studies to pinpoint its origin and mechanism are lacking. While it is widely believed that oxygen vacancies are responsible for FM ordering, surprisingly we find that annealing as-prepared samples at low temperature (high temperature) in flowing oxygen actually enhances (diminishes) the FM ordering. For these reasons, we have prepared, annealed in different environments, and measured the ensuing magnetization in micrometer and nanoscale ZnO with varying crystallinity. We further find from our magnetization measurements and ab initio calculations that a range of magnetic properties in ZnO can result, depending on the sample preparation and annealing conditions. For example, within the same ZnO sample we have observed ferro- to para- and diamagnetic responses depending on the annealing conditions. We also explored the effects of surface states on the magnetic behavior of nanoscale ZnO through detailed calculations.

No abstract

We have measured the change in the resistive T c of Bi 2 (SrCa) 2ϩn (Cu 1Ϫx M x ) 1ϩn O y whiskers due to substitution of M ϭ Ni ͑magnetic͒ or M ϭ Zn ͑nonmagnetic͒ for Cu. These whisker crystals show resistance drops for both the 2212 and 2223 (nϭ1,2) phases, with transition widths ͑10-90 %͒ of 1 K. We checked the effect of oxygenation at several values of x, and found that T c is maximum for the y obtained by growing and annealing in 1 atm of O 2 . We observed a linear depression of T c for both 2212 and 2223 phases with DϭdT c /dxϷϪ800 K for both Ni and Zn. This is in contrast with YBCO in which D Zn Ϸ3D Ni .

The bending modulus (Young’s modulus) of several chemical vapor deposition-grown multiwalled nanotubes (MWNTs) have been measured using a vibrating reed technique. Three different precursors were used to produce MWNTs with differing densities of defects in the tube walls. Individual MWNTs were electrostatically driven in air over a dark-field light microscope and the bending modulus of the nanotubes was determined from the frequency of the first vibrational resonance. A correlation between the defect density and the bending modulus was found which implies that the bending modulus is relatively more sensitive to wall defects than the nanotube diameter.

No abstract

No abstract

An unexpected presence of ferromagnetic (FM) ordering in nanostructured ZnO has been reported previously. Recently, from our detailed magnetization studies and ab initio calculations, we attributed this FM ordering in nanostructured ZnO to the presence of surface states, and a direct correlation between the magnetic properties and crystallinity of ZnO was observed. In this study, through a systematic sample preparation of both pristine and Co-doped ZnO nanostructures, and detailed magnetization and nonlinear optical (NLO) measurements, we confirm that the observed FM ordering is due to the presence of surface states.

Interface modification in transport properties of single elemental polycrystalline Bi via spark plasma sintering results in 'double-decoupling' (simultaneous decoupling of thermopower, electrical, and thermal conductivity) of otherwise coupled entities. In spark plasma sintering, the DC pulse current helps in controlling the nature and extent of surfaces of ball-milled Bi and hence results in six-fold improvement in the dimensionless figure of merit (ZT) relative to as-purchased samples.