Bulk ZnO is a diamagnetic material but ferromagnetism (FM) has been observed by several groups in its nanostructures. In order to elucidate the room temperature (RT) FM of ZnO nanostructures, magnetic property of mechanically milled and subsequently annealed nano-ZnO powder has been investigated. Sample that has been milled and then annealed at 200 °C in ambient condition shows highest value of saturation magnetization (Ms), whereas lowest value of Ms has been noticed for the sample pre-annealed at 500 °C before milling. The variation of Ms with annealing temperatures closely resembles with the variation of average positron lifetime (τav) and S-parameter reported earlier for these nano-systems. It has also been found that Ms decreases systematically for increasing average grain size of the ZnO nanoparticles. Room temperature photoluminescence of the as-milled sample shows broad defect related emission centered ∼2.23 eV. Enhancement of such emission has been observed due to 200 °C annealing. Results altogether indicate that ferromagnetism in ZnO depends critically on the nature of disorder (open volume defects as well as defect clusters) at the grain surface region. In this connection, the possible role of zinc vacancy defects has also been emphasized.

An impurity defect level formed by interstitial zinc at the surfaces of undoped ZnO nanoparticles plays a crucial role for d0 ferromagnetism.

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Metal–semiconductor hybrid nanoheterostructures have the possibility to exhibit new synergic properties other than the combination of properties from discrete components due to the interaction of metal and semiconductor components at the interfaces. Here, we have synthesized Ag–Cu2O eyeball-shaped quantum nanoheterostructures with diameter ranging between 8 and 12 nm using a single-step low-cost solvothermal process. It is observed that the presence of a minimum 3% of Ag is required for the formation of Ag–Cu2O quantum nanoheterostructures. The formation of nanoheterostructures has introduced new synergic properties like intense blue luminescence and surface-enhanced Raman scattering due to the interactions between Ag and Cu2O atoms at the interfaces. The significant presence of charge transfer through the interfaces is identified from the peak shift of Raman modes. The increase in the electron density at the metal surface due to the charge transfer and the recombination of these electrons with sp- or d-band holes of Ag could be the effective mechanism of the observed blue luminescence. The blue luminescence of Ag–Cu2O quantum nanoheterostructures together with its low band gap value (≈2.3 eV) is believed to have important applications in optoelectronic devices.

Multiferroic materials having good magnetoelectric coupling are of great interest due to their enormous variety of applications in the field of spintronic devices. Despite having good magnetoelectric coupling and multiferroic properties, gallium ferrite (GaFeO 3 ) is plagued for having a very high leakage current compared to other multiferroic materials, which shadows the possibility of its application in device fabrication. The conduction mechanism behind this exceptionally high leakage current in GaFeO 3 is still not identified properly. Here, we have carefully investigated the variation of leakage conduction with the change in morphologies, especially at the grain boundaries due to heat treatment temperature during sample preparation. A connected conductive network is detected along the fused grain boundaries, which provides the pathway for the leakage electrons to propagate through the material. Interestingly, the conductive atomic force microscopy image shows that oxygen vacancies at the near grain boundaries produce poor conductive regions. Small polaron hopping could be an effective transport mechanism present in the conductive ridges at the grain boundaries. Therefore, modification of grain boundaries could produce more effective results for the reduction in leakage current of GaFeO 3 .

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Free excitonic emission is observed for the ferromagnetic undoped ZnO nanoparticles prepared by mechanical milling.