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Journal ArticleDOI

Surface defects induced ferromagnetism in mechanically milled nanocrystalline ZnO

21 Aug 2013-Journal of Applied Physics (American Institute of Physics)-Vol. 114, Iss: 7, pp 073516
TL;DR: In this article, the magnetic properties of milling and annealing of nano-ZnO powder have been investigated and it has been found that saturation magnetization (Ms) decreases systematically for increasing average grain size of the ZnO nanoparticles.
Abstract: 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 altoge...
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TL;DR: In this paper, it was shown that ZnO samples can be magnetic even without transition-metal doping and also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZNO nanostructures.
Abstract: Extensive calculations based on density functional theory have been carried out to understand the origin of magnetism in undoped ZnO thin films as found in recent experiments. The observed magnetism is confirmed to be due to Zn, instead of O, vacancy. The main source of the magnetic moment, however, arises from the unpaired 2p electrons at O sites surrounding the Zn vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.490 to 0.740 B. Moreover, the study of vacancy-vacancy interactions indicates that in the ground state, the magnetic moments induced by Zn vacancies prefer to ferromagnetically couple with the antiferromagnetic state lying 44 meV higher in energy. Since this is larger than the thermal energy at room temperature, the ferromagnetic state can be stable against thermal fluctuations. Calculations and discussions are also extended to ZnO nanowires that have larger surface to volume ratio. Here, the Zn vacancies are found to lead to the ferromagnetic state too. The present theoretical study not only demonstrates that ZnO samples can be magnetic even without transition-metal doping but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO nanostructures. In addition, vacancy mediated magnetic ZnO nanostructures may have certain advantages over transition-metal doped systems in biomedical applications.

357 citations

Journal ArticleDOI

[...]

TL;DR: In this article, the authors investigated the intrinsic origin of defect-induced d0 ferromagnetic magnetism in undoped ZnO thin films and nanostructures and found that the main cause of controversy is rooted in a long term debate on the nature of native defects concerning the unintentional n-type conductivity in as-grown ZnOs.
Abstract: ZnO-based dilute magnetic semiconductors have attracted great interest for their promising application potential in spintronics. Observation of ferromagnetic-like behavior in oxides in general directs the recent focus to defect-rich undoped ZnO thin films and nanostructures. Such magnetic properties are generally mediated by the defects exclusive of magnetic ion doping, thus called defect-induced d0 ferromagnetism (FM). However the intrinsic origin of d0 FM in such materials is controversially reported. In this review we aim to locate the root of the controversy by revisiting the way how the defects were characterized and correlated with the d0 FM in each situation. We found that the main cause of controversy is rooted in a long term debate on the nature of native defects concerning the unintentional n-type conductivity in as-grown ZnO. It is particularly manifested in the assignment of the green luminescence center in photoluminescence spectra and electron paramagnetic resonance signals near g = 1.96 and g = 2.0. Only through X-ray-based microscopy and spectroscopy analysis, can the intrinsic origin of d0 FM in undoped ZnO be unambiguously attributed to the O 2p orbitals arising from zinc vacancies, rather than the Zn 3d orbitals and oxygen vacancies. In spite of the complex defect state in the nanostructures, certain parameters that influence the d0 FM in undoped ZnO systems can be extracted from various reports. Finally, we summarize the challenges and general conclusions on the d0 FM in undoped ZnO nanostructures, followed by outlooks on potential device application in spintronics. It is clear that an important step to promote d0 FM in ZnO for spintronics is to stabilize enough VZn in ZnO nanostructures, either through acceptor doping or epitaxial growth of strained films, without diminishing the crystalline quality of the structure. Future research focusing on this direction will hopefully produce new breakthrough in device applications.

61 citations

Journal ArticleDOI

[...]

TL;DR: In this article, a strong correlation between the inherent emission features of ZnO nanofibers and their peak positions have been detected with the grain boundaries (GBs), which is a critical parameter for improving the photocatalytic activity.
Abstract: Electrospun ZnO nanofibers (ZNF) have received increased attention as photocatalysts owing to their potential for incredible performance. However, uncertainty still exists in determining the correlation between grain boundaries (GBs) and photocatalytic activity. Therefore, effective thought has been put into engineering the GBs to convert ZNF into a promising photocatalyst. Herein, the obtained electrospun ZnO structures are composed of nanograins, which are connected to each other in an ordered manner. In-depth studies have revealed that the growth of nanograins severely altered the morphology of ZNF and GB areas at higher annealing temperatures ranging from 500 °C to 1000 °C. Based on the morphological features and their structural evolution, the obtained structures are named as ZnO nanofibers-1 (ZNF-1, 500 °C), ZnO hollow tubes (ZHT, 600 °C), ZnO nanofibers-2 (ZNF-2, 700 °C), ZnO bamboo structured fibers (ZBF, 800 °C), ZnO segmented fibers (ZSF, 900 °C) and ZnO nanoparticles (ZNP, 1000 °C). A strong correlation between the inherent emission features of ZNF and their peak positions have been detected with the GB. The comparative degradation efficiency of methylene blue (MB) has been studied and the results showed that the ZNF-1 with highly stacked nanograins containing rich grain boundaries demonstrated ∼6 times higher efficiency than other structures. In addition, it has been shown to have a strong effect towards the degradation of Rhodamine B (Rh B) and 4-nitro-phenol (4-NP). A critical parameter for improving the photocatalytic activity is found to be the GB mediated defects, which are proposed to be oxygen/zinc vacancies at nanograin fusion interfaces, while supposedly maintaining its fibrous structure, wherein no relationship has been drawn implying the direct domination of morphology, surface area and defect.

42 citations

Journal ArticleDOI

[...]

Shoupeng Shi1, Daqiang Gao1, Qiang Xu1, Zhaolong Yang1, Desheng Xue1 
TL;DR: In this paper, the magnetic properties of SnO2 powders that have been mechanically milled and subsequently annealed have been investigated, and the results indicate that saturation magnetization of the samples increases with milling time, where a high saturation magnetisation of 0.0012 emu g−1 can be obtained for the sample milled for 20 h, and saturation magnetizations for sample decreases gradually after annealing in air.
Abstract: The possibility of inducing long-range ferromagnetic order with non-transition metal ions has become a very exciting challenge in recent years. In order to elucidate the room temperature ferromagnetism of SnO2 powders, the magnetic properties of SnO2 powders that have been mechanically milled and subsequently annealed have been investigated. The results indicate that saturation magnetization of the samples increases with milling time, where a high saturation magnetization of 0.0012 emu g−1 can be obtained for the sample milled for 20 h, and saturation magnetization for the sample decreases gradually after annealing in air. Electron spin resonance results show large numbers of singly-charged oxygen vacancies on the surfaces of the SnO2 powders. Combined with X-ray photoelectron spectroscopy and room temperature photoluminescence results, this suggests that the observed ferromagnetism is related to the singly-charged oxygen vacancies.

42 citations

Journal ArticleDOI

[...]

TL;DR: In this article, the changes in magnetic properties of ZnO nanoparticles with the reduction of size by mechanical milling were investigated and it was observed that ferromagnetic ordering appeared in the sample when the particle size decreases from 39 ± 1 nm to 30 ± 1nm.
Abstract: Ferromagnetism in the nanostructures of undoped oxide semiconductors has become an exciting problem nowadays for its potential future applications in spintronics. In order to elucidate the room temperature d0 ferromagnetism of oxide semiconductors, we have investigated the changes in magnetic property of ZnO nanoparticles with the reduction of size by mechanical milling. We have observed that ferromagnetic ordering appears in the sample when the particle size decreases from 39 ± 1 nm to 30 ± 1 nm. This observation strongly supports the idea of the effect of specific grain boundaries in nanoparticles. The results of Raman scattering also support this observation. From photoluminescence spectra shifted green emissions have been found for ferromagnetic samples. This indicates clearly two different origins for green emissions that are strongly related to the changes in magnetic property. Observations from electron spin resonance spectra suggest that zinc related interstitial defects are significant to give rise to this ferromagnetic coupling. An impurity level formed by the interstitial defects at the surfaces could satisfy the Stoner criteria for the occurrence of band ferromagnetism for these samples.

35 citations

References
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Journal ArticleDOI

[...]

TL;DR: In this paper, the authors performed a comprehensive first-principles investigation of point defects in ZnO based on density functional theory within the local density approximation (LDA) as well as the $\mathrm{LDA}+U$ approach for overcoming the band-gap problem.
Abstract: We have performed a comprehensive first-principles investigation of native point defects in ZnO based on density functional theory within the local density approximation (LDA) as well as the $\mathrm{LDA}+U$ approach for overcoming the band-gap problem. Oxygen deficiency, manifested in the form of oxygen vacancies and zinc interstitials, has long been invoked as the source of the commonly observed unintentional $n$-type conductivity in ZnO. However, contrary to the conventional wisdom, we find that native point defects are very unlikely to be the cause of unintentional $n$-type conductivity. Oxygen vacancies, which have most often been cited as the cause of unintentional doping, are deep rather than shallow donors and have high formation energies in $n$-type ZnO (and are therefore unlikely to form). Zinc interstitials are shallow donors, but they also have high formation energies in $n$-type ZnO and are fast diffusers with migration barriers as low as $0.57\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$; they are therefore unlikely to be stable. Zinc antisites are also shallow donors but their high formation energies (even in Zn-rich conditions) render them unlikely to be stable under equilibrium conditions. We have, however, identified a different low-energy atomic configuration for zinc antisites that may play a role under nonequilibrium conditions such as irradiation. Zinc vacancies are deep acceptors and probably related to the frequently observed green luminescence; they act as compensating centers in $n$-type ZnO. Oxygen interstitials have high formation energies; they can occur as electrically neutral split interstitials in semi-insulating and $p$-type materials or as deep acceptors at octahedral interstitial sites in $n$-type ZnO. Oxygen antisites have very high formation energies and are unlikely to exist in measurable concentrations under equilibrium conditions. Based on our results for migration energy barriers, we calculate activation energies for self-diffusion and estimate defect-annealing temperatures. Our results provide a guide to more refined experimental studies of point defects in ZnO and their influence on the control of $p$-type doping.

2,661 citations

Journal ArticleDOI

[...]

TL;DR: In this article, a 3d transition metal-doped ZnO films (n-type Zn1−xMxO) were formed on sapphire substrates using a pulsed-laser deposition technique, and their magnetic and electric properties were examined.
Abstract: 3d-transition-metal-doped ZnO films (n-type Zn1−xMxO (x=005–025): M=Co, Mn, Cr, Ni) are formed on sapphire substrates using a pulsed-laser deposition technique, and their magnetic and electric properties are examined The Co-doped ZnO films showed the maximum solubility limit Some of the Co-doped ZnO films exhibit ferromagnetic behaviors with the Curie temperature higher than room temperature The magnetic properties of Co-doped ZnO films depend on the concentration of Co ions and carriers

1,808 citations

Journal ArticleDOI

[...]

TL;DR: In this paper, it was shown that room-temperature ferromagnetism has been observed in nanoparticles of nonmagnetic oxides such as (1), (2), (3), (4), (5), (6), (7,30), and (6,30).
Abstract: Room-temperature ferromagnetism has been observed in nanoparticles $(7--30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\phantom{\rule{0.2em}{0ex}}\mathrm{diam})$ of nonmagnetic oxides such as ${\mathrm{CeO}}_{2}$, ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, $\mathrm{ZnO}$, ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{SnO}}_{2}$. The saturated magnetic moments in ${\mathrm{CeO}}_{2}$ and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ nanoparticles are comparable to those observed in transition-metal-doped wideband semiconducting oxides. The other oxide nanoparticles show somewhat lower values of magnetization but with a clear hysteretic behavior. Conversely, the bulk samples obtained by sintering the nanoparticles at high temperatures in air or oxygen became diamagnetic. As there were no magnetic impurities present, we assume that the origin of ferromagnetism may be the exchange interactions between localized electron spin moments resulting from oxygen vacancies at the surfaces of nanoparticles. We suggest that ferromagnetism may be a universal characteristic of nanoparticles of metal oxides.

1,144 citations

Journal ArticleDOI

[...]

05 Aug 2004-Nature
TL;DR: It is shown that thin films of hafnium dioxide (HfO2), an insulating oxide better known as a dielectric layer for nanoscale electronic devices, can be ferromagnetic even without doping.
Abstract: It is generally accepted that magnetic order in an insulator requires the cation to have partially filled shells of d or f electrons. Here we show that thin films of hafnium dioxide (HfO2), an insulating oxide better known as a dielectric layer for nanoscale electronic devices, can be ferromagnetic even without doping. This discovery challenges our understanding of magnetism in insulators, because neither Hf4+ nor O2- are magnetic ions and the d and f shells of the Hf4+ ion are either empty or full.

1,006 citations

Journal ArticleDOI

[...]

TL;DR: In this article, the magnetic moment was observed in undoped semiconducting or insulating oxides to become ferromagnetic at room temperature on MgO substrates, where the magnetic field was applied parallel to the film plane.
Abstract: Remarkable room-temperature ferromagnetism was observed in undoped $\mathrm{Ti}{\mathrm{O}}_{2}$, $\mathrm{Hf}{\mathrm{O}}_{2}$, and ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ thin films. The magnetic moment is rather modest in the case of ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ films on MgO substrates (while on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrates, it is negative showing diamagnetism) when the magnetic field was applied parallel to the film plane. In contrast, it is very large in the other two cases (about 20 and $30\phantom{\rule{0.3em}{0ex}}\mathrm{emu}∕{\mathrm{cm}}^{3}$ for $200\text{\ensuremath{-}}\mathrm{nm}$-thick $\mathrm{Ti}{\mathrm{O}}_{2}$ and $\mathrm{Hf}{\mathrm{O}}_{2}$ films, respectively). Since bulk $\mathrm{Ti}{\mathrm{O}}_{2}$, $\mathrm{Hf}{\mathrm{O}}_{2}$, and ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ are clearly diamagnetic, and moreover, there are no contaminations in any substrate, we must assume that the thin film form, which might create necessary defects or oxygen vacancies, would be the reason for undoped semiconducting or insulating oxides to become ferromagnetic at room temperature.

731 citations