Magnetic Properties Of ZnO Research Articles

Structural and Magnetic Properties of ZnO:(Mn, Co) Nanoparticles

Structural and Magnetic Properties of ZnO:(Mn, Co) Nanoparticles

Effect of Li-doping on the magnetic properties of ZnO with Zn vacancies

Using first-principles calculations, we investigate the electronic and magnetic properties of Zn vacancy in ZnO with and without Li-doping. It is found that the Zn vacancy can induce magnetism but the formation energy of the system is high. We also found that the Li-dopant at either the substitutional sites or the interstitial sites and both at two sites can lower the formation energy of Zn vacancy. The total magnetic moments of the system is increased after Li doping at the substitutional site, while it is decreased after Li doping at the interstitial site. In addition, the extended tails of the wave functions of Zn vacancy make long-ranged spin couplings possible. Li atoms at the substitutional sites further stabilize the long-ranged ferromagnetism induced by Zn vacancies. Thus, it is possible to tune the magnetism of ZnO through defect engineering.

The magnetic properties of Gd doped ZnO nanowires

We present a study of the ferromagnetic properties of Gd doped ZnO nanowires (Nws) fabricated by means of a chemical vapor deposition process. The sample was grown with a Gd mole ratio 5% in a mixed Zn/Mn source under a constant O2/Ar gas mixture flowing at 580°C followed by annealing at 800°C. We found that the magnetic properties of ZnO:Gd Nws are a function of the external magnetic field and temperature. An average value of the moment per Gd atom is as high as 3278μB as compared to its atomic moment of 8μB, showing that the ZnO:Gd Nws are an intrinsic diluted magnetic semiconductor. The unprecedented colossal moment is attributed to the effective Ruderman–Kittel–Kasuya–Yosida exchanging interaction.

Investigation of the properties of ferromagnetic ZnO:Cr2O3 nanocomposites

Present work focuses on the structural, optical and magnetic properties of ZnO:Cr2O3 nanocomposites. ZnO nanoparticles were synthesized and the structure was confirmed using powder x-ray diffraction. ZnO nanoparticles was grown in the hexagonal wurtzite structure with the preferential orientation along (101) plane. ZnO:Cr2O3 composites have been synthesized by doping different concentration of Cr2O3 (1, 3 and 5wt%) into ZnO. The incorporation of Cr2O3 was confirmed using Fourier transformed infrared spectroscopy. UV–visible absorption spectra have been observed and interpreted for the determination of optical constants of ZnO:Cr2O3 composites. The optical constants like optical band gap, refractive index were determined and the effect of Cr2O3 on these constants was investigated. Relation between optical band gap and the refractive index were obtained. Magnetic studies using vibrating sample magnetometer reveal the ferromagnetism at 150K in the composites with 3 and 5wt% of Cr2O3.

Theoretical Study on Cation Vacancy Induced Magnetism in Bulk ZnO

Based on first principle FP-LAPW calculations, the electronic structure and magnetic properties of ZnO with Zn cation vacancy has been investigated. We find that the cation vacancy defect can induce a magnetic moment of about 2μB/supercell. The magnetic moment mainly comes from 2p-orbitals of O atoms which surround the Zn vacancy. We also find that the two Zn vacancies in ZnO always coupled Ferro-magnetically. Furthermore, the system shows half metallic Ferro-magnetic properties with high Curie temperature.

Optical, Structural, and Magnetic Properties of ZnO:Co Nanoparticles Prepared by a Thermal Treatment of Ball Milled Precursors

In this study, ZnO nanoparticles with different cobalt concentration were prepared by a simple and rapid method. This method is based on a short time solid state milling and calcinations of zinc acetate, cobalt acetate, and citric acid powders. The samples were characterized using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared (FTIR), photoluminescence, and UV-vis. spectroscopy. It was shown that a very low substitution of Co (less than 1% of molecular weight) has little effect on the lattice parameters of ZnO and significantly decreases the band gap (E g ) value of the synthesized ZnO:Co nanoparticles. Calculation based on the XRD data shows that the average crystallite sizes of ZnO particles are nearly 18 nm. Photoluminescence spectroscopy shows that many defects such as interstitial zinc, zinc vacancy, and exciton recombination are responsible for the observed optical properties. Magnetization measurements which were performed by using a superconducting quantum interference device (SQUID) magnetometer determine the paramagnetic behavior for all samples due to the absence of oxygen vacancy.

Effect of codoping with C or N on electronic structures and magnetic properties of ZnO:Cu

Using first-principles calculations based on density functional theory, we investigated systematically the electronic structures and magnetic properties of ZnO:Cu. The results indicate that Cu-doped ZnO prefers a ferromagnetic ground state and behaves like a half-metallic ferromagnet. The magnetic moment mainly localizes at Cu atom and the rest mainly comes from the spin polarized O atoms. It has been found that the ferromagnetic stability can be enhanced slightly by substituting an oxygen atom with one N atom; while the ferromagnetic stability can be weakened by replacing one O atom with a C atom. Due to absence of magnetic ion and the 100% spin polarization of the carriers in ZnO:Cu, one can expect that Cu-doped ZnO would be a useful half-metallic ferromagnet both in practical application and in theoretical studies.

DFT Study of Structure, Electronic, Optical and Magnetic Properties of ZnO and Cu-Doped ZnO Clusters

The structure, polarizability and magnetic properties of ZnnOn and Cu-doped Znn-1On (n=2-12, 16) clusters are theoretically investigated using density-functional theory (DFT) at B3LYP/LanL2DZ level. For pure ZnnOn clusters, Zn3O3, Zn8O8 and Zn12O12 are relatively more stable unit, and the ZnnOn clusters with bigger HOMO-LUMO gap tends to have larger <α>/2n. For Cu-doped Znn-1On clusters, the substitution of Zn by Cu atom narrows the HOMO-LUMO gaps and enhances polarizability. In single-doped CuZnn-1On clusters, the magnetic moments of O atoms bonded with Cu atoms are ferromagnetically coupled to the moments of the Cu atoms.

On intrinsic defects in Co-doped ZnO thin films

We report on electrically active point defects in Co-doped ZnO epilayers grown by molecular beam epitaxy. Using thermally stimulated current and deep level transient spectroscopy (DLTS), we identify a deep electron trap with a thermal activation energy of 0.5 eV. The density of this centre seems to decrease upon annealing of the samples in air ambient, while it remains unchanged when the annealing is performed in nitrogen ambient. It also decreases if the samples have been illuminated with UV light at a low temperature prior to the DLTS measurements, thus showing the metastable nature of the centre. We attribute this electron trap to the isolated oxygen vacancy. Comparison of our findings with what has been reported in the literature on the magnetic properties of ZnO:Co suggests a correlation between the observed ferromagnetism in these materials and the oxygen vacancy.

Indium-doped ZnO nanowires: Optical properties and room-temperature ferromagnetism

We report the optical and magnetic properties of ZnO, Zn0.97In0.03O, and Zn0.94In0.06O nanowires (NWs). All samples have similar wirelike shape with an average diameter of about 70 nm and a length of about 10 μm. The comparison of photoluminescence (PL) spectra at 10 K indicated that a new broad emission band appeared after indium doping, which is associated with donor-acceptor-pair recombination. Additionally, the intensity of oxygen-vacancies-induced visible emission increased with increasing In content, indicating that In doping can induce many oxygen vacancies. Furthermore, magnetic measurements revealed that pure ZnO NWs are diamagnetic, while indium-doped ZnO NWs exhibit intrinsic ferromagnetism at room temperature. With the increase in In content, the coercive field and the magnetic moment for indium-doped ZnO NWs increase largely. Ferromagnetic ordering can be interpreted as being due to O vacancies induced by In doping, which is in good agreement with PL results.

Photo‐EPR and magneto‐optical spectroscopy of iron centres in ZnO

AbstractZnO has attracted increasing interest as promising material for optoelectronics and spintronics. Magnetic properties of ZnO can be controlled by doping with transition metal ions among which Fe3+ is one of the interesting candidates. We report on the properties of Fe3+ centres in hydrothermally and chemical vapour transport grown ZnO single crystals investigated by photo‐electron paramagnetic resonance (EPR) and optical spectroscopy. Detailed magneto‐optical studies of Zeeman components of spin‐forbidden electric dipole transitions 4T1(G) → 6A1(6S) of Fe3+ centre in ZnO reveal the trigonal symmetry of fine structure of lowest Γ8 (4T1) excited state. The studies were performed in external magnetic fields up to 10 T and in the temperature range from 2 to 30 K. The energy positions of the Zeeman components at 8 T were measured as a function of the direction of applied magnetic field in $(1\bar {2}10)$ and (0001) planes. The detailed check of the angular variation of Zeeman lines shows two magnetically non‐equivalent Fe3+ centres. These special features were accounted by contribution of higher rank Zeeman terms of dimension BJ3. The photo‐generated EPR spectra of trigonal Fe3+ centres were detected in hydrothermally grown ZnO single crystals in addition to three types of charge compensated iron centres presented in these samples in the dark conditions.

On the oxygen vacancy in Co-doped ZnO thin films

Abstract We investigate the transport properties of Co-doped ZnO epilayers using thermally stimulated current and deep level transient spectroscopy (DLTS). We identify a defect center located at Ec−0.5 eV in all the studied materials. Annealing of our samples either in nitrogen ambient or in air reveals that the density of the center increases when the annealing is made in the nitrogen ambient. Illumination of the samples with a UV light at low temperature prior to the DLTS measurements reduces the density of the observed defect significantly, thus showing the metastable nature of the center. We therefore attribute it to the isolated oxygen vacancy. Furthermore, comparison with reports on the magnetic properties of ZnO:Co suggests a correlation between the observed ferromagnetism in these materials and the oxygen vacancy.

Subtle interplay between native point defects and magnetism in ZnO:Co

Distribution of Co ions and its effect on magnetic properties of ZnO:Co in the presence of native point defects, oxygen vacancy, and interstitial hydrogen, have been investigated using first-principles density functional calculations. The study provides a fundamental theoretical understanding on the correlation between magnetism and the distribution of magnetic ions and the native point defects in the semiconducting host. Results show that Co ions have a strong tendency toward aggregation via oxygen within ab plane in the presence of point defects. The room temperature ferromagnetism observed experimentally in ZnO:Co is mainly dominated by the interstitial hydrogen rather than oxygen vacancy.

Anderson localization enhanced ferromagnetism in Zn0.95Co0.05O

We report an enhancement in the ferromagnetic characteristics of Zn0.95Co0.05O thin films due to the localization of charge carriers. Epitaxial thin films of Zn0.95−xCo0.05GaxO (x=0–0.05) were grown on single-crystal sapphire (0001) substrates by pulsed laser deposition technique. The role of charge carrier localization on the electrical and magnetic properties of ZnO:Co was studied by introducing Ga into the system. It was observed that Ga plays a significant role in affecting both the electrical transport mechanism as well as the magnetization of the material. Electrical resistivity of Zn0.95Co0.05O at room temperature was ∼96 mΩ cm and exhibited metal-like temperature dependence, although strongly influenced by electron-electron (e-e) interactions. Strong e-e interaction was understood to arise because of the randomness introduced in the crystal potential of ZnO by the cobalt dopants. As the Ga dopants are introduced, randomness in crystal potential and hence the disorder further increases resulting in the Anderson localization of the carriers. The increase in localization was accompanied by a significant enhancement in the magnetic moment from 0.75μB/Co in Zn0.95Co0.05O films to 1.6μB/Co in Zn0.90Co0.05Ga0.05O.

Magnetic properties of ZnO:Cu thin films prepared by RF magnetron sputtering

ZnO films and ZnO:Cu diluted magnetic semiconductor films were prepared by radio frequency magnetron sputtering on Si (111) substrates, with targets of ZnO and Zn0.99Cu0.01O, respectively. The plasma emission spectra were analyzed by using a grating monochromator during sputtering. The X-ray photoelectron spectroscopy measurements indicate the existence of Zni defect in the films, and the valence state of Cu is 1+. The X-ray diffraction measurements indicate that the thin films have a hexagonal wurtzite structure and have a preferred orientation along the c-axis. The vibrating sample magnetometer measurements indicate that the sample is ferromagnetic at room temperature, and the origin of the magnetic behavior of the samples is discussed.

Substantial stabilization of ferromagnetism in ZnO:Mn induced by N codoping

Electronic structures and magnetic properties of ZnO:Mn andZnO:Mn+N systems are investigated using first-principles density functional calculations withgeneralized gradient approximation. The results provide a fundamental theoreticalunderstanding in the substantial ferromagnetic stability induced by N codoping in theZnO:Mn system observed experimentally. They demonstrate that the ferromagneticinteraction is due to the hybridization between N 2p and Mn 3d states and is very sensitiveto the geometrical configurations of dopants in the ZnO host lattice. The most stableferromagnetic configuration corresponds to the Mn–N–Mn cluster, energeticallystrong enough to lead to hole-mediated ferromagnetism at room temperature.

Influence of the substrate temperature on the electrical and magnetic properties of ZnO : N thin films grown by pulse laser deposition

We investigated the effect of the substrate temperature on the magnetic, electrical and optical properties of nitrogen-doped ZnO thin films. Experiments showed that a high substrate temperature depresses the deposition rate and produces much thinner films displaying more robust ferromagnetism. In addition, the resistivity decreased as the substrate temperature increased and all of the nitrogen-doped films, at different substrate temperatures, had larger gaps than the pure ZnO film.

Structural and magnetic properties of ZnO and Zn 1−xMn xO nanocrystals

Chemical precipitation of metal-ions from aqueous solution has been successfully used to produce Zn 1− x Mn x O nanocrystals, in the form of nano-powder. X-ray diffraction (XRD) measurements reveal that the as-prepared samples are single-phase materials and their lattice constant changes with the variation of Mn-concentration, which indicates the incorporation of Mn 2+ into the hosting ZnO. These findings are corroborated by the observation of the well defined six hyperfine lines of Mn 2+ ion in the electron paramagnetic resonance (EPR) spectra of the samples with a low Mn-concentration, and of a broad EPR line, which manifests the onset of Mn–Mn exchange interaction, in the samples with an elevated value of x.

X-ray study of the structure and magnetic properties of ZnO:(Co, Mn) powders

Mn- and Co-doped ZnO powders were prepared by sol–gel method. The XRD peak position shifts regularly with the magnetic-ions concentration of the samples. Magnetism analyzing indicates that Zn 1− x Mn x O is paramagnetic at room temperature, and at low temperature the exchange interaction between Mn ions is antiferromagnetic. Co-doped ZnO is also paramagnetic at room temperature. Ferromagnetic behavior can be found in Zn 1− x Co x O at low temperature ( T c ≈ 150 K), but the saturation magnetization is only about 0.1 μ B/Co, indicating only a fraction of Co ions participate in ferromagnetism and the rest may contribute to the observed paramagnetic signal or even be antiferromagnetic coupled.

Cross-sectional shape modulation of physical properties in ZnO and Zn1−xCoxO nanowires

We have prepared comparable-diameter ZnO and Zn1- xCoxO nanowires with both circular and hexagonal cross-sections. The average diameters are ∼113 and ∼134 nm for cylindrical and hexagonal nanowires, respectively. The as-grown nanowires have been characterized via structure, electrical conductivity and photoluminescence (PL) spectrum measurements. Pure ZnO nanowires were Co-ion implanted to make magnetic Zn1- xCoxO nanowires for magnetization studies. Bumpier edge surfaces on a nanometre scale, higher densities of stacking faults and a bending feature along the growth direction have been found in cylindrical ZnO nanowires. As compared with hexagonal nanowires, we have observed relatively higher conductivities in cylindrical nanowires, which implied large numbers of shallow donors existing in the latter nanowires. The cylindrical ZnO nanowires also displayed intensified green defect emission and considerably more stacking faults in the crystalline structure. In addition, we have found increased magnetization and stronger ferromagnetic ordering in cylindrical than in hexagonal Zn1-xCoxO nanowires, and have experimentally identified that the point defects of either Zn interstitials or O vacancies played governing roles in ferromagnetism. We conclude that the cross-sectional shape effect originating a varied point defect density can profoundly modulate the structural, electrical, optical as well as magnetic properties of ZnO and Zn1-xCoxO nanowires.