Cobalt-doped ZnO Research Articles

Magnetotransport properties of high quality Co:ZnO and Mn:ZnO single crystal pulsed laser deposition films: Pitfalls associated with magnetotransport on high resistivity materials

The electrical resistivity values for a series of pure and doped (Co, Mn, Al) ZnO epitaxial films grown by pulsed laser deposition were measured with equipment designed for determining the direct current resistivity of high resistance samples. Room-temperature resistances ranging from 7 x 10(1) to 4 x 10(8) Omega/sq were measured on vacuum-reduced cobalt-doped ZnO, (Al,Co) co-doped ZnO, pure cobalt-doped ZnO, Mn-doped ZnO, and undoped ZnO. Using a four-point collinear geometry with gold spring-loaded contacts, resistivities were measured from 295 to 5 K for resistances of < approximately 10(12) Omega/sq. In addition, magnetoresistance and Hall effect were measured as a function of temperature for select samples. Throughout the investigation, samples were also measured on commercially available instrumentation with good agreement. The challenges of transport measurements on high resistivity samples are discussed, along with some offered solutions to those challenges.

The role of Zn interstitials in cobalt-doped ZnO diluted magnetic semiconductors

Co-doped ZnO samples Zn1−xCoxO and CoyZn1−yO were prepared by the sol-gel and magnetron sputtering methods, respectively. Although the Co K-edge extended x-ray absorption fine structure spectra show that the doped Co ions are located at the Zn substitutional sites for both Zn1−xCoxO (x≤0.05) and CoyZn1−yO (y≤0.05) samples, magnetic measurements show paramagnetism in Zn1−xCoxO (x≤0.05) and high temperature ferromagnetism in CoyZn1−yO (y≤0.05). An experimental and numerical study of the O K-edge x-ray absorption near-edge structure spectra reveals that the incorporation of the Zn interstitials in the lattice is crucial to the appearance of high temperature ferromagnetism in CoyZn1−yO (y≤0.05) samples.

Low-symmetry T d-distorted Co 2+ centres in ceramic ZnO:Co

The reflectivity and photoluminescence of cobalt-doped ZnO ceramics samples have been studied by optical spectroscopy. The result reveals, in addition to the emission bands associated with intrinsic defects, weak narrow emission peaks which are ascribed to a mixed 4T 1(P), 2T 1(G), 2E(G) → 4A 2(F) transition between Co 2+ d-levels located in regular Zn 2+ sites, [Co 2+] Zn and a broad near infrared emission band centred at 710 nm which has been interpreted as belonging to Co 2+ ions located in a perturbed Zn 2+ site, [ Co 2 + ] Zn ∗ . This study shows that the annealing treatment at 800 °C activates the diffusion of point defects thus creating more [Co 2+] Zn centres.

New examination of the magnetic properties of cobalt-doped ZnO diluted magnetic semiconductors

Polycrystalline Zn1−xCoxO diluted magnetic semiconductors were prepared within a large doping concentration range from 0.5% to 10%. Thanks to a model based on the co-existence of two magnetic populations, our analysis of the magnetization measurements brings new evidence about the competition between ferromagnetic and antiferromagnetic mean field within the ZnO diamagnetic matrix. It is a temperature activated phenomenon that supports the alignment of individual Co2+ ions into superparamagnetic clusters. At the same time, the refined effective number of antiferromagnetically coupled Co2+ ions is equal to the probability of Co2+ ions belonging to a dimer.

Study of the O K-edge XANES of Cobalt-doped ZnO diluted magnetic semiconductors

Cobalt-doped ZnO samples were prepared by both sol-gel and magnetron sputtering methods, respectively. Co atoms have incorporated into the ZnO lattice and located at the substitutional sites of the Zn atoms in all samples, while the sol-gel prepared Zn1-xCoxO samples show paramagnetism and the sputtering prepared CoyZn1-yO samples appear ferromagnetism. The calculated O K-edge X-ray absorption near edge structure (XANES) can commendably reproduce all of the peaks observed in CoyZn1-yO samples when one Zn interstitial atom (Zni) nears the central O atom. Compared with the magnetic properties, it is shown that the Zn interstitial atoms play a key role to the observed ferromagnetism in CoyZn1-yO samples.

Swift heavy ion induced modifications in cobalt doped ZnO thin films: Structural and optical studies

Modifications in the structural and optical properties of 100 MeV Ni 7+ ions irradiated cobalt doped ZnO thin films (Zn 1− x Co x O, x = 0.05) prepared by sol–gel route were studied. The films irradiated with a fluence of 1 × 10 13 ions/cm 2 were single phase and show improved crystalline structure with preferred C-axis orientation as revealed from XRD analysis. Effects of irradiation on bond structure of thin films were studied by FTIR spectroscopy. The spectrum shows no change in bonding structure of Zn–O after irradiation. Improved quality of films is further supported by FTIR studies. Optical properties of the pristine and irradiated samples have been determined by using UV–vis spectroscopic technique. Optical absorption spectra show an appreciable red shift in the band gap of irradiated Zn 1− x Co x O thin film due to sp–d interaction between Co 2+ ions and ZnO band electrons. Transmission spectra show absorption band edges at 1.8 eV, 2.05 eV and 2.18 eV corresponding to d–d transition of Co 2+ ions in tetrahedral field of ZnO. The AFM study shows a slight increase in grain size and surface roughness of the thin films after irradiation.

Wurtzite-Type CoO Nanocrystals in Ultrathin ZnCoO Films

Surface x-ray diffraction experiments reveal that, in cobalt-doped ZnO films two to five monolayers thick, Wurtzite-type CoO nanocrystals are coherently embedded within a hexagonal boron-nitride- (h-BN)-type ZnO matrix, supporting the model of a phase separation. First-principles calculations confirm that, in contrast with ZnO, the formation of h-BN-type CoO is unfavorable in the ultrathin film limit. Our results are important for understanding magnetic properties of transition metal-doped semiconductors in general.

Nanosized superparamagnetic precipitates in cobalt-doped ZnO

The existence of semiconductors exhibiting long-range ferromagnetic ordering at room temperature still is controversial. One particularly important issue is the presence of secondary magnetic phases such as clusters, segregations, etc. These are often tedious to detect, leading to contradictory interpretations. We show that in our cobalt doped ZnO films grown homoepitaxially on single crystalline ZnO substrates the magnetism unambiguously stems from metallic cobalt nano-inclusions. The magnetic behavior was investigated by SQUID magnetometry, X-ray magnetic circular dichroism, and AC susceptibility measurements. The results were correlated to a detailed microstructural analysis based on high resolution X-ray diffraction, transmission electron microscopy, and electron-spectroscopic imaging. No evidence for carrier mediated ferromagnetic exchange between diluted cobalt moments was found. In contrast, the combined data provide clear evidence that the observed room temperature ferromagnetic-like behavior originates from nanometer sized superparamagnetic metallic cobalt precipitates.

Hidden ferromagnetic secondary phases in cobalt-doped ZnO epitaxial thin films

The origin of ferromagnetism is investigated in epitaxial Co:ZnO thin films which become weakly ferromagnetic after annealing in Zn vapor. Conventional characterization techniques indicate no change after treatment. However, x-ray photoelectron spectroscopy depth profiling clearly indicates the presence of Co(0) in the Zn-treated films; x-ray absorption fine structure is utilized to identify the secondary phase as ferromagnetic CoZn. This work demonstrates that the potential for ferromagnetic secondary phases must be thoroughly discounted, through painstaking materials characterization, before claims of intrinsic ferromagnetism can be made.

Lack of ferromagnetism in n-type cobalt-doped ZnO epitaxial thin films

Epitaxial thin films of cobalt-doped ZnO (Co : ZnO) were deposited by pulsed laser deposition (PLD) on both c-plane and r-plane sapphire (Al2O3). The films exhibited high structural quality with narrow x-ray diffraction (XRD) rocking curve peak widths. X-ray absorption spectroscopy (XANES and EXAFS) confirmed well-ordered Co substitution for Zn in ZnO without the formation of secondary phases. A wide range of n-type conductivities (10−4–105 Ω cm) was achieved by controlling the deposition conditions, post-annealing in vacuum, and/or addition of Al during deposition. Despite the high structural quality of the Co : ZnO thin films, no significant room temperature ferromagnetism was observed under any processing or treatment conditions. The lack of ferromagnetism indicates that itinerant conduction band electrons alone are not sufficient to induce ferromagnetism in Co : ZnO, even when the carrier concentration is a significant fraction of the magnetic dopant concentration. The implications of this observation are discussed.

Synthesis, Characterization, and Room-Temperature Ferromagnetism in Cobalt-Doped Zinc Oxide (ZnO:Co2+) Nanocrystals Encapsulated in Carbon

Carbon-encapsulated, cobalt-doped ZnO nanocrystals were synthesized by the one-step RAPET technique with Co:Zn atomic ratios of 0, 0.011, 0.037, 0.082, and 0.183. The substitution of Zn2+ by Co2+ decreased the XRD peak intensities of the Co2+-occupied lattice planes due to the lower atomic scattering factor of Co2+ with respect to Zn2+. The size of the nanocrystals varied with the variation in cobalt concentration. X-ray photoelectron spectroscopy studies revealed the presence of Co2+. The electron transfer was evident from the Zn 4s level to the unfilled higher charge density Co 3d level. In the photoluminescence spectra, band edge exciton transitions and the presence of oxygen vacancies were observed. The observed ferromagnetism was intrinsic in nature and not due to any metallic Co segregation or any cobalt oxide phase formation. The spin-split impurity band (due to hybridization between the charge carriers in Co 3d and Zn 4s) and the defect-bound carriers (from the interstitial Zni vacancies) account for the room-temperature ferromagnetism for lower and higher cobalt concentrations, respectively. The 9−15 nm graphitic carbon shell protects the semiconductor core from oxidation of the Co2+ dopant. The carbon atoms at the proximity of the ferromagnetic core get spin-polarized to contribute a weak magnetic order.

Structure and magnetism of cobalt-doped ZnO thin films

The structure and magnetic properties of Co-doped ZnO films are discussed in relation to cobalt doping levels and growth conditions. Films were deposited by pulsed-laser deposition (PLD) from ZnO targets containing cobalt concentrations from 0 to 30 at.%. The structure of the films is examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM), and optical absorption is used to infer the substitution of cobalt inside the ZnO lattice. Magnetic properties are characterized by superconducting quantum interference device (SQUID) magnetometry. Films doped with cobalt concentrations of a few per cent appear to be composed of two magnetic components: a paramagnetic component and a low-field ferromagnetic component. Films doped with 30% cobalt show a larger FM signature at room temperature with clear hysteretic shape, but films grown at low pressure are plagued by the precipitation of metallic cobalt nanoparticles within the lattice which can be easily detected by XRD. These particles are well oriented with the ZnO crystal structure. By increasing the base pressure of the vacuum chamber to pressures above 1×10−5 Torr, metallic cobalt precipitates are undetectable in XRD scans, whereas the films still show an FM signature of ∼0.08 μB/Co. Depositions in the presence of oxygen background gas at 0.02 mTorr decreases the magnetization. The decreased magnetization with oxygen suggests that the activation of ferromagnetism depends on defects, such as oxygen vacancies, created during growth. Optical absorption measurements show a sequential increase in the Co+2 absorption peaks in these films, along with an almost linearly increasing bandgap with cobalt concentration suggesting a large solubility of cobalt in ZnO. Bright-field TEM imaging and electron diffraction do not show signs of precipitation; however, dark-field imaging shows circular areas of varying contrast which could be associated with cobalt precipitation. Therefore, the possibility that ferromagnetism results from secondary phases cannot be ruled out.

Substrate orientation-induced distinct ferromagnetic moment in Co:ZnO films

Dilute (3.8 at.%) cobalt-doped ZnO thin films are deposited on LiTaO 3 (LT) substrates with three different orientations [LT(1 1 0), LT(0 1 2) and LT(0 1 8)] by direct current reactive magnetron co-sputtering. The experimental results indicate that Co atoms with 2+ chemical valence are successfully incorporated into the ZnO host matrix on various oriented substrates, and the substrate orientations have a profound influence on the crystal growth and magnetization of Co:ZnO films. A large magnetic moment of 2.42 μ B/Co at room temperature is obtained in the film deposited on LT(0 1 2), while the corresponding values of the other films deposited on LT(1 1 0) and LT(0 1 8) are 1.21 μ B/Co and 0.65 μ B/Co, respectively. Furthermore, the crystal growth mode of Co:ZnO films on various oriented LT, the relationship between the microstructures and corresponding ferromagnetic properties are also discussed.

Synthesis of Room-Temperature Ferromagnetic Co-Doped ZnO Nanocrystals under a High Magnetic Field

Transitional-metal cobalt-doped ZnO nanocrystals were synthesized under an external magnetic field. The growth behavior of cobalt-doped ZnO nanocrystals was greatly affected by the high magnetic field. The growth rate of the nanocrystals along (001) direction was significantly suppressed because of the magnetization energy differences in a, b, and c-axis. Robust room-temperature ferromagnetism of Co-doped ZnO nanocrystals can be activated by the external magnetic field. The characterization of the annealed samples combined with the analysis of Raman scattering suggested that the appearance of ferromagnetism cannot be contributed to the presence of oxygen vacancies but to the ferromagnetic coupling of Co2+ under an external magnetic field.

Structures and magnetic properties of wurtzite Zn1−xCoxO dilute magnetic semiconductor nanocomposites

Cobalt-doped ZnO dilute magnetic semiconductor nanocomposites Zn1−xCoxO with Co concentrations from 0.02 to 0.25 were prepared by the sol-gel method. The magnetic measurement shows paramagnetic behavior for all the samples. The structures of these composites were investigated by x-ray diffraction and fluorescence x-ray absorption fine structure spectroscopy. It is indicated that at low Co concentration (x⩽0.05), the Co atoms are incorporated into the ZnO lattice and located at the substitutional sites of the Zn atoms. At higher Co doping concentration (x⩾0.10), the secondary phase Co3O4 is precipitated. Correlating the magnetic behavior with the structural properties of the Zn1−xCoxO nanocomposites, the authors interpret the paramagnetism to be intrinsic in nature as a result of the low effective doping of Co in ZnO and the lack of oxygen vacancies.

Cobalt-Doped ZnO Nanorods Fabricated by a Simple Wet Chemical Route in Alcoholic Solution

The synthesis of ZnO:Co nanorods in 1-butanol using a simple hydrolysis and condensation method under normal atmosphere is shown. The rods have average diameters of about 20nm and length up to 150nm. By doping with cobalt, the one-dimensional growth is retained due to the assistance of ethylene diamine as a ligand. Using energy dispersive X-ray analysis, it is proven that Co cations are incorporated into the ZnO host lattice only when the metal precursors are added slowly to a KOH solution at elevated temperatures. The visible defect-related luminescence of ZnO is suppressed by the dopant ions on the nanoparticle surfaces.

Structure and magnetic properties ofZn1−xCoxO single-crystalline nanorods synthesized by a wet chemical method

A novel approach for the synthesis of cobalt-doped ZnO single-crystalline nanorods based on a wetchemical reaction has been developed. The as-doped ZnO nanorods have a length between 0.3 and0.6 µm and a diameter between 30 and 60 nm. Structure and composition analyses indicate that thecobalt is incorporated into the ZnO lattice, forming a solid solution without any precipitation.Magnetic property measurements reveal that there is room-temperature ferromagnetism in theZn1−xCoxO nanorodswith Tc higher than 300 K.

Magnetic circular dichroism of ferromagnetic Co2+-doped ZnO

Cobalt-doped ZnO (Co2+:ZnO) films were studied by magnetic circular dichroism (MCD) spectroscopy. A broad 300K ferromagnetic MCD signal was observed between 1.4 and 4.0eV after exposure of paramagnetic Co2+:ZnO films to zinc metal vapor, attributed to low-energy photoionization transitions originating from a spin-split donor impurity band in ferromagnetic n-type Co2+:ZnO.

Direct Kinetic Correlation of Carriers and Ferromagnetism inCo2+: ZnO

The hypothesis that high-Curie-temperature ferromagnetism in cobalt-doped ZnO (Co2+: ZnO) is mediated by charge carriers was tested by controlled introduction and removal of the shallow donor interstitial zinc. Using oriented epitaxial Co2+: ZnO films grown by chemical vapor deposition, kinetics measurements demonstrate a direct correlation between the oxidative quenching of ferromagnetism and the diffusion and oxidation of interstitial zinc. These results demonstrate controlled systematic variation of a key parameter involved in the ferromagnetism of Co2+: ZnO and, in the process, unambiguously reveal this ferromagnetism to be dependent upon charge carriers. The distinction between defect-bound and free carriers in Co2+: ZnO is discussed.

Thermal modification of magnetism in cobalt-doped ZnO nanowires grown at low temperatures

The optical and magnetic stability of hydrothermally grown Co-doped ZnO nanowires were investigated. The experimental results suggest that two different kinds of coordination of Co ions in the ZnO host, either with or without magnetic coupling, likely coexist in the as-grown samples. Low-temperature annealing resulted in the transformation of the isolated ions to magnetically coupled ones, leading to the enhancement of ferromagnetic properties. High-temperature annealing $(900\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ leads to paramagnetic behavior and improved crystallinity of the ZnO host materials due to Co migration and the formation of nonmagnetic oxide clusters, as evidenced by photoluminescence, Raman scattering, x-ray diffraction, and electron microscopy.