Ferromagnetism In ZnO Research Articles

Room temperature ferromagnetism in ZnO prepared by microemulsion

Clear room temperature ferromagnetism has been observed in ZnO powders prepared by microemulsion. The O vacancy (VO) clusters mediated by the VO with one electron (F center) contributed to the ferromagnetism, while the isolated F centers contributed to the low temperature paramagnetism. Annealing in H2 incorporated interstitial H (Hi) in ZnO, and removed the isolated F centers, leading to the suppression of the paramagnetism. The ferromagnetism has been considered to originate from the VO clusters mediated by the Hi, leading to the enhancement of the coercivity. The ferromagnetism disappeared after annealing in air due to the reduction of Hi.

FIRST-PRINCIPLES INVESTIGATION INTO FERROMAGNETISM IN C-DOPED ZINC OXIDE CLUSTERS (ZnO)n; N = 1 – 12

We report the first-principle calculations of ferromagnetism in C -doped ZnO clusters. The carbon impurities in ZnO clusters are doped at substitutional O or Zn sites and at interstitial sites and the total energy calculations suggest C at O site is more stable than that at Zn site. The substitutional C impurity is found more favorable than interstitial C impurity in these clusters. The ZnC region is mainly responsible for the observed ferromagnetism in ZnO:C systems. The average magnetic moment of Zn n O n–m C m clusters is found to be 2 μB/ C for n, m < 7. For n, m > 6 the magnetic moment decreases below 2 μB/ C . The magnetic moment in ( ZnO )n C i; i = 1 – 2 is found to be 0.1–2.0 μB/ C . The combination of substitutional and interstitial C impurities in ZnO clusters leads to magnetic moment of 0.4–1.0 μB/ C .

Role of cobalt in ZnO : Co thin films

We prepared a series of Co-doped thin films using the pulsed laser deposition method at deposition temperatures from 400 to 800 °C under oxygen pressures from 10−4 to 15 Pa. We found that the ferromagnetic signals in ZnO : Co thin films only appear simultaneously with the detectable Co 2p spectra from incompletely oxidized Co metal. Therefore, we concluded that the ferromagnetism in ZnO : Co thin films is not an inherent magnetism in the material. On the other hand, we proposed that the inhomogeneous distribution of Co in the films could improve the link of the grains and increase the carrier concentration of the films, leading to positive Hall signals although the film is n-type. Therefore, the coexistence of ferromagnetism and p-type conductivity in ZnO : Co thin films need to be re-examined carefully.

Vacancy-induced intrinsic d ferromagnetism and photoluminescence in potassium doped ZnO nanowires

Cation vacancy-induced d0 room temperature ferromagnetism was observed in nonmagnetic potassium (K) doped ZnO nanowires (NWs) synthesized within the pores of the anodic aluminum oxide template. The ferromagnetic signature was found to be significantly enhanced in the K-doped ZnO NWs with respect to the pristine ZnO NWs. The photoluminescence studies clearly indicated the presence of a large concentration of zinc vacancies in the K-doped ZnO NWs. An interesting correlation between the saturation magnetization and green luminescence intensity with the increase of K-doping has suggested that the magnetic moment originates due to Zn vacancy defects. It is expected that the incorporation of K-related defects at the Zn site might promote the formation of zinc vacancies in the system and introduce holes to stabilize the hole-mediated room-temperature ferromagnetism. For the doped ZnO NWs the ferromagnetic response was found to be a maximum at an optimum K-concentration of 4 at. %. This study demonstrates that the ferromagnetism in ZnO can be tuned by controlling the cation vacancy-defects with the proper dopant in the host semiconductors.

Effects of cobalt doping on the electrical properties of MBE-grown ZnO

We investigated n-conducting Co-doped ZnO epilayers grown by MBE with a cobalt content of nominal 4, 12, and 18 at.% by means of deep level transient spectroscopy (DLTS) and resistivity measurements. We found that in materials grown at the same substrate temperature, 500 °C, the resistivity decreases with the cobalt mole fraction. The free electron concentration accounts for most of the change in the resistivity. The DLTS measurements reveal the presence of a defect center located at Ec−0.5 eV, which is (1) metastable; (2) sensitive to annealing in oxygen-free ambient, and (3) decreasing in density in the samples with higher cobalt content. We therefore assign the center to the isolated oxygen vacancy. Furthermore, our results support the hypothesis that ferromagnetism in ZnO:Co is mediated by Co-oxygen vacancy pairs.

Intrinsic ambient ferromagnetism in ZnO:Co induced by Eu codoping

We manipulate the interaction of Co’s 3d and Eu’s 4f electrons to design and fabricate ZnO:Co+Eu, which possesses intrinsic ferromagnetism at ambient temperature. The results show that the Eu ions tend to neighboring Co ions in order to eliminate the lattice distortion caused by the larger Eu ions via strain coupling. It was also revealed that the preference of parallel spin alignment between Eu and Co ions results in ferromagnetism. The theoretical analyses and our experimental results evidenced that the induced ferromagnetism in the Eu and Co codoped ZnO is intrinsic at ambient temperature.

Influences of unintentionally doped carbon on magnetic properties in Mn–N co-doped ZnO

Carbon impurities are usually unintentionally doped in N-doped ZnO when attempting to realize p-type conductivity by metal-organic chemical vapor deposition. Mn–N co-doping technique, which is developed to realize hole-mediated room temperature ferromagnetism in ZnO, may further enhance the carbon incorporation. In this work, two kinds of Mn–N co-doped samples, grown at low temperature (400 °C) and high temperature (600 °C), respectively, have been compared to study the influences of carbon impurities. In contrast to that found on N mono-doped ZnO, an enhanced incorporation of carbon impurities is observed in the high-temperature-grown Mn–N co-doped sample with the conductivity changed from p to n type. According to X-ray photoelectron spectroscopy measurement, the compensation effect from carbon impurities is applied to elucidate the origin of the conductivity transition. Correspondingly, superconducting quantum interference device measurement certainly shows a much smaller value of the saturation magnetization for the high-temperature-grown sample. A possible effect from carbon related complexes on magnetization is also proposed to explain the decrease of the magnetic moment from the view of weakened spin polarization induced by unintentionally incorporated carbon. This assumption is further supported by the first-principle calculation on the Mn–N co-doped ZnO system with carbon incorporation.

Fabrication of Luminescent, Magnetic Hollow Core Nanospheres and Nanotubes of Cr-Doped ZnO by Inclusive Coprecipitation Method

Incorporation of Cr into ZnO lattice as a dopant in hollow core nanostructures has been achieved by coprecipitation in highly alkaline medium near room temperature. Energy dispersive spectroscopy and X-ray diffraction studies confirm the presence of Cr in the ZnO lattice and X-ray photoelectron spectroscopy (XPS) identified the dopant state to be Cr3+. Room temperature ferromagnetism in ZnO:Cr3+ along with structural investigations suggest an intrinsic nature of ferromagnetism in hollow core nanospheres. Unpaired spin in Cr3+ in zinc substitutional sites could create spin ordering and long-range ferromagnetic coupling. Higher Cr3+ concentration leads to the formation of hollow parallelepipeds and superparamagnetism. Photoluminescence studies reveal a sharp blue emission explicitly arising due to Cr doping. Such blue emitting magnetic nanocapsules can have immense potential as biocarriers and also in spintronics.

Percolation of ferromagnetism in ZnO codoped with Fe and Mg

Magnetic properties of ZnxFe1−xO nanopowder and Zn1−x−yFexMgyO ceramic samples are investigated at temperatures between T=(5–400) K, showing in the latter compound ferromagnetic (FM) interactions up to T∼330 K. When the Fe and Mg concentrations are increased, the amplitude of the Fe3+ electron spin resonance lines decrease while the FM resonance lines grow. The Curie–Weiss type temperature dependence of the magnetization M(T)∝1/T observed for Zn0.89Fe0.01Mg0.10O suggests that in this sample the magnetic interactions are caused by single polarons and small clusters. Strong growth of M(T) and room temperature ferromagnetism are observed in Zn0.70Fe0.04Mg0.26O where the magnetic susceptibility follows the scaling law [χ−1(T)−χ−1(Tc)]∼(T/TC−1)γ with exponents γ1=1.43 typical of three-dimensional (3D) Heisenberg spin systems above the critical temperature Tcr=330 K, and γ2=1.75 characteristic of 3D cluster percolation below Tcr. The estimated critical cluster radius is 40 nm. The M(B) curves are anhysteretic and no time dependence of the thermoremanent magnetization is observed.

Study of hydrogenation versus de-loading of Co and Mn doped ZnO semiconductor

The effect of substitution of Zn by transition metal atoms Co and Mn (5%) in the host diamagnetic ZnO matrix was investigated using X-ray diffraction, magnetization and photoelectron spectroscopy. The dopant atoms incorporate at the Zn site in the hexagonal wurtzite structure without forming any impurity phases or cluster formation. The Mn atoms induce weak room temperature ferromagnetism in ZnO while the Co doping could drive it to only a paramagnetic status. However, the injection of H ions causes a giant ferromagnetism in Co-doped ZnO, on the contrary, the Mn doped ZnO does not respond to hydrogenation. Interestingly, the re-heating causes the H-induced magnetism to vanish in Co-doped ZnO, however, the Mn doped ZnO shows only a marginal sensitiveness towards re-heating. The switching action between ferromagnetic and paramagnetic states in ZnO:Co, by hydrogenation and heating, respectively, is a significant finding of this work. The experimental findings strongly suggest that the ferromagnetism in doped bulk ZnO might be associated with the O vacancies.

Ferromagnetism in ZnO- and GaN-Based Diluted Magnetic Semiconductors: Achievements and Challenges

Both GaN- and ZnO-based diluted semiconductors (DMSs) have recently attracted considerable interest fueled by theoretical predictions of ferromagnetic ordering in these materials above room temperature, making them especially attractive for spintronics. The intense experimental research that followed has revealed, however, a great deal of controversy. The local structure and magnetic behavior of GaN- and ZnO-based DMSs were found to depend strongly on a preparation technique and growth conditions for the materials of the same nominal composition and the reported results varied considerably from group to group. This problem highlighted clearly the lack of theoretical understanding of physical mechanisms underlying ferromagnetisms in these materials and the inadequacy of standard characterization techniques used to probe structural and magnetic properties of the DMSs. In this paper, we report on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs and make special impact on critical discussion of experimental methods employed for investigation of their magnetic and optical properties.

Room temperature ferromagnetism in ZnO (core)/graphite (shell) nanowires fabricated by a one-step method

ZnO (core)/graphitic (shell) nanowires were successfully fabricated by a one-step method. Morphology of the as-grown nanowires was studied in detail by scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive X-ray analysis (EDS). High resolution TEM micrographs and selected area electron diffraction patterns reveal the core/shell morphology of the nanowires that grew along the c -axis of ZnO. EDS study of the nanowires confirms that there are no impurities within the detectable limit. Superconducting quantum interference device magnetometer measurements show room temperature ferromagnetic ordering in these core/shell nanowires.

Magnetism of wurtzite CoO nanoclusters

The possibility that the apparent room-temperature ferromagnetism, often measured in Co-doped ZnO, is due to uncompensated spins at the surface of wurtzite CoO nanoclusters is investigated by means of a combination of density-functional theory and Monte Carlo simulations. We find that the critical temperature extracted from the specific heat systematically drops as the cluster size is reduced, regardless of the particular cluster shape. Furthermore the presence of defects, in the form of missing magnetic sites, further reduces ${T}_{\text{C}}$. This suggests that even a spinodal decomposed phase is unlikely to sustain room-temperature ferromagnetism in ZnO:Co.

N codoping induced ferromagnetism in ZnO:Co (101¯) thin films

By first-principles density functional calculations with generalized gradient approximation ZnO:Co (101¯0) thin films have been demonstrated not to exhibit a ferromagnetic behavior in the absence of any additional carrier doping. Virtual crystal approximation is employed to introduce controlled hole concentrations by N codoping in dilute concentrations of 1–5 at. %. The results show that N codoping can change the ground state from antiferromagnetic to ferromagnetic while maximum ferromagnetic state stability occurs for a N concentration of 2 at. % which is practically achievable. Density of states analyses confirm that the substitutional N in the ZnO:Co thin film provides shallow acceptor states. N codoping is suggested to be a promising technique to realize p-type dilute magnetic semiconducting thin films in the dominantly paramagnetic ZnO:Co (101¯0) system.

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.

First-principles calculations of enhanced ferromagnetism in ZnO codoped with cobalt and nitrogen

Using first-principles calculations based on density functional theory, N-codoped ZnO:Co has been demonstrated to be potentially a p-type diluted magnetic semiconductor. By investigating 13 geometrically distinct configurations, Co and N dopants are found to have a tendency toward staying close to each other with most stable –O–Co–N–Co–O– complexes. The dominant ferromagnetic interaction is due to the hybridization between N 2p and Co 3d states, which is strong enough to lead to hole-mediated ferromagnetism at room temperature. The ferromagnetic coupling strongly relies on the distance of N from Co, while it weakly depends on the direction of aligned Co ions.

Mechanism of room temperature ferromagnetism in ZnO doped with Al

ZnO recently receives extensive interest owing to its potential applications in the dilute magnetic semiconductor. In this work, Al was deposited onto the surface of ZnO film followed by high vacuum annealing. The film showed the room temperature ferromagnetism (RTF). The saturation magnetization (Ms) highly depends on both the thickness of the Al top layer and the thickness of the ZnO film. The RTF disappeared when the film was further annealed in air atmosphere. The detailed structure characterizations (x-ray diffraction and x-ray photoelectron spectroscopy) revealed that the RTF was associated with a charge transfer between Al and Zn.

Co and Al co-doping for ferromagnetism in ZnO:Co diluted magnetic semiconductors

Co and Al co-doped ZnO diluted magnetic semiconductors are fabricated by a pulsed laser depositionand their electronic structure is investigated using x-ray absorption and emission spectroscopy. TheZn0.895Co0.100Al0.005O thin films grown under oxygen-rich conditions exhibit ferromagnetic behavior without anyindication of Co clustering. The Co L-edge and O K-edge x-ray absorption and emissionspectra suggest that most of the Co dopants occupy the substitutional sites and the oxygenvacancies are not responsible for free charge carriers. The spectroscopic results and firstprinciples calculations reveal that the ferromagnetism in Co and Al co-doped ZnOsemiconductors mainly arises from Al interstitial defects and their hybridization with Cosubstitutional dopants.

Ferromagnetism in Ni-doped ZnO films: Extrinsic or intrinsic?

Here we report a detailed study aimed on understanding the origin of ferromagnetism in Ni-doped ZnO films. A pulsed laser deposition technique was used to deposit Ni-doped (5 at. %) ZnO films on sapphire (0001) substrates under different oxygen pressures ranging from 10−6 to 0.1 Torr. Films were characterized using numerous characterization techniques including x-ray diffraction, x-ray photospectroscopy, energy dispersive x-ray spectroscopy, optical absorption spectroscopy, and electrical transport, magnetotransport, and magnetization measurements. A detailed structure-property correlation and analysis of our results revealed that the ferromagnetism in ZnO:Ni films is not an inherent property of the material but results due to a strong tendency of Ni to precipitate out in the ZnO matrix.

Activation of room-temperature ferromagnetism in Mn doped ZnO thin films by N codoping

Mn doped ZnO films with and without N codoping have been fabricated by oxidative annealing of sputtered Zn:Mn and Zn2N3:Mn films on silicon substrates in flowing O2 ambient. It was found that the ZnO:Mn films show very weak ferromagnetic behavior, while for those with N codoping, significant ferromagnetism with a moderate saturation magnetization of about 0.23—0.61 μB per Mn2+ ion was observed at room temperature. It suggests that significant ferromagnetism in ZnO:Mn films could be activated by N codoping. The results indicate that holes are favorable for ferromagnetic ordering of the doped Mn2+ ions in ZnO, in agreement with the recent theoretical studies.