Hydrogen-mediated spin-spin interaction in ZnCoO

A study of the density of states of ZnCoO:H from resistivity measurements

Understanding the electronic band structure and density of states (DOS) of a material and their relationship to the associated electronic transport properties is the starting point for optimizing the performance of a device and its technological applications. In a hydrogenated Zn_0.8Co_0.2O (ZnCoO:H) film with an inverted thin-film transistor structure, we found ambipolar behavior, which is shown in many field-effect devices based on graphene, graphene nanoribbons, and organic semiconductors. In this study, to obtain information on the DOS of ZnCoO:H to explain the ambipolar behavior in terms of the carrier density and type, resistivity and magnetoresistance measurements of a ZnCoO:H film were performed at 5 K. Our proposed DOS representation of ZnCoO:H explains qualitatively the experimental observations of carrier density modulation and ambipolar behavior. First-principles calculations of the DOS of ZnCoO:H were in good agreement with the proposed DOS representation. Through a comparison of first-principles calculations and experimental data, evidence for the existence of Co–H–Co in ZnCoO:H is suggested.

Ambipolar behavior in a hydrogenated Zn_0.8Co_0.2O (ZnCoO:H) film is investigated via resistivity and magnetoresistance measurements and first-principles calculations of the DOS. Evidence for the existence of Co–H–Co in ZnCoO:H is suggested.

Bound magnetic polaron in Zn-rich cobalt-doped ZnSe nanowires

The micro-luminescence spectra of the diluted magnetic semiconductor (DMS) can reflect the spin-exciton interaction and related relaxation process. Here the micro-photoluminescence (micro-PL) spectra and PL lifetime measurements have been done on an individual ferromagnetic (FM)-coupled cobalt (Co) doped zinc selenide (ZnSe) nanowire. There occurs a double-peak profile in its near bandedge emission spectrum: the first peak is from free exciton (FX) and the second comes from magnetic polaron (MP). In their temperature dependent PL spectra, the MP emission peak demonstrates obviously temperature-independent behavior, in contrast to the behaviors of FX and reported exciton MP in nanobelt. It is found that in this Co(II) doped ZnSe nanowires, this MP's temperature-independent emission is related to the coupling between exciton and a FM nanocluster (↑↑↓). The nanocluster is likely due to the interaction of Se vacancies of the wide bandgap semiconductors with the antiferromagnetic (AFM) arrangement transition metal (TM) ions in these Se-deficient Co doped ZnSe nanowires. These results reflect that the AFM coupling TM ions pair can give rise to FM behavior with the involvement of positive charge defect, also indicating that the micro-luminescence detection can be used to study the magnetic coupling in DMS.

Formation of ferromagnetic Co-H-Co complex and spin-polarized conduction band in Co-doped ZnO

Magnetic oxide semiconductors with wide band gaps have promising spintronic applications, especially in the case of magneto-optic devices. Co-doped ZnO (ZnCoO) has been considered for these applications, but the origin of its ferromagnetism has been controversial for several decades and no substantial progress for a practical application has been made to date. In this paper, we present direct evidence of hydrogen-mediated ferromagnetism and spin polarization in the conduction band of ZnCoO. Electron density mapping reveals the formation of Co–H–Co, in agreement with theoretical predictions. Electron spin resonance measurement elucidates the ferromagnetic nature of ZnCoO by the formation of Co–H–Co. We provide evidence from magnetic circular dichroism measurements supporting the hypothesis that Co–H–Co contributes to the spin polarization of the conduction band of hydrogen-doped ZnCoO.

Ferromagnetism and Conductivity in Hydrogen Irradiated Co-Doped ZnO Thin Films

Impressive changes in the transport and ferromagnetic properties of Co-doped ZnO thin films have been obtained by postgrowth hydrogen irradiation at temperatures of 400 °C. Hydrogen incorporation increases the saturation magnetization by one order of magnitude (up to ∼1.50 μB/Co) and increases the carrier density and mobility by about a factor of two. In addition to the magnetic characterization, the transport and structural properties of hydrogenated ZnO:Co have been investigated by Hall effect, local probe conductivity measurements, micro-Raman, and X-ray absorption spectroscopy. Particular care has been given to the detection of Co oxides and metal Co nanophases, whose influence on the increase in the transport and ferromagnetic properties can be excluded on the ground of the achieved results. The enhancement in ferromagnetism is directly related to the dose of H introduced in the samples. On the contrary, despite the shallow donor character of H atoms, the increase in carrier density n is not related to the H dose. These apparently contradictory effects of H are fully accounted for by a mechanism based on a theoretical model involving Co-VO (Co-O vacancy) pairs.

Gate voltage-dependent magnetoresistance of Zn0.8Co0.2O:H

The magnetoresistance (MR) of ZnCoO:H was measured at 7 K to verify the MR dependency on carrier density. It was found that MR increased with negative gate voltage. This increase in MR is not caused by an increase in pMR, but by a decrease in nMR.

Control of magneto-transport characteristics of Co-doped ZnO by electron beam irradiation

Electron beam irradiation can be used to remove shallow donor type hydrogen located in Zn(Co)–O bonding centers in Co-doped ZnO, which enables to modify the conduction band and the magneto-transport characteristics of Co-doped ZnO.

Magnetic domains in H-mediated Zn0.9Co0.1O microdisk arrays

We have fabricated and studied magnetic domains in the periodic ZnCoO microdisk structures at room temperature with MFM technique. The z-component of the remanent magnetic moment is uniform even though the value is much smaller than the saturation magnetic moment.

Room temperature ferromagnetic Zn0.98Co0.02O powders with improved visible-light photocatalysis

The realization of ferromagnetic diluted magnetic semiconductors requires the fine tuning of the energy levels of the band structure, which might have important applications in photocatalysis.

Study on the formation of magnetic nanoclusters and change in spin ordering in Co-doped ZnO using magnetic susceptibility

The temperature-dependent magnetic susceptibility measurement could provide a useful methodological approach as well as experimental clues for identifying the origin of magnetism in magnetic semiconductor.

p-type conductivity generated by ferromagnetic ordering via percolative anionic H chain formation in ZnCoO

We report on the simultaneous realization of p-type conductivity and strong ferromagnetism in heavily Co-doped ZnO thin films in the presence of a high concentration of hydrogen impurities. Through ab initio calculations, we find that the microscopic origin of hole carrier generations and ferromagnetic ordering is due to the partially occupied band of the percolative structures wherein the carrier-induced magnetic interactions can stabilize the strong spin-parallel state.

Fabrication of ZnCoO nanowires and characterization of their magnetic properties

Hydrogen-treated ZnCoO shows magnetic behavior, which is related to the formation of Co-H-Co complexes. However, it is not well known how the complexes are connected to each other and with what directional behavior they are ordered. In this point of view, ZnCoO nanowire is an ideal system for the study of the magnetic anisotropy. ZnCoO nanowire was fabricated by trioctylamine solution method under different ambient gases. We found that the oxidation of trioctylamine plays an essential role on the synthesis of high-quality ZnCoO nanowires. The hydrogen injection to ZnCoO nanowires induced ferromagnetism with larger magnetization than ZnCoO powders, while becoming paramagnetic after vacuum heat treatment. Strong ferromagnetism of nanowires can be explained by the percolation of Co-H-Co complexes along the c-axis.

Bistability of hydrogen in ZnO: origin of doping limit and persistent photoconductivity

Substitutional hydrogen at oxygen site (HO) is well-known to be a robust source of n-type conductivity in ZnO, but a puzzling aspect is that the doping limit by hydrogen is only about 10(18) cm(-3), even if solubility limit is much higher. Another puzzling aspect of ZnO is persistent photoconductivity, which prevents the wide applications of the ZnO-based thin film transistor. Up to now, there is no satisfactory theory about two puzzles. We report the bistability of HO in ZnO through first-principles electronic structure calculations. We find that as Fermi level is close to conduction bands, the HO can undergo a large lattice relaxation, through which a deep level can be induced, capturing electrons and the deep state can be transformed into shallow donor state by a photon absorption. We suggest that the bistability can give explanations to two puzzling aspects.

Effects of substitutional Li on the ferromagnetic response of Li co-doped ZnO:Co nanoparticles

Li co-doped ZnO:Co (Zn0.96-yCo0.04LiyO , y ≤ 0.1) nanoparticles were synthesized by the sol-gel technique and the correlation between the structural, electronic and magnetic properties was investigated. All the samples show a single phase hexagonal (wurtzite) ZnO structure and no secondary phases were detected. Variational trends in lattice parameters suggest the incorporation of Li in the ZnO:Co system in both substitutional and interstitial sites. Detailed electronic studies have been performed by high-resolution x-ray photoelectron spectroscopy (XPS) to determine the states of Zn, O, Co and Li. It was determined that Co substitutes at Zn sites (CoZn) while the O vacancy and Zn defects did not show much variation with increasing Li concentration. Deconvolution of the Li XPS peak showed a clear non-monotonic trend in the variation of the substitutional Li (LiZn) and interstitial Li (Lii) defects with increasing Li concentration in the particles. The magnetization study of the samples showed that the variation of the moment closely followed the trend of variation of the LiZn defects. The data are interpreted in terms of substitutional Li acting as a hole dopant and optimizing the conditions for ferromagnetism in Co-doped ZnO. Interstitial Li is not seen to be playing this role.

Ferromagnetism in ZnO:Co originating from a hydrogenated Co-O-Co complex

The effects of hydrogen interstitials and oxygen vacancies on the overall ferromagnetic behaviour of Co doped ZnO (ZnO:Co) have been closely examined using different density functional calculations. The results demonstrate the importance of correcting the bandgap problem of the ZnO host as well as the lack of correlation in Co's 3d states which can severely affect the coupling of H and Co's impurity bands. Our results show that in hydrogenated ZnO:Co, hydrogen interstitial can also stabilize the ferromagnetic interaction at low Co concentrations, but this requires the formation of the in-plane O-H-Co-O-Co complex. In this structure, the hydrogen interstitial forms an anionic complex with the neighbouring oxygen, which polarizes the surrounding oxygen to mediate the ferromagnetism through the superexchange mechanism. An oxygen vacancy by itself would not cause ferromagnetism in ZnO:Co. On the other hand, in the presence of hydrogen interstitials, oxygen vacancies can significantly enhance the magnetic coupling between H and Co-O-Co as a shallow donor if it is far away from the in-plane O-H-Co-O-Co complex. However, the total energy results show that this is much more favoured when the oxygen vacancy is near the in-plane O-H-Co-O-Co complex, which can inhibit the ferromagnetic interaction between Co ions.

Intrinsic impurity-band Stoner ferromagnetism in C60Hn

We identify Stoner ferromagnetism in fcc C60H(n) (n=odd) by using a local density approximation in the framework of the density functional theory. Hydrogen chemisorption on fullerenes creates quasilocalized π electrons on the fullerene surface, overlapping of their wave functions giving rise to a narrow half filled impurity band in the fcc C60H(n). The Stoner-type ferromagnetic exchange between the itinerant electrons leads to spin-split impurity bands. The magnetic moment per C60H(n) molecule is 1  μ(B) (for n=odd) or 0 (for n=even, including zero), only one of the hydrogens contributing to the spin-split states. Direct overlapping of the quasilocalized π-electron orbitals is essential for the ferromagnetism.

Ferromagnetic interactions in hosted bipartite materials--generalized-double-exchange and generalized-superexchange interactions

Defect-induced magnetism in dilute magnetic semiconductors challenges our understanding of magnetism in solids. Theories based on conventional superexchange or double-exchange interactions cannot explain long range magnetic order at concentrations below the percolation threshold in these materials. On the other hand, the codoping-induced magnetism, which can explain magnetic interactions below the percolation threshold, has eluded explanation. In this work we propose that defect-induced magnetism in codoped non-magnetic materials can be viewed within a molecular generalization of the atomic double-exchange and superexchange interactions applied to an arbitrary bipartite lattice hosted by (or embedded in) defect-free non-magnetic materials. In this view, the crucial factor for the development of magnetism appears to be the defect complementarity of the codopants. We demonstrate this by taking ZnO and GaN (the most widely studied doped oxide and nitride magnetic semiconductors, respectively) as host materials and perform theoretical calculations using ab initio methods after codoping them with transition metal impurities for a variety of configurations. Our results indicate that the magnetic coupling among the induced and/or doped magnetic moments takes the form of an interaction among spin-polarized molecular units which is facilitated by the formation of the hosted bipartite codopant structures. The universality of the proposed mechanism is further supported by earlier results referring to the rhombohedral C(60)-based polymers.

A ten-year perspective on dilute magnetic semiconductors and oxides

Over the past ten years, the search for compounds combining the properties of semiconductors and ferromagnets has evolved into an important field of materials science. This endeavour has been fuelled by many demonstrations of remarkable low-temperature functionalities in the ferromagnetic structures (Ga,Mn)As and p-(Cd,Mn)Te, and related compounds, and by the theoretical prediction that magnetically doped, p-type nitride and oxide semiconductors might support ferromagnetism mediated by valence-band holes to above room temperature. Indeed, ferromagnetic signatures persisting at high temperatures have been detected in a number of non-metallic systems, even under conditions in which the presence of spin ordering was not originally anticipated. Here I review recent experimental and theoretical developments, emphasizing that they not only disentangle many controversies and puzzles accumulated over the past decade but also offer new research prospects.

Predominant role of defects in magnetic interactions in codoped ZnO:Co

The roles of codoping ions (Li, Ga and Cu) and defects (oxygen vacancy and hydrogen impurity) in magnetic interactions in ZnO:Co systems have been studied systematically using an ab initio method with density functional theory and the standard molecular field model. The results show that where defects are not included in ZnO's lattice carrier mediated magnetism is only achievable in shallow p-type codoping, such as ZnO:Co + Cu. However, in deep p-type codoping (ZnO:Co + Li) and deep n-type codoping (ZnO:Co + Ga), the carriers generally do not induce spontaneous magnetism. It was also found that the oxygen vacancy, due to its deep donor nature, has a minor favoring effect on ferromagnetic ordering among Co ions. The observed ferromagnetism in such systems can be attributed to the interaction of Co ions with unintentional hydrogen contamination rather than codopants or oxygen vacancies.

Hydrogen multicenter bond mediated magnetism in Co doped ZnO

Magnetism in Co doped ZnO (ZnO:Co) is strongly affected by the presence of the ZnO's extrinsic impurities, such as unintentional hydrogen dopants. Our ab initio investigation reveals that in ZnO:Co the formation of substitutional H (H(O)) with four-fold hydrogenic bonds is favored over interstitial hydrogen (H(I)) by 0.4 eV. It is found that H(O) is trapped by Co ions to form highly stable Co-H(O)-Co complexes. H(O) also mediates a strong short-ranged ferromagnetic interaction between Co dopants via short-range exchange interaction which induces room temperature ferromagnetism.

First-principles study of the diffusion of hydrogen in ZnO

Zinc oxide, a wide-gap semiconductor, typically exhibits n-type conductivity even when nominally undoped. The nature of the donor is contentious, but hydrogen is a prime candidate. We present ab initio calculations of the migration barrier for H, yielding a barrier of less than approximately 0.5 eV. This indicates isolated hydrogen is mobile at low temperature and that thermally stable H-related donors must logically be trapped at other defects. We argue this is also true for other oxides where H is a shallow donor.