Evidence of defect-mediated magnetic coupling on hydrogenated Co-doped ZnO
Highlights
► High-quality Co-doped ZnO samples were prepared by solid state reaction method. ► Structural defects were introduced by a post-annealing (Ar-95% + H2-5%, 3 h/600 °C). ► After the annealing a room temperature ferromagnetic (RTFM) phase is observed. ► The magnetization saturation presents a linear dependence on Co content. ► RTFM is interpreted in the scope of BMP theory.
Introduction
The theoretical prediction of room temperature ferromagnetism (RTFM) in transition metal (TM) doped large band gap semiconductors [1] promoted the research on such kind of systems as one of the most active and attractive topic in materials science and condensed-matter physics. The manipulation of both charge and spin of carriers in semiconductors turns the development of spintronic devices with all its new functionalities attainable. In this direction, a huge effort has been concentrated on TM-doped oxide semiconductors as ZnO, TiO2 and SnO2 [2], [3], [4]. In spite of the extensive investigations, the origin of the observed RTFM remains inconclusive and controversial. Early works on such systems attributed the observed RTFM to a carrier-mediated mechanism [5]. However, there is a growing consensus that defects play an important role to drive the ferromagnetic behavior. In this scenario, the main theoretical models proposed to describe the origin and properties of ferromagnetism suppose that electrons introduced by donor defects into the conduction band [6] or forming bound magnetic polarons (BMP) [7] mediate ferromagnetic couplings between TM ions. Another important model, attempting to explain the observed RTFM in undoped systems [8], assigns the ferromagnetic response to spins of electrons residing on point or extended defects, the called d0 ferromagnetism [9].
In the present work we report a study of the structural and the magnetic properties of hydrogenated Zn1−xCoxO bulk samples with Co molar concentrations up to 15 at.%. Our previous work showed that the presence of substitutional Co on ZnO matrix was not a sufficient condition to achieve RTFM [10]. Besides, recent reports presented direct evidences of the correlation between the concentration of oxygen vacancies (VO) and the observed RTFM [11], [12], [13], [14]. As pointed by Kohan et al., oxygen/zinc vacancy is the main defect in the ZnO matrix under zinc/oxygen-rich conditions [15]. Therefore, VO can be introduced into the system by annealing the samples in oxygen-poor atmospheres. This effect can be enhanced using a reduction gas to perform the annealing; in the case of using hydrogen gas, the annealing process is called hydrogenation [16]. The hydrogenation of our paramagnetic set of samples [10] added a ferromagnetic phase, confirming reports of robust enhancement of magnetization at room temperature, especially for Co-doped ZnO systems, under this annealing process [17], [18]. The hydrogenation is intended to be responsible to introduce a properly density of defects in the structure of the samples that couples ferromagnetically the magnetic moments of the dopants.
Section snippets
Experiment
Polycrystalline Zn1−xCoxO (x = 0.08, 0.12 and 0.15) bulk samples were prepared by standard solid state reaction method following procedures described in Ref. [10]. These samples were here labeled as-prepared samples and only present a paramagnetic behavior. The as-prepared samples were then annealed in an atmosphere of Ar (95%) and H2 (5%) for 3 h at 600 °C. The effects of hydrogenation on the structural properties were investigated by X-ray diffraction (XRD) recorded in the range of 2θ = 30–70° with
Results and discussion
Fig. 1(a) shows the X-ray diffraction (XRD) results for the whole set of samples with the refined Rietveld pattern. The observed peaks correspond to those expected for polycrystalline wurtzite ZnO. Furthermore, the line-widths of the diffraction peaks are relatively quite narrow, revealing the good crystallinity quality of the samples. No additional phases were observed within the XRD detection limit (Fig. 1(b)). The Rietveld refinement initiated with Zn+2 and O−2 atoms located at (1/3, 2/3, 0)
Conclusion
In summary, in the absence of Co-rich nanocrystals and segregated secondary magnetic phases, the observed magnetic order in our samples is consistent with defect-induced ferromagnetism related to the hydrogenation process, consistent to the BMP theory developed to TM-doped oxides. The linear increasing of saturation magnetic moment as a function of Co concentration for same annealing process for all samples leads to conclusion that both conditions are necessary to achieve RTFM: the substitution
Acknowledgements
Support from agencies FAPEMIG, CNPq and FAPESP are gratefully acknowledged. The authors also acknowledge LNLS for the XAS measurements and the Electron Microscopy Laboratory (LME) of the Brazilian Nanotechnology National Laboratory (LNNano) for the HRTEM microscopy facilities.
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