Vacancy-Mediated Magnetism in Pure Copper Oxide Nanoparticles
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Room temperature ferromagnetism (RTF) is observed in pure copper oxide (CuO) nanoparticles which were prepared by precipitation method with the post-annealing in air without any ferromagnetic dopant. X-ray photoelectron spectroscopy (XPS) result indicates that the mixture valence states of Cu1+ and Cu2+ ions exist at the surface of the particles. Vacuum annealing enhances the ferromagnetism (FM) of CuO nanoparticles, while oxygen atmosphere annealing reduces it. The origin of FM is suggested to the oxygen vacancies at the surface/or interface of the particles. Such a ferromagnet without the presence of any transition metal could be a very good option for a class of spintronics.
KeywordsCuO Nanoparticles X-ray photoelectron spectroscopy Room temperature ferromagnetism
Recently, integration of semiconductor with ferromagnetic function has been focused on in spintronics because of the difficulties associated with the injection of spins from magnetic metal into nonmagnetic semiconductors in conventional spintronic devices. Many groups have found RTF in transition or rare earth metal–doped compound semiconductors such as TiO2, ZnO , SnO2, and In2O3. When groups of people try to explain the FM in transition metal–doped semiconducting oxides, an unexpected FM was reported in pure HfO2 thin film , which challenges the understanding of magnetism for the researchers. Hong et al. suggested that oxygen vacancies were key factors in introducing FM to HfO2, while Pemmaraju and Sanvito  suggested that the origin of RTF in HfO2 was due to defects on Hf sites. Hu et al.  presented that RTF observed in pure MgO results from cation vacancies. Elfimov et al.  suggested that a small concentration of Ca vacancies in CaO can also induce FM based on results of the ab initio electronic structure calculation, which could be a path to new ferromagnets. Recently, similar FM has been reported in other pure semiconductor materials, such as TiO2, ZnO, SnO2, In2O3, Al2O3, and CeO2, where the origin of FM is believed to be oxygen defects [10, 11].
CuO, as a narrow band gap p-type semiconductor, has been recognized as an industrially important material for a variety of practical applications, such as catalysis, batteries, solar energy conversion, gas sensing, and field emission [12, 13, 14]. Therefore, the synthesis and study of CuO nanostructures should be of practical and fundamental importance. Apart from this, if one can find FM without any magnetic impurity doping, this may bring a new opportunity to the field of spintronics because there will be no issues due to clustering or precipitation of dopants. Indeed, Punnoose et al. and Mishra et al. [15, 16] represented the presence of an exchange interaction between the ferromagnetic surface and the antiferromagnetic core. Recently, Xiao et al. and Shang et al. [17, 18] reported the observation of RTF in CuO nanostructures. However, the FM in CuO remains controversial because most groups suggested that Cu atoms have no clustering tendency and Cu-based oxides are not ferromagnetic [19, 20]. Here, we synthesize CuO nanoparticles by a simple co-precipitation method to avoid the influences of substrate and the interface between film and substrate . We found the CuO nanoparticles show RTF, and the origin of the FM is discussed.
CuO nanoparticles were prepared by the precipitation technique with the post-oxidation annealing in air. Briefly, 3 g highly pure Cu (NO3)26H2O was dissolved in 50 ml de-ionized water, and the NH4 OH solution was added into it gradually until the pH level reached 10. The mixture was stirred for 4 h at room temperature and then dried at 50°C for 6 h. In the end, the precursor was annealed at 800°C for 1 h in air. The morphologies of the nanoparticles were obtained by transmission electron microscopy (TEM, JEM-2010). X-ray diffraction (XRD, X’ Pert PRO PHILIPS with Cu Kα radiation) was employed to study the structure of the particles. The doping levels and the bonding characteristics were determined by X-ray Photoelectron Spectroscopy (XPS, VG ESCALAB 210). The compositions of the particles were analyzed by inductively coupled plasma atomic emission spectrometer (IRIS, ER/S). The measurements of magnetic properties were made using Quantum Design MPMS magnetometer based on superconducting quantum interference device (SQUID) and the vibrating sample magnetometer (VSM, Lakeshore 7304).
Results and Discussions
Several publications show contamination is a possible source of the FM in HfO2, TiO2, SnO2, etc. [22, 23]. In explaining the origin of FM in the CuO nanoparticles, a careful consideration whether the contamination is responsible for the FM has to be undertaken. All the processes of experiments were carried out very carefully. And the capsules used to hold the samples during the magnetic measurements were also checked carefully and showed no ferromagnetic signals. The result of inductively coupled plasma (ICP) indicates that elements such as Fe (Co) exists in our sample are about 2 × 10−6 at.%, which may come from the precursor material of Cu(NO3)2º6H2O. However, magnetic measurement indicates that the precursor particles are PM, and it did not influence the magnetic properties of the CuO nanoparticles in agreement with other reports . Therefore, we suggest that the observed FM is intrinsic in all samples. So we must reconsider the possibility origin of FM which was previously assumed for the other ferromagnetic undoped oxides: FM due to oxygen vacancies and/or defects [5, 6, 7, 8, 9, 10, 11].
In summary, CuO nanoparticles were prepared by co-precipitation method with the post-annealing in air. The clear hysteresis loop is observed at room temperature in CuO nanoparticles, and the extrinsic impurity origin is excluded. The results of repeating annealing CuO nanoparticles in different conditions indicate that oxygen vacancies at the surface of the particles are likely to be responsible for the FM. Further theoretic investigations into the defects introducing FM are expected and our work is on the way.
This work is supported by NSFC (Grant No.50671046 and No.50801033), National Science Fund for Distinguished Young Scholars (Grant No. 50925103) and the Fundamental Research Funds for the Central Universities (Grant No. Lzujbky-2009-162).
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