Ferromagnetic Ordering at Room Temperature in ZnO:Co Nanoparticles
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Ferromagnetic Ordering at Room Temperature in ZnO:Co Nanoparticles Sujeet Chaudhary, Kanwal Preet Bhatti, Shankhmala Kundu, Subhash C. Kashyap, and Dinesh K. Pandya Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, 110016, India
ABSTRACT Intrinsic room temperature ferromagnetism is reported in sequentially sintered (in air ambient) nanocrystalline ZnO:Co (1 to 10 at% Co) samples prepared by chemical route. The Curie temperature of the ZnO:Co ferromagnetic samples is determined to be ~495°C. The saturation magnetization of the ferromagnetic phase is found to first increase with Co concentration (up to 5%) and then decrease. It is also found to decrease with increase in sintering temperature up to 800°C; there after it remains unaffected till 1000°C. The d-d band transitions in the optical spectrum confirm that Co2+ substitutes Zn2+ in the ZnO lattice. A plausible explanation of the observed ferromagnetic ordering is presented in terms of Bound Magnetic Polarons model. INTRODUCTION The progress in the emerging field of spintronics is inherently associated with the search for new semiconductor materials, which should exhibit ferromagnetic behavior at room temperature. Such semiconductors are expected to add new functionalities to the existing electronic and photonic devices as well as open new device possibilities. The challenge is to modify the oxides of Zn, Sn, Ti, etc., so that they exhibit intrinsic and stable room temperature ferromagnetism (RTFM). A fundamental understanding of these new ferromagnetic systems is currently under investigation. The ZnO:Co system is intriguing, since most of the reports on bulk samples do not corroborate RTFM, though invariably evidenced in several reports on the thin films. In case of polycrystalline ZnO:Co bulk samples, prepared by solid state reaction method, most of the groups have observed paramagnetic behavior at RT [1]. RTFM has, however, been reported when the samples were either co-doped with CuO [2] or hydrogenated [3-4]. The observed RTFM in hydrogenated samples was attributed either to the appearance of Co clusters [3] or oxygen vacancies [4]. Nanocrystals of ZnO:Co are reported to be paramagnetic [5,6], ferromagnetic [7,8] or superparamagnetic [9]. In the present work, we report the observation and possible explanation of intrinsic RTFM in chemically synthesized nanocrystalline ZnO:Co samples having cobalt concentration ranging from 1 to 10 at%. Particularly, the effect of cobalt concentration and processing parameters on the magnetization (M), structure and phase-purity of nanocrystalline powder samples is presented in this paper. EXPERIMENT Nanocrystalline powders of ZnO:Co system, with 1-10 at% Co (named as C1, C3, C5, C7 and C10), were synthesized by chemical method using zinc acetate, cobalt acetate and polyvinyl pyrrolidane ((PVP) as precursors. For preparing the samples appropriate quantities of the
constituents were dissolved in DI water and stirred for 1h. The solution was then dried at 90°C, and the dried mass w
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