Room Temperature Ferromagnetism and Band Gap Engineering in Mg Doped ZnO RF/DC Sputtered Films
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Room Temperature Ferromagnetism and Band Gap Engineering in Mg Doped ZnO RF/DC Sputtered Films Sreekanth K. Mahadeva1,2, Zhi-Yong Quan 1,3, Jin-Cheng Fan1,4, Hasan B. Albargi5, Gillian A Gehring5 , Anastasia V. Riazanova1, Lyubov M. Belova1 and K. V. Rao1 1. Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, SE 100 44, Sweden 2. Department of Physics, Amrita Vishwa Vidyapeetham University, Amritapuri Campus, Kollam 690 525, Kerala, India 3. Key Laboratory of Magnetic Molecules and Magnetic Information Materials of Ministry of Education, Shanxi Normal University, Linfen 041004, China 4. School of Materials and Engineering, Anhui University of Technology, Maanshan, 243002, China 5. Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, U. K. ABSTRACT Mg doped ZnO thin films were prepared by DC/RF magnetron co-sputtering in (Ar+O2) ambient conditions using metallic Mg and Zn targets. We present a comprehensive study of the effects of film thickness on the structural, optical and magnetic properties. Room temperature ferromagnetism was observed in the films and the saturation magnetization (MS) increases at first as the film’s thickness increases and then decreases. The MS value as high as ~15.76 emu/cm3 was achieved for the Mg-doped ZnO film of thickness 120 nm. The optical band gap of the films determined to be in the range 3.42 to 3.52 eV. INTRODUCTION Many studies have reported room temperature ferromagnetism (RTFM) in undoped and doped metal oxides especially including dilute magnetic semiconductors (DMSs) and dilute metal oxides (DMOs). Conventionally RTFM in diamagnetic II-VI materials have been achieved by the introduction of the atoms of magnetic materials into metal oxides host lattice. These materials are the key for developing magneto-optic and spin electronics devices [1,2]. To realize spintronics devices, materials that are ferromagnetic above room temperature are essential. With wide direct band gap (Eg = 3.37 eV) and large exciton binding energy (~60 meV) at room temperature, ZnO thin films were predicted to be a suitable host material to achieve RTFM [3-4]. Extensive studies show that defects and non-magnetic impurities are playing an important role in inducing RTFM in ZnO [5-7]. The RTFM in non-transition metal doped and un-doped ZnO films may be attributed to the different types of defects, such as oxygen vacancies (VO), zinc vacancies (VZn), zinc interstitial (Zni), film thickness, intrinsic strain and chemisorbed oxygen among others. Yi et al. reported RTFM in Li doped ZnO and suggested that the origin of ferromagnetism was associated with VZn produced by the induction of Li doping [7]. Also, RTFM has been reported in both pristine and doped MgO films [8,9]. MgO is almost an insulator with a band gap of 7.8 eV, but with exciton energy comparable to that of ZnO. The wide tunability of the band gap in Mg incorporated ZnO films opens the door for the realization of novel optoelectronic devices especially short wavelength light emitters an
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