Room Temperature Ferromagnetism and Band Gap Investigations in Mg Doped ZnO RF/DC Sputtered Films
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Room Temperature Ferromagnetism and Band Gap Investigations in Mg Doped ZnO RF/DC Sputtered Films Sreekanth K. Mahadeva1,2, Zhi-Yong Quan1,3, J. C. Fan1, Hasan B Albargi4, Gillian A Gehring4, Anastasia Riazanova1, L. Belova1, K. V. Rao1 1. Department of Materials Science, Royal Institute of Technology, Stockholm, SE100 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. Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK ABSTRACT Mg@ZnO thin films were prepared by DC/RF magnetron co-sputtering in (N2+O2) ambient conditions using metallic Mg and Zn targets. We present a comprehensive study of the effects of film thickness, variation of O2 content in the working gas and annealing temperature on the structural, optical and magnetic properties. The band gap energy of the films is found to increase from 4.1 to 4.24 eV with the increase of O2 partial pressures from 5 to 20 % in the working gas. The films are found to be ferromagnetic at room temperature and the saturation magnetization increases initially with the film’s thickness reaching a maximum value of 14.6 emu/cm3 and then decreases to finally become diamagnetic beyond 95 nm thickness. Intrinsic strain seems to play an important role in the observed structural and magnetic properties of the Mg@ZnO films. On annealing, the as-obtained ‘mostly amorphous’ films in the temperature range 600 to 800oC become more crystalline and consequently the saturation magnetization values reduce. INTRODUCTION Dilute Magnetic semiconductors obtained by doping magnetic impurities into host semiconductors, mostly II - VI and III - V compounds, are the key materials for developing magneto-optic and spin electronics devices [1,2]. With wide direct band gap (Eg = 3.37 eV) and large exciton binding energy (~60 MeV) at room temperature (RT), ZnO thin films were predicted to be promising host materials to achieve room temperature ferromagnetism (RTFM) [3-5]. Extensive studies show that defects and non-magnetic impurities like Li, play an important role in inducing RTFM in ZnO [6, 7]. Various types of intrinsic and extrinsic defects in nontransition metal doped and un-doped ZnO have been attributed to give rise to RTFM. However, the experimental results on obtaining RTFM by different approaches have raised more questions because the properties of the materials obtained are highly dependent on the complexities of sample preparation and processing conditions. For example, P. Zhan et al. observed the FM of un-doped ZnO was induced by the singly occupied oxygen vacancies and provided a way to further enhance its ferromagnetic property [8]. Also, RTFM has been reported in both pristine and doped MgO films [9, 10]. The wide tunability of Eg in Mg incorporated ZnO films open the door for tailoring novel optoelectronic devices especially short wavelen
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