The effect of Oxygen partial pressure during deposition in the magnetic properties of ZnO thin film
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The effect of Oxygen partial pressure during deposition in the magnetic properties of ZnO thin film
Anis Biswas, Wang Shirong, Sandeep Nagar, L. Belova and K. V. Rao Department of Materials Science-Tmfy-MSE, The Royal Institute of Technology, S- 100 44, Stockholm, Sweden
ABSTRACT We have studied the magnetic properties of 100 nm thick ZnO thin films prepared by magnetron sputtering in different oxygen partial pressures (ratio of oxygen pressure to total pressure in deposition chamber, POxy). Only the films fabricated at POxy below ~ 0.5 show room temperature ferromagnetism. The saturation magnetization at room temperature is initially found to increase as POxy increases and reaches maximum value of ~ 5 emu/gm at POxy ~ 0.3 and then starts to decrease and becomes diamagnetic for POxy > 0.5. From small angle XRD study of structural properties of the films, we find that the lattice stress developed in the film along c-axis also exhibits a similar behavior with the variation of POxy. Thus, both the room temperature ferromagnetism and lattice stress appear to originate from the intrinsic defects present in the sample. INTRODUCTION Search of ferromagnetism above room temperature (RTFM) in different oxides has been an active area in which considerable research activities are being carried out. One of the major breakthroughs in this pursuit is the discovery of robust ferromagnetism above room temperature in Mn-doped ZnO thin films [1]. However, such RTFM has also been observed even in undoped thin films and nanoparticles of these oxides [2-10]. In their bulk form, those oxides are all diamagnetic. Thus, it is expected that RTFM should in principle be film thickness dependent. Another parameter that could affect the onset of RTFM is the partial oxygen pressure during the film deposition by sputtering or pulsed laser deposition Theoretical calculation of Wang et al., indicates that magnetic moment in pure ZnO arises from 2p unpaired electrons at O sites surrounding a Zn vacancy (VZn) [11]. There is a high probability of formation of vacancies in the surface layer than in the sub-surface layer i.e., lower formation energy of vacancies on the surface than in the bulk. Therefore, in principle, RTFM should be observable only in thin films, nanoparticles and perhaps highly defective bulk samples [2-10], A systematic study of the role of vacancies on RTFM in oxides is best carried out in thin films
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In this paper we explore how the magnetic and structural properties modify due to the variation of deposition conditions in ZnO thin films produced by sputtering. We first carry out a systematic study of the variation of the saturation magnetization value Ms with film thickness of the films deposited under a fixed partial oxygen pressure and determine the optimum thickness t opp (found to be ~ 100 nm ) for which the maximum Ms value is obtained. We then monitor the structural and magnetic properties of topp (~ 100 nm) thick films deposited at room temperature at various oxygen partial pressures.Poxy We observe that POxy has sign
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