Concentration and Defect Dependent Ferromagnetism above Room Temperature in Co Doped ZnO Films Prepared by Metalorganic
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Concentration and Defect Dependent Ferromagnetism above Room Temperature in Co Doped ZnO Films Prepared by Metalorganic Decomposition P. Kharel1, C. Sudakar1, G. Lawes1, R. Suryanarayanan1*, R. Naik1 and V. M. Naik2 1 Department of Physics and Astronomy, Wayne State University, Detroit, MI 48201 2 Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, MI 48128 *Permanent address: LPCES, Université Paris-Sud, 91405 Orsay, France
ABSTRACT Zn1-xCoxO (x = 0.0 − 0.047) thin films (thickness ~0.5 – 1 µm) have been prepared on sapphire substrates using metalorganic decomposition (MOD) method. The X-ray diffraction and Raman scattering studies indicate films to be polycrystalline ZnO with wurtzite structure. The optical absorption spectra show an expected bandgap of ~3.2 eV. The magnetization studies show that the as prepared films lack the room temperature ferromagnetic order, whereas the films when vacuum annealed at a temperature 500 – 600 oC acquire ferromagnetic ordering at room temperature. Further, the observed ferromagnetism (FM) appears only for a limited range of Co concentration, 0.03 < x < 0.10 (after heat treating in vacuum at 550 oC), and it reversibly disappears upon re-annealing in air. The data presented here seem to suggest that the appearance of ferromagnetic order is dictated by both the oxygen defects and the critical concentration of Co, and thus may lend support to a recent model proposed by Coey et al. [Nature Materials 4, 173 (2005)]. INTRODUCTION Dilute magnetic semiconductors (DMS) are attractive candidates for applications in spintronic devices.1 Ferromagnetic properties in these materials at and above room temperature are necessary for any practical device application.2 Earlier reports based on III–V semiconductors such as GaAs, doped with Mn, revealed that Tc hardly exceeded 150 K, rendering them unsuitable for viable room temperature applications.3 Theoretical studies predicted that the wide band-gap compound semiconductors, such as GaN and ZnO doped with 3d transition metal, such as Mn, Co, Ni, etc., may show high-temperature ferromagnetic transition.4 The observation of ferromagnetism with Tc > 300 K in Co-doped TiO2 films prepared by pulsed laser deposition (PLD) by Matsumoto et al5 turned the attention of researchers to the oxide semiconductors. Following this, several reports have appeared on the occurrence of ferromagnetism above room temperature in other materials such as TiO2 doped with Fe,6,7 ZnO doped with Co,8,9 GaN doped with Mn,10 AlN doped with Cr,11 and SnO2 doped with Co,12 or with Fe.13 The ferromagnetic samples examined were prepared using different preparative techniques such as oxygen-plasma-assisted molecular beam epitaxy (MBE),14 sputtering,15 ion implantation16 and laser MBE17 as well as PLD.7,11,12 Though all these samples did show unambiguous signs of ferromagnetic behavior at 300 K or above, it was difficult to exclude the possibility of formation of clusters of dopants and thus attribute the observed magnetism entirely to 3d eleme
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