Oxygen Content and Crystallinity Effects in Pulsed Laser Deposited Lanthanum Manganite Thin Films
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Oxygen Content and Crystallinity Effects in Pulsed Laser Deposited Lanthanum Manganite Thin Films Srinivas V. Pietambaram, D. Kumar, Rajiv K. Singh and C. B. Lee' Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611-6400, U.S.A. 'Department of Electrical Engineering, North Carolina A & T University, Greensboro, North Carolina 27411, U.S.A. ABSTRACT Eventhough colossal magnetoresistance in Lanthanum calcium manganite (LCMO) thin films was known for a long time, the effect of oxygen content and crystallinity on the properties of these films is not clearly understood. It is in this context that we have performed a systematic study of these effects by annealing the films in various ambients. A series of LCMO thin films have been grown in situ on (100) LaAIO3 substrates using a pulsed laser deposition technique under identical conditions. Microstructural characterization carried out on these films has shown that the films are smooth, single phase and highly textured. The films were subjected to the following post deposition treatments: (i) annealing in oxygen at 900'C for 4 hrs, (ii) annealing in argon at 900'C for 4 hrs, (iii) annealing in oxygen at 500'C for 12 hrs, (iv) annealing in argon for 12 hrs and (v) annealing in vacuum at 850'C for half-an-hour. As deposited LCMO films show a transition temperature of 260 K and a magnetoresistance ratio (defined as [R(0)R(H)/R(H)]) of 190% at 260 K in 5T magnetic field. The samples subjected to 500'C oxygen and Ar anneal have shown no change in the transition temperature and MR ratio. The films subjected to a 900'C annealing in Ar ambient have shown marginal improvement in transition temperature but a drastic improvement in the MR ratio (525%). 900'C oxygen annealed films have shown an improvement in the transition temperature (290 K) and MR ratio (225%) over as deposited films. Vacuum annealed samples have shown deteriorated properties. These results indicate that the metal-insulator transition is related to the oxygen content of the films while the MR ratio is related to the domain size. INTRODUCTION Colossal magnetoresistance (CMR) was first observed in single crystal La 06Pb 0.4 Mn0 3 by Searle and Wang [I]. Recent interest in these materials was sparked off by the observation of large magnetoresistance in epitaxial thin films near room temperature [2]. Thin films with large room temperature magnetoresistance open up new possibilities for applications in diverse areas of technology such as magnetic random access memories and read heads for hard disk drives. The effects of oxygen in manganite film have been a focus since the discovery of extraordinary magnetotransport in this kind of materials. There is no quantitative relation between the oxygen content and the magnetic and transport properties in thin films to date due to the difficulty in the determination and control of oxygen content. Substrate temperature, oxygen partial pressure, and deposition rate could have an effect on the oxygen content of the film severely [3-5]. Va
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