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J. Li and C.K. Ong Center for Superconducting and Magnetic Materials and Department of Physics, National University of Singapore, Singapore 119260 (Received 2 July 2002; accepted 30 July 2002)

The microstructure of Ag-doped La2/3Sr1/3MnO3 (LSMO) thin films deposited on (001) LaAlO3 single-crystal substrates was systematically investigated in cross section and plan view by high-resolution electron microscopy and analytical electron microscopy. The results showed that the films deposited at 750 °C were perfectly epitaxial with or without Ag-doping. No Ag in the doped film was detected. On the other hand, the LSMO films deposited at 400 °C were less perfect. With increasing Ag-doping level, the shape of LSMO grains became irregular, and the grain size increased gradually. Large polycrystalline clusters consisting of LSMO, AgO, and Ag grains formed in the doped films, and the amount and size of them increased with increasing Ag-doping level. Ag existed at the LSMO grain boundaries in its elemental state. A growth process for the LSMO-Ag system is discussed based on the experimental results. The enhancement of the magnetic spin disorders at the grain boundaries and interfaces caused by doping Ag could result in an improvement of low-field magnetoresistance.

I. INTRODUCTION

Mixed valence manganites with perovskite structure exhibit two kinds of magnetoresistance (MR): intrinsic or intragrain MR observed in the vicinity of magnetic transition temperature Tc and extrinsic or intergrain MR occurring over a wide temperature range below Tc.1,2 The well-known colossal magnetoresistance (CMR), explained by the double exchange interactions,3 is an intrinsic effect. However, the sharp drop in resistance was achieved only in a high magnetic field in the Tesla range, thus severely limiting the potential applications of CMR materials. On the contrary, the intergrain magnetoresistance usually obtained in an applied field of a few hundred Oersted has attracted special interest.4 –7 In particular, polycrystalline samples have shown the significant low-field effect, which becomes increasingly important at low temperatures. It was shown that lowfield MR is dominated by the grain boundaries.5,6 Spinpolarized tunneling 4 or spin-dependent scattering 6 through or at the grain boundaries is believed to be responsible for it. Researchers explore various methods for enhancing the low-field MR as highly as possible, such as polycrystalline bulk,4,8 polycrystalline thin films,5,9 artificially induced grain boundaries,10 and trilayer tunnel junctions.11 It seems that introducing weak-link 2712

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J. Mater. Res., Vol. 17, No. 10, Oct 2002 Downloaded: 14 Mar 2015

grain boundaries and interfaces is a promising way to enhance the low-field MR value in manganite films.12 Since the physical properties, in particular the MR effect, are sensitive to the structure of perovskite-related manganites, it is expected that the extrinsic MR would be enhanced by doping to modify the microstructure of materials. Ag has been widely us