A scanning tunneling microscopy and potentiometry study of epitaxial thin films of La 0.7 Ca 0.3 MnO 3
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A scanning tunneling microscopy and potentiometry study of epitaxial thin films of La0.7Ca0.3MnO3 Mandar Paranjape1, K. Shantha Shankar1, A.K. Raychaudhuri1, N.D. Mathur and M.G. Blamire Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, U.K.CB2 3QZ. 1 Department of Physics, Indian Institute of Science, Bangalore, India 560012. ABSTRACT To investigate the role of grain boundaries and other growth related microstructure in manganite films, a scanning tunneling microscope is used to simultaneously probe surface topography and local potential distribution under current flow at nanometer level in films of epitaxial thin films of La0.7Ca0.3MnO3 deposited on single crystal SrTiO3 and NdGaO3 substrate by laser ablation. We have studied two types of films strained and strain relaxed. Thin (50nm) films (strained due to lattice mismatch between substrate and the film) show step growth (unit cell steps) and have very smooth surfaces. Relatively thicker films (strain relaxed, thickness 200nm) do not have these step growths and show rather smooth well connected grains. Charge transport in these films is not uniform on the nanometer level and is accompanied by potential jumps at the internal surfaces. In particular scattering from grain boundaries results in large variations in the local potential resulting in fields as high as 104-105V/cm located near the grain boundaries. We discuss the role of local strain and strain inhomogeneties in determining the current transport in these films and their resistance and magnetoresistivity. In this paper we attempt to correlate between bulk electronic properties with microscopic electronic conduction using scanning tunneling microscopy and scanning tunneling potentiometry. INTRODUCTION Colossal magnetoresistance (CMR) in rare earth perovskite manganites has attracted considerable current attention. These materials with general chemical formula Re1-xAxMnO3 (Re = rare earth like La, Nd, Pr etc. and A= Sr, Ca, Ba, Pb etc.) show transition from a paramagnetic insulating (PMI) phase to a ferromagnetic metallic (FMM) phase at temperatures around TC ≈ 200 K- 350 K for x ≈ 0.2-0.4. On application of a magnetic field the resistivity ρ of the material is severely suppressed leading to the phenomenon of CMR [1,2]. It has been realized that the electrical transport in general and the value of MR in particular in these materials depend strongly on the transport through the grain boundaries (GB). This is particularly true for thin films. For instance, films of the same CMR material grown epitaxially on different substrates can have very different ρ, TP and TC and consequently the magnetoresistance (MR). It has been proposed that the presence of biaxial strain, εbi stabilizes the Jahn-Teller distortion around the Mn3+ ions and reduces TP and increases ρ [3]. The effect of εbi is expected to be important in thin films of manganites. Recent experiments have established this [4]. In thin films of CMR materials, the strain and microstructure together determine the nat
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