Introduction

The last decade has seen the emergence of epitaxial metal-oxide films as one of the most attractive subjects for the condensed matter community. The emergence of such interest was primarily stimulated by the discovery of high temperature superconductors (HTSC) and more recently by the discovery of the colossal magnetoresistance (CMR) effect in thin films of manganese oxides Re1-XAXMnO3 (where Re is a rare earth and A is an alkaline earth) [1-5]. CMR materials exhibit large changes in electrical resistance when an external magnetic field is applied [6,7].

The doped manganites are mixed-valence with Mn3+ (3d4) and Mn4+ (3d3). For the octahedral site symmetry of MnO6 the configurations become tfgeg for Mn3+ and t|g for Mn4+. In the double-exchange mechanism, the eg electrons are considered as mobile charge carriers interactions with localized Mn4+ (S = 3/2) spins. The carriers hoping avoids the strong on-site Hund rule exchange energy Jex when the spins are aligned ferromagnetically. (Note that if the Mn spins are not parallel or if the Mn-O-Mn bond is bent, the electron transfer becomes more difficult and mobility decreases.) Jex is much larger that the eg bandwidth, and thus the conduction electrons are highly spin-polarized in the ground state. With this idea, correlations of the half ferromagnetic character of the CMR materials were found [8]. Theoretical and experimental studies indicate that the small polaron effects including Jahn-Teller distorsion also play important roles for the transition and transport measurements as well [9].

These oxide materials are important from a fundamental point of view since they offer a chemical flexibility that enables new structures and new properties to be generated and, consequently, the relations between the structure, electronic, magnetic, and transport properties to be studied.

Since most technological applications require thin films on substrates, the ability to prepare such films and understand their properties is of prime importance. The synthesis of the first HTSC oxide thin films almost 15 years ago generated great interest in the thin films community. This resulted in the development of various techniques, guided by the importance of preparing high quality thin films of superconductor compounds, including sputtering, molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD), but the most popular technique is probably pulsed laser deposition (PLD) [10]. This latter method is used extensively to synthesize cuprates and HTSCs, which are now routinely made in laboratories, and it has been easily and rapidly adapted for manganites. Another reason for this quick-transfer technology is that these oxide materials crystallize in a perovskite structure as the HTSCs, and in some sense, they are quite similar [11]. Moreover, the man-ganite oxides are highly sensitive to any kind of pertubation, and strain effects in particular. This offers the possibility of studying its influence upon various properties such as insulator-to-metal transition temperature (TIM), Curie temperature (TC), structure, microstructure/morphology, etc.

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 10: Pages (107-124)

Thus, the renewed interest in the manganite materials has resulted in a large volume of published research in this field.

In the present chapter, the deposition procedure and its influence (through deposition temperature, oxygen pressure, postannealing, substrate type, ...) upon the magnetotransport properties will briefly presented. We will particularly discuss the work related to the strain effects such as the substrate-induced strain and thickness dependence of both the structure and their properties.

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