Substrate Induced Strains

The first important parameter for successful thin film growth is undoubtedly the substrate. For CMR materials, the same substrates as used for the HTSC compounds were utilized. The most commonly used substrates to grow man-ganite perovskites are MgO (cubic, a = 0.4205 nm), SrTiO3 (STO, cubic, a = 0.3905 nm), LaAlO3 (LAO, pseudocubic, a = 0.3788 nm), NdGaO3 (NGO, orthorhombic with a = 0.5426 nm, b = 0.5502 nm, and c = 0.7706 nm), and Si (cubic, a = 0.543 nm). Many authors have investigated the strain effects of the substrate by growing various films on different substrates [102]. They have experimentally [87,103] or theoretically [104] studied the effect of strains on the magne-toresistive properties of La07Sr03MnO3 and La0 7Ca0.3MnO3 or on the surface flatness [105] for many substrates. The physical properties of these materials depend on the overlap between the manganese d orbitals and oxygen p orbitals, which are closely related to the Mn-O-Mn bond angle and the Mn-O distance. As the unit cell of the thin film is modified with respect to the bulk material, the Mn-O distances and Mn-O-Mn angles are altered, inducing variations in the electronic properties. We will review the main characteristics, such as the structure and the physical properties, which are affected by the substrate-induced strains.

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