The polycrystalline La 1Ϫx Ce x MnO 3 manganites do not exist in single phase in bulk under the preparation conditions so far employed, but their polycrystalline and epitaxial films deposited by the pulsed laser deposition ͑PLD͒ technique form readily in single phase. The cerium oxide (CeO 2 ) remains partially unreacted when the bulk sample is prepared through the solid state reaction route. The resistivity of the bulk La 0.7 Ce 0.3 MnO 3 sample shows a broad metal insulator transition ͑MIT͒ clearly resolved into two peaks, suggesting the presence of a second ͑impurity͒ phase, which is identified as unreacted CeO 2 by the intensity analysis of the x-ray diffraction ͑XRD͒ data. However, when prepared as thin films by PLD, La 0.7 Ce 0.3 MnO 3 forms in single phase, as corroborated by the uniqueness and sharpness of the MIT peak and also by the XRD patterns of the polycrystalline films. We also performed a detailed study of the epitaxial films by a high-resolution XRD system with a four-circle goniometer and did not find any impurity phase. The magnetization data shows a very sharp transition followed by a sharp MIT in resistivity at the same temperature in the epitaxial thin film. These results suggest that PLD can be used as a useful technique to synthesize unconventional compounds, which do not form easily in bulk.

La 0.7 Ce 0.3 MnO 3 is a relatively new addition to the family of colossal magnetoresistive manganites, in which the cerium ion is believed to be in the Ce4+ state. In this article, we report the magnetotransport properties of laser ablated La0.7Ce0.3MnO3 films on LaAlO3, and the effect of varying the ambient oxygen pressure during growth and the film thickness. We observe that the transport and magnetic properties of the film depend on the oxygen pressure, surface morphology, film thickness, and epitaxial strain. The films were characterized by x-ray diffraction using a four-circle goniometer. We observe an increase in the metal-insulator transition temperature with decreasing oxygen pressure. This is in direct contrast to the oxygen pressure dependence of La0.7Ca0.3MnO3 films and suggests the electron doped nature of the La0.7Ce0.3MnO3 system. With decreasing film thickness we observe an increase in the metal-insulator transition temperature. This is associated with a compression of the unit cell in the a-b plane due to epitaxial strain. On codoping with 50% Ca at the Ce site, the system (La0.7Ca0.15Ce0.15MnO3) is driven into an insulating state suggesting that the electrons generated by Ce4+ are compensated by the holes generated by Ca2+, thus making the average valence at the rare-earth site 3+ as in the parent material LaMnO3.

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