Zoey Warecki

Project abstract

Perovskite metal oxides are materials that are predicted to play as big a role in future electronic technologies as silicon does in today's semiconductor based electronic technologies. Mixed valent rare earth manganites are perovskite metal oxides containing rare earth/alkaline earth atoms, manganese and oxygen. Research in thin films of manganites in the past has largely been focused on the hole-doped compositions that exhibit the phenomenon of colossal magnetoresistance. Hole-doped manganites are derived from rare earth manganese oxides (such as LaMnO3) by partial substitution of the trivalent rare earth site (such as La3+) by a divalent alkaline earth element (such as Ca2+) which leads to the formation of Mn4+ ions to replace some of the Mn3+ ions. In contrast, electron-doped manganites can be obtained from alkaline earth manganese oxide (such as CaMnO3) by introducing Mn3+ ions to replace Mn4+ ions, which can be achieved either by partially substituting Ca with elements of higher valency, or by introducing oxygen vacancies. CaMnO3 as well as its electron doped derivatives have potential applications in several energy technologies.We are currently investigating the properties of thin films of these electron-doped manganites. We use the technique of Pulsed Laser Deposition to grow the thin films. The films are grown epitaxially on LaAlO3 substrates, whose lattice parameters are larger than that of CaMnO3, thus causing the films to be under tensile stress. By decreasing the film thickness we can increase the tensile strain. We have studied structural and electrical properties of CaMnO3 films under tensile strain, by means of X-ray diffraction, temperature dependent resistivity measurements, and characterization of the film surface morphology using atomic force microscopy. Our results indicate that tensile strain causes CaMnO3 to be more susceptible to the formation of oxygen vacancies, thus reducing electrical resistivity. This result agrees with recent theoretical predictions correlating strain and oxygen vacancies, and has important implications for technological applications.