Abstract
We report our studies of the thickness dependence of electrical resistivity and lattice constants in strained epitaxial thin films of calcium manganese oxide. Our results indicate the potential of bi-axial lattice mismatch strain as a handle for modulating electrical resistivity. We observe thickness dependence of lattice constants consistent with what is expected for strain relaxation for films thicker than 400 Å. At lower thickness values, anomalies are observed suggestive of reduced oxygen stoichiometry. We observe a remarkable decrease in electrical resistivity with decreasing film thickness. The resistivity of our thinnest films (5–7 nm) is about three orders of magnitude lower than the resistivity of bulk CaMnO3. Resistivity increases as the film thickness increases, along with the progression of strain relaxation. It is noteworthy that the thickness dependence of resistivity we observe in CMO thin films is the opposite of what has been reported for their hole-doped rare earth manganite counterpart La0.67Ca0.33MnO3 (LCMO), where tensile lattice mismatch strain suppresses metallicity, leading to the increase in resistivity with film thickness. We believe that the enhanced conductivity in our thinnest films is related to the possible oxygen deficiency promoted by tensile strain. Recent x-ray absorption measurements have revealed reduced oxygen content and associated changes in Mn valence states in tensile-strained CMO thin films, as also predicted by density functional theory calculations. This report is the first observation of electrical transport behavior possibly indicative of strain–oxygen stoichiometry coupling.
Highlights
CaMnO3 (CMO) is one of the end members of the series of rare earth (RE)/alkaline earth (AE) manganites belonging to the ABO3 perovskite family
Manganites are well known for the rich array of electronic and magnetic phases resulting from the mixed occupation of the A site by rare earth (RE) or alkaline earth (AE) elements as in (RE)1−x(AE)xMnO3
While we do not have quantitative data on the oxygen content in our films to support the hypothesis of strain-induced oxygen deficiency, it is important to note that our results are consistent with the observation of systematic increase in oxygen deficiency with lattice mismatch strain, as evidenced by Hard X-ray Photoelectron Spectroscopy (HAXPES) and X-ray Absorption Spectroscopy (XAS) studies in strained CMO thin films
Summary
CaMnO3 (CMO) is one of the end members of the series of rare earth (RE)/alkaline earth (AE) manganites belonging to the ABO3 perovskite family. The main result of our study is the observation that, for epitaxial films of CMO grown on (100) LaAlO3, a substrate that induces tensile lattice mismatch strain, the electrical resistivity decreases dramatically as the film thickness decreases. Our results point to a possible coupling between tensile strain and oxygen stoichiometry, consistent with density functional theory calculations and recent x-ray absorption and x-ray photoelectron spectroscopy studies of tensilestrained CMO thin films.. Our results point to a possible coupling between tensile strain and oxygen stoichiometry, consistent with density functional theory calculations and recent x-ray absorption and x-ray photoelectron spectroscopy studies of tensilestrained CMO thin films.12 To our knowledge, this is the first report of electrical resistivity modulations arising as a possible consequence of such strain–oxygen stoichiometry coupling. Electrical resistivity was measured using the DC four-probe Van der Pauw method
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