Effect of Post-Deposition Fluorination Treatment on the Structure and Electrical Transport Properties of Epitaxial La_0.67Ca_0.33MnO_3-y Thin Films

La_0.67Ca_0.33MnO₃(LCMO) is a perovskite material belonging to the family of hole-doped manganites well known for electronic phenomena such as insulator-metal transitions, charge ordering and colossal magnetoresistance. These phenomena are sensitive to the compositional variations at the Rare Earth and Alkaline Earth sites leading to a rich phase diagram because of the change in Mn valence states. Variations in oxygen stoichiometry also cause changes in Mn valence states as well as structural changes, both of which lead to interesting variations in electrical properties such as enhanced low field magnetoresistance. Although oxygen stoichiometry variations are easier to realize in thin films compared to bulk, the enhanced oxygen mobility also leads to instability over time, which is not conducive to harnessing the useful properties. While the effects of cation stoichiometry variations through substitutional doping has been well studied, there has been very little research on anionic substitution at the oxygen site. We will present the results of our experiments designed to partially substitute oxygen by fluorine in epitaxial thin films of LCMO grown by Pulsed Laser Deposition. We subject the LCMO films to a post-deposition heat treatment in the presence of a fluorine-containing polymer- a process we refer to as “fluorination”. We observe structural changes as well as pronounced changes in resistivity upon subjecting the films to the fluorination process. The changes are sensitive to the oxygen stoichiometry of the as-grown film which we vary by changing the oxygen partial pressure during film growth. We observe that fluorination enhances conductivity and promotes insulator-to-metal transition in oxygen deficient films. Through well-designed control experiments, we have established that the effects are related to possible fluorine incorporation and not merely the result of the post-deposition heat treatment. Our results are consistent with previous fluorination experiments on other metal oxide films[1,2]. We will discuss our results in the light of possible charge doping and structural changes introduced as a result of the differences in the valence state and ionic sizes of oxygen and fluorine.<br/>Acknowledgement: This work was supported by the NSF Grant DMR 1709781 & Fisher Endowment Grant from the Fisher College of Science and Mathematics at Towson University<br/>References:<br/>E.J. Moon, Y. Xie, E.D. Laird, D.J. Keavney, C.Y. Li and S.J. May, J. Am. Chem. Soc 136 2224 (2014)<br/>E.J Moon, A.K Choquette, A.Houn, S.Z. Kulesa, D. Barbash and S.J. May, APL Mater. <b>3</b>, 062511 (2015) doi: 10.1063/1.4921579