La1-xSrxCoO3 has received considerable attention in bulk form. This is due to interest in the fundamental magnetic properties (spin-state transitions and magnetic phase separation) as well as potential applications in ferroelectric memory and solid-oxide fuel cells. The structure and properties in thin film form are not well understood, and the influence of dimensional confinement on effects such as magnetic phase separation is unknown. Here, we report a comprehensive investigation of structure, magnetism, and transport in strained epitaxial La 0.5Sr0.5CoO3 (001) films deposited on SrTiO3 (001) substrates by reactive dc magnetron sputtering. The crystalline quality, phase purity, strain state, oxygen stoichiometry, morphology, and magnetic and electronic properties of the epilayers are all probed and are found to be particularly sensitive to the total sputtering gas pressure and the ratio of reactive to inert gas (PO2/PAr). The various structure-property relationships are discussed in detail, particularly with respect to the degree of oxygenation and oxygen-induced resputtering. The films are strained and tetragonally distorted due to the 1.9% lattice mismatch with SrTiO3. Significant strain relaxation occurs at thicknesses around 200 Å, resulting in a crossover from two-dimensional-like to three-dimensional growth. Polarized neutron reflectometry was combined with x-ray reflectometry to obtain chemical and magnetic depth profiles, which are compared with cross-sectional scanning transmission electron microscopy. The results indicate a thin (∼10 Å) layer at the film/substrate interface with significantly different structural properties to the bulk of the film, as well as a strongly graded magnetic and chemical profile at the film surface due to the significant roughness. The Curie temperature was found to decrease very slowly as the thickness is reduced down to ∼50 Å, at which point a rapid decrease occurs, almost coincident with a sharp decrease in saturation magnetization. At this point, the temperature dependence of the resistivity shows a crossover from metallic to insulating, accompanied by dramatic changes in the magnetoresistance. The magnetoresistance has a negative contribution peaking around the Curie point (similar to that seen in bulk), a second negative contribution occurring at low temperature (only for the thinnest samples), as well as a large anisotropic magnetoresistance, which vanishes at the Curie point. Remarkably, the low temperature contribution in the thinnest x=0.5 films bears a striking resemblance to that seen in the insulating phase (x<0.17) in bulk, suggesting the formation of a nonmetallic phase at low thickness that is similar to the low doping bulk phase, i.e., magnetic phase separation near the interface with SrTiO3.
Bibliographical noteFunding Information:
This work was supported by NSF Division of Materials Research through Grant No. 0509666 and DoE BES through Grant No. DE-FG02-06ER46275. Research at ORNL (STEM by M.V.) was sponsored by the Division of Materials Science and Engineering, U.S. DoE. M.V. wishes to thank Julia Luck for specimen preparation for STEM. We gratefully acknowledge use of facilities at the UMN characterization facility and the Los Alamos Neutron Sciences Center. We would like to thank Jeff Parker and Mike Manno for assistance, as well as Jing Wu for initial film depositions.