Abstract
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 La0.5Sr0.5CoO3 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 x0.17 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.