  The double perovskite La2NiMnO6 (LNMO) exhibits complex magnetism due to the
competition of magnetic interactions that are strongly affected by structural
and magnetic inhomogeneities. In this work, we study the effect of oxygen
annealing on the structure and magnetism of epitaxial thin films grown by
pulsed laser deposition. The key observations are that a longer annealing time
leads to a reduction of saturation magnetization and an enhancement in the
ferromagnetic transition temperature. We explain these results based upon
epitaxial strain and oxygen defect engineering. The oxygen enrichment by
annealing caused a decrease in the volume of the perovskite lattice. This
increased the epitaxial strain of the films that are in-plane locked to the
SrTiO3 substrate. The enhanced strain caused a reduction in the saturation
magnetization due to randomly distributed anti-site defects. The reduced oxygen
defects concentration in the films due to the annealing in oxygen improved the
ferromagnetic long-range interaction and caused an increase in the magnetic
transition temperature.
