  The ferromagnetic properties of epitaxial La$_{0.5}$Sr$_{0.5}$CoO$_3$ thin
films have been studied. The magnetic transition is affected by both strain and
finite thickness. We have used a series of films of different thickness and on
different substrates in order to quantitatively determine the change in Curie
temperature contributed by each effect. The phase diagram of T$_C$ versus
in-plane strain suggests that the ferromagnetic transition temperature is
suppressed by tensile strain and enhanced by compressive strain. The general
method of separating strain and finite thickness effects should be applicable
to any ordering phase transition in thin films.
