Separation of the strain and finite size effect on the ferromagnetic properties of La$_{0.5}$Sr$_{0.5}$CoO$_3$ thin films

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. A leading theory for the ferromagnetism in La$_{0.5}$Sr$_{0.5} $CoO$_3$ is the double exchange mechanism. This model relies upon Co-O-Co electron hopping so that a strong dependence on bond length is expected. Our recent EXAFS results will examine whether the double exchange mechanism quantitatively predicts the strain dependence we have measured.