Strain dependence of antiferromagnetic interface coupling in La_0.7Sr_0.3MnO₃/SrRuO₃ superlattices

We have investigated the magnetic response of La_0.7Sr_0.3MnO₃/SrRuO₃ superlattices to biaxial in-plane strain applied in situ. Superlattices grown on piezoelectric substrates of 0.72PbMg_1/3Nb_2/3O₃-0.28PbTiO₃(001) (PMN-PT) show strong antiferromagnetic coupling of the two ferromagnetic components. The coupling field of mu H-0(AF) = 1.8 T is found to change by mu(0)Delta H-AF/Delta epsilon similar to -520 mT %(-1) under reversible biaxial strain Delta epsilon at 80 K in a [La_0.7Sr_0.3MnO₃(22 angstrom)/SrRuO₃(55 angstrom)]₁₅ superlattice. This reveals a significant strain effect on interfacial coupling. The applied in-plane compression enhances the ferromagnetic order in the manganite layers, which are under as-grown tensile strain, leading to a larger net coupling of SrRuO₃ layers at the interface. It is thus difficult to disentangle the contributions from strain-dependent antiferromagnetic Mn-O-Ru interface coupling and Mn-O-Mn ferromagnetic double exchange near the interface for the strength of the apparent antiferromagnetic coupling. We discuss our results in the framework of available models.