Strain dependence of interfacial antiferromagnetic coupling in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrRuO$_{3}$ superlattices*

We have investigated the magnetic response of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrRuO$_{3}$ superlattices to biaxial in-plane strain applied \textit{in-situ}. Superlattices grown on piezoelectric substrates of 0.72PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$-0.28PbTiO$_{3}$(001) (PMN-PT) show strong antiferromagnetic coupling of the two ferromagnetic components. The coupling field of $\mu_{0}H_{AF\thinspace }=$ 1.8 T is found to change by $\mu_{0}\Delta H_{AF}/\Delta \varepsilon \sim $ -520 mT {\%}$^{-1}$ under reversible biaxial strain ($\Delta \varepsilon )$ at 80 K in a [La$_{0.7}$Sr$_{0.3}$MnO$_{3}$(22 {\AA})/SrRuO$_{3}$(55 {\AA})]$_{15}$ 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. 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, since the enhanced magnetic order of Mn spins leads to a larger net coupling of SrRuO$_{3}$ layers at the interface. We discuss our experimental findings taken into account both the strain-dependent orbital occupation in a single--ion picture and the enhanced Mn order at the interface.