Magnetic coupling at the interface between ultrathin tetragonal CuO and La0.7Sr0.3MnO3

High-symmetry rocksalt type tetragonal CuO (T-CuO) does not exist in bulk but can be synthesized via thin film epitaxy limited to a few unit cells (3, 4) thick and above which it relaxes to its bulk tenorite structure. Direct probe into magnetic properties of T-CuO layer has been a challenge because of its ultrathin limit. Here, we demonstrate the interfacial magnetic coupling between ultrathin T-CuO and ferromagnetic (${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$) layers in an epitaxial $\phantom{\rule{0.16em}{0ex}}\mathrm{CuO}\text{/}{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ bilayer grown on (001)-oriented ${\mathrm{SrTiO}}_{3}$. We observe a positive exchange bias shift of $\ensuremath{\sim}30$ Oe at 2 K in $\mathrm{CuO}\text{/}{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ bilayer. The observation of positive exchange bias indicates that there exists antiferromagnetic exchange coupling between Mn and Cu moments at the interface. Notably, the exchange bias vanishes at 5 K and it is discussed in view of the proposed spin structure revealed from low-energy muon spin rotation and x-ray magnetic circular dichroism study [Phys. Rev. B 103, 224429 (2021)]. Furthermore, an enhanced Gilbert damping, linewidth broadening and larger inhomogeneous $4\ensuremath{\pi}{M}_{\mathrm{eff}}$ value from in-plane ferromagnetic resonance measurements, are the direct consequence of antiferromagnetic exchange coupling at the $\mathrm{CuO}\text{/}{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ interface. Combining both static and dynamic magnetic characterization, we establish an understanding of interfacial exchange coupling in $\mathrm{CuO}\text{/}{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ bilayer.