Investigation of structure transformation in ultra-thin SrRuO₃ films driven by SrTiO₃ capping layer

In a cubic perovskite oxide of ABO₃, the BO₆ octahedral rotation and tilting result in a structural distortion and transform the cubic structure into tetragonal or orthorhombic phases, where the lowering of the system symmetry may result in changes in electronic and physical properties. In this presentation, we demonstrated that a transformation from tetragonal (T-) phase to orthorhombic (O-) phase in ultra-thin films of SrRuO₃ (SRO) can be achieved by an additional SrTiO₃ (STO) capping layer. A series of nominal STO (t nm)/SRO (~3 nm) films with t ranging from 0 to 3 nm were grown on STO (001) substrates using an oxide molecular beam system with adsorption controlled growth technique. From rigorous high precision X-ray measurements, a clear transition from T-phase to O-phase was observed when the STO capping layer thickness increases from 0 to 3 nm, inferring a favorable multiple RuO₆ rotation and tilting with an additional STO capping layer. In addition, detailed analysis also revealed the structural orientations of SRO [001]_o along STO [010]_c for O-phase and SRO [001]_T along STO [001]_c for T-phase. The corresponding anomalous Hall measurements showed minor variations, suggesting the robustness of the ferromagnetism in SRO thin films that appear to be insensitive to the orthorhombic-phase to tetragonal-phase transition.