Giant Polar Displacements via Strain Relaxation in Itinerant Ferromagnet SrRuO ₃ Freestanding Films

Polar displacements are conventionally seen as incompatible with metallicity due to charge screening. However, in perovskite oxide heterostructures, cation off-centring distortions can be achieved through interface engineering strategies-such as ferroelectric proximity and substrate strain-even in centrosymmetric metallic systems. It remains unexplored whether such off-centring displacements can be stabilized in freestanding oxide films, where substrate-induced strain is intentionally eliminated. Here, the discovery of substantial Ru off-centre displacements exceeding 30 pm in freestanding SrRuO₃ films is reported, achieved by properly controlling the strain relaxation process to engineer an expanded lattice. It is further demonstrated that these Ru off-centre displacements effectively tune the electronic structure and consequently modify the magnetic properties of SrRuO₃ thin films. This work shows that tailored strain relaxation strategies can be used to design magnetic polar metals with desired electronic states in otherwise centrosymmetric perovskite oxides.