A Correlated Polar Ferromagnetic Metal by Design

Polar metals with a combination of conflicting polarization and metallicity have garnered increased interests recently because of their promising functionalities. Adding ferromagnetism into this unique state would offer exciting opportunities to obtain exotic quantum states, analogous to magnetoelectric multiferroics with coupled polarization and magnetization. However, such an intrinsic ferromagnetic polar metal remains elusive, especially in strongly correlated electron systems. Here, we report the experimental realization of coexisting ferromagnetism, polarity, and metallicity in a new 3d transition metal oxide Ca₃Co₃O₈. Here, structural oxygen vacancies order to form a periodic stacking of oxygen tetrahedral monolayers alternating with octahedral bilayers. The magnetic metallic state is confined within the quasi-two-dimensional CoO₆ octahedra layers, while the broken inversion symmetry arises simultaneously from Co displacements, which is further enhanced by the onset of magnetism, indicating an intrinsic strong magnetoelectric coupling. This coupled response and dual absence of spatial-inversion and time-reversal symmetries produce an intrinsic magnetochiral anisotropy with exotic magnetic field-free nonreciprocal electrical resistivity. We also observe an extraordinarily robust topological Hall effect that persists over a broad temperature-field phase space arising from a Rashba-type spin-orbit coupling. Our work not only provides a rich platform to harvest the fascinating coupling between polar and magnetic states, but also defines a novel design strategy to access a new class of oxide materials with rich properties through ordered oxygen vacancies.