Engineering Polar Magnetic Metals: A Journey from Ferromagnetic to Antiferromagnetic Correlated States

The combination of structural polarity and magnetism creates multiferroics, a fertile field with exciting applications. However, prior studies have largely focused on insulating materials to facilitate the polarity. Polar magnetic metals are exceptionally rare, yet they offer a unique playground for discovering novel physics and properties because their conductivity, spin order and lattice distortions can interact in unexpected ways. In this talk, I will present our strategy for designing such exotic materials, beginning with our discovery of a correlated ferromagnetic polar metal, Ca3Co3O8.1 Its structure features an alternating stack of oxygen tetrahedral CoO4 monolayers and octahedral CoO6 bilayers. The ferromagnetic metallic state is confined within the quasi-two-dimensional CoO6 layers, while the broken inversion symmetry arises specifically from the Co displacements. This interplay leads to pronounced nonreciprocal charge transport and a large topological Hall effect. Building on this, I will further introduce our discovery of a polar antiferromagnetic metal in the double-layered perovskite Sr3Co2O7.2 In this material, a geometric imbalance in the cobalt sublattice simultaneously breaks inversion symmetry and stabilizes an A-type antiferromagnetic ground state, all while maintaining metallicity. The most striking consequence is the observation of a pronounced zero-field anomalous Hall effect in the absence of a net magnetization. This phenomenon is attributed to the collective breaking of parity-time-reversal symmetry by the intertwined polar and antiferromagnetic orders, a mechanism distinct from that in ferromagnets. We envision these findings open exciting avenues for exploring emergent electronic states and developing next-generation spintronic devices that leverage the synergy between polarity, magnetism, and metallicity.