Magnetically driven loss of centrosymmetry in metallic Pb2CoOsO6

We report magnetic, transport, neutron diffraction, and muon spin rotation data showing that Pb${}_2$CoOsO${}_6$, a newly synthesized metallic double-perovskite with a centrosymmetric space group at room temperature, exhibits a continuous second-order phase transition at 45 K to a magnetically ordered state with a non-centrosymmetric space group. The absence of inversion symmetry is very uncommon in metals, particularly metallic oxides. In contrast to the recently reported ferroelectric-like structural transition in LiOsO${}_3$, the phase transition in Pb${}_2$CoOsO${}_6$ is driven by a long-range collinear antiferromagnetic order, with propagation vector $$\mathbf{\text{k}}$ = (\frac{1}{2},0,\frac{1}{2})$, which relieves the frustration associated with the symmetry of themagnetic exchanges. This magnetically-driven loss of inversion symmetry represents a new frontier in the search for novel metallic behavior.