Magnetic control of electric polarization in the noncentrosymmetric compound (Cu,Ni)B₂O₄

The weak ferromagnetic moment in Ni-doped CuB₂O₄ can be rotated by applying an electric field. While this implies spin-driven ferroelectricity, no direct evidence of electric polarization has been reported to date. Here we report the induction and control of polarization in the borate in the presence of an external magnetic field. Applying a magnetic field along the [110] or [11̄0] axis induces electric polarization along the [001] axis. The polarization is reversed by switching the magnetic-field direction between the [110] and [11̄0] axes. The mechanism by which electric polarization emerges can be well explained in the framework of spin-dependent metal-ligand hybridization.