We have discovered the appearance of ferroelectricity below the Neel temperature of 6.7K in the antiferromagnetic Ba2CoGe2O7 with square magnetic lattice. The ferroelectric polarization is rather small in magnitude and aligned along the c axis, but its magnitude increases remarkably and its direction rotates away smoothly from the c axis when magnetic fields are applied along the c axis. The electric polarization’s behavior of smooth rotation, rather than a sudden flip, is unique. It also accompanies a linear increase in magnetization and a smooth change of dielectric constant with little hysteresis, which may be important for certain technological applications.

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