Multiferroicity in the square-lattice antiferromagnet of Ba ₂CoGe₂O₇

We have discovered the appearance of ferroelectricity below the Neel temperature of 6.7 K in the antiferromagnetic Ba₂CoGe ₂O₇ 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.