Figure 1: Magnetic field-induced phase transition in multiferroic Ba₂CoGe₂O₇.

(a) Structure of Ba₂CoGe₂O₇ as perceived from a direction perpendicular to the ab-plane. Two Co²⁺ ions sit at the (0,0,0) and (0.5,0.5,0) positions of a unit cell. Co²⁺ ions are placed inside a oxygen tetrahedra that is tilted from the [110] direction by angles +κ and−κ. (b) Magnetization as a function of the magnetic field along the c-axis. Magnetization saturates above H_c=37.1 T. Inset shows the canted spin structure at H<H_c (top left) and fully saturated spin structure at H≥H_c (bottom right). (c) Dielectric constant (ε) measured at 1 kHz. (d) Electric polarization (P) along the a-axis as a function of the magnetic field applied along the c-axis. Magnetic field-sweep rates for ε and P measurements are 0.0083 and 0.033 T s⁻¹, respectively.