Successive magnetic transitions of Ca$_2$CoSi$_2$O$_7$ in high magnetic fields

Magnetic and dielectric properties of \aa kermanite Ca$_2$CoSi$_2$O$_7$ single crystals were investigated in pulsed high magnetic fields. In magnetic fields along the $c$ axis, this material shows a magnetization plateau in a wide range of field below the saturation. Magnetization processes for fields along the $a$ and $b$ axes show multiple anomalies but different traces to each other, indicating the breaking of four-fold symmetry. Measurements of the magnetoelectric effects exhibit the changes in electric polarization according to the changes in the spin system. The experimentally determined quadratic magnetoelectric tensor is consistent with that expected in the crystal symmetry of orthorhombic $P2_12_12$.