Magnetic and dielectric properties of A₂CoSi₂O ₇ (A=Ca, Sr, Ba) crystals

We have investigated the magnetic and dielectric properties of A ₂CoSi₂O₇ (A=Ca, Sr, and Ba) crystals with a two-dimensional network of CoO₄ and SiO₄ tetrahedra connected with each other through the corners. In Ca₂CoSi ₂O₇, a weak ferromagnetic transition occurs at 5.7 K, where the dielectric constant parallel to the c axis shows a concomitant anomaly. The large magnetocapacitance effect is observed below 5.7 K; Δ(H)/(0) [(H) - (0)]/(0) reaches 13 % at 5.1 K. These results indicate a strong coupling between the magnetism and dielectricity in Ca ₂CoSi₂O₇. Sr₂CoSi₂O ₇ shows a similar magnetoelectric behavior to that of Ca ₂CoSi₂O₇. In contrast, in Ba ₂CoSi₂O₇, which has the different arrangement of SiO₄ and CoO₄ tetrahedra from that of Ca ₂CoSi₂O₇, the magnetocapacitance is hardly observed. The key for the magnetocapacitance effect of A₂CoSi ₂O₇ lies in the quasi-two-dimensional crystal structure.