Antiferromagnetic-to-ferrimagnetic phase transition in frustrated polar magnet CaBaCo₄O₇

Magnetic frustration often plays important roles in multiferroic properties as is exemplified by the electric polarization induced by the spiral magnetic order. CaBaCo₄O₇ belongs to the orthorhombic space group Pbn2₁. Magnetic Co ions form Kagomé- and triangular- lattice layers, which alternately stack along the c-axis. CaBaCo₄O₇ shows a ferrimagnetic transition at T_c ∼ 60 K at zero magnetic field. CaBaCo₄O₇ is a multiferroic material hosting magnetic frustration[1]. It exhibits an electric polarization change of about 17 mC/m² at the ferrimagnetic transition which is the largest among all the multiferroic materials so far[2]. The specific heat and the electric permittivity of single crystal CaBaCo₄O₇ exhibits another anomaly at T_N∼69 K slightly higher than T_c in zero magnetic field, while the origin of the anomaly was not clarified[2].
We investigate the phase transition of CaBaCo₄O₇. An antiferromagnetic phase is formed between T_c ∼ 60 K and T_N ∼ 69 K in our magnetization measurement and single-crystal neutron diffraction measurement. The large electric polarization change in CaBaCo₄O₇ should be attributed to the antiferromagnetic-to-ferrimagnetic transition.
[1] Zhe Qu, et al., Solid State Commun. 151, 917-919 (2011).
[2] V. Caignaert, et al., Phys. Rev. B 88, 174403 (2013).