Magnetoelastic coupling in multiferroic CaBaCo₄O₇

A large change in magnetic field induced polarization of ΔP ~8 mC/m² around T_C was discovered in multiferroic CaBaCo₄O₇ suggesting a giant linear magnetoelectric effect. The crystal structure of CaBaCo₄O₇ contains alternating triangular and kagome layers along c axis with distorted corner shared CoO₄ tetrahedra that makes these oxides a unique class of geometrically frustrated systems. It is assumed that the large magnetoelectric coupling arises from their atypical crystal structure and the mixed valence state of Co cations. It is well known that specific spin-lattice interaction can control the direction of the electrical polarization with a magnetic field. Although strong magnetoelastic coupling is expected in these improper ferroelectrics, there are hardly any studies to investigate the same. In this work we carry out a detailed investigation on magnetoelastic coupling using temperature dependent Raman measurements. We observe anomalous deviation of phonon frequencies and line-width from anharmonicity in the magnetic ordered phase. The maximum deviation is observed across Tc. This study suggests a strong magnetoelastic coupling in this compound in the ferrimagnetic phase and indicates the role of lattice in the gigantic spin induced polarization along c axis.