LAUSR

We investigate phase transitions of a geometrically frustrated polar magnet ${\mathrm{CaBaCo}}_{4}{\mathrm{O}}_{7}$, which exhibits the largest magnetoelectric (ME) effect in multiferroic compounds reported so far. Measurements of magnetization, sound velocity, and neutron diffraction of single crystals reveal an antiferromagnetic phase between the ferrimagnetic transition temperature ${T}_{\mathrm{C}}\ensuremath{\sim}60$ $\mathrm{K}$ and ${T}^{*}\ensuremath{\sim}69$ $\mathrm{K}$. In the antiferromagnetic phase, the ordered Co moments on the triangular layers remain nearly disordered. We demonstrate that a large electric-polarization change emerges upon the antiferromagnetic-to-ferrimagnetic transition. The large ME coupling can be attributed to the exchange striction.