Competing exchange interactions in multiferroic and ferrimagnetic CaBaCo4O7

Competing exchange interactions can produce complex magnetic states together with spin-induced electric polarizations. With competing interactions on alternating triangular and kagome layers, the swedenborgite CaBaCo4O7 may have one of the largest measured spin-induced polarizations of ∼1700 nC/cm2 below its ferrimagnetic transition temperature at 70 K. Upon rotating our sample about c=[0,0,1] while the magnetic field is fixed along [1,0,0], the threefold splitting of the spin-wave frequencies indicates that our sample is hexagonally twinned. Magnetization measurements then suggest that roughly 20% of the sample is in a domain with the a axis along [1,0,0] and that 80% of the sample is in one of two other domains with the a axis along either [-1/2,3/2,0] or [-1/2,-3/2,0]. Powder neutron-diffraction data, magnetization measurements, and terahertz (THz) absorption spectroscopy reveal that the complex spin order in each domain can be described as a triangular array of bitetrahedral c-axis chains ferrimagnetically coupled to each other in the ab plane. The electric-field dependence of bonds coupling those chains produces the large spin-induced polarization of CaBaCo4O7.