Frustrated magnetism of Co₄(BO₂)₆O

Using a hydrothermal method, single crystals of Co₄(BO ₂)₆O were synthesized. The DC susceptibility revealed a Curie-Weiss behavior at high temperature. The Weiss temperature was estimated at -190 K, showing very strong antiferromagnetic interactions. However, the susceptibility does not show any phase transitions down to 2 K, although there is a broad maximum in the vicinity of 14 K. This demonstrates an effect of the geometric frustration arising from the crystal structure based on isolated Co₄ tetrahedral clusters. The magnetization curve exhibits an onset of a transition to higher magnetization state at around 5 T. The result suggests that the Co₄ cluster has a nonmagnetic ground state with a spin gap and a magnetic field induces a transition into a magnetic state above the spin gap.