Magnetism and charge order in the ladder compound Co3O2BO3

There are two known ludwigites containing a single transition metal element, Fe3O2BO3 and Co3O2BO3. The structure of these materials has low-dimensional units in the form of three-legged ladders (3LL) that confer to each of them unique magnetic and electronic properties. Fe3O2BO3 presents a staggered charge density wave (CDW) transition in the ladders near room temperature and two magnetic transitions. It has remained a mystery why the other compound Co3O2BO3 behaves so conventionally, with a single magnetic transition and no CDW in spite of similar structural and electronic configurations. Neutron diffraction results presented here in this system finally unravel these differences. Far from a trivial explanation, we uncover a coexistence of low and high spin Co ions in well-defined octahedral sites. Our results allow one to solve the contrasting behavior of the Fe and Co ludwigites in terms of a subtle and unique charge ordering mechanism occurring at the microscopic level of the rungs of the 3LL.Support from the Brazilian agencies CNPq and FAPERJ is gratefully acknowledged. We are grateful to Institut Laue-Langevin for neutron beam time allocation. J.A.R.-V. acknowledges CSIC for a JAEdoc contract.Peer Reviewe