Magnetic Frustration and Magnetoelectric Coupling in CoSe$_2$O$_5$

We present structural and magnetic measurements on CoSe$_2$O$_5$, a compound with one dimensional chains of irregular edge-shared CoO$_6$ octahedra, separated by Se$_2$O$_5$ units. Below 8.5\,K low-field magnetic susceptibility and heat capacity measurements show long range antiferromagnetic order develops. The magnetic structure of this ordered state has been determined by neutron diffraction to consist of moments aligned antiparallel along the length of the chain as well as antiparallel to neighboring chains. The magnetic ordering becomes more complex when the compound is cooled under strong fields, with highly non-linear $M-H$ behavior below the ordering temperature. Measurement of the dielectric constant and pyrocurrent has also shown exposure to fields larger than 3\,T results in a spontaneous electric polarization of the order of 1.5\,$\mu$C m$^{-2}$ which develops below the magnetic transition temperature.