Magnetic structure and susceptibility of CoSe2O5: An antiferromagnetic chain compound

CoSe2O5 has a crystal structure consisting of zigzag chains of edge shared CoO6 octahedra running along the c axis with the chains separated by Se2O52- units. Magnetic-susceptibility measurements indicate a transition at 8.5 K to an ordered state. We investigate the nature of this magnetic order using magnetization and specific-heat measurements, in addition to powder neutron diffraction. A transition to long-range antiferromagnetic order is found below T-N=8.5 K as identified by magnetic-susceptibility and heat-capacity measurements, and the emergence of magnetic Bragg reflections with a propagation vector k=0. The low-field magnetic-susceptibility data indicate the presence of weak ferromagnetism in the antiferromagnetic ordered state which saturates in very small fields. Heat-capacity measurements at low temperatures indicate that only about half the configurational entropy is released at T-N. The functional form of the heat capacity below TN suggests the presence of a gap of 6.5 K in the magnon dispersion. The refined magnetic structure shows that the moments align perpendicular to the c axis, and tilting alternately by +14 degrees and -14 degrees with respect to the a axis.