Two- and three-dimensional magnetic order in the layered cobalt oxychloride Sr2CoO3Cl

The temperature dependence of the nuclear and magnetic structure of the cobalt oxychloride Sr2CoO3Cl has been studied using neutron powder diffraction. The material crystallizes with a structure related to K2NiF4 and contains two-dimensional (2D) layers of CoO5 square pyramids that are segregated along z by alternate rocksalt SrCl and SrO blocks. The development of magnetic Bragg scattering indicates that the compound orders antiferromagnetically with a T-N=330(5) K. The phase adopts a collinear magnetic structure related to the nuclear cell by the propagation vector k=(1/2, 1/2, 0) with the cobalt spins aligned along the a axis of the magnetic cell. The ordered moment mu=2.82(3)mu(B), refined at 3 K, is consistent with a high-spin (t(2g)(4)e(g)(2)) electron configuration for the Co(III) ions. The onset of long-range magnetic order is characterized by a three-dimensional transition and is accompanied by anomalous behavior in the Co environment with distinct magnetostriction effects observed in the interlayer Co to Co exchange pathways. The transition is preceded by diffuse magnetic scattering arising from short-range in-plane correlations, with significant diffuse intensity observed up to the maximum temperature studied of 378 K. Magnetic susceptibility measurements indicate that the onset of significant 2D interactions occurs at Tapproximate to500 K. The diffuse intensity can be fitted using the Warren function to give a maximum in the 2D correlation length xi of 40(4) Angstrom just above T-N. Below T-N diffuse scattering coexists with magnetic Bragg scattering, indicating that the transition to long-range order is hindered most probably due to the presence of stacking disorder between the antiferromagnetic sheets.