Neutron diffraction study on Co-based melilite compounds

Abstract

Layered transition metal oxides have attracted much attention owing to their interesting electronic and magnetic properties. The melilite structure A2XB2O7 (A = alkali metals and lanthanides, X = transition metals, B = Si, Ge) generally has a tetragonal structure and XO4 tetrahedra form two-dimensional network on the ab plane with B atoms between layers. They have a wide variety of magnetic structures, ranging from long-ranged commensurate magnetic orders to incommensurate magnetic orders [1]. Furthermore, these compounds are known to exhibit multiferroic properties with magnetic origins due to tilted antiferromagnetism and spiral magnetic structures induced by Dzyaloshinskii-Moriya interaction (DMI) associated with inversion symmetry broken structure.

Sr2CoSi2O7 and Sr2CoGe2O7 have the C-type antiferromagnetic magnetic structures with magnetic moments pointing to the ab-plane [2,3]. Recent neutron diffraction experiments on single crystalline samples of Sr2CoSi2O7 [3] and isostructural Ba2CoGe2O7 [4] have revealed the tilted antiferromagnetic structures, but the R factors for two magnetic space groups Cm′m2′ and P212′12′ are quite comparable and thus the magnetic structure has not yet been refined. Electric polarization measurements would be required to determine the intrinsic magnetic space group in these materials.

In this study, we performed neutron powder diffraction to refine magnetic structures of Sr2CoB2O7 (B = Si, Ge), which is expected to have a magnetic structure canted by DMI due to its non-centrosymmetric structure. Using the powder diffractometer ECHIDNA at ANSTO, magnetic reflections were observed at the base temperature, 3 K. All magnetic reflections can well be indexed by the magnetic wavevector qm = (0,0,0) r.l.u., and the magnetic space group was used for the magnetic structure refinement. In this presentation, we will give an overview, our experimental findings of the neutron powder diffraction and magnetic structure analysis, and will discuss the refined magnetic structures and their relations to DMI.