Figure 1

Crystallographic structure and temperature (T) dependence of magnetic properties. (a,b) Views of the crystal structure of Co₄Ta₂O₉ (a space group P\(\overline{3}\)c1, No. 165) with the Co²⁺ moments in a selected magnetic domain from the top (a) and the side (b). Orange and pink spheres with arrows represent two inequivalent Co²⁺ ions and their spin directions, and light grey and yellow spheres denote nonmagnetic Ta⁵⁺ and O²⁻ ions, respectively. The grey box with the rhombic cross-section represents the crystallographic unit cell. (c) T dependence of magnetic susceptibility, χ = M/H, at magnetic field H = 0.1 T applied along the three inequivalent crystallographic orientations a, b^* and c. (d) T dependence of specific heat divided by the temperature, C/T, measured at H = 0 T. A dashed grey line indicates the Néel temperature, T_N = 20.5 K.