Highly anisotropic 1/3-magnetization plateau in a ferrimagnet Cs 2 Cu 3 (SeO 3 ) 4 ·2H 2 O: topology of magnetic bonding necessary for magnetization plateau.

We prepared Cs 2 Cu 3 (SeO 3 ) 4 ·2H 2 O composed of Cu 2+ ions at square-planar coordination sites and characterized its structural and magnetic properties, to show that Cs 2 Cu 3 (SeO 3 ) 4 ·2H 2 O is a ferrimagnet exhibiting a highly anisotropic 1/3-magnetization plateau. This unprecedented anisotropy in a magnetization plateau is the consequence of three effects, namely, the orthogonal arrangements of the corner-sharing CuO 4 square planes, the nearest-neighbour antiferromagnetic exchange, and the anisotropic g -factor of the Cu 2+ ions at square-planar coordination sites. By analyzing the topology of magnetic bonding, we found why magnetic plateaus are observed only for certain ferrimagnets and antiferromagnets.