Magnetic anisotropies in the Ising spin-chain compound BaCo2V2O8

The effective Ising spin 1/2 system BaCo2V2O8 consists of CoO6 octahedra that form screw chains along the crystallographic c axis which are separated by nonmagnetic Ba2+ and V5+ ions. Long-range antiferromagnetic order is observed below TN = 5.5 K in zero magnetic field with the spins aligned along c. Within the tetragonal ab-plane competing exchange interactions are present causing a lower symmetry of the stabilized magnetic structure. Therefore, magnetic domains occur with ferromagnetic alignment of the spins along [100] and an antiferromagnetic one along [010], or vice versa. High resolution zero-field thermal-expansion data show, that the system undergoes a structural transition from tetragonal to orthorombic while entering the Neel phase. Here, the frustration within the ab-plane is lifted by a small orthorhombic splitting (a-b)/(a+b) ≠ 0, that is below the resolution of diffraction techniques. Moreover, detailed investigations of e.g. magnetization, thermal expansion, specific heat, and thermal conductivity reveal an additional magnetic-field anisotropy within the ab-plane. The corresponding phase diagrams for magnetic fields along [100] and [110] are presented.