Magnetic and structural investigations on NiI2 and CoI2

Measurements of the magnetic susceptibility of NiI2 show an anisotropic behaviour and a magnetic ordering temperature of 75 K. The nuclear and magnetic structures are examined with X-ray and neutron diffraction. The nuclear symmetry changes at 60 K from trigonal to monoclinic. The magnetic structure of NiI2 is an incommensurate helix of type 1, with propagation vector Qx=0.1384a∗; Qy=0; Qz=1.457c∗. The anisotropy of the magnetic susceptibility of CoI 2 is due to single-ion anisotropy. The magnetic structure is a commensurate helix of type 1, with Qx=18a∗; Qy=0; Qz 12c∗. 129I-Mössbauer data of NiI2 and CoI2 are in good agreement with the paoposed magnetic structures. The stability of magnetic structures in trigonal layer compounds is discussed. It is found that the structures of CoI2 and NiI2 are close to the phase boundary between helix-1 type structures and structures with ferromagnetic layers.