Crystal growth and anisotropic magnetic properties of quasi-two-dimensional (Fe1-xNix)2P2S6

We report the crystal growth by chemical vapor transport together with a thorough structural and magnetic characterization of the quasi-2D magnets (Fe1-xNix)(2)P2S6 with x(Ni) = 0, 0.3, 0.5, 0.7, 0.9, and 1. As-grown crystals exhibit a layered morphology with weak van der Waals interactions between layers parallel to the crystallographic ab plane of the monoclinic structure in the space group C2/m (No. 12). Magnetization measurements reveal an antiferromagnetic ground state for all grown crystals. In the ordered state, the magnetization along different crystallographic directions is in agreement with an Ising anisotropy for 0 <= x <= 0.9 and only for Ni2P2S6 a different behavior is observed which is in line with an anisotropic Heisenberg or XXZ model description. This striking abrupt change of anisotropy at a critical Ni concentration is coupled to a change in the width of the maximum in the magnetization around T-max. Furthermore, all intermediate compounds 0 < x <= 0.9 exhibit an anisotropic magnetization already in the paramagnetic state similar to the parent compound Fe2P2S6.