Weakly (x=0) and randomly (x=0.033) coupled Ising antiferromagnetic planes in (Li_1-3xFe_x)NiPO₄ compounds

Neutron diffraction and magnetic susceptibility studies of (Li_1-3xFe_x)NiPO₄(x=0 and x=0.033) compounds reveal remarkable differences between the magnetic properties of pure LiNiPO₄ and those of its lightly iron-doped derivative. The spin system associated with the Ni²⁺ ions (S=1) in the pure compound undergoes a collinear antiferromagnetic ordering at T_N=19.1±0.5K, with the characteristics of weakly coupled two-dimensional (2D) Ising square planes. By contrast, randomly intercalated iron spins (in Li sites) between Ni²⁺ planes comprise a spin-glass-like subsystem which, despite their minute amount, drives the antiferromagnetic transition to higher temperatures T_N=25.2±0.5K, and significantly modifies the critical behavior of the 2D Ni²⁺ system. It is argued that the doped compound can serve as a model system for studying the randomly coupled planar Ising model.