Weakly (x=0) and randomly (x=0.033) coupled Ising antiferromagnetic planesin (Li1-3xFex)NiPO4 compounds

Neutron diffraction and magnetic susceptibility studies of (Li1-3xFex)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 der ivative. The spin system associated with the Ni2+ ions (S = 1) in the pure compound undergoes a collinear antiferromagnetic ordering at T-N = 19.1 +/- 0.5 K, with the characteristics of weakly coupled two-dimensional (2D) Isi ng square planes. By contrast, randomly intercalated iron spins (in Li site s) between Ni2+ planes comprise a spin-glass-like subsystem which, despite their minute amount, drives the antiferromagnetic transition to higher temp eratures T-N= 25.2 +/- 0.5 K; and significantly modifies the critical behav ior of the 2D Ni2+ system. It is argued that the doped compound can serve a s a model system for studying the randomly coupled planar Ising model.