Neutron diffraction and magnetic susceptibility studies of $\left({\mathrm{Li}}_{1-3x}{\mathrm{Fe}}_x\right){\mathrm{NiPO}}_4\hspace{0.5em}(x=0\mathrm{}$ and $x=0.033)$ compounds reveal remarkable differences between the magnetic properties of pure ${\mathrm{LiNiPO}}_4$ and those of its lightly iron-doped derivative. The spin system associated with the ${\mathrm{Ni}}^{2+}$ ions $\mathrm{}(S=1\mathrm{})\mathrm{}$ in the pure compound undergoes a collinear antiferromagnetic ordering at $T_N=19.1\mathrm{}\pm 0.5\hspace{0.5em}\mathrm{K},$ with the characteristics of weakly coupled two-dimensional (2D) Ising square planes. By contrast, randomly intercalated iron spins (in Li sites) between ${\mathrm{Ni}}^{2+}$ planes comprise a spin-glass-like subsystem which, despite their minute amount, drives the antiferromagnetic transition to higher temperatures $T_N=25.2\mathrm{}\pm 0.5\hspace{0.5em}\mathrm{K},$ and significantly modifies the critical behavior of the 2D ${\mathrm{Ni}}^{2+}$ system. It is argued that the doped compound can serve as a model system for studying the randomly coupled planar Ising model.

• Received 16 October 1998

DOI:https://doi.org/10.1103/PhysRevB.60.1100