Magnetic cluster formation in LixNi1-xO compounds: Experiments and numerical simulations

The magnetic properties of LixN1-xO compounds with x ranging between 0.4 an d 0.49 are investigated. Magnetization and ac susceptibility measured at te mperatures between 2 K and 300 K reveal a high sensitivity to x, the excess lithium concentration. We introduce a percolation model describing the for mation of Ni clusters and use an Ising model to simulate their magnetic pro perties. Numerical results, obtained by a Monte Carlo technique, are compar ed to the experimental data. We show the existence of a critical concentrat ion, x(c) = 0.432, locating the Ni percolation threshold. The system is sup erparamagnetic for x > x(c), while it is ferrimagnetic for x < x(c). The 18 0 degrees Ni-O-Ni interplane superexchange coupling J(perpendicular to) sim ilar or equal to -110 K is confirmed to be the predominant magnetic interac tion. From the low-temperature behavior, we find a clear indication of a 90 degrees Ni-O-Ni intraplane antiferromagnetic interaction J(parallel to) si milar or equal to -1.5 K, which implies magnetic frustration.