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

The magnetic properties of LixN1-xO compounds with x ranging between 0.4 and 0.49 are investigated. Magnetization and ac susceptibility measured at temperatures between 2 K and 300 K reveal a high sensitivity to x, the excess lithium concentration. We introduce a percolation model describing the formation of Ni clusters and use an Ising model to simulate their magnetic properties. Numerical results, obtained by a Monte Carlo technique, are compared to the experimental data. We show the existence of a critical concentration, x(c) = 0.432, locating the Ni percolation threshold. The system is superparamagnetic for x > x(c), while it is ferrimagnetic for x < x(c). The 180 degrees Ni-O-Ni interplane superexchange coupling J(perpendicular to) similar or equal to -110 K is confirmed to be the predominant magnetic interaction. From the low-temperature behavior, we find a clear indication of a 90 degrees Ni-O-Ni intraplane antiferromagnetic interaction J(parallel to) similar or equal to -1.5 K, which implies magnetic frustration.