Percolation effects and magnetic properties of the randomly diluted fcc system ${\mathrm{Co}}_{\mathrm{p}}$${\mathrm{Mg}}_{1\mathrm{-}\mathrm{p}}$O

Temperature-dependent, low-field (50 Oe) dc magnetic susceptibility (χ) studies are reported for the randomly diluted fcc system ${\mathrm{Co}}_{\mathrm{p}}$${\mathrm{Mg}}_{1\mathrm{-}\mathrm{p}}$O. The studies, using powder samples, covered the temperature range 1.6–300 K and the composition range 0.10≤p<1. Variations of the Néel temperature $T_N$, the Curie-Weiss temperature Θ, the Curie constant C, the low-temperature magnetization M, and the lattice constant $a_0$ with p are reported. The observed percolation threshold $p_c$=0.13 agrees well with the predicted value of $p_c$=0.136 for fcc system with nearest-neighbor and next-nearest-neighbor interactions included. The variation of Θ(p) and $C_M$(p) with p is found to be linear whereas that of t=$T_N$(p)/$T_N$(1) is more complex, showing a crossover near p=0.45, where $d^2$t/${\mathrm{dp}}^2$ changes sign. Calculations are needed to explain this observed t versus p variation. For $p_c$χ below $T_N$ depends upon how the sample is cooled through $T_N$, and the hysteresis and time dependence (relaxation) of χ are observed at 4.2 K if the sample is cycled through a higher field (6 kOe). These observations are not fully understood. An estimate of the exchange constants for the ${\mathrm{Co}}_{\mathrm{p}}$${\mathrm{Mg}}_{1\mathrm{-}\mathrm{p}}$O system is given.