Percolation effects and magnetic properties of the randomly diluted fcc system CopMg1-pO

Temperature-dependent, low-field (50 Oe) dc magnetic susceptibility (χ) studies are reported for the randomly diluted fcc system Co_pMg_1-pO. 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 a₀ 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 d²t/dp² changes sign. Calculations are needed to explain this observed t versus p variation. For p_c<p<0.45, 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 Co_pMg_1-pO system is given.