Effect of Co-Substitution on Magnetic Properties in Spinels GeNi2O4 Systems

The magnetic properties of spinel GeNi2-2xCo2xO4 systems in the range 0 ≤ x ≤ 1 are studied by mean Geld theory and high-temperature series expansions. The nearest neighbouring and the next-neighbouring super-exchange interactions J1(x) and J2(x) are evaluated for these systems in the range 0 ≤ x ≤ 1, by using the first theory. The intra-planar and the inter-planar interactions and the exchange energy are deduced. The second theory is applied in the spinel GeNi2-2xCo2xO4 systems, combined with the Padé approximants method, to determine the magnetic phase diagrams (Tn versus dilution x) in the range 0 ≤ x ≤ 1. The obtained theoretical results are in agreement with experimental data obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility γ and the correlation lengths v are deduced. (condensed matter: electronic structure, electrical, magnetic, and optical properties)