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

The magnetic properties of spinel GeNi_2-2xCo_2xO₄ systems in the range 0≤x≤1 are studied by mean field 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 GeNi_2-2xCo_2xO₄ systems, combined with the Padé approximants method, to determine the magnetic phase diagrams (T_N 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 ν are deduced.