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

The magnetic properties of spinel GeNi(2-2x)Co(2x)O(4) 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 J(1)(x) and J(2)(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(2-2x)Co(2x)O(4) systems, combined with the Pade 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 gamma and the correlation lengths nu are deduced.