Doping through the percolation limit in GeNi2−xCoxO4

The solid solutions GeNi2−xCoxO4 have been prepared to examine the difference in ground states between integer and non-integer spins on the corner-shared tetrahedra lattice. It is found that there is a change in the evolution of the Weiss constant, and the magnetic transition temperatures near the percolation limit for the pyrochlore lattice. Substitution with Ni2+ results in a filling of the t2g orbitals and acts like a non-magnetic impurity. The sensitivity of the GeNi2O4 magnetic ground state to impurities is demonstrated by a change in sign of the Weiss constant for very small Co2+ doping (5%). A third magnetic transition in GeNi2O4 at T~11 K, in addition to previous transitions at TN1 = 12.1 K and TN2 = 11.5 K, is noted through dc susceptibility measurements. Evidence for phase separation in GeNi2O4 is observed.