A Study of the Magnetic Properties of the Frustrated Spinels GeNi2O4 and GeCo2O4

The spinels GeNi$_{2}$O$_{4}$ and GeCo$_{2}$O$_{4}$, in which the spin-1 Ni$^{2+}$ or spin-3/2 Co$^{2+}$ ions are located on the vertices of a lattice of corner-sharing tetrahedra, exhibit interesting magnetic and structural properties. GeNi$_{2}$O$_{4}$ has a double N\'{e}el transition (T$_{N1}$ = 12.13 K and T$_{N2}$ = 11.46 K), but the crystal structure remains cubic in the N\'{e}el state. In contrast, GeCo$_{2}$O$_{4 }$has a single N\'{e}el transition (T$_{N}$ = 20.6 K) that coincides closely with a cubic to tetragonal structural phase transition, below which $c$/$a >$ 1. In the past we have used magnetic susceptibility, heat capacity, synchrotron x-ray, and neutron powder diffraction to study these materials. In this talk we will describe selected recent results of elastic and inelastic neutron scattering measurements of polycrystalline and single crystalline samples, the latter grown by the floating zone technique at AIST.