Structure and properties of the integer-spin frustrated antiferromagnet ${\mathrm{GeNi}}_2{\mathrm{O}}_4$

We report the results of magnetic susceptibility, specific heat, synchrotron x-ray, and neutron powder diffraction measurements for the normal spinel ${\mathrm{GeNi}}_2{\mathrm{O}}_4,$ which becomes antiferromagnetic below a Néel temperature ${(T}_N)$ of 12 K. The Néel transition occurs in two discrete steps, separated in temperature by 0.6 K. The total magnetic entropy evaluated from the specific heat data is only $\sim 1/2$ of the expected $2R\ln 3$ per mole of ${\mathrm{GeNi}}_2{\mathrm{O}}_4.$ The specific heat data also suggest the presence of both gapless and gapped excitations within the Néel state. ${\mathrm{GeNi}}_2{\mathrm{O}}_4$ remains cubic to temperatures well below $T_N.$