Frustrated minority spins in GeNi₂O₄

Recently, two consecutive phase transitions were observed, upon cooling, in an antiferromagnetic spinel GeNi₂O₄ at T _N1=12.1 K and T_N2=11.4 K, respectively (Crawford M. K. et al., Phys. Rev. B, 68 (2003) 220408(R)). Using unpolarized and polarized elastic neutron scattering we show that the two transitions are due to the existence of frustrated minority spins in this compound. Upon cooling, at T_N1 the spins on the 〈111〉 kagome planes order ferromagnetically in the plane and antiferromagnetically between the planes (phase I), leaving the spins on the 〈111〉 triangular planes that separate the kagome planes frustrated and disordered. At the lower T_N2, the triangular spins also order in the 〈111〉 plane (phase II). We also present a scenario involving exchange interactions that qualitatively explains the origin of the two purely magnetic phase transitions.