Magnetic Frustration In The Spinel Compounds Geco2o4 And Geni2o4

In both spinel compounds GeCo2O4 and GeNi2O4 which order antiferromagnetically (at T-N=23.5 K and T-N1=12.13 K/T-N2=11.46 K) with different Curie-Weiss temperatures (T-CW=80.5 K and -15 K), the usual magnetic frustration criterion f=parallel to T-CW parallel to/T-N > 1 is not fulfilled. Using neutron powder diffraction and magnetization measurements up to 55 T, both compounds are found with a close magnetic ground state at low temperature and a similar magnetic behavior (but with a different energy scale), even though spin anisotropy and first neighbor exchange interactions are quite different. This magnetic behavior can be understood when considering the main four magnetic exchange interactions. Frustration mechanisms are then enlightened.