Abstract
Cubic spinel GeNi2O4 and GeCo2O4 compounds exhibit field-induced magnetic anisotropy below the antiferromagnetic transition temperature. GeNi2O4 has two antiferromagnetic like transitions at 12.1 K and 11.4 K. Structural phase transitions (PT) which may lower the symmetry and relief frustration of magnetic spins are expected at the temperatures. However, this cubic material does not undergo a structural PT. On the other hand, GeCo2O4 has a structural PT from high-temperature cubic symmetry to low-temperature tetragonal one at the temperature. Although GeCo2O4 demonstrates antiferromagnetic ordering, derived Weiss temperature is ferromagnetic (11.9 K). In order to understand those intriguing ground states in detail, we have measured AC susceptibility below 1 K. The results show there are no other phase transitions above 150 mK for both compounds.
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