Experimental evidence for competition of antiferromagnetic and   ferromagnetic correlations in HgCr2S4
  A detailed study of poly- and single crystalline samples of the normal spinel HgCr2S4 is reported. The structural refinement reveals enhanced values of the atomic displacements suggesting closeness to a structural instability. Magnetization, electron-spin resonance and specific-heat studies document strong ferromagnetic fluctuations close to 50 K and the occurrence of complex antiferromagnetic order at TN = 22 K. We found highly unconventional behavior resembling properties of a non-collinear antiferromagnet and soft ferromagnet dependent on temperature and magnetic field. Even weak external magnetic fields disturb the antiferromagnetic order and strongly enhance the ferromagnetic correlations. The conductivity shows an insulating character with non-monotonous temperature behavior and a metal-to-insulator like transition at 80 K well above the magnetic transition. The observed anomalies are related to bond frustration due to competing magnetic exchange interactions between the Cr ions. 
V. Tsurkan, J. Hemberger, A. Krimmel, H.-A. Krug von Nidda, P.   Lunkenheimer, S. Weber, V. Zestrea, and A. Loidl