Experimental evidence for competition between antiferromagnetic and ferromagnetic correlations in HgCr2S4

A detailed study of polycrystalline and single crystalline samples of the normal spinel HgCr2S4 is reported. The structural refinement reveals enhanced values of the atomic displacements suggesting the 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 T-N=22 K. We found highly unconventional behavior resembling properties of a noncollinear antiferromagnet and of a soft ferromagnet dependent on temperature and magnetic field. Even weak external magnetic fields disturb the antiferromagnetic order and strongly enhance the ferromagnetic correlations. The observed anomalies are related to bond frustration due to competing magnetic exchange interactions between the Cr ions.