We report on x-ray diffraction, magnetic susceptibility, electron-spin resonance and heatcapacity studies of Co[Al 1-x Co x ] 2 O 4 for Co concentrations 0 ≤ x ≤ 1. In this spinel system only the A-site Co 2+ cation is magnetic, while the non-magnetic B-site Al 3+ is substituted by the lowspin non-magnetic Co 3+ , and it is possible to investigate the complete phase diagram from

A detailed study of poly-and single crystalline samples of the normal spinel HgCr 2 S 4 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 T N = 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.

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Using surface acoustic waves, the elastomagnetic coupling could be studied in thin single crystalline plates of the ferrimagnetic semiconductor FeCr 2 S 4 by measuring the attenuation and the frequency tracking in the temperature range 4.2 K to 200 K. The data clearly display the anomalies found in low-field magnetization measurements.