The magnetization of the sigma-phase Fe(0.53)Cr(0.47) and Fe(0.52)V(0.48) alloys was studied as a function of temperature and field. The experiments show that both materials behave magnetically as re-entrant spin glass systems. Field versus temperature diagrams were obtained where the locations of the paramagnetic phase, the intermediate ferromagnetic-like phase and the spin glass fundamental state were displayed. These diagrams are in qualitative agreement with the predictions of the mean field theory for the interplay between the ferromagnetic and spin glass orderings. The critical phenomenology near the para-ferromagnetic transition could be investigated. It was found that the paramagnetic susceptibility is quite well described by the extended scaling scheme, where the reduced temperature is written as τ = (T - T(c))/T. The value obtained for the susceptibility critical exponent γ is intermediate between the prediction of the 3D Heisenberg universality class and the large values observed in spin glasses, as previously found in other re-entrant systems. The data do not confirm the validity of the extended scaling in the ferromagnetic-like phase. Using either the conventional or extended scaling protocols, the exponents β and δ were found to have values close to those reported for spin glass transitions. Despite the relevance of disorder and the anomalous values determined for β, γ and δ, the Widom scaling relation holds as an equality.

The magnetic behaviour of the antiferromagnet FeTa 2 O 6 was investigated through heat capacity, magnetic susceptibility and magnetization measurements. The broad magnetic susceptibility curve (peaked at 15 K) presents an inflection typical of two-dimensional systems. This inflection is observed near 10 K, the temperature at which early reported Mössbauer measurements indicate the onset of magnetic order. The heat capacity measurements have shown a clear transition at 8.7 K, where Mössbauer measurements show complete ordering. Magnetization curves, for temperatures between 1.4 K and 5 K, display a noticeable change in slope, suggesting a spin-flop transition induced by a field higher than 10 T. It is argued that this transition, which disappears for T > 6 K, is evidence of two-dimensional behaviour.

A detailed study of the magnetic properties of the tetragonal rare earth compound DyRu2Si2 is presented. Magnetization measurements on a single crystal as well as specific heat and resistivity measurements on a polycrystal show features which are typical of a long-period commensurate system in the presence of a huge uniaxial anisotropy. Multistep metamagnetism occurs below the Neel temperature TN=29 K, in particular at low temperatures. In the low-field low-temperature region, a new magnetic phase has been discovered with an anomalous behaviour.

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