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We define the concentration range where a dilute alloy of transition impurities in a noble matrix is a spin glass, by the possibility of representing its properties through universal functions of the T/c and H/c variables. The susceptibility of a spin glass contains one reversible part χr which shows a sharp peak at a temperature TM ~ c. This sharp peak is due to the presence of a remanent magnetization which appears when T < TM. The remanent magnetization of a spin glass is represented by an universal curve in the reduced diagram σr = f (T/c). Its properties are like those of the remanent magnetization of monodomains. We interpret it by supposing that at T << TM a spin glass (in which the magnetic moments randomly distributed are frozen at T = 0 in random directions) is spontaneously divided in regions, each one containing in average n impurities and having a resulting moment Mg described by a Gaussian distribution ([MATH]2 = ηµ20, where µ0 is the individual magnetic moment)

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A study of magnetic properties in the insulating spinel ZnCrl 6Gao 404 (chromium ions in octahedral sites) has been made by carrying out low-field dcand ac-susceptibility measurements as well as neutron diffraction, Mossbauer, and EPR experiments, which all give evidence of spin-glass-like behavior. The topological frustration, connected with the high ground-state degeneracy of the antiferromagnetic spinel octahedral sublattice, is the origin of the spin-glass ordering. Strong nearestneighbor antiferromagnetic interactions are dominant in the system, leading to short-range magnetic order which appears at temperatures much higher than the freezing temperature defined by the acsusceptibility cusp [T(v=17 Hz}=2.50 K is frequency dependent]. The thermoremanent magnetization does not vanish at Tf, suggesting the existence of uncompensated clusters blocked above Tf.A paramagnetic behavior is observed below T=1 K, providing evidence for the presence of unfrozen spins below Tf (entropic clusters).

Si clathrates ͑Ba 1−x Sr x ͒ 8 Si 46 with x ഛ 0.75 have been successfully synthesized and their superconductivity studied both experimentally and by ab initio calculations. The linear decrease of the cubic lattice parameter with the Sr content shows the continuous shrinkage of the constituting ͑Ba/ Sr͒ @ Si20 and ͑Ba/ Sr͒ @ Si24 cages, while the critical superconducting transition temperature T c decreases from 8 K to 4.1 K at 75% of substitution. The experimentally extrapolated value for the T c of the hypothetical Sr 8 Si 46 is in good agreement with the calculated one of 3 K. The lower contribution of the Sr d-electrons to the density of states at the Fermi energy explains the observations.