Spin-glass versus "blocking" in dilute Eu_xSr _1-xS

The properties of this dilute insulator have been studied by two groups, either on single crystals or on powders. In single crystals, spin-glass properties (maximum of the susceptibility at Tf ∝ x) are observed down to x = 0.05. They are attributed to an essential role of the dipolar interaction. Below this concentration the blocking of isolated ferromagnetic pairs at Tp becomes predominant since Tp (concentration independent) becomes larger than Tf. In powders, an identical behavior is observed below x = 0.05. A more complex behavior, described by the blocking of magnetic clusters, is observed for 0.05 < x < 0.13 and spin-glass properties are only observed for x ≳ 0.13. Thus, the observation of a "dipolar spin-glass" in a dilute insulator depends on the sample preparation; it requires confirmation.