Anomalous low-temperature magnetic behaviour in DyRu

The body-centred tetragonal compounds DyRu2Si2 and ${\rm DyRu_2Ge_2}$ order within a collinear modulated magnetic structure at $T_{\rm N} = 29$ and 22 K, respectively. Below $\sim 5\;\rm K$, both compounds present a low-field low-temperature magnetic state which exhibits several non-conventional features. The most spectacular one is a double-peak structure in the temperature dependence of the specific heat, associated with a giant variation of the magnetic entropy. Other anomalous characteristics include an unexpected change of behaviour in the magnetization processes and an unusual variation of the resistivity in this temperature range. The nature of this low-temperature phase is a puzzle