Low-temperature specific heat of the diluted magnetic semiconductor ${\mathrm{Hg}}_{1\mathrm{-}\mathit{x}\mathrm{-}\mathit{y}}$${\mathrm{Cd}}_{\mathit{y}}$${\mathrm{Fe}}_{\mathit{x}}$Se

The specific heat of the diluted magnetic semiconductors ${\mathrm{Hg}}_{1\mathrm{-}\mathit{x}\mathrm{-}\mathit{y}}$ ${\mathrm{Cd}}_{\mathit{y}}$ ${\mathrm{Fe}}_{\mathit{x}}$Se has been measured for temperatures below 25K. The excess specific heat (${\mathit{C}}_{\mathit{m}}$) has been extracted by scaling of nonmagnetic lattice contributions. The resulting ${\mathit{C}}_{\mathit{m}}$ of these systems reveals a broad maximum at T≊10 K, whereas for low temperatures a Schottky-type exponential decay of ${\mathit{C}}_{\mathit{m}}$ can be observed. These phenomena can be fairly well understood on the basis of a simple crystal-field model and a random distribution of ${\mathrm{Fe}}^{2+}$. On the other hand, no drastic effect of the energy gap is clearly reflected by our data and therefore we suggest that only ${\mathrm{Fe}}^{2+}$ states involving the valence bands dominate the antiferromagnetic d-d interactions, analogously to the ${\mathrm{Mn}}^{2+}$ case.