The magnetic-field dependence of the specific-heat measurements on ${\mathrm{Pb}}_2$${\mathrm{Sr}}_2$${\mathrm{TbCu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ with 0.5 K<T<30 K, are reported in order to study the magnetic order in this compound. Anomalies in the specific heat C(T) of ${\mathrm{Pb}}_2$${\mathrm{Sr}}_2$${\mathrm{TbCu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ are observed at ${\mathit{T}}_{\mathit{N}1}$∼5.3 K and ${\mathit{T}}_{\mathit{N}2}$∼2.4 K, which are related to two antiferromagnetic transitions in the Tb sublattice. While the anomaly seen in the specific heat as well as in the magnetic susceptibility at ${\mathit{T}}_{\mathit{N}1}$∼5.3 K is suppressed, the peak position is unaffected with increasing magnetic fields. On the other hand, the anomaly at ${\mathit{T}}_{\mathit{N}2}$∼2.4 K is quickly suppressed by a low field. The entropy change associated with the magnetic transitions deduced from the specific heat of ${\mathrm{Pb}}_2$${\mathrm{Sr}}_2$${\mathrm{TbCu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ by subtracting the data from ${\mathrm{Pb}}_2$${\mathrm{Sr}}_2$${\mathrm{YCu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ up to T=12 K is about 5.8 J/mol K (∼R ln2), indicative of a doublet ground state for Tb in the crystalline electric fields. In addition, the magnetic specific heat far below the transitions has the form ${\mathit{C}}_{\mathit{M}}$(T)=${\mathit{AT}}^2$+B/${\mathit{T}}^2$, suggesting contributions from two-dimensional magnon and nuclear Schottky, respectively. These results are discussed in comparison to those of related compounds, such as R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{Pb}}_2$${\mathrm{Sr}}_2$R${\mathrm{Cu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$.

• Received 22 April 1994

DOI:https://doi.org/10.1103/PhysRevB.50.9619