Systematic dc magnetization studies using the Banerjee criterion, Kouvel-Fisher, and magnetocaloric effect methods provide physical insights into the origin of the magnetic anomaly and the tunneling effect of europium on the ferromagnetic ordering in Eu${}_8$Ga${}_{16}$Ge${}_{30}$ type-I clathrates. We show that Eu${}_8$Ga${}_{16}$Ge${}_{30}$ undergoes a second-order magnetic transition (SOMT) at $T_C$ ∼ 35 K, resulting from the magnetic interaction between the Eu${}^{2+}$ ions at the Eu2 sites, followed by a secondary magnetic transition at $T_L$ ∼ 10 K (indicated as a magnetic anomaly in previous studies), as a result of the magnetic interaction between the Eu${}^{2+}$ ions at the Eu1 and Eu2 sites. The critical exponent β = 0.388 is close to that predicted from the three-dimensional Heisenberg model (β = 0.365), while the critical exponent γ = 0.956 is close to that predicted from the mean-field model (γ = 1). The substitution of Sr${}^{2+}$ for Eu${}^{2+}$ retains the SOMT but largely reduces the transition temperatures ($T_C$ ∼ 15 K and $T_L$ ∼ 5 K), with the critical exponents β = 0.521 and γ = 0.917 close to those predicted from the mean-field model (β = 0.5 and γ = 1). These results point to the important fact that the tunneling of Eu${}^{2+}$ between the four equivalent sites in the tetrakaidecahedral cage tends to prevent the occurrence of a long-range ferromagnetic ordering in the type-I clathrate materials.

• Received 17 September 2010

DOI:https://doi.org/10.1103/PhysRevB.84.054436