Origin of the magnetic anomaly and tunneling effect of europium on the ferromagnetic ordering in Eu8-xSrxGa16Ge30 (x=0,4) type-I clathrates

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 Eu8Ga16Ge30 type-I clathrates. We show that Eu8Ga16Ge30 undergoes a second-order magnetic transition (SOMT) at T-C similar to 35 K, resulting from the magnetic interaction between the Eu2+ ions at the Eu2 sites, followed by a secondary magnetic transition at T-L similar to 10 K (indicated as a magnetic anomaly in previous studies), as a result of the magnetic interaction between the Eu2+ ions at the Eu1 and Eu2 sites. The critical exponent beta = 0.388 is close to that predicted from the three-dimensional Heisenberg model (beta = 0.365), while the critical exponent gamma = 0.956 is close to that predicted from the mean-field model (gamma = 1). The substitution of Sr2+ for Eu2+ retains the SOMT but largely reduces the transition temperatures (T-C similar to 15 K and T-L similar to 5 K), with the critical exponents beta = 0.521 and gamma = 0.917 close to those predicted from the mean-field model (beta = 0.5 and gamma = 1). These results point to the important fact that the tunneling of Eu2+ 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.