Magnetic moment of Eu-dopant and superconductivity in Eu-substitutional Ba8Al5Si41 clathrates

Si clathrates Ba8−xEuxAl5Si41 with nominal x = 0, 0.1, 0.2, 0.3 and 0.4 have been synthesized by arc melting and acid washing so as to delete impure phases. Eu substitution for Ba in the clathrate results in introduction of magnetic moment and modification of crystalline structure. The decrease of the cubic lattice parameter a with the Eu content implies the incorporation of the Eu into the cages. The critical superconducting transition temperature TC decreases from 6.8 to 4.5 K as x increases from 0 to 0.3. For sample with nominal x = 0.4, no diamagnetic behavior indicative of superconductivity was found down to 2 K. These suggest that due to the introduction of the magnetic moment of Eu, the superconductivity was suppressed. In addition, the ferromagnetic interaction was also observed at the same time. For the Eu-free sample, both M-T and M-H curves show the typical behavior of type-II superconductivity. With the increases of Eu content, the superconducting lower critical field HC1 decreases and Weiss temperature increases. As x reaches to 0.4, superconductivity is totally suppressed and the sample shows only ferromagnetic behavior at low temperature.