Magnetic polaron effect in Sr8−xEuxGa16Ge30clathrates probed by electron spin resonance

摘要

Single crystals of the type-I clathrate series Sr8-xEu xGa16Ge30 (x=0.01-8) were grown by Ga flux and systematically studied by magnetic susceptibility, electrical resistivity, and electron spin resonance (ESR) measurements. Both magnetic transition temperatures (Tc and T*) were mapped as a function of x. We found that the Curie temperature (Tc) and the magnetic anomaly temperature (T*) decrease systematically as Eu is substituted by Sr until no ordering is observed above 2 K for x=1.0. For samples where the rattling Einstein temperature of θE≈25-60 K lies above Tc, we have observed no features in the ESR data, suggesting that the Eu2+ 4f spin dynamics is weakly coupled to the rattling modes. As the temperature is lowered and the magnetic field is increased, the ESR linewidth (ΔH) behavior points to the formation of magnetic polarons. © 2013 American Physical Society.