Enhanced cryogenic magnetocaloric effect in Eu8Ga16Ge30 clathrate nanocrystals

We observe an enhanced magnetic entropy change ( D S M ) at cryogenic temperatures (T < 20 K) in Eu 8 Ga 16 Ge 30 clathrate (type-I) nanocrystals prepared by a ball milling method. With reduction in the crystal size to 15 nm, D S M is enhanced at low temperatures, reaching the highest value ( 10 J/kg K) at 5 K for a field change of 5 T. For all samples investigated, there is a cross-over tem- perature ( 25 K) in D S M (T) above which D S M decreases with crystal size, opposite to that observed at low temperatures. A careful analysis of the magnetic and magnetocaloric data reveals that as the crystal size decreases the magnetic interaction between Eu 2 þ ions on the Eu2 site gov- erning the primary ferromagnetic transition at 35 K becomes gradually weaker, in effect, altering the interaction between Eu 2 þ ions occupying the Eu1 and Eu2 sites responsible for the secondary ferromagnetic transition at 15 K. As a result, we have observed a strong change in magnetization and the enhancement of D S M at low temperature.