Enhanced Cryogenic Magnetocaloric Effect in Eu8Ga16Ge30 Clathrate Nanocrystals

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We observe an enhanced magnetic entropy change (−ΔSM) at cryogenic temperatures (T < 20 K) in Eu8Ga16Ge30 clathrate (type-I) nanocrystals prepared by a ball milling method. With reduction in the crystal size to 15 nm, −ΔSM 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 temperature (∼25 K) in −ΔSM (T) above which −ΔSM 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 Eu2+ ions on the Eu2 site governing the primary ferromagnetic transition at ∼35 K becomes gradually weaker, in effect, altering the interaction between Eu2+ 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 −ΔSM at low temperature.

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Journal of Applied Physics, v. 117, issue 3, art. 033903