Sujet de stage / Master 2 Internship

The aim of this internship is to find new rare-earth garnets with better magnetocaloric performances as an alternative to liquid helium.

Alternate coolants are needed to replace the use of increasingly scarce liquid helium, required, for instance, to cool the superconducting magnets used in medical resonance imaging. Magnetocaloric materials, with their entropically driven cooling power when cycled in a magnetic field, are such a replacement. The gadolinium-based garnets developed recently show amongst the largest magnetocaloric effects (MCE) ; yet the cooling power of those materials peaks below 2 K, too low for many applications of liquid helium.

The aim of this internship is to find new rare-earth garnets with better magnetocaloric performances : to this date, much remains to be explored and understood about the optimization of the MCE in Ln3Ga5O12 garnets. All the more so that the garnet structure is adaptable enough to offer a rich playground for substitutions : the cation sites can be divided in three distinct environments, a dodecahedral one for the rare-earth ions on the hyperkagome network, and an octahedral (B) and a tetrahedral (C) environment.

The internship will focus on the investigation of high-entropy garnet oxides to achieve this goal. High-entropy oxides are single-phased materials that contain five or more principal cations in equimolar amounts on a given site, so as to minimize the Gibbs energy of mixing and favours the thermodynamic stabilization of a single phase with multiple cations. Chemical disorder is a known cause of sluggish magnetic phase transitions during cooling, which often result in an enhanced MCE. Therefore, high-entropy garnets are potentially very attractive candidates as magnetic refrigerants.