Doan Nguyen, post-doc dans l’équipe « Croissance et propriétés de systèmes hybrides en couches minces » de l’INSP

En visioconférence

Abstract

We report fabrications of magnetocaloric thick films with the first and second order phase transitions. For the second transition material, we present gadolinium (Gd) free-standing flexible films prepared by sputtering on Silicon substrates. Tantalum is used as a buffer and capping layer. Microstructure, magnetic and magnetocaloric properties are measured before and after substrate removal. Results show a good agreement with data from high purity bulk Gd. Moreover, the free-standing film is flexible and magnetic properties are preserved under strain. The choice of the capping layers, deposition conditions and thickness have been carried to obtain a high quality, smooth surfaces which are suitable for energy conversion applications, where heat exchange takes place through solid-solid contact.

FeRh was selected as the main material for the first order phase transition. It shows a very high adiabatic temperature change due to its first order antiferromagnetic (AFM) to ferromagnetic transition, making it one of the best magnetocaloric materials known. Recently, this material has been revisited because of potential application in spintronics, sensing and energy harvesting. It is also interesting as a model system to explore the magnetostructural change in magnetocaloric materials. However, the composition range having the first order transition is narrow, from 48 to 55 at. % Rh. In this work, we succeed in preparing FeRh thick films with the desired AFM phase using a combinatorial approach. Magnetic and structural characterisation was made as a function of composition on compositionally graded films. The phase transition temperature of thick films was modulated by ion irradiation.