Giant Magnetocaloric Materials: Investigations of defects induced by ion collisions

The magnetic refrigeration is among the most promising environment-friendly refrigeration technologies, alternative to common techniques. This new technology exploits the magnetocaloric effect, i.e. the temperature variation occurring in materials when they are subjected to a magnetic field, to produce refrigeration without requiring the compression and expansion of a gas. Materials characterized by a giant magnetocaloric effect are the best candidates for the fabrication of next-generation magnetic refrigerators. However, they are characterized by a first order phase transition which, on one hand, guarantees a larger refrigeration power but, on the other, is associated with a thermal hysteresis, which introduces a loss of refrigeration efficiency. Nowadays the research activity is focused on finding a way to remove this hysteresis effect, in order to preserve the giant magnetocaloric effect on such materials. Since one of the recently proposed solutions is the irradiation of the material with accelerated ions, the aim of this project is to study the defects induced by ion collisions. This research activity can be divided into two parts, referring to two different types of samples used: MnAs-thin films and MnFePSi-powders. MnAs is a well-known magnetocaloric material, the magnetic properties of which were already characterized in the past by ASUR (French acronym for “Clusters and Surfaces under Intense Excitation”) research team at the Paris Institute of Nanosciences (INSP). The new contributions, I developed during my stay at the INSP, allow an accurate characterization of irradiated samples for a deeper understanding of the processes involved in the thermal hysteresis suppression. More precisely, my activity concerned the accurate study of the temperature dependency of the magnetization in samples irradiated by ions with different masses and velocities. In particular, I studied the evolution of the thermal hysteresis area of magnetization curves as a function of the ion fluence and the induced nuclear collisions in the sample. In order to further investigate the nature of the induced defects, I studied the modifications of irradiated MnAs film properties against a temperature treatment (annealing) to test the robustness of the irradiation-induced effects. MnFePSi is a new magnetocaloric material extensively studied also in the private sector for industrial magnetic refrigeration applications. Starting from the MnFePSi-sample preparation, the complete procedure for the irradiation-induced modification and study of the magnetic properties of the material will be presented.