Physical behaviour and properties at the first order phase transition of magnetocaloric materials

This Ph.D. project was mainly devoted to the study of the connection between magnetocaloric properties and first order phase transitions in ferromagnetic materials based on the La(Fe,Si)13 compound. The magneto caloric effect (MCE) and its application in magnetic cooling cycles rely on the reversible magnetization and demagnetization of a magnetic material by an external magnetic field, resulting in a temperature change that is maximal at temperatures close to a magnetic phase transition. The possibility to improve the performance of the active refrigerator materials, depends on many factors: the need of a Curie temperature close to ambient temperature, a low magnetic and thermal hysteresis and a high magnetic entropy variation for magnetic fields below two Tesla. The latter requisite can be found in first order magnetic phase transitions that, unfortunately, are accompanied by intrinsic thermo-magnetic hysteresis. This drawback for magneto cooling cycles, motivates the present study on the phase transitions dynamics. On the other hand, the investigation of magneto-thermal phenomena in magnetic materials is of great importance also for solving fundamental problems of magnetism and solid state physics, for example, it is recognized that the properties of interest of such functional materials are intimately linked to the detailed micro structure, however, the nature of this link itself is not understood very often. In this Ph.D. project, thermo-magnetic phase transitions in La(Fe,Si)13 compounds were investigated through the comparison of various experimental techniques within a collaboration between the applied superconductivity group of Politecnico of Torino and the electromagnetism division of INRiM (National Institute of Metrological Research). To achieve a proper physical understanding of the connection between thermo-magnetic hysteresis at the microscopic level and the microstructure, a magneto optical method was applied to samples of La-F-Si-13 with cobalt substitutions, so to allow the dynamical visualisation of the phase boundaries motion in a first order phase transition. These type of experiments have been compared with low rate calorimetry data and, from the experimental work, it has been found that the presence of avalanches is a characteristic feature of these alloys and it is related to their thermal hysteresis. The difference between first and second order phase transition dynamics were highlighted thanks to the employment of different techniques, which also favoured the separation of the general aspects of hysteresis, common to all irreversible processes, from features more strictly dependent on specific microstructural properties. For the aim of this Ph.D., other techniques were also used to observe temperature induced magnetic phase transitions in functional magnetic materials. Among them an in-temperature ferromagnetic resonance method was implemented for the study of the magnetization dynamics in canted spin structures. The present research activity has been partially related to the European Project DRREAM [1] (a collaborative research project funded by the EC under the Seventh Framework Program 2013-2015), whose goal is to reduce the use of rare earth elements in the life cycle of technologies that use magnetic phase change materials, in particular magnetic refrigerators.