Thermomagnetic studies on transition metal pnictides

Abstract

The magnetocaloric effect (MCE) consists, in simple terms, in the heating of a magnetic material by the application of an external magnetic field. This can be easily understood if one but imagines a magnetic material with randomly applied spins; as a magnetic field is applied the spins will tend to alight with this field and as a result the overall Entropy of the system decreases, which consequently results in an exchange in Heat. This idea then becomes analogous to the vapor compression cycle used in our current household refrigerators, making the possibility of assembling a magnetic refrigerator quite tangible. The current thesis is in part an ample exploratory research of several material families with the aim of determining their magnetocaloric potential. In this scope the (Mn,Fe)3(Si,P), (Mn,Co)3(Si,P), (Fe,Co)3(Si,P) and (Mn,Fe)2(P,Ge) systems are studied and characterized. The study in (Mn,Fe)3(Si,P) resulted in the full magnetostructural mapping of this system, revealing many possible applications in a wide variety of physical areas. The (Mn,Co)3(Si,P) and (Fe,Co)3(Si,P), although not wielding positive results for MCE applications, still revealed the existence of other related material systems with extremely relevant physical properties, such as the inverse MCE. Finally the (Mn,Fe)2(P,Ge) revealed a very promising MCE potential, with the added discovery of a definite potential for permanent magnet applications. Finally, the current thesis also covers the assembly of microcalorimetry setup using commercial Xensor chips designed for the measurement of key physical properties for the understanding of the transition phenomenon in materials of MCE interest.