Efeito Magnetocalórico em sistemas competitivos

Abstract

This work aims to study the main concepts for the understanding of the magnetocaloric effect (MCE) and to develop models to investigate this phenomenon in geometrically frustrated systems. This effect employs isothermal magnetization and adiabatic demagnetization processes with the purpose of inducing variations in temperature and magnetic entropy. Therefore, through magnetocaloric effect, magnetic refrigeration can be designed, an alternative that allows greater efficiency and less damage to the environment in relation to current refrigeration processes (steam cycle). However, to replace the vapor cycle cooling method with a system based on magnetocaloric materials, it is necessary to focus on magnetic materials with a high magnetocaloric effect. In this sense, frustrated magnetic systems have shown promise, due to their peculiar properties such as high entropy even at low temperatures. Thus, we propose to help clarify the relationship between the magnetocaloric effect and the frustration due to the geometry of the lattice. We applied the Ising spin model to two lattice configurations: the kagome lattice, which has geometrical frustration, and the hexagonal lattice, which does not. In both cases, we consider the formation of finite clusters in the presence of a magnetic field. The intra-cluster interactions were calculated via exact numeration, while the inter-cluster interactions were treated using self-consistent equations via mean-field theory with clusters. From the solutions obtained, we elaborate phase diagrams at low temperatures and extract thermodynamic quantities, such as magnetization, entropy and magnetocaloric effect, based on the high degeneracy fundamental states. We observed that in the kagomé lattice there is a notable isothermal variation in entropy for low-intensity magnetic fields, geometric frustration being the predominant factor behind this behavior. Nevertheless, in the hexagonal lattice, the absence of geometric frustration, no substantial magnetocaloric effects were observed. Therefore, we found that the kagome lattice demonstrates a considerable capacity to improve the efficiency and economic viability of the magnetic refrigeration process based on the high magnetocaloric effect present.