Comportamento magnético e efeito magnetocalórico de nanoestruturas frustradas de Ising

Abstract

In this work, we investigate the magnetic and magnetocaloric properties of two frustrated Ising clusters in the presence of a longitudinal magnetic field: the centered tetrakis hexahedron structure and the hexagonal bipyramidal structure. These clusters were recently found as metallic structures formed by iron ions in recently synthesized compounds. We investigate the magnetism of these clusters by considering the exact enumeration technique. The centered tetrakis hexahedron cluster contains 15 sites, for which we consider four exchange interactions (J1, J2, J3, and J4) between Ising spin-1/2 particles. For the hexagonal bipyramidal cluster, consisting of 8 sites, we consider Ising spin-5/2 particles and two exchange interactions (Jt and Jr). We obtain the ground-state phase diagram, magnetization, entropy, and isentropic curves for both systems. A rigorous analysis was conducted on the entropic behavior of the two clusters to discuss the magnetocaloric effect (MCE). Additionally, we perform a detailed study of the magnetic behavior and magnetic susceptibility of the centered tetrakis hexahedron cluster, due to exotic phenomena occurring in specific scenarios. Our results show that the centered tetrakis hexahedron cluster exhibits five phases in the ground state within the evaluated interaction spectrum. Furthermore, we observe an unusual temperature-dependent magnetization behavior, which displays a local minimum. Investigating the magnetic susceptibility of this cluster, we also found interesting behaviors such as a divergence at low temperatures and a double-peak structure. Evaluating the entropic behavior, we note a significant convergence of isentropic curves towards critical fields at the boundaries of the ground-state phase diagram. This behavior indicates a significant MCE in the system, where the centered tetrakis hexahedron cluster can be driven to very low temperatures by reducing the longitudinal magnetic field. For the hexagonal bipyramidal cluster, we find three ground states and scenarios with a large number of degenerate states. Furthermore, the isentropic curves indicate that this system exhibits a very high MCE. In summary, both studied clusters show an enhanced MCE. For appropriate values of exchange interactions, absolute zero can be achieved by removing the external magnetic field. We stress that the hexagonal bipyramidal structure exhibits an enhanced MCE, with the highest-value isentropes reaching low temperatures under zero field. This high MCE is associated with the high degeneracy of the ground state of the system, which is introduced by frustration.