Propiedades magnéticas de agregados de metales 3d adsorbidos sobre superficies no magnéticas
Hernández Vázquez, Esther Elena
DirectorMorán López, José Luis
The magnetic elements of the 3d row of the periodic table, have been extensively studied from an experimental and theoretical points of view. The study of these systems has been encouraged because their possible applications in electronics and the magnetic recording industry. Mn is one of the most intriguing elements of this series because it has a quite peculiar magnetic and crystallographic behavior, which depends on its environment. Also, at the atomic scale, it has been observed that the aggregates have physical properties that depend on their size. It is important to highlight that the size of the structures, geometry, and mobility are influenced by the intrinsic defects of the surface and by the temperature. In this work, phenomena related to the absorption of elements of the 3d series on noble metal surfaces and perfect graphene with defects were studied. In the first case, we report the results of the calculation of the stability, electronic and magnetic structure of the adsorption of a Mn trimer, forming a linear chain and when they are adsorbed in triangular form, on non-magnetic metal surfaces (Cu and Au) on the surface (111), using the functional theory of density. As a result, we find that the physical properties of the trimer depend on both the interaction between the Mn atoms and the surface of the substrate. Which could be studied from a qualitative analysis of the distribution of the electronic charges of the Mn/Cu(111) system [Mn/Au(111)]. The atoms that actively participate in adsorption energy are mainly those in the vicinity of the trimer. In the case of the Cu (111) substrate, there is also an important interaction with those neighbors of the second layer. On the other hand, we find that the most stable magnetic state is that in which the atoms in the trimer couple antiferromagnetically. The non-collinear and ferromagnetic states are close in energy but are less bounded. In the case of the adsorbed trimer in a triangular arrangement, two geometric configurations are described; we call them Delta and Hexagonal. We found that the Delta configuration has lower energy states compared to their counterparts in H. The energies differ energetically between 230 and 350 meV. The second system describes the interaction of an adsorbed Ni atom in a pristine graphene network and with a simple vacancy. For this, a large cell (8$\times$8) was used. The stability, electronic and magnetic structure was calculated and the influence of the adsorbed Ni atom at different sites in the network, both on the perfect graphene and in the presence of defects, was analyzed. For this, a study of the electronic localization function, charge distribution, and magnetic charge of the Ni/graphene system (pristine and vacancy) was carried out. To understand the mobility of Ni adatom on its surface, the energy barriers associated with its mobility were calculated and the characteristics of the potential energy surface along several trajectories were calculated. As a result, we find that a simple vacancy in graphene generates a total magnetic moment of 1.15 muB, which is mainly due to the magnetic contribution of the atoms with a loose bond, which suffer a break in symmetry due to the effect Jahn-Teller On the other hand, when the Ni adatom occupies the vacancy, the magnetic moment of the surface is almost null. To study the migration of Ni adatoms through the graphene network, calculations of energy barriers were performed across different points using the elastic band method (cNEB). As a result, we found that in pristine graphene the smallest barrier has a value of 224 meV. While in graphene with a simple vacancy, we found that, around the vacancy, the energy barriers are a few tens of meV, and in areas farther from the vacancy these increases to several hundreds of meV. These barriers are small enough to guarantee the mobility of the Ni adatom at room temperature. Finally, we report the results of the growth of metallic Ni clusters based on the porosity of a graphene surface and the temperature of the system using a semi-classical molecular dynamics method. For this study, a graphene surface of 10$\times$10 nm was used with a defect concentration of 0.5% (21 single vacancies) and 200 Ni atoms smoothly deposited for 2 ns. We found that at low temperatures the islands of Ni are formed mainly by monomers. In contrast, as the temperature rises to 1600 K, the average cluster size of up to 40 atoms also increases. It should be noted that despite the easy mobility of the clusters through the network, when they find a vacancy they anchor and remain fixed, even at temperatures above 2000 K.
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