Electronic and Magnetic Properties of Rough Surfaces of Transition Metals (Fe, Co and Ni) and Their Alloyed Interfaces with Metal (Ag, Cu and Au) Substrates

Abstract

The real space technique, “the Augmented Space Formalism (ASF) coupled with Recur-sion method and Density Functional Theory (DFT) based Tight-Binding Linear Muffin-Tin Orbitals (TB-LMTO) method” is applied to carry out the layerwise electronic and magnetic properties of transition metals (Fe, Co and Ni) rough surfaces and their alloyed interfaces with metal (Ag, Cu and Au) substrates. The potential parameters are generated by TB-LMTO method. These parameters are used to set-up Augmented space Hamilto-nian. Finally the density of states are calculated by recursion technique. The relativistic self-consistent calculation is based on local spin density approximation (LSDA). There are two parts in the thesis work. The first part deals with the surface electronic and magnetic properties of only transition metal layers. The second part includes the electronic and magnetic properties of interface of transition metals with metal substrates. The surface properties are carried out for bcc Fe(001), fcc Co(001) and Ni(001) transition metals. And the interface properties are carried out for Fe, Co and Ni transition metals on (001) surfaces of Ag, Cu and Au metal substrates. We have considered both smooth and rough surfaces and alloyed interfaces. ASF is a most successful method to study disordered alloys. We have considered here a rough surface. Roughness is due to randomly replacement of transition metal atoms by empty spheres (vacancies). Interface is taken as an alloy of transition metal overlayer and metal substrate. Therefore in both the cases, surface and interfaces, resemble as disordered systems. Hence ASF is an appropriate technique to study surfaces and interfaces. We have also shown that ASF can be extended to study almost smooth surfaces and interfaces. We have defined our systems as twelve atomic layers of the transition metals along (001) direction. We have considered two layers of empty spheres above the surface to take care of charge leakage into the vacuum. Relaxation of the top most layer is considered using minimum energy principle. Two types of roughening are considered. One with roughening of the top layer only with 10% & 20% empty spheres. The second one is by roughening the first four layers with 20%, 15%, 10%, & 5% from top layer respectively. The second type resembles a more realistic surface as observed in any experimentally grown surface. We have made a comparisons between these two types of rough surfaces. For four layered rough surface, the bulk properties of the systems are obtained at the 9th layer from the top in the case of Fe(001) whereas at 8th layer in the cases of Co(001) & Ni(001). The trend in the variation of the width and the structure of DOS among the layers changes when a realistic surface is considered in comparison to a smooth surface. With the change in roughness in different layers, the appearance of new peaks in DOS for all these systems, corresponds to disorderedness. The magnetic moment of the top layer is maximum for Fe(001) and Co(001) and of 3rd layer for Ni(001) among all the roughed layers. Layered based magnetic moments differ between both type of rough surfaces. Work functions are found to be almost same for both type of rough surfaces. We have extended ASF to study almost smooth surface to compare our result with other existing theoretical results. We have considered nine layers of bcc Fe(001), fcc Co(001) and fcc Ni(001) to carry out layerwise properties. The lattice relaxation of the top most layer is carried out using energy minimization procedure. For bcc Fe(001), fcc Co(001) and fcc Ni(001), the lattice relaxations of the surface layer are obtained at 5%, 16% and 9% respectively. Surface magnetic moment has been found to be higher than that of the bulk and in different layers below. Magnetic moments show Friedel oscillations in agreement with other studies. Work functions of these systems have been found to agree with experimental values. The orbital resolved density of states show the significant contribution of d-orbital towards the surface as well as bulk magnetic moments. The bulk magnetic property is attained at the 5th layer down the top most surface layer in the case of Fe(001) and at the 4th layer in the cases of Co(001) and Ni(001).

We have carried out the layerwise interface electronic and magnetic properties of rough and sharp transition metals interfaces with metal substrate along (001). The potential parameters of the top layer are generated by considering surface lattice relaxation using energy minimization procedure. We have carried out the electronic and magnetic prop-erties for one monolayer, two monolayers of transition metals to compare our result with other theoretical studies. We have also considered a more realistic interface, that is, an alloyed interface consisting of two layers of transition metals and two layers of metal sub-strates. We have also carried out the properties for three layers of transition metals for sharp and rough interfaces to compare our results with few existing experimental studies. For one smooth monolayer of transition metal deposited on Ag and Au substrate, the magnetic moment of the overlayer enhances compared to its bulk value. But in case of Cu substrate, the magnetic moment enhances for Fe overlayer but not for Co and Ni. For all the systems, with one ML of transition metals on metal substrate, the bulk electronic and magnetic properties is attained at the fourth substrate layer from the interface. We have also calculated the interface properties for single monolayer of transition metal with 5%and 10% interdiffusion of atoms to compare our result with previous existing work. We have compared these results with the sharp interface properties. For one ML of transition metal with 5% interdiffusion of atoms at the interface, the average magnetic moment of the top layer decreases and the average induced magnetic moment of the top substrate layer increases than that of without interdiffusion case, except for Ni/Ag(001). With increasing the amount of interdiffusion from 5% to 10%, the average magnetic moment of the overlayer further decreases and that for the substrate further increases, except for Ni/Au(001). In case of two and three ML of transition metals on Ag and Cu substrates, the bulk properties of the substrate are obtained at the third substrate layer from the interface. But in case of Au substrate, the bulk properties are attained at the fourth layer from the interface. The properties of three layers of Fe/Ag with rough interface agrees with few available experimental results.In case of more realistic four layered rough interfaces, the layerwise magnetic moment and density of states show the effect of roughness at the interface. The magnetic moment of the transition metal as well as the average magnetic moment of these interface layers gradually decreases towards the substrate for all the systems. The effect of hybridization on the magnetic properties are discussed.