Density functional theory study of structural and magnetic properties of low dimensional

Abstract

This is the synopsis of the thesis entitled "Density Functional Theory Study of Structural and Magnetic Properties of Low Dimensional Systems" by Mighfar Imam. The work presented in this thesis involves the study of magnetic nanostructures in low dimensions, using density functional theory. Materials in low dimensions are at the heart of current scientific and technological investigations. In particular, their magnetic properties constitute an exciting field of research, being stimulating both for fundamental physics and for many potential applications. With the reduction of dimensionality, the coordination number of atoms gets reduced. This change in atomic environment from the bulk state results in an enhancement in many magnetic properties. Effects that are either not present or only weakly present in the bulk state now get manifested strongly. Thus, nanomagnetism is a field of great interest today. The systems studied in this thesis consist of thin layers of magnetic materials (either a magnetic material, or an alloy composed of a magnetic element and a "non-magnetic" element), deposited on a substrate of another metal. We have also investigated the way in which these properties can be modified by the subsequent deposition of a self assembled monolayer of organic molecules. In order to see whether we can gauge trends as a function of lattice constant and dimensionality, and to evaluate the effects of deposition on a substrate, wc have also studied hypothetical model systems consisting of monoatomic wires and freostanding two dimensional monolayers of atoms. We have mainly focused on the magnetic properties of these systems, viz., the magnetic moments and the magnetic anisotropy energy (MAE). The MAE, which serves as a measure of the ease of flipping the orientation of magnetization, is of vital technological importance, for applications in Ttiagnetic memory storage. For the case of alloys, we have also evaluated the enthalpy of mixing, i.e., wc have seen whether it is favorable for the alloy to form (as opposed to phase segregate), and tried to understand the different effects responsible for this. All of the work in this thesis has been motivated by the attempt to understand specific experimental data and/or guide future experiments. We have split the thesis work in to seven chapters. A brief description of the chapters is outlined as follows: In Chapter 1, we provide a general introduction and the outline of the thesis. Chapter 2 describes the theoretical method, namely density functional theory (DFT), in the plane-wave pseudopotential approach, which has been used in all our work. After introducing the standard Kohn-Sham approach of DFT for the nonmagnetic systems, with standard approximations for the exchangecorrelation, we have described how to treat collinear and non-collincar magnetism, and how to include the spin-orbit interaction, as is necessary in order to calculate the MAR. In our approach, this is done by making use of fully relativistic pseudopotentials. In Chapter 3, wc present a brief review of some magnetic properties related to our work and describe some general concepts related to magnetism and magnetic anisotropy. Wc also describe some experimental and theoretical methods for the measurement and calculation of magnetic anisotropy. In Chapter 4, wc study surface alloys of the type MN/S, where M is a magnetic clement, A'^ is a ''non-magnetic'' clement, and 5 is a substrate (which, in our case, is restricted to Rh(lll)). This work was in part motivated by experiments by Thayer et al. that showed that Ag-Co/Ru(0001) did not form an atomically mixed alloy, despite expectations to the contrary. In our work, we study the structural and magnetic properties of quasi two-dimensional magnetic surface alloys of the type Mj.Ai_j., (M=Fe, Co, Ni; A^=Pt, Au, Ag, Cd, Pb; and the concentration x = 0,0.25.0.33,0 5.0.67,0.75,1), on a Rh(lll) substrate. The choice of elements is made keeping in mind the Humc-Rothcry alloying criterion for bulk alloys, which may, however, not hold at the surface. Many compositions and geometric patterns of the surface alloys are considered, in order to study their mixing as well as magnetic behavior. Wo grouped all the patterns into two types, the linear chain type and "Chinese checkerboard" type, and compared their mixing and magnetic properties. We find that some of the combinations of these magnetic and nonmagnetic elements result in mixing (even though they are immiscible in the bulk), while some show no or very small mixing. Both chemical and clastic contributions to mixing arc found to be important. We find that the greater the number the valence electrons in N, the smaller the magnetic moments in the alloy; this finding can be explained by simple density-of-states arguments. We identify suitable candidates that would be appropriate for future experiments. In Chapter 5, wc study the structural and magnetic properties of thin films of Co on Au(lll), as well as how these arc modified upon the subsequent adsorption of methane thiolate (CH3S) on the Co/Au(111) substrate. With a clever choice of our unit cell we could simulate the reconstructed Co film on Au(lll), which mimics the experimental structure rather well. We checked the stability of clean and thiol-adsorbed Co/Au(lll) for various stackings sequences. We also studied the properties of pseudomorphic Co layers on Au(lll). In all cases, we find an out-of-plane easy axis for the system. We find that the adsorption of methane thiolate slightly reduces the MAE when three Co layers are present; however, preliminary results suggest that this effect is considerably reduced when the number of Co layers is increased. These investigations were motivated by unpublished experimental investigations by Rousset, Repain et al.