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.