Dissertationen
https://kobra.uni-kassel.de:443/handle/123456789/2012010940259
Fri, 04 Dec 2020 07:16:41 GMT2020-12-04T07:16:41ZFirst-principles electronic theory of non-collinear magnetic order in transition-metal nanowires
https://kobra.uni-kassel.de:443/handle/123456789/2015052748393
The structural, electronic and magnetic properties of one-dimensional 3d transition-metal (TM)
monoatomic chains having linear, zigzag and ladder geometries are investigated in the frame-work of
first-principles density-functional theory. The stability of long-range magnetic order along the
nanowires is determined by computing the corresponding frozen-magnon dispersion relations as a
function of the 'spin-wave' vector q.
First, we show that the ground-state magnetic orders of V, Mn and Fe linear chains at the equilibrium
interatomic distances are non-collinear (NC) spin-density waves (SDWs) with characteristic equilibrium
wave vectors q that depend on the composition and interatomic distance. The electronic and
magnetic properties of these novel spin-spiral structures are discussed from a local perspective
by analyzing the spin-polarized electronic densities of states, the local magnetic moments and the
spin-density distributions for representative values q. Second, we investigate the stability
of NC spin arrangements in Fe zigzag chains and ladders. We find that the non-collinear SDWs are
remarkably stable in the biatomic chains (square ladder), whereas ferromagnetic order (q =0)
dominates in zigzag chains (triangular ladders). The different magnetic structures are interpreted in terms of the
corresponding effective exchange interactions J(ij) between the local
magnetic moments μ(i) and μ(j) at atoms i and j.
The effective couplings are derived by fitting a classical Heisenberg model to the
ab initio magnon dispersion relations. In addition they are analyzed in the
framework of general magnetic phase diagrams having arbitrary first, second,
and third nearest-neighbor (NN) interactions J(ij).
The effect of external electric fields (EFs) on the stability of NC magnetic order has been
quantified for representative monoatomic free-standing and deposited chains. We find that an
external EF, which is applied perpendicular to the chains, favors non-collinear order in V chains,
whereas it stabilizes the ferromagnetic (FM) order in Fe chains. Moreover, our calculations reveal a change
in the magnetic order of V chains deposited on the Cu(110) surface in the presence of external EFs.
In this case the NC spiral order, which was unstable in the absence of EF, becomes the most
favorable one when perpendicular fields of the order of 0.1 V/Å are applied.
As a final application of the theory we study the magnetic interactions within monoatomic TM chains deposited
on graphene sheets. One observes that even weak chain substrate hybridizations can
modify the magnetic order. Mn and Fe chains show incommensurable NC spin configurations. Remarkably, V chains
show a transition from a spiral magnetic order in the freestanding geometry to FM order when they are deposited
on a graphene sheet. Some TM-terminated zigzag graphene-nanoribbons, for example V and Fe
terminated nanoribbons, also show NC spin configurations. Finally, the magnetic anisotropy energies (MAEs) of TM
chains on graphene are investigated. It is shown that Co and Fe chains exhibit significant MAEs and orbital
magnetic moments with in-plane easy magnetization axis. The remarkable changes in the magnetic properties of chains
on graphene are correlated to charge transfers from the TMs to NN carbon atoms.
Goals and limitations of this study and the resulting perspectives of future investigations are
discussed.
Wed, 27 May 2015 00:00:00 GMThttps://kobra.uni-kassel.de:443/handle/123456789/20150527483932015-05-27T00:00:00ZTanveer, MuhammadThe structural, electronic and magnetic properties of one-dimensional 3d transition-metal (TM)
monoatomic chains having linear, zigzag and ladder geometries are investigated in the frame-work of
first-principles density-functional theory. The stability of long-range magnetic order along the
nanowires is determined by computing the corresponding frozen-magnon dispersion relations as a
function of the 'spin-wave' vector q.
First, we show that the ground-state magnetic orders of V, Mn and Fe linear chains at the equilibrium
interatomic distances are non-collinear (NC) spin-density waves (SDWs) with characteristic equilibrium
wave vectors q that depend on the composition and interatomic distance. The electronic and
magnetic properties of these novel spin-spiral structures are discussed from a local perspective
by analyzing the spin-polarized electronic densities of states, the local magnetic moments and the
spin-density distributions for representative values q. Second, we investigate the stability
of NC spin arrangements in Fe zigzag chains and ladders. We find that the non-collinear SDWs are
remarkably stable in the biatomic chains (square ladder), whereas ferromagnetic order (q =0)
dominates in zigzag chains (triangular ladders). The different magnetic structures are interpreted in terms of the
corresponding effective exchange interactions J(ij) between the local
magnetic moments μ(i) and μ(j) at atoms i and j.
The effective couplings are derived by fitting a classical Heisenberg model to the
ab initio magnon dispersion relations. In addition they are analyzed in the
framework of general magnetic phase diagrams having arbitrary first, second,
and third nearest-neighbor (NN) interactions J(ij).
The effect of external electric fields (EFs) on the stability of NC magnetic order has been
quantified for representative monoatomic free-standing and deposited chains. We find that an
external EF, which is applied perpendicular to the chains, favors non-collinear order in V chains,
whereas it stabilizes the ferromagnetic (FM) order in Fe chains. Moreover, our calculations reveal a change
in the magnetic order of V chains deposited on the Cu(110) surface in the presence of external EFs.
In this case the NC spiral order, which was unstable in the absence of EF, becomes the most
favorable one when perpendicular fields of the order of 0.1 V/Å are applied.
As a final application of the theory we study the magnetic interactions within monoatomic TM chains deposited
on graphene sheets. One observes that even weak chain substrate hybridizations can
modify the magnetic order. Mn and Fe chains show incommensurable NC spin configurations. Remarkably, V chains
show a transition from a spiral magnetic order in the freestanding geometry to FM order when they are deposited
on a graphene sheet. Some TM-terminated zigzag graphene-nanoribbons, for example V and Fe
terminated nanoribbons, also show NC spin configurations. Finally, the magnetic anisotropy energies (MAEs) of TM
chains on graphene are investigated. It is shown that Co and Fe chains exhibit significant MAEs and orbital
magnetic moments with in-plane easy magnetization axis. The remarkable changes in the magnetic properties of chains
on graphene are correlated to charge transfers from the TMs to NN carbon atoms.
Goals and limitations of this study and the resulting perspectives of future investigations are
discussed.Magnetic interactions between transition metal impurities and clusters mediated by low-dimensional metallic hosts: A first principles theoretical investigation
https://kobra.uni-kassel.de:443/handle/123456789/2015033147887
The magnetic properties and interactions between transition metal (TM) impurities and clusters in low-dimensional metallic hosts are studied using a first principles
theoretical method. In the first part of this work, the effect of magnetic order in 3d-5d systems is addressed from the perspective of its influence on the enhancement of the magnetic anisotropy energy (MAE). In the second part, the possibility of using external electric fields (EFs) to control the magnetic properties and interactions between nanoparticles deposited at noble metal surfaces is investigated.
The inﬂuence of 3d composition and magnetic order on the spin polarization of the
substrate and its consequences on the MAE are analyzed for the case of 3d impurities
in one- and two-dimensional polarizable hosts. It is shown that the MAE and easy-
axis of monoatomic free standing 3d-Pt wires is mainly determined by the atomic
spin-orbit (SO) coupling contributions. The competition between ferromagnetic (FM) and antiferromagnetic (AF) order in FePtn wires is studied in detail for n=1-4 as a function of the relative position between Fe atoms. Our results show an oscillatory behavior of the magnetic polarization of Pt atoms as a function of their distance from the magnetic
impurities, which can be correlated to a long-ranged magnetic coupling of the Fe
atoms. Exceptionally large variations of the induced spin and orbital moments at the
Pt atoms are found as a function of concentration and magnetic order. Along with
a violation of the third Hund’s rule at the Fe sites, these variations result in a non
trivial behavior of the MAE.
In the case of TM impurities and dimers at the Cu(111), the effects of surface charging
and applied EFs on the magnetic properties and substrate-mediated magnetic interactions have been investigated. The modifications of the surface electronic structure, impurity local moments and magnetic exchange coupling as a result of the EF-induced metallic screening and charge rearrangements are analysed. In a ﬁrst study, the properties of surface substitutional Co and Fe impurities are investigated as a function of
the external charge per surface atom q. At large inter-impurity distances the effective magnetic exchange coupling ∆E between impurities shows RKKY-like oscillations
as a function of the distance which are not signiﬁcantly affected by the considered values of q. For distances r < 10 Å, important modifications in the magnitude of ∆E, involving changes from FM to AF coupling, are found depending non-monotonously on the value and polarity of q. The interaction energies are analysed from a local perspective. In a second study, the interplay between external EF effects, internal magnetic order and substrate-mediated magnetic coupling has been investigated for Mn dimers on Cu(111). Our calculations show that EF (∼ 1eV/Å) can induce a switching from AF to FM ground-state magnetic order within single Mn dimers. The relative coupling between a pair of dimers also shows RKKY-like oscillations as a function of the inter-dimer distance. Their effective magnetic exchange interaction
is found to depend significantly on the magnetic order within the Mn dimers and on
their relative orientation on the surface. The dependence of the substrate-mediated
interaction on the magnetic state of the dimers is qualitatively explained in terms
of the differences in the scattering of surface electrons. At short inter-dimer distances, the ground-state conﬁguration is determined by an interplay between exchange interactions and EF effects. These results demonstrate that external surface charging and applied EFs offer remarkable possibilities of manipulating the sign and strength of the magnetic coupling of surface supported nanoparticles.
Tue, 31 Mar 2015 00:00:00 GMThttps://kobra.uni-kassel.de:443/handle/123456789/20150331478872015-03-31T00:00:00ZJuárez Reyes, Lucila MaitreyaThe magnetic properties and interactions between transition metal (TM) impurities and clusters in low-dimensional metallic hosts are studied using a first principles
theoretical method. In the first part of this work, the effect of magnetic order in 3d-5d systems is addressed from the perspective of its influence on the enhancement of the magnetic anisotropy energy (MAE). In the second part, the possibility of using external electric fields (EFs) to control the magnetic properties and interactions between nanoparticles deposited at noble metal surfaces is investigated.
The inﬂuence of 3d composition and magnetic order on the spin polarization of the
substrate and its consequences on the MAE are analyzed for the case of 3d impurities
in one- and two-dimensional polarizable hosts. It is shown that the MAE and easy-
axis of monoatomic free standing 3d-Pt wires is mainly determined by the atomic
spin-orbit (SO) coupling contributions. The competition between ferromagnetic (FM) and antiferromagnetic (AF) order in FePtn wires is studied in detail for n=1-4 as a function of the relative position between Fe atoms. Our results show an oscillatory behavior of the magnetic polarization of Pt atoms as a function of their distance from the magnetic
impurities, which can be correlated to a long-ranged magnetic coupling of the Fe
atoms. Exceptionally large variations of the induced spin and orbital moments at the
Pt atoms are found as a function of concentration and magnetic order. Along with
a violation of the third Hund’s rule at the Fe sites, these variations result in a non
trivial behavior of the MAE.
In the case of TM impurities and dimers at the Cu(111), the effects of surface charging
and applied EFs on the magnetic properties and substrate-mediated magnetic interactions have been investigated. The modifications of the surface electronic structure, impurity local moments and magnetic exchange coupling as a result of the EF-induced metallic screening and charge rearrangements are analysed. In a ﬁrst study, the properties of surface substitutional Co and Fe impurities are investigated as a function of
the external charge per surface atom q. At large inter-impurity distances the effective magnetic exchange coupling ∆E between impurities shows RKKY-like oscillations
as a function of the distance which are not signiﬁcantly affected by the considered values of q. For distances r < 10 Å, important modifications in the magnitude of ∆E, involving changes from FM to AF coupling, are found depending non-monotonously on the value and polarity of q. The interaction energies are analysed from a local perspective. In a second study, the interplay between external EF effects, internal magnetic order and substrate-mediated magnetic coupling has been investigated for Mn dimers on Cu(111). Our calculations show that EF (∼ 1eV/Å) can induce a switching from AF to FM ground-state magnetic order within single Mn dimers. The relative coupling between a pair of dimers also shows RKKY-like oscillations as a function of the inter-dimer distance. Their effective magnetic exchange interaction
is found to depend significantly on the magnetic order within the Mn dimers and on
their relative orientation on the surface. The dependence of the substrate-mediated
interaction on the magnetic state of the dimers is qualitatively explained in terms
of the differences in the scattering of surface electrons. At short inter-dimer distances, the ground-state conﬁguration is determined by an interplay between exchange interactions and EF effects. These results demonstrate that external surface charging and applied EFs offer remarkable possibilities of manipulating the sign and strength of the magnetic coupling of surface supported nanoparticles.Many-body theory of laser-induced ultrafast demagnetization and angular momentum transfer in ferromagnetic transition metals
https://kobra.uni-kassel.de:443/handle/123456789/2014080745830
An electronic theory is developed, which describes the ultrafast demagnetization in itinerant ferromagnets following the absorption of a femtosecond laser pulse. The present work intends to elucidate the microscopic physics of this ultrafast phenomenon by identifying its fundamental mechanisms. In particular, it aims to reveal the nature of the involved spin excitations and angular-momentum transfer between spin and lattice, which are still subjects of intensive debate. In the first preliminary part of the thesis the initial stage of the laser-induced demagnetization process is considered. In this stage the electronic system is highly excited by spin-conserving elementary excitations involved in the laser-pulse absorption, while the spin or magnon degrees of freedom remain very weakly excited. The role of electron-hole excitations on the stability of the magnetic order of one- and two-dimensional 3d transition metals (TMs) is investigated by using ab initio density-functional theory. The results show that the local magnetic moments are remarkably stable even at very high levels of local energy density and, therefore, indicate that these moments preserve their identity throughout the entire demagnetization process. In the second main part of the thesis a many-body theory is proposed, which takes into account these local magnetic moments and the local character of the involved spin excitations such as spin fluctuations from the very beginning. In this approach the relevant valence 3d and 4p electrons are described in terms of a multiband model Hamiltonian which includes Coulomb interactions, interatomic hybridizations, spin-orbit interactions, as well as the coupling to the time-dependent laser field on the same footing. An exact numerical time evolution is performed for small ferromagnetic TM clusters. The dynamical simulations show that after ultra-short laser pulse absorption the magnetization of these clusters decreases on a time scale of hundred femtoseconds. In particular, the results reproduce the experimentally observed laser-induced demagnetization in ferromagnets and demonstrate that this effect can be explained in terms of the following purely electronic non-adiabatic mechanism: First, on a time scale of 10–100 fs after laser excitation the spin-orbit coupling yields local angular-momentum transfer between the spins and the electron orbits, while subsequently the orbital angular momentum is very rapidly quenched in the lattice on the time scale of one femtosecond due to interatomic electron hoppings. In combination, these two processes result in a demagnetization within hundred or a few hundred femtoseconds after laser-pulse absorption.
Thu, 07 Aug 2014 00:00:00 GMThttps://kobra.uni-kassel.de:443/handle/123456789/20140807458302014-08-07T00:00:00ZTöws, WaldemarAn electronic theory is developed, which describes the ultrafast demagnetization in itinerant ferromagnets following the absorption of a femtosecond laser pulse. The present work intends to elucidate the microscopic physics of this ultrafast phenomenon by identifying its fundamental mechanisms. In particular, it aims to reveal the nature of the involved spin excitations and angular-momentum transfer between spin and lattice, which are still subjects of intensive debate. In the first preliminary part of the thesis the initial stage of the laser-induced demagnetization process is considered. In this stage the electronic system is highly excited by spin-conserving elementary excitations involved in the laser-pulse absorption, while the spin or magnon degrees of freedom remain very weakly excited. The role of electron-hole excitations on the stability of the magnetic order of one- and two-dimensional 3d transition metals (TMs) is investigated by using ab initio density-functional theory. The results show that the local magnetic moments are remarkably stable even at very high levels of local energy density and, therefore, indicate that these moments preserve their identity throughout the entire demagnetization process. In the second main part of the thesis a many-body theory is proposed, which takes into account these local magnetic moments and the local character of the involved spin excitations such as spin fluctuations from the very beginning. In this approach the relevant valence 3d and 4p electrons are described in terms of a multiband model Hamiltonian which includes Coulomb interactions, interatomic hybridizations, spin-orbit interactions, as well as the coupling to the time-dependent laser field on the same footing. An exact numerical time evolution is performed for small ferromagnetic TM clusters. The dynamical simulations show that after ultra-short laser pulse absorption the magnetization of these clusters decreases on a time scale of hundred femtoseconds. In particular, the results reproduce the experimentally observed laser-induced demagnetization in ferromagnets and demonstrate that this effect can be explained in terms of the following purely electronic non-adiabatic mechanism: First, on a time scale of 10–100 fs after laser excitation the spin-orbit coupling yields local angular-momentum transfer between the spins and the electron orbits, while subsequently the orbital angular momentum is very rapidly quenched in the lattice on the time scale of one femtosecond due to interatomic electron hoppings. In combination, these two processes result in a demagnetization within hundred or a few hundred femtoseconds after laser-pulse absorption.Theoretical study of magnetism, structure and chemical order in transition-metal alloy clusters
https://kobra.uni-kassel.de:443/handle/123456789/2012022740836
Research on transition-metal nanoalloy clusters composed of a few atoms is fascinating
by their unusual properties due to the interplay among the structure, chemical order and magnetism.
Such nanoalloy clusters, can be used to construct nanometer devices
for technological applications by manipulating their
remarkable magnetic, chemical and optical properties. Determining the nanoscopic features
exhibited by the magnetic alloy clusters signifies the need for a systematic global and local exploration of
their potential-energy surface in order to identify all the relevant
energetically low-lying magnetic isomers.
In this thesis the sampling of the potential-energy surface has been performed
by employing the state-of-the-art spin-polarized density-functional theory
in combination with graph theory and the basin-hopping global optimization techniques.
This combination is vital for a quantitative analysis of the quantum mechanical energetics.
The first approach, i.e., spin-polarized density-functional theory together with
the graph theory method, is applied to study the Fe$_m$Rh$_n$ and Co$_m$Pd$_n$
clusters having $N = m+n \leq 8$ atoms. We carried out a thorough and systematic sampling of
the potential-energy surface by taking into account all possible initial cluster topologies,
all different distributions of the two kinds of atoms within the cluster,
the entire concentration range between the pure limits,
and different initial magnetic configurations such as ferro- and anti-ferromagnetic coupling.
The remarkable magnetic properties shown by FeRh and CoPd nanoclusters are
attributed to the extremely reduced coordination number together with the charge transfer
from 3$d$ to 4$d$ elements.
The second approach, i.e., spin-polarized density-functional theory together with
the basin-hopping method is applied to study the small Fe$_6$, Fe$_3$Rh$_3$ and Rh$_6$ and
the larger Fe$_{13}$, Fe$_6$Rh$_7$ and Rh$_{13}$ clusters
as illustrative benchmark systems. This method is able to identify the
true ground-state structures of Fe$_6$ and Fe$_3$Rh$_3$ which were not obtained by
using the first approach. However, both approaches predict a similar cluster for the
ground-state of Rh$_6$. Moreover, the computational time taken by this approach is found
to be significantly lower than the first approach. The ground-state structure of Fe$_{13}$
cluster is found to be an icosahedral structure,
whereas Rh$_{13}$ and Fe$_6$Rh$_7$ isomers relax into cage-like and layered-like structures, respectively.
All the clusters display a remarkable variety of structural and magnetic behaviors.
It is observed that the isomers having similar shape with
small distortion with respect to each other can exhibit quite different magnetic moments.
This has been interpreted as a probable artifact of spin-rotational symmetry breaking
introduced by the spin-polarized GGA. The possibility of combining the
spin-polarized density-functional theory with some other global optimization
techniques such as minima-hopping method could be the next step in this
direction. This combination is expected to be an ideal sampling approach
having the advantage of avoiding efficiently the search over irrelevant regions of the potential energy surface.
Mon, 27 Feb 2012 00:00:00 GMThttps://kobra.uni-kassel.de:443/handle/123456789/20120227408362012-02-27T00:00:00ZMokkath, Junais HabeebResearch on transition-metal nanoalloy clusters composed of a few atoms is fascinating
by their unusual properties due to the interplay among the structure, chemical order and magnetism.
Such nanoalloy clusters, can be used to construct nanometer devices
for technological applications by manipulating their
remarkable magnetic, chemical and optical properties. Determining the nanoscopic features
exhibited by the magnetic alloy clusters signifies the need for a systematic global and local exploration of
their potential-energy surface in order to identify all the relevant
energetically low-lying magnetic isomers.
In this thesis the sampling of the potential-energy surface has been performed
by employing the state-of-the-art spin-polarized density-functional theory
in combination with graph theory and the basin-hopping global optimization techniques.
This combination is vital for a quantitative analysis of the quantum mechanical energetics.
The first approach, i.e., spin-polarized density-functional theory together with
the graph theory method, is applied to study the Fe$_m$Rh$_n$ and Co$_m$Pd$_n$
clusters having $N = m+n \leq 8$ atoms. We carried out a thorough and systematic sampling of
the potential-energy surface by taking into account all possible initial cluster topologies,
all different distributions of the two kinds of atoms within the cluster,
the entire concentration range between the pure limits,
and different initial magnetic configurations such as ferro- and anti-ferromagnetic coupling.
The remarkable magnetic properties shown by FeRh and CoPd nanoclusters are
attributed to the extremely reduced coordination number together with the charge transfer
from 3$d$ to 4$d$ elements.
The second approach, i.e., spin-polarized density-functional theory together with
the basin-hopping method is applied to study the small Fe$_6$, Fe$_3$Rh$_3$ and Rh$_6$ and
the larger Fe$_{13}$, Fe$_6$Rh$_7$ and Rh$_{13}$ clusters
as illustrative benchmark systems. This method is able to identify the
true ground-state structures of Fe$_6$ and Fe$_3$Rh$_3$ which were not obtained by
using the first approach. However, both approaches predict a similar cluster for the
ground-state of Rh$_6$. Moreover, the computational time taken by this approach is found
to be significantly lower than the first approach. The ground-state structure of Fe$_{13}$
cluster is found to be an icosahedral structure,
whereas Rh$_{13}$ and Fe$_6$Rh$_7$ isomers relax into cage-like and layered-like structures, respectively.
All the clusters display a remarkable variety of structural and magnetic behaviors.
It is observed that the isomers having similar shape with
small distortion with respect to each other can exhibit quite different magnetic moments.
This has been interpreted as a probable artifact of spin-rotational symmetry breaking
introduced by the spin-polarized GGA. The possibility of combining the
spin-polarized density-functional theory with some other global optimization
techniques such as minima-hopping method could be the next step in this
direction. This combination is expected to be an ideal sampling approach
having the advantage of avoiding efficiently the search over irrelevant regions of the potential energy surface.