A spin dynamics approach to solitonics

It is spatial dispersion which is exclusively responsible for the emergence of exchange interaction and magnetic ordering. In contrast, magneto-crystalline anisotropy present in any realistic material brings in a certain non-linearity to the equation of motion. Unlike homogeneous ferromagnetic ordering a variety of non-collinear ground state configurations emerge as a result of competition among exchange, anisotropy, and dipole-dipole interaction. These particle-like states, e.g. magnetic soliton, skyrmion, domain wall, form a spatially localised clot of magnetic energy. In this paper we explore topologically protected magnetic solitons that might potentially be applied for logical operations and/or information storage in the rapidly advancing filed of solitonics (and skyrmionics). An ability to easily create, address, and manipulate such structures is among the prerequisite forming a basis of -onics technology, and is investigated in detail here using numerical and analytical tools

Atomistic spin dynamics and relativistic effects in chiral nanomagnets

In this thesis, studies based on magnetization dynamics on atomic length scales are presented for a number of magnetic systems, where Dzyaloshinskii-Moriya (DM) interaction is present. First-principle methods, based on density functional theory (DFT), have been used to study the pairwise magnetic interactions, such as Heisenberg exchange and DM interaction, which are the crucial parameters for the helimagnetic systems. The first part of this thesis concerns the theoretical background: basics of DFT, atomistic spin dynamics and magnetic skyrmions. The second part concerns the ground state and dynamical properties of helimagnets. Magnetic interaction parameters have been calculated for heterostructures, such as Co/Ni/Co on heavy metal non-magnetic substrates. These parameters are strongly dependent on the material of the substrate. Furthermore, the magnetization dynamics of domain wall and skyrmion are studied and our results show that motion is influenced by the spin-Hall effect (SHE) which arises from the non-magnetic substrate. Similar studies of magnetic interaction parameters have been made for several half-Heusler compounds MnZSn (Z=Tc, Ru, Rh, Os, Ir and Pt) and the phase diagram of the MnPt0.99Ir0.01Sn alloy proves the existence of skyrmions in a wide range of temperature and external magnetic field.  The manipulation of low-dimensional magnetic structures (skyrmions and solitons) with spin transfer torques have been investigated. The nucleation and annihilation processes of skyrmion, by the use of spin polarised current, are essential and the impact of different edges (antiferromagnetic, magnetically softer and stiffer) on both processes is studied. When the edge is magnetically softer, less current is required for skyrmion nucleation and annihilation. Furthermore, one-dimensional magnetic solitons are used to explore concepts of logical operations in a prototype majority gate device, since they are stable and can be easily created and manipulated by spin currents. Lastly, edge dislocations in FeGe helimagnet have been studied. These dislocations described in terms of thermally driven dynamics by the use of atomistic spin dynamics approach and possibly explain some unusual jumps of the spiral wavelength observed by time-dependent experiments

Realization of either physisorption or chemisorption of 2H-tetraphenylporphyrin on the Cu(111) from density functional theory

The adsorption of organic molecules to surfaces is a central issue to achieve fully-functional molecular devices, for which porphyrins are well-studied due to their chemical stability and functional diversity. Herein, we investigate both the physical and the chemical adsorption of the free-base tetraphenylporphyrin 2H-TPP on the Cu(111) surface within the framework of density functional theory and find that the most stable physisorbed configuration is more weakly bound by -0.36 eV than the chemisorbed configuration. We use the electron localization function to investigate the difference in binding mechanisms between strong physisorption and weak chemisorption. We have computed a reaction barrier of 0.12 eV in going from physical binding to chemical bonding to the surface, and a barrier of 50 meV in going between neighboring physical binding sites. Our results support the possibility of realizing free-base porphyrins either physisorbed or chemisorbed on Cu(111) depending on the deposition procedure and experimental conditions.Validerad;2024;Nivå 2;2024-03-25 (joosat);License full text: CC BY</p

Direct light-induced spin transfer between different elements in a spintronic Heusler material via femtosecond laser excitation

Heusler compounds are exciting materials for future spintronics applications because they display a wide range of tunable electronic and magnetic interactions. Here, we use a femtosecond laser to directly transfer spin polarization from one element to another in a half-metallic Heusler material, Co2MnGe. This spin transfer initiates as soon as light is incident on the material, demonstrating spatial transfer of angular momentum between neighboring atomic sites on time scales &lt; 10 fs. Using ultrafast high harmonic pulses to simultaneously and independently probe the magnetic state of two elements during laser excitation, we find that the magnetization of Co is enhanced, while that of Mn rapidly quenches. Density functional theory calculations show that the optical excitation directly transfers spin from one magnetic sublattice to another through preferred spin-polarized excitation pathways. This direct manipulation of spins via light provides a path toward spintronic devices that can operate on few-femtosecond or faster time scales