Atomistic spin dynamics and relativistic effects in chiral nanomagnets

  • Datum: 27 februari, kl. 09.00
  • Plats: Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala
  • Doktorand: Koumpouras, Konstantinos
  • Om avhandlingen
  • Arrangör: Materialteori
  • Kontaktperson: Koumpouras, Konstantinos
  • Disputation

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.