Doctoral defence in Chemistry - Mariia Potkina

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Ph.D. student: Mariia Potkina

Dissertation title: Stability and dynamics of chiral magnetic structures in ferro- and antiferromagnets

Opponents:
Dr. Gísli Hólmar Jóhannesson, Division Director at Keilir, Iceland
Dr. Sergei A. Egorov, Professor at the University of Virginia, USA

Advisor: Dr. Hannes Jónsson, Professor at the Faculty of Physical Sciences, University of Iceland

Doctoral committee:
Dr. Valery M. Uzdin, Professor at the ITMO University, St. Petersburg
Dr. Pavel F. Bessarab, Researcher at the Science Institute of the University of Iceland

Chair of Ceremony: Dr. Oddur Ingólfsson, Professor and Vice Head of the Faculty of Physical Sciences, University of Iceland

Abstract:

Reduction of the size of magnetic bits for data storage, transfer and processing of information down to the nanoscale would lay the foundation for large advances in information technology, both in terms of processing speed and energy efficiency. Such a development requires analysis of the stability of localized magnetic states with respect to thermal fluctuations and random external influences. In the dissertation, these issues are investigated in the framework of harmonic transition state theory. The lifetime of skyrmions and antiskyrmions, two examples of chiral magnetic structures, is calculated using various values of materials parameters, such as the ratio of the lattice constant to the skyrmion size. The energy barrier for skyrmion collapse is found to approach the Belavin-Polyakov lower bound of the energy of a topological soliton in the sigma-model, and the entropy contribution to the pre-exponential factor in the Arrhenius rate expression for the lifetime is found to approach a constant. As a result, the skyrmion lifetime can, for large enough number of spins, correspond to thermal stability at room temperature even without magnetic dipole–dipole interaction. Calculations of skyrmions in antiferromagnets are also presented, and their properties compared with skyrmions in corresponding ferromagnets. The rates of skyrmion collapse and escape through the boundary of a track, as well as the binding to and collapse at a non-magnetic impurity, are calculated as a function of an applied magnetic field. The lifetime of antiskyrmions in an Mn–Pt–Sn tetragonal Heusler material has been calculated using an atomic scale representation including nearly a million spins. The long lifetime observed experimentally at room temperature is found to result from energetic effects rather than entropic effects in this system. The properties of antiskyrmions in ferromagnetic and antiferromagnetic thin films are analyzed using an energy conserving transformation of

skyrmion spin configuration and vectors in the extended Heisenberg Hamiltonian. The dynamics as well as thermal stability of antiskyrmions and corresponding skyrmions can easily be compared using the transformation.

About the doctoral candidate:

Mariia Potkina was born in Saint Petersburg, Russia, in 1993 and grew up in Leningrad region. She studied theoretical physics in St. Petersburg State University, where she comleted her Bachelor's and Master's degree. In 2017 she entered PhD program at the University of Iceland during which she studied the stability of chiral magnetic states for magnetic memorу devices. In her free time, she enjoys learning neuroscience, languages and swimming.