Universität zu Köln

Hardt, Dennis ORCID: 0000-0002-2854-0303 (2025). Dynamics of Active Magnetic and Non-Equilibrium Systems. PhD thesis, Universität zu Köln.

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Abstract

Many everyday phenomena, including climate patterns, traffic flow, and biological processes such as metabolic activity, are fundamentally out of equilibrium. The latter prevents the equilibration of an organism and makes life as we know it possible. Another instance is the relatively new field of 'active matter', which focuses on systems composed of self-propelled units that consume energy to generate motion and exhibit collective behaviours, often leading to complex and dynamic patterns. These examples, and many more, motivate the study of non-equilibrium phenomena. Due to its complex systems, technological relevance, and emergent phenomena, condensed matter physics and magnetism, in particular, offer fertile ground for non-equilibrium research. In order to reach a non-equilibrium state in a material, some external force has to be applied to the system. This can be done, for example, by creating temperature gradients, applying external fields or mechanical stress, or using ultrafast laser pulses. In this thesis, we investigate the model of a ferrimagnet driven out of equilibrium by an oscillating magnetic field. The model hosts ferromagnetic order in the z-direction and antiferromagnetic order in the xy-plane. We show that the oscillating magnetic field activates the rotational Goldstone mode, which describes a rotation of the magnetisation with constant speed in the xy-plane. A crucial point of the construction is that the sign of the ferromagnetic component determines the direction of rotation. Necessarily, dynamical frustration builds up at a domain wall, where the different senses of rotation meet. A direct consequence of the frustration is that a static domain wall is no longer stable. One way for the system to lift the frustration is via the active movement of the domain walls with a velocity proportional to the amplitude of the driving field. This drastically changes many properties of the system. We analyse different properties not only in one, two and three dimensions.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Hardt, Dennis hardt@thp.uni-koeln.de orcid.org/0000-0002-2854-0303 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-783341
Date:	2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute for Theoretical Physics
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Active Magnetic Matter English Driven Magnet English Domain Walls English Non-Equilirbium English Landau-Lifshitz-Gilbert English Oscillating Magnetic Field English Goldstone Mode Activation English Dynamical Frustration English Phase Transition English Kibble-Zurek English Coarsening English
Date of oral exam:	14 May 2025
Referee:	Name Academic Title Rosch, Achim Prof. Dr. Rizzi, Matteo Prof. Dr. Hemberger, Joachim Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/78334