Doctoral defence in Physics - Kristbjörg Anna Þórarinsdóttir

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Ph.D. student: Kristbjörg Anna Þórarinsdóttir

Dissertation title: The order in disorder: Magnetism in amorphous cobalt-based thin films and heterostructures

Opponents: 
Dr. Eric Fullerton, Professor at the University of California San Diego, USA
Dr. Sarah Thompson, Professor at the University of York, UK

Advisor: Dr. Friðrik Magnus, Professor at the Faculty of Physical Sciences, University of Iceland

Doctoral committee: 
Dr. Snorri Þorgeir Ingvarsson, Professor at the Faculty of Physical Sciences, University of Iceland
Dr. Björgvin Hjörvarsson, Professor at Uppsala University, Sweden

Chair of Ceremony: Dr. Einar Örn Sveinbjörnsson, Professor and the Head of the Faculty of Physical Sciences, University of Iceland

Abstract:
The work in this thesis is focused on understanding the emergent properties of amorphous magnetic materials. Amorphous materials lack long-range structural ordering, characteristic of crystalline materials, but instead they exhibit medium-to-short range order. Due to the disordered structure the magnetic properties of amorphous films are non-homogeneous, and can therefore be elusive. First, we examine how the magnetic properties of the ferromagnet Co are affected by alloying with the non-magnetic compound AlZr. We find a surprising manifestation of the disorder in this amorphous alloy, where local changes in the composition result in competing anisotropies. From the competition between the anisotropy terms we can define quantitatively a cobalt composition distribution on the nanoscale. This shows how sensitive amorphous alloys are to small changes in composition.

We also study hybrid structures composed of multilayers of alternating high- and low- T_c CoAlZr and bilayers of TbCo and CoAlZr with crossed perpendicular and in-plane anisotropy. In the CoAlZr multilayers, we find that within the low-T_c layer there is a non-zero magnetization at three times its intrinsic ordering temperature which extends through at least 10~nm due to a magnetic proximity effect. In the TbCo/CoAlZr bilayers, we investigate the exchange coupling between two layers with crossed magnetic anisotropies. We find that a 7.5~nm interface layer of the CoAlZr is strongly exchange coupled to the TbCo with a magnetization perpendicular to the plane and switches in unison with the TbCo layer. These structures could for example be used in magnetic storage devices or magnetic nano-oscillators.

About the doctoral candidate:

Kristbjörg Anna Þórarinsdóttir started her B.Sc.studies in Physics at the University of Iceland in 2013 and graduated in 2016. After graduation she started a M.Sc. degree studies at Århus Universitet in Denmark and graduated in 2018 and soon after she started her doctoral studies at the University of Iceland.