Doctoral defence in Physics - Einar Baldur Þorsteinsson

Doctoral candidate: Einar Baldur Þorsteinsson

Title of thesis: Growth and characterization of Mn-based magnetic MAX phases: The quest for room temperature ferromagnetism

Opponents:
Dr. Michael Farle, Professor at the Faculty of Physics, Experimental Physics, Universität Duisburg-Essen, Germany
Dr. Hans Högberg, Associate Professor at the Department of Physics, Chemistry and Biology (IFM), Linköping Universitet, Sweden

Administrative supervisor: Dr. Einar Örn Sveinbjörnsson, Professor at the Faculty of Physical Sciences, University of Iceland

Advisor: Dr. Friðrik Magnus, Research professor at the Science Institute, University of Iceland

Other members of the doctoral committee:
Dr. Unnar Bjarni Arnalds, Professor at the Faculty of Physical Sciences, University of Iceland
Dr. Árni Sigurður Ingason, Managing Director at Grein Research

Chair of Ceremony: Dr. Birgir Hrafnkelsson, Professor and Head of the Faculty of Physical Sciences, University of Iceland

Abstract

MAX phases are a family of naturally nanolaminated materials, with the chemical formula Mn+1AXn, the most common being M2AX. Traditionally they are composed of a transition metal (M), an A-group element (A), and either carbon or nitrogen (X). However in recent years, the elements available at each site have been expanding as new phases continue to be discovered. The basic structural ordering of M2AX is distinct layers of elements with the order M-A-M-X-M-A-M-X. In this thesis the MAX phase Mn2GaC is used as a base material to branch out from by substituting manganese (Mn) with chromium (Cr), iron (Fe) and scandium (Sc), with the aim of investigating the magnetic properties of the resulting compounds. The magnetic properties of epitaxially grown Mn2GaC thin films on MgO(111) are studied, where the low-temperature magnetization at 5 T field is not found to change significantly with temperature, contradicting the existing literature. This is found to stem from insufficient subtraction of the large non-linear MgO substrate background signal. Investigations into the magnetic anisotropy of Mn2GaC using two films with different crystal growth orientations, find that the (000l) crystallographic planes are magnetic easy planes. Scandium is used to obtain a different chemical ordering from the standard MAX phase, referred to as an i-MAX phase, which has an additional kagome-like in-plane ordering. This is formed when 1/3 of the Mn is substituted for Sc, forming (Mn2/3Sc1/3 )2GaC. The films are grown epitaxially on MgO(111), Al2O3(0001) and SiC4H(001) substrates, with SiC giving the highest crystal quality. The magnetic properties of this phase are investigated and it is found to be antiferromagnetic. Substituting Cr for Mn, forming (Mn1−xCrx)2GaC with x ≤ 0.29, results in a significant ferromagnetic response at room temperature, with critical temperatures reaching up to 489 K. The strongest moment measured was at x = 0.12, with a saturation magnetization and remanent magnetization of 370 kA/m and 176 kA/m respectively, and a coercive field of 16.8 mT at room temperature. This is the first time a strong ferromagnetic response at and above room temperature is reported on for MAX phases, which is a significant milestone. The (Mn1−xCrx)2GaC films are grown both epitaxially on MgO(111) substrates and polycrystalline textured on Si/SiO2 substrates, both resulting in similar magnetic properties. This opens up the possibilities of utilizing them in practical applications. Investigating the solid solution of (Mn1−xFex)2GaC, for 0.05 ≤ x ≤ 0.38, shows that Fe seems to successfully integrate into the MAX phase according to x-ray measurements. However, even for the lowest concentration of x = 0.05, a large amount of the antiperovskite phase (Mn,Fe)3GaC is also formed. The addition of Cr to make (Mn1−x−yFexCry)2GaC, has a stabilizing effect on the MAX phase, allowing almost a phase pure x = 0.11, y = 0.21 sample to be synthesized with only a trace of the antiperovskite competing phase appearing. This sample has similar magnetic characteristics to a Cr y = 0.29 sample with no Fe, however with a slightly lower critical temperature.

About the doctoral candidate

Einar Baldur Þorsteinsson was born in 1988 in Reykjavík. He graduated from the Commercial College of Iceland in 2008 with a specialization in natural sciences. He graduated with a BSc. in Physics from the University of Iceland in 2012 and went on to complete an MSc. in Engineering Physics from the University of Iceland in 2018. After that he started doctoral studies. Alongside his studies he has worked on various other research projects, as well as being a teacher assistant at the University of Iceland.