Date of Graduation

12-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Laurent Bellaiche

Committee Member

William Oliver

Second Committee Member

Sergey Prosandeev

Third Committee Member

Hugh Churchill

Keywords

Ferrimagnet, Magnetization compensation temperature, Magnetoelectric coupling, Multiferroic

Abstract

This dissertation contains several investigations on the cross-coupling between structural and spin degrees of freedom in multiferroic and ferrimagnetic compounds by means of first-principles calculations and ab-initio-based Monte-Carlo simulations. We start with the reviews of magnetoelectricity, ferrimagnetism, strain engineering, followed by a brief introduction to first-principles computational methods, magnetic effective Hamiltonians, and other techniques that are utilized here. The results section of the dissertation can be divided into two parts. The first half focuses on magnetoelectric effects arising from different sources, while the second half is about the ferrimagnetic nature of materials. In the first part, we examine the epitaxial strain effect on magnetoelectric coupling through lattice mediation and study the underlying mechanism behind the magnetic domain-wall-induced magnetoelectric effect in a non-polar cubic structure. Through the investigation of epitaxial strain effect in the multiferroic Sr0.5Ba0.5MnO3 compound, a large enhancement of linear magnetoelectric coupling coefficient was found at the edge of the so-called morphotropic phase boundary. Such enhancement was studied (at the microscopic level) and found to be related to the large enhancement in the electric susceptibility tensor at this morphotropic phase boundary. Furthermore, we investigate the magnetoelectric effect arising from the magnetic domain wall in Rare-earth Iron Garnet systems. Our results reveal that such domain-wall induced magnetoelectric effect neither requires the existence of magnetism at the rare-earth sites nor non-collinear magnetism to exist, which is in contrast to what was previously proposed in various studies. It is rather found to originate from a (magnetoelectric) symmetric exchange-striction mechanism involving ferromagnetic interactions between two different iron sublattices at the domain wall. In the second half, we study the epitaxial strain effect on magnetic properties (e.g. the magnetization compensation temperature) of ferrimagnetic Rare-earth Iron Garnets and investigate magnetic and topological properties of anti-perovskite ferrimagnet Mn4N. The introduction of the epitaxial strain effect in Rare-earth Iron Garnets is found to significantly affect its magnetic properties and our results reveal that one can tune the magnetization compensation temperature to be at room temperature using a common substrate, which is beneficial for application purposes. Furthermore, our study on the anti-perovskite ferrimagnet Mn4N shows that there is a previously overlooked magnetization compensation temperature in this system and nano-metric sized topological states were also identified from our simulations. Such topological states were found to be stabilized by frustrated exchange coupling interactions between long-distance Mn pairs.

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