Date of Graduation

5-2018

Document Type

Thesis

Degree Name

Master of Science in Physics (MS)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Laurent Bellaiche

Committee Member

Sergey Prosandeev

Second Committee Member

Hugh Churchill

Keywords

Epitaxial Strain, First-principles, Magnetoelectric Coupling, Multiferroic

Abstract

Magnetoelectric multiferroics, that possess coupled magnetic and electric degrees of freedom, have been receiving ever renewed attention for more than 15 years since they hold promise for the design of novel devices exploiting their cross-coupling.

In this thesis, we present the results of first-principles studies on physical properties of multiferroic Sr0.5Ba0.5MnO3 films under epitaxial compressive and tensile strains, and chemical ordering. We start by reviewing multiferroic materials, a magnetoelectric coupling mechanism and then we give a brief introduction to the first-principles computational methods that are involved in this study.

Here, we report that Sr0.5Ba0.5MnO3 (SBM) films under compressive strain become strongly polar ferromagnet with a large axial ratio and with its properties being controllable by an external knob such as a magnetic field or strain. Furthermore, we investigated SBM films subject to epitaxial strain continuously varying from relatively large compressive to relatively large tensile values and surprisingly found, in addition to previously documented tetragonal and orthorhombic states, a novel phase that has been overlooked in the recent intensive literature on SBM systems. This latter phase adopts a monoclinic symmetry and allows the polarization to rotate continuously between out-of-plane and in-plane directions, which results in giant physical responses such as large piezoelectricity. Moreover, the strain boundaries separating tetragonal, monoclinic and orthorhombic phases are predicted to be rather sensitive to the magnetic ordering (e.g., they significantly differ between G-type antiferromagnetic and ferromagnetic spin arrangements), which therefore hints at the exciting possibility of inducing structural phase transitions (e.g., from tetragonal to monoclinic or orthorhombic to monoclinic) by applying a magnetic field. Such latter effect constitutes another novel and giant magnetoelectric effect.

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