Interplay Between the Lattice and Spin Degrees of Freedom in Magnetoelectric and Magnetic Materials

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

Properties of Epitaxial Sr0.5Ba0.5MnO3 Films from First-Principles

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

Ab initio amorphous spin Hamiltonian for the description of topological spin textures in FeGe

Topological spin textures in magnetic materials such as skyrmions and hopfions are interesting manifestations of geometric structures in real materials, concurrently having potential applications as information carriers. In the crystalline systems, the formation of these topological spin textures is well understood as a result of the competition between interactions due to symmetry breaking and frustration. However, in systems without translation symmetry such as amorphous materials, a fundamental understanding of the driving mechanisms of non-trivial spin structures is lacking owing to the structural and interaction complexity in these systems. In this work, we use a suite of first-principles-based calculations to propose an ab initio spin Hamiltonian that accurately represents the diversity of structural and magnetic properties in the exemplar amorphous FeGe. Monte Carlo simulations of our amorphous Hamiltonian find emergent skyrmions that are driven by frustrated geometric and magnetic exchange, consistent with those observed in experiment. Moreover, we find that the diversity of local structural motifs results in a large range of exchange interactions, far beyond those found in crystalline materials. Finally, we observe the formation of large-scale emergent structures in amorphous materials, far beyond the relevant interaction length-scale in the systems, suggesting a new route to emergent correlated phases beyond the crystalline limit

Quantifying the Topology of Magnetic Skyrmions in three Dimensions

Magnetic skyrmions have so far been treated as two-dimensional spin structures characterized by a topological winding number describing the rotation of spins across the skyrmion. However, in real systems with a finite thickness of the material being larger than the magnetic exchange length, the skyrmion spin texture extends into the third dimension and cannot be assumed as homogeneous. Using soft x-ray laminography we reconstruct with about 20nm spatial (voxel) resolution the full three-dimensional spin texture of a skyrmion in an 800 nm diameter and 95 nm thin disk patterned into a trilayer [Ir/Co/Pt] thin film structure. A quantitative analysis finds that the evolution of the radial profile of the topological skyrmion number and the chirality is non-uniform across the thickness of the disk. Estimates of local micromagnetic energy densities suggest that the changes in topological profile are related to non-uniform competing energetic interactions. Theoretical calculations and micromagnetic simulations are consistent with the experimental findings. Our results provide the foundation for nanoscale magnetic metrology for future tailored spintronics devices using topology as a design parameter, and have the potential to reverse-engineer a spin Hamiltonian from macroscopic data, tying theory more closely to experiment.Comment: 18 pages, 4 figure