Author ORCID Identifier

https://orcid.org/0000-0003-0672-5355

Date Available

6-30-2024

Year of Publication

2023

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department/School/Program

Physics and Astronomy

First Advisor

Dr. Ambrose Seo

Abstract

In layered 5d transition metal oxides, like Sr2IrO4, strong spin-orbit interaction comparable to the relevant interactions such as electron correlation and crystal field splitting provides fertile ground for emergent non-trivial phenomena. Some of these phenomena include metal-insulator transition, high-temperature superconductivity, and spin-orbit Mott insulator. The ground state is governed by the spin-orbit-entangled pseudospins Jeff =1/2, which have a significant orbital contribution. Therefore, magnetic exchange interactions and anisotropies are highly susceptible to external stimuli such as strain and heterointerface. As a result, it has a significant impact on the magnetic properties of the system. However, tunability of the lattice has been constrained to a minimal value using strain devices and a limited number of conventional insulating perovskite substrates, which also limit the heterointerfaces to insulating substrates. In this dissertation, the magnetic properties of the epitaxial single-layered iridates, Sr2IrO4, thin films have been investigated in various unconventional circumstances, including the application of the considerable biaxial anisotropic strain using lab-grown unconventional 4d transition metal oxide substrates and epitaxial heterointerface with metallic and insulating substrates. Magnetic properties were investigated using resonant x-ray techniques, including x-ray resonant magnetic scattering, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and resonant inelastic x-ray scattering. These techniques are chosen for their polarization dependence, element and orbital specificity, bulk sensitivity, and the ability to work with small sample volumes.

In the first part, x-ray resonant magnetic scattering experiments revealed emergent La2CuO4-like interlayer antiferromagnetic order in Sr2IrO4 thin films. This emergent magnetic order was realized by subjecting the Sr2IrO4 thin films to substantial biaxial anisotropic strain to induce orthorhombic distortion, which was achieved by depositing Sr2IrO4 thin films on Ca3Ru2O7 single crystal. This distinct antiferromagnetic order is explained by the competition between the anisotropic interlayer exchange interaction and the interlayer pseudo-dipolar interaction, highlighting the importance of interlayer interaction in understanding the quasi-two-dimensional system.

In the second part, we investigated uniaxial magnetic anisotropy in Sr2IrO4 thin film by inducing a pronounced uniaxial distortion along the direction of oxygen octahedral edges. Remarkably-divergent hysteresis responses along the easy and hard magnetic axes were unveiled through x-ray magnetic circular dichroism (XMCD) measurements under magnetic fields. Specifically, the spin-flop transition observed when the magnetic field is parallel to the hard axis allows us to estimate the magnetic anisotropy energy. This energy is significantly lower than the extrapolated value derived from the theoretical estimation of the strain-magnetic anisotropy relationship. Consequently, this lower magnetic anisotropy underscores that the spin-orbit entangled Jeff =1/2 pseudospin states remain well-defined even at the large anisotropic lattice. The findings from x-ray absorption spectroscopy (XAS) and XMCD measurements further validate this observation.

In the final part, resonant inelastic x-ray scattering experiments revealed significant softening of the single magnon peak at the (π/2, π/2) zone boundary when the substrate is in a metallic state, while it remains unchanged for the substrate in the insulating form. The conventional wisdom of external stimuli such as strain, doping, and proximity effects cannot explain this nontrivial phenomenon. The theoretical fitting of the magnon dispersion shows decreased first-nearest-neighbor exchange interaction with increased second and third- nearest-neighbor exchange interaction for the metallic substrate. This change may be attributed to the electron-phonon interaction at the interface between the metal substrate and Sr2IrO4 thin film. These phonons then interact with the magnons present in the Sr2IrO4 thin film. This study demonstrates that the metallicity of the adjacent substrate can also be an effective knob to manipulate the terahertz magnon of the spin-orbit-coupled Mott insulator.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2023.466

Funding Information

This study was supported by the National Science Foundation grant no. DMR-1454200 from 2018 to 2021.

This study was supported by the National Science Foundation grant no. DMR-2104296 from 2021 to 2023.

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