Graduation Year

2024

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Denis Karaiskaj, Ph.D.

Committee Member

Jiangfeng Zhou, Ph.D.

Committee Member

Garrett Matthews, Ph.D.

Committee Member

Ashwin Parthasarathy, Ph.D.

Keywords

TR-MOKE, NOPA, Ferromagnets, Kagome lattice, Magnetic skyrmion

Abstract

The interaction between light and matter plays a crucial role in numerous systems, including optoelectronicdevices such as Photodiode (PD), Laser diode (LD), LED, solar cells to even biological components like photosystem II, and potential upcoming quantum technologies. Light is absorbed or emitted usually at a scale smaller than a nanometer, with associated processes happening within attosecond to picosecond durations. It is known that in a vacuum, static magnetic fields do not affect light. On the other hand, the magnetic field or magnetization of a material affects the light that passes through or is reflected by the material. The magneto-optic effect (MOE) is the name given to this phenomenon. There are two different ways to apply a magnetic field, the Faraday configuration, which applies the magnetic field parallel to the direction in which light propagates, and the Voigt configuration, which applies the magnetic field perpendicular to the direction of light propagation. The Faraday effect or configuration rotates light polarization (Faraday rotation) and produces elliptically polarized light useful for Magnetic circular dichroism (MCD) measurements. Conversely, the Voigt configuration produces the Cotton-Mouton effect which induces magnetic birefringence. MOE effect in the reflection geometry is called the magneto-optical Kerr effect (MOKE). The Kerr effect was discovered by the Scottish physicist John Kerr in 1877[7]. A linearly polarized electromagnetic wave becomes elliptical when it is reflected on a metal surface with the presence of an electric or magnetic field. The rotation of the polarization is proportional to the magnetization M and the thickness of the medium. The microscopic genesis (Zeeman effect) is based on spin-orbit interaction and relativistic processes. While MOKE effects have been known for over 100 years, their practical applications have only recently emerged, with the majority appearing in the last three decades.

Spintronics, also known as magneto-electronics, is a novel area that integrates microscopic magnetic components with semiconductor electronics to create devices with greater functionality. This revolutionary technique uses the electron’s inherent spin, magnetic moment, and electrical charge to create solid-state electronics. MOKE is a tool not only to make optoelectronic devices, but also a powerful technique to study the electronic structure and physical properties of materials. Currently, mass-storage technologies demand rapid read-write capabilities, as well as substantial storage capacities. While magnetic storage, such as hard disks, offers speed, its lifespan is finite. Optical storage provides stability, but suffers from sluggish readwrite speeds and restricted usage cycles. Magneto-optical storage emerges as a promising solution for mass storage due to its combined benefits. MOKE spectra have been explored in magnetic materials containing 3d, 4f, and 5f elements [8, 9, 10], not solely for data storage applications, but also to enhance comprehension of the electronic structures within these materials. Optical transitions represent dipole transitions from occupied to unoccupied states below and above the Fermi energy. With advancements in first-principles calculations driven by enhanced computing capabilities over the past two decades, MOKE spectra have garnered attention for their potential to offer more intricate insights into the electronic structure of magnetic materials. Since the Kerr effect results from the interplay of two circular polarizations, precise electronic structure calculations are indispensable for MOKE analysis. Comparing the computed optical and magnetooptical spectra with experimental data validates the accuracy of band structure calculations. The exploration of MOKE not only holds fundamental physical significance, but also guides enhancing magneto-optic materials. The magneto-optic effects result from the diverse interactions of materials with two circularly polarized states. Spectroscopic MOKE relies on optical transitions, which are intricately linked to the electronic band structures of materials. Spectroscopic optical assessments ascertain a material’s response (reflection, transmission, absorption) relative to incident photon energy (wavelength). MOKE serves as an invaluable spectroscopic tool for examining the electronic structures of magnetic materials. A quantum mechanical approach to optical transitions is essential for understanding the origins of MOKE. The underlying mechanisms of the magneto-optic effects stem from the spin-orbit interaction and exchange interaction at the microscopic level.

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