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Spin-state switching has been investigated for several decades using a variety of methods and magnetizable materials. Modern electronic devices pose a need for faster information processing methods. Additionally, the advent of spintronic (a portmanteau of "spin transport electronic") devices combine the application benefits of utilizing both the charge and spin of an electron has garnered interest in this field. Logic gates and magnetoresistive random access memory (MRAM) devices based on spin-transfer switching are examples of devices incorporating electronic spin to achieve increased data storage capacities. These devices require less power and represent a lower cost than their traditional, non-spin incorporated counterparts. To achieve manipulation of spin within these devices, spin injection is traditionally used. A spin-neutral current is passed through a ferromagnetic material to spin-polarize the current. This can then be injected into another, non-magnetic device for further use (i.e., logic operations). Another approach relies on placing a non-magnetic material in close proximity with a ferromagnetic insulator (FMI). This induces a magnetization within the non-magnetic material due to the overlapping of the wave functions between the localized magnetic moments in the FMI and non-magnetic material through the magnetic proximity effect. Unfortunately, many methods of magnetization manipulation require temperatures far below room temperature, limiting their practical applications. To circumvent this restriction, we investigate the potential for optically-induced, spin-state switching as demonstrated in magnetic metal oxides as spintronic device candidates. While optical excitation leading to spin-state switching has been performed successfully for various materials (e.g., Prussian Blue analogues), the necessity for multi-functional materials that can operate in ambient conditions motivates our work by exploiting the favorable properties of magnetic metal oxides.For this investigation, we have chosen the ferrimagnetic insulators cobalt and nickel ferrite. These semiconducting thin films are promising materials for other spintronic applications and their electronic structure indicates their potential for optically-induced spin-state switching as compared to Prussian Blue analogues. Because the materials chosen are bimetallic, we require a method that can selectively probe the different elemental and oxidation states of Fe, Co, and Ni while also being spin-state sensitive. Additionally, the spin-state switching observed in Prussian Blue analogues is typically initiated by a charge-transfer process on a timescale of hundreds of femtoseconds to a few picoseconds. Therefore, the tabletop extreme-ultraviolet (XUV) reflection absorption (RA) spectroscopy that our lab has designed and built is uniquely equip to investigate these materials, providing the same capabilities as traditional x-ray spectroscopies (element-, oxidation-, geometric-, and spin-state sensitivity) on the ultrafast timescale required for these measurements. Furthermore, we have developed a semi-empirical method for interpreting XUV-RA spectra based on ligand-field multiplet simulations that has successfully been applied to monometallic and metal oxide heterojunctions. However, the bimetallic systems of cobalt and nickel ferrite require modification to this method to simulate a spectrum with a high degree of accuracy. The details of this development are provided in Chapter 2.With the ability to properly simulate bimetallic systems, we proceed by investigating the charge and spin dynamics in cobalt and nickel thin films. We find that optical excitation of these materials initiate a charge transfer process the leads either to a direct metal-to-metal charge-transfer state or proceeds through a ligand-to-metal charge transfer state first. In both cases, the systems relax into a state whereby either optically excited Co or Ni atoms undergo a spin flip from a high- to low-spin state on a timescale of several hundreds of femtoseconds. This confirms our hypothesis that these spintronic materials can change their spin states via optical excitation at ambient temperatures indicating that magnetic materials in the class of ferrimagnetic inverse spinel ferrite can support efficient optically induced spin-state switching. Detailed analysis of cobalt and nickel ferrite thin films are provided in Chapters 3 and 4, respectively.Finally, we seek to extend the ability of our XUV instrument by incorporating circularly polarized XUV light for future experiments. While we have shown that we can change the spin state of a material via optical excitation, we are interested in investigating the effect this has on the magnetization of the material. To achieve this, we introduce a four-mirror, XUV circular polarizer into our existing beamline along with developing a method to apply a magnetic field that we can efficiently alternate. The methods to accomplish this and other instrumental considerations are provided in Chapter 5 along with preliminary measurements on nickel-oxide-layered yttrium iron garnet (NiO-YIG).The primary motivation of the work detailed herein is to understand and properly assign the charge and spin dynamics of metal oxide thin films. Using both static and time-resolved XUV-RA spectroscopy, this dissertation outlines the research that has been accomplished in pursuit of this motivation. With the discoveries provided here, we hope to provide further insight to the fields of spintronic and of spin-crossover materials. Additionally, with increasing the capabilities of our instrument, we hope to inspire future students to explore and investigate other spin-related phenomena. |
