Voltage-Controlled Spintronics and Fundamentals of Magnetoelectricity in Magnetoelectric Antiferromagnetic Chromia

Spintronics, which exploits both the spin and charge of electrons, offers a path to more energy-efficient and functionally enhanced devices compared to traditional CMOS electronics. A significant development in this field is voltage-controlled spintronics, where electric fields manipulate magnetic and spin states without the large power dissipation seen in current-based technologies. Among the materials explored for this purpose, magnetoelectric (ME) antiferromagnetic (AFM) materials show promise for energy-efficient, non-volatile memory technologies. Controlling AFM ordering via voltage application is essential for realizing these devices. This dissertation focuses on ME AFM materials, specifically chromia (Cr2O3), to advance voltage-controlled spintronics. However, pristine Cr2O3 encounters two main challenges: 1) it requires an external magnetic field to switch the Néel vector, and 2) it lacks high thermal stability, which is crucial for integration with CMOS technologies, where operational temperatures often exceed 350 K. Boron doping in chromia (B:Cr2O3) addresses these issues by enabling non-volatile Néel vector rotation without a magnetic field and enhancing thermal stability, raising the Néel temperature from 307 K to approximately 400 K. This has been confirmed through cold neutron depth profiling (cNDP), X-ray photoemission spectroscopy (XPS) depth profiling, and magnetotransport measurements, which verify boron segregation at the surface. In addition to technological advancements in spintronics, this research aims to develop a material relevant for investigating axion fields, significant in both condensed matter and high-energy physics. This connection arises from the shared mathematical structure between the axion electrodynamics Lagrangian and the parity and time-reversal symmetry-breaking term associated with axions. Insights into axions and dynamic axion fields are crucial for understanding topological insulators and linear magnetoelectric materials like Cr2O3. The research seeks to create a magnetoelectric material exhibiting a non-zero axion component, which would display an isotropic magnetoelectric response and enhance our understanding of these fundamental physics concepts. In conclusion, this dissertation advances voltage-controlled spintronic devices and provides deeper insights into fundamental physics, positioning B:Cr2O3 as a promising material for energy-efficient computing and advancing the understanding of axion physics in solid-state systems. Advisor: Christian Binek