Engineered Spin and Orbital Currents in Epitaxial IrO2 Thin Film Heterostructures.

다국어 초록 (Multilingual Abstract)

Spintronic devices that utilize the spins of electrons as an additional degree of freedom for logic, memory, sensor, and other technologies are a promising avenue for highly efficient low power consumption electronics. Understanding the relationship between the crystal structure and spin transport relationship is critical for developing highly efficient spintronic materials. Additionally, studying orbital current may be just as important for technological spintronic advances due to the intricate relationship between the two properties. Many different material platforms have shown unique spin transport phenomena such as heavy metals (Pt, W, etc.) for highly efficient charge-spin conversion, low symmetry materials such as transition metal dichalcogenides and antiferromagnets for unconventional spin-orbit torque, and light metals (Ti, Cr, etc.) have shown large orbital currents that have similar properties and applications as spin currents. However, studying all three of these properties and the relationship to crystallographic symmetries has not been achieved in a single material before. In this thesis, I present a detailed study on IrO2, a heavy semimetal oxide demonstrating large spin-charge conversion and is able to generate unconventional spin and orbital currents making it an ideal platform for understanding and developing next generation spintronic devices.Crystal symmetries can restrict the polarization of spin currents to only be along certain directions. However, for applications such perpendicular magnetic switching that requires the spin to be polarized out-of-plane, high symmetry materials won't work. We show that using epitaxial design in higher symmetry materials, where the crystal orientation and relative crystal symmetries can be controlled, can lead to large unconventional spin-orbit torques. This work, discussed in Chapter 3 of this thesis, highlights which crystal symmetries to avoid in spintronic materials to generate unconventional spin currents by studying IrO2 in the (001), (110), and (111) orientations. Additionally, we can predict the conventional and unconventional spin Hall conductivity for any orientation (i.e. (110), (101), (111)) with high accuracy using the experimental results from the high symmetry orientations (001) and (100). This work, which is discussed in Chapter 4, demonstrates that the spin Hall conductivity truly is an intrinsic property of IrO2 and follows the crystalline symmetries as we would expect, which has not been demonstrated before. Orbital currents have recently been shown in several material platforms including light element metals which have dominating orbital currents compared to spin currents. However, few to no studies have looked at orbital currents in materials with high spin-charge conversion. Additionally, no studies have demonstrated unconventional orbital currents. We show evidence for large conventional as well as unconventional spin and orbital currents in IrO2. These results, discussed in Chapter 5, agree with theoretical calculations and demonstrate the interplay between spin and orbital currents. Field-free switching of perpendicular magnetic materials has promising applications for highly efficient and low power consumption spintronics devices. Field-free switching have been achieved in low symmetry materials such as antiferromagnets, transition metal dichalcogenides, magnetic trilayers, and other low crystalline symmetry materials. However, the z-spin polarized spin-orbit torque that is required to switch out-of-plane magnetic moments have typically been small leading to large current densities which is a disadvantage for commercial applications. Chapter 6 demonstrates field-free perpendicular magnetic switching using IrO2(111)/[Pt/Co]N/Pt heterostructures.  .