Event
Ph.D. Research Proposal Exam: Brendan Jordan
Wednesday, April 23, 2025
2:30 p.m.
AVW 1146
Maria Hoo
301 405 3681
mch@umd.edu
ANNOUNCEMENT: Ph.D. Research Proposal Exam
Name: Brendan Jordan
Committee:
Professor Kevin Daniels (Chair)
Professor Neil Goldsman
Professor Thomas Murphy
Date/time: Wednesday April 23, 20225 at 2:30pm
Location: AVW 1146
Title: CVD Thin Films of Kondo Insulator SmB6 Growth, Fabrication, and Applications to Spintronics
Abstract:
Samarium hexaboride (SmB6) attracted significant interest over the past decade as a predicted Kondo topological insulator, the first of its kind. Topological insulators have already been shown as promising candidates in spintronics, however some issues can arise due to in gap states due to impurities. The gap in a Kondo insulator such as SmB6 is more robust due to being cause by strong electron interaction meaning it may improve certain bulk resistance limitations of other topological insulators. Verification of the topological nature, however, has proven difficult due to issues involving polarized surfaces and the small Kondo gap of only ~20meV that is difficult to be resolved by most ARPES measurements.
Initial research successfully proved low temperature surface states, and further research has shown other signs of topological states, such as spin-momentum locking in transport measurements. Additionally, a bulk Fermi surface has been measured by de Haas van Alphen oscillations requiring further investigation of the robustness of the bulk insulating state. Intrinsic explanations have been brough forward, as well as evidence for extrinsic causes such as impurity atoms and vacancies that may break the hybridization between f-orbital and conduction electrons resulting in local conducting states. Further testing is needed to understand the effects of different sources of disorder on the Kondo gap. Measurements have also been done to test preliminary performance in spintronic applications. SmB6 has been shown to produce charge currents from magnetic polarization via the inverse Edelstein effect, and thin films have been shown to have promise in use for spin orbit torque devices.
This research focuses on the scalable synthesis of SmB6¬ by chemical vapor deposition (CVD) to be fabricated into test structures. Most research has been done on bulk crystals grown though Al based flux or float zone methods. Thin films of 50-500nm have also been grown using magnetron sputtering and pulsed laser deposition. Recently, CVD-grown nanostructures were developed which consisted of single crystal rectangular rods roughly a few hundred nanometers thick.
In this work we are focusing on thin films grown by chemical vapor deposition. This can allow better control of crystal quality and thus remove additional sources of local disorder. It will also still allow attempts at scalable fabrication that is limited by bulk or nanocrystalline growth methods. Measurements such as XRD and Raman suggest better crystallinity compared to sputtered thin films with a strong preference for growth in the (100) direction. However, surface roughness and uniformity remain issues. Resistivity measurements show a low temperature to high temperature ratio (R_3K/R_300K ) of ~70 in the middle of reported bulk and nanostructured/sputtered samples. Magnetoresistance measurements also show linear behavior at 3K, suggesting surface transport. The thickness of the films varies over the sample but are on the order of 1-3"μ" m.
The proposed work would focus on fabricating Permalloy/ SmB6 stacks to directly measure the spin-hall angle and how it changes below the Kondo transition. This measurement will be done using spin-torque ferromagnetic resonance (ST-FMR) as have been done on other materials showing spin orbit torque such as Platinum. This would be measured at a range of temperatures to see how the transition effects the spin-orbit properties. Additionally, we can use the CVD material as a new medium to study the effects of grain boundaries on the electronic structure through transport measurements and low temperature Raman spectroscopy. This would further understanding of not just SmB6 but also other materials in the intersection of Kondo lattices with strong interaction and non-trivial topology and how they can be applied to spin-based devices.
