New Physics from Structures Containing van der Waals Magnets and Antiferromagnets

Non-technical Abstract: This project is focused on experimental studies of two related magnetic systems, both built by peeling off atomically-thin layered materials and stacking them to make structures with ultra-clean interfaces. Antiferromagnets are materials in which atomic-scale magnets are arranged in a cancelling pattern so that there is no net magnetism. One activity studies the high-speed dynamics of such antiferromagnets, with the goal of learning to detect and control these dynamics, to enable operations at much higher frequencies than existing technologies based on conventional ferromagnets. A second activity is studying an interesting recently-discovered family of electronic states called quantized anomalous Hall effects, some of which allow electrons to flow without resistance even though they are not superconductors. One goal of this part of the work is to expand the range of temperatures at which these effects can be realized to well above liquid-helium temperature. Graduate students and undergraduates involved in this research are mastering frontier technical skills and receiving training in scientific communication and the soft skills important for successful science and engineering careers. The project also includes an education component in which the principal investigator is organizing and posting for free graduate-level lecture notes for students and teachers of solid state physics, about modern advances in tools and techniques for performing structural characterization of materials. Technical Abstract: This research consists of experimental studies of two related magnetic systems, both built by mechanical stacking of van der Waals layers. The first set of experiments is studying high-frequency spin dynamics in antiferromagnets, by developing improved electrical techniques for both detecting and controlling these dynamics. Objectives include to develop the capability to tune the effective damping of an antiferromagnet through zero to negative values so as to excite steady-state oscillations with large-angle precession, and then to understand the nonlinear physics governing this little-explored large-angle precessional regime. The second set of experiments is studying structures which integrate topological insulators with magnetic layers to induce an exchange gap in the topological surface state, thereby generating a quantized anomalous Hall conductivity. Research supported by previous NSF funding found the surprising result that the temperature scale for the quantized anomalous Hall conductivity in mechanically-stacked topological insulator/magnet bilayers is at least 100 times greater than if the same materials stack is grown by conventional deposition techniques. The objectives of this part of the project are to create high-temperature Chern insulator states and axion insulator states in trilayer devices with the structure magnet/topological insulator/magnet, to explore to how high in temperature one can create quantized anomalous Hall effects by varying materials and applying pressure, and to investigate the properties of dissipationless edge channels as they are turned on and off by switching the magnetization orientations within the trilayers. The methods employed in both sets of experiments include device fabrication by mechanical stacking and etching, followed by cryogenic measurements of spin dynamics, electron transport, and electron chemical potential as a function of gating, applied current, magnetic field, temperature, and pressure. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria. NSF Award ID: 2611101 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Daniel Ralph | Institution: Cornell University, ITHACA, NY | Award Amount: $679,240 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2611101 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2611101.html