Developing materials with high magnetoresistance (MR) and understanding their underlying physical mechanisms have always been one of the key areas of research in materials science. In the past decade, an unusual MR effect was discovered and quickly gained widespread attention from the scientific community. The observed MR is extremely large and remains nonsaturating even under a high magnetic field, which is a highly desirable property for the design of high-field magnetic sensors. Nevertheless, there is still a lack of clear understanding of the physical mechanism, despite extensive experimental efforts in the field. Furthermore, the number of magnetic materials exhibiting this phenomenon is extremely limited, and their MR performance is found to be significantly lower than that of the non-magnetic host materials that have been frequently explored. However, it is precisely these magnetic hosts that allow for the interplay between magnetic order and the topological electronic state, which holds the most promise for novel spintronic applications, such as information storage, high-field sensing, and efficient spin-charge conversion.

Recently, the PI (Shiming Lei) at HKUST has put forth a new mechanism, based on Weyl nodal ring (NR) states, to generate large, nonsaturating MR in magnetic compounds. To move forward, this project aims to systematically study the role of the Weyl NR states and their tunability in the MR performance toward functional devices. To this end, we propose the development of a series of magnetic Weyl NR semimetals guided by wallpaper group symmetry analysis, featuring px/py orbitals on the nondistorted square lattice, px/py orbitals on the distorted square lattice and beyond, to eventually establish a design principle for high-MR magnetic materials. The proposed research covers several intimately connected topics, including growth of high-quality bulk single-crystalline Weyl NR semimetals, characterization of their electrical and magnetic properties, and creation of prototype functional devices. It is anticipated that this project will advance the research of magnetic Weyl NR semimetal materials and expand the frontiers of research on materials with high MR performance.