Frustration and Crystallization of Vortices in Artificial Spins / Superconductor Hybrids

Northern Illinois University, Dekalb IL

Investigators

Abstract

Non-technical: Artificial spin ices are arrays of lithographically created nanoscale single domain bar magnets. They are introduced to mimic the behavior of electron spins naturally occurring in magnetic materials. Instead of focusing on bar magnets, each of the nanobars can be 'broken up' into a positive and negative magnetic charges, enabling to design an artificial spin structure that produces a magnetic charge ice with reconfigurable charge configurations. When an artificial-spin-ice structure is placed onto a superconducting thin film, its magnetic charges strongly interact with the flux quanta in the superconductor, enabling the realization of various disordered and crystallized flux quanta states. The different states have measurable effects on the superconducting critical current profile, which can be reconfigured by precise selection of the spin ice magnetic state through application of an external magnetic field. This research provides a foundation for understanding and manipulating the new complex and collective effects that are expected in the flux quanta system in a controllable way. This project also supports the training of graduate and undergraduate students, enhancing their academic knowledge and improving their likelihood to succeed in future careers in academia, industry, or government. Technical: Geometric frustration emerges when local interaction energies cannot be simultaneously minimized, resulting in numerous degenerate states. A paradigm system with geometric frustration is spin ice materials such as pyrochlores with 'two spins in, two spins out' spin configuration, which are magnetic analogues of the old problem of how hydrogen atoms order in the water ice lattice. Recently, an artificial spin structure has been developed to produce magnetic charge ices with in-situ tunable long-range ordering. This project is to investigate a hetero-structure consisting of such an artificial-spin structure on top of a superconducting thin film. Certain magnetic charge configurations of the spin structure lead to ordered arrangements of vortices in the superconducting film, whereas others frustrate the vortex ensemble. This research is to establish a new front in geometric frustration research by pursuing the experimental realization and novel properties of frustrated vortex matter. This project can lead to new approaches for in-situ tunable critical currents in a superconductor and novel superconducting devices such as vortex diodes. The coupling of an artificial-spin-structure with other functional materials provides a new setting for designing and controlling geometric frustrations in magnetic 'particle' systems, such as magnetic skyrmions and magnetic colloids. It also leads to an effective way to design in-situ reconfigurable functionalities and electronic devices in a wide range of electronic materials, such as graphene, topological insulators, and quantum wells. 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.