ICEBERG: a bottom-up approach towards three-dimensional artificial spin ice devices

Microwave devices are critical components in our communication technology. While each device has a specific function, there is interest in producing reconfigurable devices whose properties can be actively modified by external stimuli, from a simple toggle feature to completely different characteristics. A promising reconfigurable system is a collection of magnetic nanoparticles called artificial spin ices (ASIs). However, their microwave properties are deficient because of the weak coupling between particles that limits the information propagation in the form of spin waves. In this proposal, we aim to solve this issue by numerically investigating three-dimensional ASI designs targeted to increase the coupling between nanoparticles and, therefore, extend the spin wave propagation. The first design features an ASI patterned over a collection of soft magnetic particles that will mediate magnetic coupling. This design is expected to produce a reconfigurable microwave frequency comb with a micrometer footprint. The second design is a 3D-stacked ASI, similar to a brickwall stacking. This design will decrease the spacing between magnetic particles to increase the coupling and is expected to support tunable spin wave modes, acting as a directional microwave waveguide. The numerical results will be supported by a semi-analytical model that will allow for a rapid exploration of the parameter space and device design. The project will assess the potential functionality of three-dimensional ASI devices and chart the course toward their application. This project will also provide research opportunities to graduate and undergraduate students at the forefront of magnetism research. In particular, undergraduate researchers will participate in summer programs, research dissemination, and industry partnerships to develop their careers and excite other undergraduate students to follow a similar career path. Such an effort is envisioned to increase undergraduate research in our institution and contribute to the development of an American STEM workforce. Artificial spin ices (ASIs) are reconfigurable magnetic metamaterials based on a geometric arrangement of nanomagnets. Ferromagnetic resonance in ASIs has been shown to depend on their long-range magnetization state, suggesting ASIs as potential reconfigurable magnonic crystals. However, ASI-based magnonic devices remain elusive because of the weak dynamic coupling between nanomagnets, which leads to coherence loss in a handful of unit cells. In this proposal, we will investigate a method to increase the dynamic coupling in ASIs based on three-dimensional stacking. We propose two alternative designs: a vertex-modified ASI where circular soft-magnetic particles are placed under the nanomagnets’ vertices in order to mediate coupling by magnons; and a 3D-stacked ASI where coupling is enhanced by vertical proximity of the nanomagnets’ edges. We will numerically investigate both basic magnetic characterization, magnon characteristics, and potential devices, namely, reconfigurable frequency combs and directional magnon transport. We consider this approach “bottom-up,” in which the underlying physics will inform potential applications. We will also develop a fully nonlinear model based on the existing Gænice framework that will aid parameter optimization. This research will immediately impact the field of magnonics and the evolving field of three-dimensional magnonics. Our project provides ample research opportunities for graduate and undergraduate students. In particular, we aim to tackle the student retention problem at the host institution by providing undergraduate students with year-long as well as summer research opportunities. These will be complemented by dissemination activities including presentations, panel discussions, and industry partnerships. The goals of these activities are to develop the participating students’ skills and excite other undergraduate students to participate in the future. Therefore, our undergraduate students will gain a better understanding of their career options and prospects for their professional life, further developing the American STEM workforce. 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.