UC Riverside

Spintronics research bears great potential for advancing information technologies and strengthen this rapidly developing sector of the economy. The ever-growing thirst for smaller and faster information technologies is accompanied by the urgent need to design them to be energy-efficient. This challenge is met by developing novel concepts that may promise substantial performance improvement of future spintronics applications.

In this work, the two-magnet paradigm is explored. Its main idea lies in combining two magnetic materials within one spintronic device to tailor its magnetic properties/parameters in a way that would not be achievable with a single magnetic material. Generally, a combination of magnetic materials within one heterostructure is considered to complicate the spin physics of such a device significantly and thus typically avoided.

Recent developments in spin-orbitronics have pushed metallic ferromagnets into the focus of research due to the plethora of spin-charge effects with unusual symmetries. These effects may enable the designing of novel spintronics applications that have been considered unrealistic. On the other hand, magnetic insulators have proven to be beneficial spintronic materials by dint of their low magnetic damping, lack of electrical shunting, and magnetic tunability by growth parameters.

The spin physics of the two-magnet heterostructures is explored by directly interfacing magnetic insulator with a metallic ferromagnet in a nanoscale multilayer device. By carrying out microwave spectroscopy on the nanodevices, spin-wave modes are observed that, as supported by micromagnetic simulations, delocalize over both layers and show features of hybridized spin dynamics. By applying the temperature gradient across the nanodevice layers, thermal spin currents are studied, resulting in magnetic auto-oscillations. These auto-oscillations are a manifestation of a thermally driven condensation of hybrid magnons – a phenomenon that is novel from both experimental and theoretical standpoint. The auto-oscillations are converted into sizeable electric microwave signals under large spin-charge effects inherent to ferromagnetic metals. The results indicate that the two-magnet paradigm brings significant performance improvements for spintronics applications such as spin-torque oscillators, magnetic memory, and neuromorphic networks.

The paradigm is further explored by studying the effect of large thermal and spin-orbit torques on the hybridized spin dynamics of the two-magnet heterostructures. A critical phenomenon is observed that manifests through the formation of a solitonic dynamic mode in the heterostructure. While a satisfying theoretical model is yet to be developed, the experimental data suggest that a breathing domain wall forms in the nanodevice. The soliton oscillates at microwave frequencies below but comparable to the spin-wave frequencies and results in microwave emission with powers that exceed those of a spin torque oscillator by about three orders of magnitude. This observation suggests that the two-magnet devices could be used in magnetic switching and magnetic memory applications.

The two-magnet paradigm is further advanced by exchanging metallic ferromagnets with novel van der Waals two-dimensional magnetic layers. The 2D magnets are usually deposited by exfoliation, which results in laterally micrometer size confined flakes. Thus, an approach for designing and fabricating microscale two-magnet heterostructures has been developed. Moreover, inductive microwave spectroscopy technique with a sensitivity allowing for exploring such microstructures has been developed and tested. The data suggest hybridization of spin-wave modes in the magnetic insulator with critical spin fluctuations of the 2D magnetic subsystem, occurring near the van der Waals magnet's magnetic phase transitions. The results open new avenues for research on two-magnet heterostructures with 2D magnets to advance future spintronics technologies.

This work does not give an exhaustive answer to the question on the future of the two-magnet heterostructures in spintronics technologies. However, it presents preliminary data that points out the paradigm's potential and seeks to advance understanding of the related spin physics that may be critical and instructive for the research to come.