Synthetic antiferromagnets are emerging as a flexible and powerful material platform for fundamental research inquiries into how magnons can be engineered to interact. The growing field of quantum hybrid magnonics[1], where information transduction relies on the interaction between magnons and other quasiparticles, will eventually require tunability of these interactions for practical device technologies. The oft-emulated archetype of a S-AFM consists of two ferromagnetic thin films separated by a non-magnetic spacer film, e.g., an antiferromagnetic bilayer. The spacer provides an effective antiferromagnetic exchange interaction via the Ruderman-Kittel-Kasuya-Yosida interaction, allowing both long wavelength acoustic and optical magnons to exist at the GHz scale. Thus, with only a modest external field antiferromagnetic magnons have been recently controlled by tuning the interactions between acoustic and optical magnons. So far, the application of symmetry breaking external fields[2] - or - the reliance on interlayer dipolar interactions[3] has led to the induction and control of magnon-magnon interactions in S-AFMs.
By re-imagining the structure of synthetic antiferromagnets to consist of additional magnetic layers, or by altering individual interlayers, new opportunities to engineer interactions into S-AFMs occur. From a structural standpoint, we have synthesized S-AFM bilayers, tetralayers, hexalayers, octalayers, and decalayers. In tetralayers, we experimentally observe energy level repulsion effects in the magnon spectrum that are consistent with dynamic fieldlike torques arising from spin pumping. Our results are explained by a newly developed theory of spin pumping for non-collinear magnetic materials. Furthermore, micromagnetic simulations suggest a viable path forward to electrically control this new magnon-magnon interaction[4]. Hexalayers, in contrast, exhibit level attraction between magnon branches suggesting that dampinglike torques, generated by spin pumping, can alter the magnon energy spectrum.
By structurally inducing energy level attraction effects into S-AFMs, it becomes possible to engineer exceptional points (EP) into the magnon energy spectrum. EPs can be used to trigger dynamical phase transitions in S-AFMs[5], enabling a new approach to spin torque oscillators based on antiferromagnetism. This seminar will conclude with practical considerations, informed by micromagnetic simulations[6], which should be accounted for when attempting to engineer the appearance of EPs within the magnon energy spectrum
[1] Li et al. Journal of Applied Physics 128, 130902 (2020).
[2] Sud et al. Phys. Rev. B 102, 100403(R) (2020).
[3] Shiota et al. Phys. Rev. Lett. 125, 017203 (2020).
[4] Sklenar et al. Phys. Rev. Applied 15, 044008 (2021).
[5] Deng et al. arXiv preprint arXiv:2205.02308 (2022).
[6] Jeffrey et al. Appl. Phys. Lett. 118, 202401 (2021)
BIO: Joe Sklenar worked between Northwestern University and Argonne National Laboratory over the duration of his graduate work. He received his PhD in 2015 from Northwestern. Joe was a post-doctoral research associate at the University of Illinois from 2015-2019. He started his current position as an Assistant Professor at Wayne State University in 2019.
