Steering magnonic dynamics and permeability at exceptional points in a parity-time symmetric waveguide

Abstract: Tuning the low-energy magnetic dynamics is a key element in designing novel magnetic metamaterials, spintronic devices and magnonic logic circuits. This study uncovers a new, highly effective way of controlling the magnetic permeability via shaping the magnonic properties in coupled magnetic waveguides separated by current carrying spacer with strong spin-orbit coupling. The spin-orbit torques exerted on the waveguides leads to an externally tunable enhancement of magnetic damping in one waveguide and a decreased damping in the other, constituting so a magnetic parity-time (PT) symmetric system with emergent magnetic properties at the verge of the exceptional point where magnetic gains/losses are balanced. In addition to controlling the magnetic permeability, phenomena inherent to PT-symmetric systems are identified, including the control on magnon power oscillations, nonreciprocal magnon propagation, magnon trapping and enhancement as well as the increased sensitivity to magnetic perturbation and abrupt spin reversal. These predictions are demonstrated analytically and confirmed by full numerical simulations under experimentally feasible conditions. The position of the exceptional points and the strength of the spontaneous PT symmetry breaking can be tuned by external electric and/or magnetic fields. The roles of the intrinsic magnetic damping, and the possibility of an electric control via Dzyaloshinskii-Moriya interaction are exposed and utilized for mode dispersion shaping and magnon amplification and trapping. The results point to a new route to designing optomagnonic waveguides, traps, sensors, and circuits.