Active nonreciprocal cloaking for pseudo-Hermitian magnons

Cloaking has key applications but entails sophisticated control of signal propagation and scattering characteristics. Here, we show that invisibility for magnon signals is achievable in a nonreciprocal and electrically controlled way by engineering the magnonic channels such that they exhibit parity-time ($PT$) symmetry. This is accomplished by attaching current-carrying heavy-metal contacts to the magnon waveguides and exerting fields from an attached bias layer. Tuning the current density in the metal layer, the magnons in this setup experience electrically controlled, compensated gain and loss due to spin-orbit torque, which renders the setup $PT$ symmetric. The magnon dynamics is then shown to be pseudo Hermitian with exceptional points (EPs) determined actively by an external electric field. We analyze the magnon scattering from single and periodic $PT$-symmetric regions and identify the conditions necessary for the formation of unidirectional invisibility, which can be steered by specific combinations of bias layers and current amplitudes in the heavy metal as to reach the EP. The unidirectional invisibility at EP is found to be extended for a periodic $PT$-symmetric region. Intrinsic damping on $PT$-symmetric unidirectional invisibility is shown to be marginal confirming the experimental feasibility. It is shown how the unidirectional magnons can be utilized to amplify and generate magnonic orbital angular-momentum states in coupled magnetic rings demonstrating an alternative path for manipulating magnon propagation and processing.