  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.
