  Magnons serve as a testing ground for fundamental aspects of Hermitian and
non-Hermitian wave mechanics and are of high relevance for information
technology. This study presents setups for realizing spatio-temporally driven
parity-time (PT) symmetric magnonics based on coupled magnetic waveguides and
magnonic crystals. A charge current in a metal layer with strong spin-orbit
coupling sandwiched between two insulating magnetic waveguides leads to gain or
loss in the magnon amplitude depending on the directions of the magnetization
and the charge currents. When gain in one waveguide is balanced by loss in the
other waveguide a PT-symmetric system hosting non-Hermitian degeneracies (or
exceptional points (EPs)) is realized. For AC current multiple EPs appear for a
certain gain/loss strength and mark the boundaries between the preserved
PT-symmetry and the broken PT-symmetry phases. The number of islands of broken
PT-symmetry phases and their extensions is tunable by the frequency and the
strength of the spacer current. At EP and beyond, the induced and amplified
magnetization oscillations are strong and self-sustained. In particular, these
magnetization auto-oscillations in broken PT-symmetry phase occur at low
current densities and do not require further adjustments such as tilt angle
between electric polarization and equilibrium magnetization direction in
spin-torque oscillators, pointing to a new design of these oscillators and
their utilization in computing and sensoric. It is also shown how the periodic
gain/loss mechanism allows for the generation of high-frequency spin waves with
low-frequency currents. For spatially-periodic gain/loss acting on a magnonic
crystal, magnon modes approaching each other at the Brillouin-zone boundaries
are highly susceptible to PT-symmetry, allowing for a wave-vector-resolved
experimental realization at very low currents.
