  We investigate instabilities of the magnetic ground state in ferromagnetic
metals that are induced by uniform electrical currents, and, in particular, go
beyond previous analyses by including dipolar interactions. These instabilities
arise from spin-transfer torques that lead to Doppler shifted spin waves. For
sufficiently large electrical currents, spin-wave excitations have negative
energy with respect to the uniform magnetic ground state, while remaining
dynamically stable due to dissipative spin-transfer torques. Hence, the uniform
magnetic ground state is energetically unstable, but is not able to dynamically
reach the new ground state. We estimate this to happen for current densities $
j\gtrsim (1-D/D_c)10^{13} \mathrm{A/m^2} $ in typical thin film experiments,
with $ D $ the Dzyaloshinskii-Moriya interaction constant, and $ D_c $ the
Dzyaloshinskii-Moriya interaction that is required for spontaneous formation of
spirals or skyrmions. These current densities can be made arbitrarily small for
ultrathin film thicknesses at the order of nanometers, due to surface- and
interlayer effects. From an analogue gravity perspective, the stable negative
energy states are an essential ingredient to implement event horizons for
magnons -- the quanta of spin waves -- giving rise to e.g. Hawking radiation
and can be used to significantly amplify spin waves in a so-called black-hole
laser.
