Dynamically stable negative-energy states induced by spin-transfer torques

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