  The quanta of magnetic excitations - magnons - are known for their unique
ability to undergo Bose-Einstein condensation at room temperature. This
fascinating phenomenon reveals itself as a spontaneous formation of a
macroscopic coherent state under the influence of incoherent stimuli. Spin
currents have been predicted to offer electronic control of magnon
Bose-Einstein condensates, but this phenomenon has not been experimentally
evidenced up to now. Here we experimentally show that current-driven
Bose-Einstein condensation can be achieved in nanometer-thick films of magnetic
insulators with tailored dynamic magnetic nonlinearities and minimized
magnon-magnon interactions. We demonstrate that, above a certain threshold,
magnons injected by the spin current overpopulate the lowest-energy level
forming a highly coherent spatially extended state. By accessing magnons with
essentially different energies, we quantify the chemical potential of the
driven magnon gas and show that, at the critical current, it reaches the energy
of the lowest magnon level. Our results pave the way for implementation of
integrated microscopic quantum magnonic and spintronic devices.
