  Spin waves are ideal candidates for wave-based computing, but the
construction of magnetic circuits is blocked by a lack of an efficient
mechanism to excite long-running exchange spin waves with normalised
amplitudes. Here, we solve the challenge by exploiting the deeply nonlinear
phenomena of forward-volume spin waves in 200 nm wide nanoscale waveguides and
validate our concept with microfocused Brillouin light scattering spectroscopy.
An unprecedented nonlinear frequency shift of >2 GHz is achieved, corresponding
to a magnetisation precession angle of 55{\deg} and enabling the excitation of
exchange spin waves with a wavelength of down to ten nanometres with an
efficiency of >80%. The amplitude of the excited spin waves is constant and
independent of the input microwave power due to the self-locking nonlinear
shift, enabling robust adjustment of the spin wave amplitudes in future on-chip
magnonic integrated circuits.
