  We present an experimental study of time refraction of spin waves propagating
in microscopic waveguides under the influence of time-varying magnetic fields.
Using space- and time-resolved Brillouin light scattering microscopy, we
demonstrate that the broken translational symmetry along the time coordinate
can be used to in- or decrease the energy of spin waves during their
propagation. This allows for a broadband and controllable shift of the
spin-wave frequency. Using an integrated design of spin-wave waveguide and
microscopic current line for the generation of strong, nanosecond-long,
magnetic field pulses, a conversion efficiency up to 39% of the carrier
spin-wave frequency is achieved, significantly larger compared to photonic
systems. Given the strength of the magnetic field pulses and its strong impact
on the spin-wave dispersion relation, the effect of time refraction can be
quantified on a length scale comparable to the spin-wave wavelength.
Furthermore, we utilize time refraction to excite spin-wave bursts with pulse
durations in the nanosecond range and a frequency shift depending on the pulse
polarity.
