  Hybrid systems combining magnons and superconducting quantum circuits have
attracted increasing interest in recent years. Magnonic crystals (MCs) are one
of the building blocks of room-temperature magnonics and can be used to create
devices with an engineered band structure. These devices, exhibiting tunable
frequency selectivity and the ability to store travelling excitations in the
microwave regime, may form the basis of a set of new tools to be used in the
context of quantum information processing. In order to ascertain the
feasibility of such plans, MCs must be demonstrated to work at the low
temperatures required for microwave-frequency quantum experiments. We report
the first measurements of the transmission of microwave signals through an MC
at 20 mK and observe a magnonic bandgap in both continuous-wave and pulsed
excitation experiments. The spin-wave damping at low temperatures in our
yttrium iron garnet MC is higher than expected, indicating that further work is
necessary before the full potential of quantum experiments using magnonic
crystals can be realised.
