  Establishing a way to control magnetic dynamics and elementary excitations
(magnons) is crucial to fundamental physics and the search for novel phenomena
and functions in magnetic solid-state systems. Electromagnetic waves have been
developed as means of driving and sensing in magnonic and spintronics devices
used in magnetic spectroscopy, non-volatile memory, and information processors.
However, their millimeter-scale wavelengths and undesired cross-talk have
limited operation efficiency and made individual control of densely integrated
magnetic systems difficult. Here, we utilize acoustic waves (phonons) to
control magnetic dynamics in a miniaturized phononic crystal micro-cavity and
waveguide architecture. We demonstrate acoustic pumping of localized
ferromagnetic magnons, where their back-action allows dynamic and
mode-dependent modulation of phononic cavity resonances. The phononic crystal
platform enables spatial driving, control and read-out of tiny magnetic states
and provides a means of tuning acoustic vibrations with magnons. This
alternative technology enhances the usefulness of magnons and phonons for
advanced sensing, communications and computation architectures that perform
transduction, processing, and storage of classical and quantum information.
