  Cooling the motion of a massive mechanical oscillator into its quantum ground
state plays an essential role in observing macroscopic quantum effects in
mechanical systems. Here we propose a measurement-based feedback cooling
protocol in cavity magnomechanics that is able to cool the mechanical vibration
mode of a macroscopic ferromagnet into its ground state. The mechanical mode
couples to a magnon mode via a dispersive magnetostrictive interaction, and the
latter further couples to a microwave cavity mode via the magnetic-dipole
interaction. A feedback loop is introduced by measuring the amplitude of the
microwave cavity output field and applying a force onto the mechanical
oscillator that is proportional to the amplitude fluctuation of the output
field. We show that by properly designing the feedback gain, the mechanical
damping rate can be significantly enhanced while the mechanical frequency
remains unaffected. Consequently, the vibration mode can be cooled into its
quantum ground state in the unresolved-sideband regime at cryogenic
temperatures. The protocol is designed for cavity magnomechanical systems using
ferromagnetic materials which possess strong magnetostriction along with large
magnon dissipation.
