  We show how to generate tripartite entanglement in a cavity magnomechanical
system which consists of magnons, cavity microwave photons, and phonons. The
magnons are embodied by a collective motion of a large number of spins in a
macroscopic ferrimagnet, and are driven directly by an electromagnetic field.
The cavity photons and magnons are coupled via magnetic dipole interaction, and
the magnons and phonons are coupled via magnetostrictive (radiation
pressure-like) interaction. We show optimal parameter regimes for achieving the
tripartite entanglement where magnons, cavity photons, and phonons are
entangled with each other, and we further prove that the steady state of the
system is a genuinely tripartite entangled state. The entanglement is robust
against temperature. Our results indicate that cavity magnomechanical systems
could provide a promising platform for the study of macroscopic quantum
phenomena.
