  We describe the quantum dynamics of a magnetic rigid rotor in the mesoscopic
scale where the Einstein-De Haas effect is predominant. In particular, we
consider a single-domain magnetic nanoparticle with uniaxial anisotropy in a
magnetic trap. Starting from the basic Hamiltonian of the system under the
macrospin approximation, we derive a bosonized Hamiltonian describing the
center-of-mass motion, the total angular momentum, and the macrospin degrees of
freedom of the particle treated as a rigid body. This bosonized Hamiltonian can
be approximated by a simple quadratic Hamiltonian that captures the rich
physics of a nanomagnet tightly confined in position, nearly not spinning, and
with its macrospin anti-aligned to the magnetic field. The theoretical tools
derived and used here can be applied to other quantum mechanical rigid rotors.
