  Control over transition rates between spin states of emitters is crucial in a
wide variety of fields ranging from quantum information science to the
nanochemistry of free radicals. We present an approach to drive a both electric
and magnetic dipole-forbidden transition of a spin emitter by placing it in a
nanomagnonic cavity, requiring a description of both the spin emitter beyond
the point dipole approximation and the vacuum magnetic fields of the
nanomagnonic cavity with a large spatial gradient over the volume of the spin
emitter. We specifically study the SiV$^-$ defect in diamond, whose
Zeeman-split ground states comprise a logical qubit for solid-state quantum
information processing, coupled to a magnetic nanoparticle serving as a model
nanomagnonic cavity capable of concentrating microwave magnetic fields into
deeply subwavelength volumes. Through first principles modeling of the SiV$^-$
spin orbitals, we calculate the spin transition densities of magnetic
dipole-allowed and -forbidden transitions and calculate their coupling rates to
various multipolar modes of the nanomagnonic cavity. We envision using such a
framework for quantum state transduction and state preparation of spin qubits
at GHz frequency scales.
