  Motivated by the presence of an unquenched orbital angular momentum in CoO, a
team at Chalk River, including a recently hired research officer Roger Cowley,
performed the first inelastic neutron scattering experiments on the classic
Mott insulator [Sakurai $\textit{et al.}$ 1968 Phys. Rev. $\mathbf{167}$ 510].
Despite identifying magnon modes at the zone boundary, the team was unable to
parameterise the low energy magnetic excitation spectrum below $T\rm{_{N}}$
using conventional pseudo-bosonic approaches. It would not be for another 40
years that Roger, now at Oxford and motivated by the discovery of the
high-$T_{c}$ cuprate superconductors [Bednorz & Muller 1986 Z. Phys. B
$\mathbf{64}$ 189], would make another attempt at the parameterisation of the
magnetic excitation spectrum that had previously alluded him. Upon his return
to CoO, Roger found a system embroiled in controversy, with some of its most
fundamental parameters still remaining undetermined. Faced with such a
formidable task, Roger performed a series of inelastic neutron scattering
experiments in the early 2010's on both CoO and a magnetically dilute
structural analogue MgO. These experiments would prove instrumental in the
determination of both single-ion [Cowley $\textit{et al.}$ 2013 Phys. Rev. B
$\mathbf{88}$ 205117] and cooperative magnetic parameters [Sarte $\textit{et
al.}$ 2018 Phys. Rev. B $\mathbf{98}$ 024415] for CoO. Both these sets of
parameters would eventually be used in a spin-orbit exciton model [Sarte
$\textit{et al.}$ 2019 Phys. Rev. B $\mathbf{100}$ 075143], developed by his
longtime friend and collaborator Bill Buyers, to successfully parameterise the
complex spectrum that both measured at Chalk River almost 50 years prior. The
story of CoO is of one that has come full circle, one filled with both
spectacular failures and intermittent, yet profound, little victories.
