Two dimensional honeycomb ferromagnets host massless Dirac magnons which are a bosonic analogue of Dirac fermions in graphene. The Dirac magnons may become massive and topological when the time reversal symmetry breaks and an energy gap opens up at the Dirac point, which was experimentally observed in \(\hbox {Cr}^{3+}\) -based van der Waals magnets. Here, we investigate the spin wave excitations in the 3d magnetic oxide \(\hbox {FeTiO}_3\) with \(\hbox {Fe}^{2+}\) electrons ( \(3d^4\) ). Using inelastic neutron scattering, we observe two magnon bands separated by a 1.2-meV gap at the Dirac points indicating that its Dirac magnons are massive. Using the linear spin-wave and density functional theory calculations, we find that the spin-orbit-coupled antisymmetric Dzyaloshinskii-Moriya exchanges can best account for the observed Dirac gap opening. The associated Berry curvature and Chern number ( \(C^\pm = \pm 1\) ) indicate that \(\hbox {FeTiO}_3\) hosts topological spin excitations via time-reversal symmetry breaking of Dirac magnons.
