Collective excitations in the tetravalent lanthanide honeycomb antiferromagnetNa2PrO3

Thermomagnetic and inelastic neutron-scattering measurements on ${\mathrm{Na}}_{2}\mathrm{Pr}{\mathrm{O}}_{3}$ are reported. This material is an antiferromagnetic honeycomb magnet based on the tetravalent lanthanide ${\mathrm{Pr}}^{4+}$ and has been proposed to host dominant antiferromagnetic Kitaev interactions. These measurements reveal magnetic fluctuations in ${\mathrm{Na}}_{2}\mathrm{Pr}{\mathrm{O}}_{3}$ below an energy of 2 meV as well as crystal-field excitations around 230 meV. The latter energy is comparable to the scale of the spin-orbit interaction and explains both the very small effective moment of around $1.0\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ per ${\mathrm{Pr}}^{4+}$ and the difficulty to uncover any static magnetic scattering below the ordering transition at ${T}_{\mathrm{N}}=$ 4.6 K. By comparing the low-energy magnetic excitations in ${\mathrm{Na}}_{2}\mathrm{Pr}{\mathrm{O}}_{3}$ to those of the isostructural spin-only compound, ${\mathrm{Na}}_{2}\mathrm{Tb}{\mathrm{O}}_{3}$, a microscopic model of exchange interactions is developed that implicates dominant and surprisingly large Heisenberg exchange interactions $J\ensuremath{\approx}1.1(1)$ meV. Although antiferromagnetic Kitaev interactions with $\mathit{\text{K}}\ensuremath{\le}0.2J$ cannot be excluded, the inelastic neutron-scattering data of ${\mathrm{Na}}_{2}\mathrm{Pr}{\mathrm{O}}_{3}$ is best explained with a $\mathrm{\ensuremath{\Delta}}=1.22$ easy-axis XXZ exchange anisotropy.