Unraveling the magnetic ground state in the alkali-metal lanthanide oxide Na2PrO3

A comprehensive set of muon spin spectroscopy and neutron scattering measurements supported by ab initio and model Hamiltonian simulations have been used to investigate the magnetic ground state of Na2PrO3. mu SR reveals a Neel antiferromagnetic order below T-N similar to 4.9 K, with a small static magnetic moment mstatic m(static) <= 0.22 mu(B)/Pr collinearly aligned along the c axis. Inelastic neutron measurements reveal the full spectrum of crystal field excitations and confirm that the Pr4+ ground-state wave function deviates significantly from the Gamma(7) limit that is relevant to the Kitaev model. Single- and two-magnon excitations are observed in the ordered state below T-N = 4.6 K and are well described by nonlinear spin wave theory from the Neel state using a magnetic Hamiltonian with Heisenberg exchange J = 1 meV and symmetric anisotropic exchange Gamma/J = 0.1, corresponding to an XY model. Intense two magnon excitations are accounted for by g-factor anisotropy g(z)/g(+/-) = 1.29. A fluctuating moment delta m(2) = 0.57(22) mu(2)(B)/Pr extracted from the energy and momentum integrated inelastic neutron signal is reduced from expectations for a local J = 1/2 moment with average g factor g(avg) approximate to 1.1. Together, the results demonstrate that the small moment in Na2PrO3 arises from crystal field and covalency effects and the material does not exhibit significant quantum fluctuations.