Magnetic ordering in the Ising antiferromagnetic pyrochlore Nd₂ScNbO₇

The question of structural disorder and its effects on magnetism is relevant to a number of spin liquid candidate materials. Although commonly thought of as a route to spin glass behaviour, here we describe a system in which the structural disorder results in long-range antiferromagnetic order due to local symmetry breaking. Nd₂ScNbO₇ is shown to have a dispersionless gapped excitation observed in other neodymium pyrochlores below T _N = 0.37 K through polarized and inelastic neutron scattering. However the dispersing spin waves are not observed. This excited mode is shown to occur in only 14(2)% of the neodymium ions through spectroscopy and is consistent with total scattering measurements as well as the magnitude of the dynamic moment 0.26(2) μ _B. The remaining magnetic species order completely into the all-in all-out Ising antiferromagnetic structure. This can be seen as a result of local symmetry breaking due disordered Sc⁺³ and Nb⁺⁵ ions about the A-site. From this work, it has been established that B-site disorder restores the dipole-like behaviour of the Nd⁺³ ions compared to the Nd₂B₂O₇ parent series. (paper)