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 behavior, here we describe a system in which the structural disorder results in long-range antiferromagnetic order due to local symmetry breaking. Nd$_2$ScNbO$_7$ 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) $\mu_B$. 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 Nd$_2$B$_2$O$_7$ parent series.