Magnetic Ordering in the Ising Antiferromagnetic Pyrochlore Nd2ScNbO7

Abstract

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) $\hat{I}{¼}_B$. 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.