Beyond magnons in Nd2ScNbO7: An Ising pyrochlore antiferromagnet with all in all out order and random fields

We report the low temperature magnetic properties of Nd${}^{3+}$ pyrochlore $\mathrm{N}{\mathrm{d}}_2\mathrm{S}\mathrm{c}\mathrm{N}\mathrm{b}{\mathrm{O}}_7$. Susceptibility and magnetization show an easy-axis moment, and heat capacity reveals a phase transition to long range order at $T_N=371(2)$ mK with a fully recovered $\mathrm{\Delta }S=R\ln (2)$, 53% of it recovered for $T>T_N$. Elastic neutron scattering shows a long-range all-in-all-out magnetic order with low-$Q$ diffuse elastic scattering. Inelastic neutron scattering shows a low-energy flat-band, indicating a magnetic Hamiltonian similar to $\mathrm{N}{\mathrm{d}}_2\mathrm{Z}{\mathrm{r}}_2{\mathrm{O}}_7$. Nuclear hyperfine excitations measured by ultra-high-resolution neutron backscattering indicates a distribution of static electronic moments below $T_N$, which may be due to B-site disorder influencing Nd crystal electric fields. Analysis of heat capacity data shows an unexpected $T$-linear or $T^{3\mathrm{/}2}$ term which is inconsistent with conventional magnon quasiparticles, but is consistent with fractionalized spinons or gapless local spin excitations. We use legacy data to show similar behavior in $\mathrm{N}{\mathrm{d}}_2\mathrm{Z}{\mathrm{r}}_2{\mathrm{O}}_7$. Comparing local static moments also reveals a suppression of the nuclear Schottky anomaly in temperature, evidencing a fraction of Nd sites with nearly zero static moment, consistent with exchange-disorder-induced random singlet formation. Taken together, these measurements suggest an unusual fluctuating magnetic ground state which mimics a spin-liquid – but may not actually be one.