Frustrated Magnetism in Charge Disordered Ln2ScNbO7 Pyrochlores

Abstract

The pyrochlore structure is well studied in the field of frustrated magnetism as it provides a platform for investigating many magnetic cations on a corner-sharing tetrahedral network. Here the rare earth charge-disordered Ln$_2$ScNbO$_7$ pyrochlores are studied as well as the last remaining member of the titanate series, Sm$_2$Ti$_2$O$_7$, for an in depth investigation of its magnetic ground state. The charge disordered pyrochlores provide an extreme example of non-magnetic chemical disorder in a frustrated rare earth system and can provide insight into the effects of disorder on underlying magnetic ground states. In the Ln$_2$ScNbO$_7$ (Ln = La, Pr, Nd, Sm, Gd, Tb, Dy) pyrochlores the non-magnetic B-site is occupied by a mixture of Sc$^{+3}$ and Nb$^{+5}$ ions, while leaving the magnetic sublattice nominally unchanged. Structural investigation by x-ray and neutron, diffraction and total scattering measurements show that there are significant short-range correlations between Sc$^{+3}$ and Nb$^{+5}$ ions that are not evident in the crystallographic results. These correlations approach charge ice correlations in the large cation members of the series, Ln = La, Pr, Nd and the correlations appear to fall off for smaller cation members of the series (with Ln = Tb as the only viable example).

Magnetic studies are presented on Ln = Nd, Gd, Dy and Sm$_2$Ti$_2$O$_7$. Ising antiferromagnetism is observed in Nd$_2$ScNbO$_7$ alongside diffuse scattering indicative of moment fragmentation. A glassy magnetic ground state is observed in Gd$_2$ScNbO$_7$ although emergent xy spin ordering, similar to the parent compounds, is observed on a short length scale. Differing from the parent compounds, Dy$_2$ScNbO$_7$ is not a classical spin ice, and instead freezes into a glassy ground state with on average isotropic spins. Despite nominally having the single ion symmetry requirements for moment fragmentation, Sm$_2$Ti$_2$O$_7$ does not show signatures of the phenomenon; instead a well ordered Ising antiferromagnetic ground state is observed. Although discussions of non-magnetic disorder are usually limited to structural distortions altering exchange interactions, these systems show that in rare earth pyrochlores the distortion of the local crystal field environments is equally relevant to structural distortions and can fundamentally change the single ion symmetry and anisotropy of the rare earth ions. The lack of a universal single ion symmetry changes how these rare earth systems need to be approached in comparison to how rare earth systems are normally treated. The single ion symmetry and anisotropy typically restricts the possible magnetic ground states. Comparisons to spin-glass systems can be drawn in the Gd$_2$ScNbO$_7$ and Dy$_2$ScNbO$_7$ cases. However, in Dy$_2$ScNbO$_7$ the mechanistic differences to conventional diluted magnetic ion spin-glasses may necessitate study into something akin to anisotropy-disordered magnetic glasses, which to our knowledge have not been studied theoretically in rare earth pyrochlore systems.