Classical Spin Nematic Transition in LiGa$_{0.95}$In$_{0.05}$Cr$_4$O$_8$

We present the results of a combined $^{7}\mathrm{Li}$-NMR and diffraction study on ${\mathrm{LiGa}}_{0.95}{\mathrm{In}}_{0.05}{\mathrm{Cr}}_{4}{\mathrm{O}}_{8}$, a member of the ${\mathrm{LiGa}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}{\mathrm{Cr}}_{4}{\mathrm{O}}_{8}$ ``breathing'' pyrochlore family. Via specific heat and NMR measurements, we find that the complex sequence of first-order transitions observed for ${\mathrm{LiGaCr}}_{4}{\mathrm{O}}_{8}$ is replaced by a single second-order transition at ${T}_{f}=11\text{ }\text{ }\mathrm{K}$. Neutron and x-ray diffraction rule out both structural symmetry lowering and magnetic long-range order as the origin of this transition. Instead, reverse Monte Carlo fitting of the magnetic diffuse scattering indicates that the low-temperature phase may be described as a collinear spin nematic state, characterized by a quadrupolar order parameter. This state also shows signs of short-range order between collinear spin arrangements on tetrahedra, revealed by mapping the reverse Monte Carlo spin configurations onto a three-state color model.