The ferrimagnetic insulator ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$, which features a Curie temperature $T_c$ at 78 K and a delicate yet consequential magnetic frustration, exhibits colossal magnetoresistance (CMR) when the magnetic field is applied along the magnetic hard axis, surprisingly inconsistent with existing precedents [Y. Ni, H. Zhao, Y. Zhang, B. Hu, I. Kimchi, and G. Cao, Phys. Rev. B 103, L161105 (2021)]. This discovery motivates a thorough single-crystal neutron diffraction study in order to gain insights into the magnetic structure and its hidden correlation with the new type of CMR. Here we report a noncollinear magnetic structure below the $T_c$ where the moments lie predominantly within the basal plane but tilt toward the $c$ axis by $\approx {10}^{\circ }$ at ambient conditions. A substantial magnetic diffuse scattering decays slowly and persists well above the $T_c$. The evolution of the spin correlation lengths agrees well with the electrical resistivity, underscoring the role of spin fluctuation contributing to the magnetoresistivity near the transition. Application of magnetic field along the $c$ axis renders a swift occurrence of CMR but only a slow tilting of the magnetic moments toward the $c$ axis. The unparalleled changes indicate a nonconsequential role of magnetic polarization.

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• Colossal magnetoresistance
• Magnetism
• Neutron diffraction