In this talk, I will briefly introduce single crystal neutron diffraction program at the Spallation Neutron Source of the Oak Ridge National Laboratory. A specific example of using neutron diffraction to investigate the evolution of magnetic structure of recent discovered colossal magnetoresistance (CMR) Mn3Si2Te6 will be followed. In stark contrast with other CMR manganite and pyrochlore materials, the resistivity in Mn3Si2Te6 drops by orders of magnitude leading to an insulator-metal transition with an applied magnetic field above 9 T. The CMR occurs only when the field is applied along the hard axis, while such effect is conspicuously absent with the field applied in the basal plane, where magnetization is fully saturated. Single crystal neutron study reveals the ground state spin structure in zero field and applied field along the c-axis are full characterized. A prominent magnetic diffuse scattering is observed above the magnetic transition and persists at temperature much higher than TC. The relevant mechanisms including spin fluctuation and the orbital loop-current will be discussed.