Colossal magnetoresistance (CMR) refers to a large change in electrical conductivity induced by a magnetic field in the vicinity of a metal-insulator transition and has inspired extensive studies for decadescite{Ramirez1997, Tokura2006}. Here we demonstrate an analogous spin effect near the Neel temperature $T_{rm{N}}$=296 K of the antiferromagnetic insulator CrO. Using a yttrium iron garnet YIG/CrO/Pt trilayer, we injected a spin current from the YIG into the CrO layer, and collected via the inverse spin Hall effect the signal transmitted in the heavy metal Pt. We observed a change by two orders of magnitude in the transmitted spin current within 14 K of the Neel temperature. This transition between spin conducting and nonconducting states could be also modulated by a magnetic field in isothermal conditions. This effect, that we term spin colossal magnetoresistance (SCMR), has the potential to simplify the design of fundamental spintronics components, for instance enabling the realization of spin current switches or spin-current based memories.
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