  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 decades\cite{Ramirez1997,
Tokura2006}. Here we demonstrate an analogous spin effect near the N\'eel
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 N\'eel
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.

Materials Science