  Materials with strong magnetoresistive responses are the backbone of
spintronic technology, magnetic sensors, and hard drives. Among them, manganese
oxides with a mixed valence and a cubic perovskite structure stand out due to
their colossal magnetoresistance (CMR). A double exchange interaction underlies
the CMR in manganates, whereby charge transport is enhanced when the spins on
neighboring Mn3+ and Mn4+ ions are parallel. Prior efforts to find different
materials or mechanisms for CMR resulted in a much smaller effect. Here we show
an enormous CMR at low temperatures in EuCd2P2 without manganese, oxygen, mixed
valence, or cubic perovskite structure. EuCd2P2 has a layered trigonal lattice
and exhibits antiferromagnetic ordering at 11 K. The magnitude of CMR (104
percent) in as-grown crystals of EuCd2P2 rivals the magnitude in optimized thin
films of manganates. Our magnetization, transport, and synchrotron X-ray data
suggest that strong magnetic fluctuations are responsible for this phenomenon.
The realization of CMR at low temperatures without heterovalency leads to a new
regime for materials and technologies related to antiferromagnetic spintronics.
