  We report the observation of a very large negative magnetoresistance effect
in a van der Waals tunnel junction incorporating a thin magnetic semiconductor,
CrI3, as the active layer. At constant voltage bias, current increases by
nearly one million percent upon application of a 2 Tesla field. The effect
arises from a change between antiparallel to parallel alignment of spins across
the different CrI3 layers. Our results elucidate the nature of the magnetic
state in ultrathin CrI3 and present new opportunities for spintronics based on
two-dimensional materials.
