  Magnetic multilayer devices that exploit magnetoresistance are the backbone
of magnetic sensing and data storage technologies. Here we report novel
multiple-spin-filter magnetic tunnel junctions (sf-MTJs) based on van der Waals
(vdW) heterostructures in which atomically thin chromium triiodide (CrI3) acts
as a spin-filter tunnel barrier sandwiched between graphene contacts. We
demonstrate tunneling magnetoresistance which is drastically enhanced with
increasing CrI3 layer thickness, reaching a record 19,000% for magnetic
multilayer structures using four-layer sf-MTJs at low temperatures. These
devices also show multiple resistance states as a function of magnetic field,
suggesting the potential for multi-bit functionalities using an individual vdW
sf-MTJ. Using magnetic circular dichroism measurements, we attribute these
effects to the intrinsic layer-by-layer antiferromagnetic ordering of the
atomically thin CrI3. Our work reveals the possibility to push magnetic
information storage to the atomically thin limit, and highlights CrI3 as a
superlative magnetic tunnel barrier for vdW heterostructure spintronic devices.
