  All van der Waals (vdW) Fe3GeTe2/Cr2Ge2Te6/graphite magnetic heterojunctions
have been fabricated via mechanical exfoliation and stacking, and their
magnetotransport properties are studied in detail. At low bias voltages large
negative junction magnetoresistances have been observed and are attributed to
spin-conserving tunneling transport across the insulating Cr2Ge2Te6 layer. With
increasing bias, a crossover to Fowler-Nordheim tunneling takes place. The
negative sign of the tunneling magnetoresistance (TMR) suggests that the bottom
of conduction band in Cr2Ge2Te6 belongs to minority spins, opposite to the
findings of some first-principles calculations. This work shows that the vdW
heterostructures based on 2D magnetic insulators are a valuable platform to
gain further insight into spin polarized tunneling transport, which is the
basis for pursuing high performance spintronic devices and a large variety of
quantum phenomena.
