  We examined what interactions control the sign and strength of the interlayer
coupling in van der Waals ferromagnets such as Fe3-xGeTe2, Cr2Ge2Te6, CrI3 and
VI3, to find that high-spin orbital interaction across the van der Waals gaps
are a key to understanding their ferromagnetism. Interlayer ferromagnetic
coupling in Fe3-xGeTe2 Cr2Ge2Te6, and CrI3 is governed by the high-spin
two-orbital two-electron destabilization, but that in VI3 by the high-spin
four-orbital two-electron stabilization. These interactions explain a number of
seemingly puzzling observations in van der Waals fer-romagnets.
