  Layered magnets have recently received tremendous attention, however,
spin-exchange coupling mechanism across their interlayer regions is yet to be
revealed. Here, we report a Bethe-Slater-curve (BSC) like behavior in nine
transition metal dichalcogenide bilayers (MX2, M=V, Cr, Mn; X=S, Se, Te) and
established interlayer spin-exchange coupling mechanisms at their van der Waals
gaps using first-principle calculations. The BSC-like behavior offers a
distance-dependent interlayer anti-ferromagnetic (AFM) to ferromagnetic (FM)
transition. This phenomenon is explained with the spin-exchange coupling
mechanisms established using bilayer CrSe2 as a prototype in this work. The Se
pz wavefunctions from two adjacent interfacial Se sublayers overlap at the
interlayer region. The spin alignment of the region determines interlayer
magnetic coupling. At a shorter interlayer distance, Pauli repulsion at the
overlapped region dominates and thus favors anti-parallel oriented spins
leading to interlayer AFM. For a longer distance, kinetic energy gain of
polarized electrons across the bilayer balances the Pauli repulsion and the
bilayer thus prefers an interlayer FM state. In light of this, the AFM-FM
transition is a result of competition between Pauli and Coulomb repulsion and
kinetic energy gain. All these results open a new route to tune interlayer
magnetism and the revealed spin-exchange coupling mechanisms are paramount
additions to those previously established ones.
