Bethe-Slater-curve-like behavior and interlayer spin-exchange coupling mechanisms in two-dimensional magnetic bilayers

TL;DR

This study reveals a distance-dependent transition between anti-ferromagnetic and ferromagnetic interlayer coupling in 2D magnetic bilayers, explained by spin-exchange mechanisms involving wavefunction overlap and energy competition.

Contribution

It uncovers a Bethe-Slater-curve-like behavior in transition metal dichalcogenide bilayers and establishes the underlying spin-exchange coupling mechanisms through first-principles calculations.

Findings

Interlayer AFM to FM transition depends on interlayer distance.

Spin-exchange mechanisms are driven by wavefunction overlap and energy competition.

The results enable tuning of interlayer magnetism in 2D materials.

Abstract

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.