Giant Biquadratic Exchange in 2D Magnets and its Role in Stabilizing Ferromagnetism of NiCl2 Monolayer

TL;DR

This study reveals that giant biquadratic exchange interactions in 2D NiCl2 monolayers are essential for understanding their ferromagnetism, challenging the traditional bilinear models and highlighting the importance of higher-order magnetic interactions.

Contribution

The paper provides the first quantitative evidence of giant biquadratic exchange in 2D magnets and demonstrates its crucial role in stabilizing ferromagnetism in NiCl2 monolayers.

Findings

Giant biquadratic exchange interactions are present in monolayer NiX2.

Biquadratic interactions are crucial for ferromagnetic ground state in NiCl2.

Large hopping between orbitals causes the biquadratic exchange.

Abstract

Two-dimensional (2D) van der Waals (vdW) magnets provide an ideal platform for exploring, on the fundamental side, new microscopic mechanisms and for developing, on the technological side, ultra-compact spintronic applications. So far, bilinear spin Hamiltonians have been commonly adopted to investigate the magnetic properties of 2D magnets, neglecting higher order magnetic interactions. However, we here provide quantitative evidence of giant biquadratic exchange interactions in monolayer NiX2 (X=Cl, Br and I), by combining first-principles calculations and the newly developed machine learning method for constructing Hamiltonian. Interestingly, we show that the ferromagnetic ground state within NiCl2 single layers cannot be explained by means of bilinear Heisenberg Hamiltonian; rather, the nearest-neighbor biquadratic interaction is found to be crucial. Furthermore, using a three-orbitals Hubbard model, we propose that the giant biquadratic exchange interaction originates from large hopping between unoccupied and occupied orbitals on neighboring magnetic ions. On a general framework, our work suggests biquadratic exchange interactions to be important in 2D magnets with edge-shared octahedra.