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Two-dimensional (2D) van der Waals (vdW) magnets provide an ideal platformfor exploring, on the fundamental side, new microscopic mechanisms and fordeveloping, on the technological side, ultra-compact spintronic applications.So far, bilinear spin Hamiltonians have been commonly adopted to investigatethe magnetic properties of 2D magnets, neglecting higher order magneticinteractions. However, we here provide quantitative evidence of giantbiquadratic exchange interactions in monolayer NiX2 (X=Cl, Br and I), bycombining first-principles calculations and the newly developed machinelearning method for constructing Hamiltonian. Interestingly, we show that theferromagnetic ground state within NiCl2 single layers cannot be explained bymeans of bilinear Heisenberg Hamiltonian; rather, the nearest-neighborbiquadratic interaction is found to be crucial. Furthermore, using athree-orbitals Hubbard model, we propose that the giant biquadratic exchangeinteraction originates from large hopping between unoccupied and occupiedorbitals on neighboring magnetic ions. On a general framework, our worksuggests biquadratic exchange interactions to be important in 2D magnets withedge-shared octahedra.

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