Tunable, Ferroelectricity-Inducing, Spin-Spiral Magnetic Ordering in Monolayer FeOCl

Spin spirals (SS) are a special case of noncollinear magnetism, where the magnetic-moment direction rotates along an axis. They have generated interest for novel phenomena, spintronics applications, and their potential formation in monolayers, but the search for monolayers exhibiting SS has not been particularly fruitful. Here, we employ density functional theory calculations to demonstrate that SS form in a recently synthesized monolayer, FeOCl. The SS wavelength and stability can be tuned by doping and uniaxial strain. The SS-state band gap is larger by 0.6 eV compared to the gap of both the ferromagnetic and antiferromagnetic state, enabling bandgap tuning and possibly an unusual formation of quantum wells in a single material via magnetic-field manipulation. The SS-induced out-of-plane ferroelectricity enables switching of the SS chirality by an electric field. Finally, forming heterostructures, for example, with graphene or boron nitride, maintains SS ordering and provides another method of modulation and a potential for magnetoelectric devices.

Evidence for a single-layer van der Waals multiferroic

Multiferroic materials have attracted wide interest because of their exceptional static^1-3 and dynamical^4-6 magnetoelectric properties. In particular, type-II multiferroics exhibit an inversion-symmetry-breaking magnetic order that directly induces ferroelectric polarization through various mechanisms, such as the spin-current or the inverse Dzyaloshinskii-Moriya effect^3,7. This intrinsic coupling between the magnetic and dipolar order parameters results in high-strength magnetoelectric effects^3,8. Two-dimensional materials possessing such intrinsic multiferroic properties have been long sought for to enable the harnessing of magnetoelectric coupling in nanoelectronic devices^1,9,10. Here we report the discovery of type-II multiferroic order in a single atomic layer of the transition-metal-based van der Waals material NiI₂. The multiferroic state of NiI₂ is characterized by a proper-screw spin helix with given handedness, which couples to the charge degrees of freedom to produce a chirality-controlled electrical polarization. We use circular dichroic Raman measurements to directly probe the magneto-chiral ground state and its electromagnon modes originating from dynamic magnetoelectric coupling. Combining birefringence and second-harmonic-generation measurements with theoretical modelling and simulations, we detect a highly anisotropic electronic state that simultaneously breaks three-fold rotational and inversion symmetry, and supports polar order. The evolution of the optical signatures as a function of temperature and layer number surprisingly reveals an ordered magnetic polar state that persists down to the ultrathin limit of monolayer NiI₂. These observations establish NiI₂ and transition metal dihalides as a new platform for studying emergent multiferroic phenomena, chiral magnetic textures and ferroelectricity in the two-dimensional limit.

Non-collinear magnetism & multiferroicity: the perovskite case

The most important types of non-collinear magnetic orders that are realized in simple perovskite oxides are outlined in relation to multiferroicity. These orders are classified and rationalized in terms of a mimimal spin Hamiltonian, based on which the notion of spin-driven ferroelectricity is illustrated. These concepts find direct application in reference materials such as BiFeO3, GdFeO3 and TbMnO3 whose multiferroic properties are briefly reviewed.