Dimensional crossover and local strain induced deflection of the spin spiral state in multiferroic NiI2

Abstract

Low-dimensional multiferroics hold great promise for integrated magnetoelectric devices. Spin spiral state has recently been shown to induce ferroelectricity in single-layer van der Waals (vdW) material NiI2. However, how this state evolves and can be tuned towards the two-dimensional limit remain unclear. Here, we combine spin-polarized scanning tunneling microscopy, layer-by-layer film growth, and multi-scale theoretical modeling to investigate the spin spirals in NiI2 thin films. As the film thickness increases from 1 to 7 monolayers, we observed a continuous increase of spin-spiral wavelength and a rotation of wavevector from near [110] to [1-10] direction, which evidences a dimensional crossover primarily driven by enhanced interlayer exchange energy. Moreover, we find that the film wrinkles can cause deflection of the spin spiral wavevector, which is caused by local curvature induced modification of exchange interactions. Our findings establish thickness and local strain as two tuning methods for engineering non-collinear helical magnetism and accompanied electric polarization in vdW multiferroics.

References in corpus (14)

• Magnetic 2D materials and heterostructures
• Evidence for a single-layer van der Waals multiferroic
• Possible persistence of multiferroic order down to bilayer limit of van der Waals material NiI${}_2$
• Microscopic origin of multiferroic order in monolayer NiI${}_2$
• Realistic Spin Model for Multiferroic NiI${}_2$
• Atomic-scale visualization of multiferroicity in monolayer NiI${}_2$
• Microscopic origin of magnetism in monolayer $3d$ transition metal dihalides
• Stacking-engineered ferroelectricity and multiferroic order in van der Waals magnets
• Dzyaloshinskii-Moriya interaction and magnetic skyrmions induced by curvature
• Competing multiferroic phases in monolayer and few-layer NiI${}_2$
• Arc-shaped structure factor in the $J_1$-$J_2$-$J_3$ classical Heisenberg model on the triangular lattice
• Electric field control of moiré skyrmion phases in twisted multiferroic NiI${}_2$ bilayers
• General time-reversal equivariant neural network potential for magnetic materials
• Controlling the Magnetic Properties of the van der Waals Multiferroic Crystals Co${}_{1-x}$Ni${}_x$I${}_2$