Moiré-Induced Magnetoelectricity in Twisted Bilayer NiI_{2}

Area of Science:

• Condensed Matter Physics
• Materials Science
• Computational Physics

Background:

• Twisted van der Waals materials are promising for multiferroic engineering.
• Modeling large-scale moiré superlattices in these materials presents significant challenges.

Purpose of the Study:

• To develop and apply a machine learning potential for modeling spin-lattice coupled systems in twisted bilayer NiI2.
• To investigate the emergence of multiferroic properties, including spin-driven polarization and ferroelectricity, as a function of twist angle and lattice relaxation.

Main Methods:

• Development of a comprehensive interatomic machine learning potential using the SpinGNN++ framework.
• Application of the potential to twisted bilayer NiI2, including structural relaxation.
• Incorporation of the generalized Katsura-Nagaosa-Balatsky mechanism for precise spin-driven polarization calculations.

Main Results:

• Structural relaxation induces moiré-periodic modulations in interlayer spacing and ionic shifts.
• The machine learning potential accurately captures magnetic configurations and spin interactions.
• Cooperative ionic and spin-driven ferroelectricity is observed for twist angles between 1.89° and 2.45°, creating rich polarization textures.
• Lattice relaxation is crucial for generating polar-magnetic topologies like skyrmions.
• Near 60° twist angles, stacking-dependent ferroelectric displacements lead to polar meron-antimeron networks.

Conclusions:

• Twisted bilayer NiI2 exhibits cooperative ionic and spin-driven ferroelectricity.
• Lattice relaxation plays a critical role in enabling multiferroic phenomena in these systems.
• Twisted van der Waals magnets serve as adaptable platforms for developing tunable multiferroic devices.