[Paper Review] Atomic-scale visualization of multiferroicity in monolayer NiI$_2$

Mohammad Amini, Adolfo O. Fumega+5 more|arXiv (Cornell University)|Sep 20, 2023

2D Materials and Applications48 references4 citations

TL;DR

This study uses atomic-resolution scanning tunneling microscopy (STM) and density functional theory (DFT) to directly visualize type-II multiferroic order in monolayer NiI₂, revealing that spin-spiral order combined with strong spin-orbit coupling induces a ferroelectric polarization detectable via electrostatic potential modulation. The key result is the direct experimental observation and electric-field manipulation of multiferroic domain walls, confirming magnetoelectric coupling at the atomic scale.

ABSTRACT

Progress in layered van der Waals materials has resulted in the discovery of ferromagnetic and ferroelectric materials down to the monolayer limit. Recently, evidence of the first purely two-dimensional multiferroic material was reported in monolayer NiI$_2$. However, probing multiferroicity with scattering-based and optical bulk techniques is challenging on 2D materials, and experiments on the atomic scale are needed to fully characterize the multiferroic order at the monolayer limit. Here, we use scanning tunneling microscopy (STM) supported by theoretical calculations based on density functional theory (DFT) to probe and characterize the multiferroic order in monolayer NiI$_2$. We demonstrate that the type-II multiferroic order displayed by NiI$_2$, arising from the combination of a magnetic spin spiral order and a strong spin-orbit coupling, allows probing the multiferroic order in the STM experiments. Moreover, we directly probe the magnetoelectric coupling of NiI$_2$ by external electric field manipulation of the multiferroic domains. Our findings establish a novel point of view to analyse magnetoelectric effects at the microscopic level, paving the way towards engineering new multiferroic orders in van der Waals materials and their heterostructures.

Motivation & Objective

• To experimentally characterize the atomic-scale origin of multiferroicity in monolayer NiI₂, a proposed 2D type-II multiferroic.
• To overcome the limitations of bulk and optical techniques in probing multiferroic order in 2D materials by using local probe microscopy.
• To establish direct evidence of magnetoelectric coupling in monolayer NiI₂ through external electric field manipulation of multiferroic domains.
• To provide a microscopic understanding of the emergent ferroelectric polarization and spin-spiral vector in this van der Waals material.
• To develop a generalizable strategy for probing and engineering multiferroic order in 2D van der Waals heterostructures.

Proposed method

• Employing low-temperature scanning tunneling microscopy (STM) to image the local electronic structure and electrostatic potential modulation in monolayer NiI₂.
• Using non-collinear density functional theory (DFT) calculations to model the magnetic spin-spiral order and predict the associated ferroelectric polarization.
• Applying voltage pulses via the STM tip to locally manipulate multiferroic domain boundaries and observe their motion in real space.
• Identifying and tracking defects (neutral and charged) as spatial references to quantify domain wall displacement during electric field manipulation.
• Measuring band bending in STM spectra to estimate the magnitude of the electric polarization in the material.
• Correlating experimental STM images with theoretical predictions of spin-spiral periodicity and polarization direction to extract the spin-spiral wavevector q and exchange interaction ratio J₃/J₁.

Experimental results

Research questions

• RQ1 Can the multiferroic order in monolayer NiI₂ be directly visualized at the atomic scale using scanning tunneling microscopy?
• RQ2 What is the relationship between the spin-spiral order and the emergent ferroelectric polarization in monolayer NiI₂?
• RQ3 Can the magnetoelectric coupling in monolayer NiI₂ be experimentally demonstrated through local electric field control of multiferroic domains?
• RQ4 What is the magnitude of the electric polarization in monolayer NiI₂, and how does it relate to the observed electrostatic potential modulation?
• RQ5 How do charged and neutral defects behave during electric field-induced domain wall motion, and can they serve as reliable spatial references?

Key findings

• The STM experiments reveal a periodic modulation of the local density of states with half the periodicity of the spin spiral, allowing direct determination of the spin-spiral wavevector q = (0.069, 0.041, 0) in reciprocal lattice units.
• The observed band bending of 100–200 meV corresponds to an estimated electric polarization of P ∼ 10⁻¹² C/m, consistent with theoretical predictions.
• Voltage pulses from the STM tip induce reproducible motion of the multiferroic domain wall, with the domain boundary shifting away from the tip location, demonstrating direct electric field control of the magnetic and ferroelectric order.
• Charged defects are mobile under electric field perturbation, while neutral defects remain stationary, enabling their use as spatial references for tracking domain wall displacement.
• The measured ratio of third-neighbor to first-neighbor exchange interactions is J₃/J₁ = -0.263, consistent with the spin-spiral model derived from DFT calculations.
• The combination of STM imaging and DFT calculations confirms that the multiferroic order in monolayer NiI₂ arises from spin-orbit coupling-induced ferroelectricity in a spin-spiral ground state, establishing a robust mechanism for 2D multiferroicity.