[Paper Review] Spin-resolved imaging of atomic-scale helimagnetism in monolayer NiI2

Mao-Peng Miao, Nanshu Liu+4 more|arXiv (Cornell University)|Sep 28, 2023

2D Materials and Applications4 citations

TL;DR

This study achieves atomic-scale spin-resolved imaging of helimagnetism in monolayer NiI2 using spin-polarized scanning tunneling microscopy (SP-STM), revealing a canted spin spiral with a Q vector of (0.2203, 0, 0) distinct from bulk values. The observed spin spiral exhibits collective switching under magnetic fields due to incommensurability with the lattice, confirming intrinsic noncollinear order and paving the way for type-II multiferroic and 2D spintronic applications.

ABSTRACT

Identifying intrinsic noncollinear magnetic order in monolayer van der Waals (vdW) crystals is highly desirable for understanding the delicate magnetic interactions at reduced spatial constraints and miniaturized spintronic applications, but remains elusive in experiments. Here, we achieved spin-resolved imaging of helimagnetism at atomic scale in monolayer NiI2 crystals, that were grown on graphene-covered SiC(0001) substrate, using spin-polarized scanning tunneling microscopy. Our experiments identify the existence of a spin spiral state with canted plane in monolayer NiI2. The spin modulation Q vector of the spin spiral is determined as (0.2203, 0, 0), which is different from its bulk value or its in-plane projection, but agrees well with our first principles calculations. The spin spiral surprisingly indicates collective spin switching behavior under magnetic field, whose origin is ascribed to the incommensurability between the spin spiral and the crystal lattice. Our work unambiguously identifies the helimagnetic state in monolayer NiI2, paving the way for illuminating its expected type-II multiferroic order and developing spintronic devices based on vdW magnets.

Motivation & Objective

• To identify intrinsic noncollinear magnetic order in monolayer van der Waals magnets, which remains experimentally elusive despite theoretical predictions.
• To resolve the magnetic ground state of monolayer NiI2 at atomic scale, particularly its spin spiral characteristics and spin texture.
• To investigate the origin of magnetic response under external magnetic fields in atomically thin magnetic systems.
• To establish a direct experimental link between observed spin textures and first-principles calculations in 2D magnetic materials.
• To explore the potential of monolayer NiI2 as a platform for type-II multiferroic and spintronic devices.

Proposed method

• Spin-polarized scanning tunneling microscopy (SP-STM) was employed to image the spin texture at atomic resolution in monolayer NiI2 grown on graphene/SiC(0001).
• The magnetic structure was probed by measuring spin-polarized tunneling current, enabling direct visualization of spin orientation across the atomic lattice.
• The spin spiral wavevector Q was extracted from Fourier analysis of the spin-resolved imaging data, yielding Q = (0.2203, 0, 0).
• First-principles density functional theory (DFT) calculations were performed to validate the observed spin spiral and its magnetic anisotropy.
• Magnetic field-dependent SP-STM measurements were conducted to study the response of the spin spiral to external fields.
• The incommensurability between the spin spiral and the underlying crystal lattice was analyzed to explain the observed collective switching behavior.

Experimental results

Research questions

• RQ1 What is the true magnetic ground state of monolayer NiI2 at atomic scale, and does it exhibit noncollinear spin order?
• RQ2 How does the spin spiral wavevector in monolayer NiI2 compare to its bulk counterpart or in-plane projections?
• RQ3 What causes the collective spin switching behavior observed under magnetic fields in monolayer NiI2?
• RQ4 To what extent does the incommensurability between the spin spiral and the crystal lattice influence the magnetic response?
• RQ5 Can the observed spin spiral state in monolayer NiI2 support type-II multiferroic order?

Key findings

• The spin spiral in monolayer NiI2 exhibits a wavevector Q = (0.2203, 0, 0), which differs from both the bulk value and its in-plane projection, indicating strong two-dimensional magnetic confinement.
• The observed spin spiral state is confirmed to be intrinsic and stable, with excellent agreement between experimental SP-STM data and first-principles DFT calculations.
• Under applied magnetic fields, the spin spiral undergoes collective switching, a behavior attributed to the incommensurability between the spin spiral and the crystal lattice.
• The spin texture is canted in the plane, confirming the presence of a noncollinear helimagnetic ground state in the monolayer.
• The results provide unambiguous experimental evidence for intrinsic helimagnetism in a 2D van der Waals magnet, supporting its potential for multiferroic and spintronic applications.
• The work establishes SP-STM as a powerful tool for probing complex magnetic textures at the atomic scale in 2D materials.