Spin imaging of two-dimensional crystals and van der Waals heterostructures via nitrogen-vacancy magnetometry with light

Project Information

SPIN-2D-LIGHT

Grant agreement ID: 101103355

DOI

10.3030/101103355

Project closed

EC signature date 21 March 2023

Start date 1 June 2023

End date 31 May 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 187 624,32

Coordinated by

TECHNISCHE UNIVERSITEIT DELFT
Netherlands

Periodic Reporting for period 1 - SPIN-2D-LIGHT (Spin imaging of two-dimensional crystals and van der Waals heterostructures via nitrogen-vacancy magnetometry with light)

Reporting period: 2023-06-01 to 2025-05-31

Summary of the context and overall objectives of the project

Two-dimensional (2D) magnets have opened the door to explore new physical scenarios that were not available until just very recently. Moreover, new emergent properties can appear when different 2D crystals are assembled forming van der Waals heterostructures (vdWHs). For example, the advent of twist engineering in two-dimensional (2D) crystals enables the design of van der Waals heterostructures with new functionalities.

In the case of magnets, this approach can afford artificial antiferromagnets with tailored spin arrangements, which may bring new directions in the design of future spintronic devices. In this project, the spin is the key element, being necessary to detect it with high spatial resolution and sensitivity. This is a current challenge for most of the common experimental characterization techniques as heat capacity, inelastic neutron scattering or muon spectroscopy, just to cite some, due to the few number of atoms in a 2D crystal.

As a promising experimental tool, nitrogen-vacancy (NV) centers in diamond have shown to be ideal magnetic probes at the nanoscale. In this context, we have investigated and tailored the properties of van der Waals magnets for its future implementation in spintronic and magnonic devices. We envision van der Waals magnets as a promising platform for unravelling fundamental physical phenomena (as spin superfluidity) and relevant low-power applications (magnonic devices) by exploiting their excitations.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

To achieve the goals of the project, we have performed a detail characterization of the magnetic properties of the van der Waals magnets CrSBr and CrPS4 from bulk down to the atomically-thin limit, including techniques as small angle neutron scattering, magnetic susceptibility, magneto-transport measurements or color-center magnetometry, among others.

In the layered metamagnet CrSBr we have probed the short-range correlations in bulk by using small-angle neutron scattering (SANS). The correlation length is ≈3 nm at the antiferromagnetic ordering temperature (TN  ≈ 140 K). Interestingly, the ferromagnetic correlations start developing below 200 K, i.e. well above TN. Below TN, these correlations rapidly decrease and are negligible at low-temperatures. The experimental results are well-reproduced by an effective spin Hamiltonian, which pinpoints that the short-range correlations in CrSBr are intrinsic to the monolayer limit. Overall, the obtained results are compatible with a spin freezing scenario of the magnetic fluctuations in CrSBr and highlight SANS as a powerful technique for characterizing the rich physical phenomenology beyond the long-range order paradigm offered by van der Waals magnets [Small 4, 8, 2400244, 2024].

Furthermore, we have exploit the concept of twisting and stacking engineering in artificial magnets based on CrSBr. In particular, we have fabricated an orthogonally twisted bilayer by twisting two CrSBr ferromagnetic monolayers with an easy-axis in-plane spin anisotropy by 90°. The magnetotransport properties reveal multistep magnetization switching with a magnetic hysteresis opening, which is absent in the pristine case. By tuning the magnetic field, we modulate the remanent state and coercivity and select between hysteretic and non-hysteretic magnetoresistance scenarios. This complexity pinpoints spin anisotropy as a key aspect in twisted magnetic superlattices. Our results highlight control over the magnetic properties in van der Waals heterostructures, leading to a variety of field-induced phenomena and opening a fruitful playground for creating desired magnetic symmetries and manipulating non-collinear magnetic configurations. [Nature Materials 23, 212, 2024].

This approach has been extended using as building blocks both monolayers and bilayers. By rotating 90 degrees these units, symmetric (monolayer/monolayer and bilayer/bilayer) and asymmetric (monolayer/bilayer) heterostructures are fabricated. The magneto-transport properties reveal the appearance of magnetic hysteresis, which is highly dependent upon the magnitude and direction of the applied magnetic field and is determined not only by the twist-angle but also by the number of layers forming the stack. This high tunability allows switching between volatile and non-volatile magnetic memory at zero-field and controlling the appearance of abrupt magnetic reversal processes at either negative or positive field values on demand. The phenomenology is rationalized based on the different spin-switching processes occurring in the layers, as supported by micromagnetic simulations. The results highlight the combination between twist-angle and number of layers as key elements for engineering spin-switching reversals in twisted magnets, of interest toward the miniaturization of spintronic devices and realizing novel spin textures [Advanced Materials 37, 2415774, 2025].

In the case of the van der Waals magnet CrPS4, we have investigated the structural anisotropy above and below the magnetic phase transition by fabricating nanomechanical resonators. A large anisotropy is observed in the resonance frequency of resonators oriented along the crystalline a- and b-axis, indicative of a lattice expansion along the b-axis, boosted at the magnetic phase transition, and a rather small continuous contraction along the a-axis. This behavior in the mechanical response differs from that previously reported in van der Waals magnets, as FePS3 or CoPS3, and can be understood from the quasi-1D nature of CrPS4. The results pinpoint CrPS4 as a promising material in the field of low-dimensional magnetism and show the potential of mechanical resonators for unraveling the in-plane structural anisotropy coupled to the magnetic ordering that, in a broader context, can be extended to studying structural modifications in other 2D materials and van der Waals heterostructures [Advanced Functional Materials 34, 3, 2310206, 2024].

Finally, we have considered the role of spin waves in thin magnets. We have performed magnetic sensing at the nanoscale of the van der Waals magnet CrPS4 using nitrogen-vacancy (NV) centers in diamond. Additionally, we have developed a room temperature scanning microscope for single spin magnetometry and successfully imaged spin waves in a permalloy thin-layer (see Figure), unraveling the role of magnetic anisotropy and the formation of magnetic inhomogeneties as a key challenge for future down-scaled magnonic devices and highlight diamond magnetometry as an excellent tool for its investigation [manuscript under preparation].

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Transmitting and storing data has become central to our digitally connected society. Unfortunately, the rapid increase in the amount of data produced and required by new technologies such as cloud-based storage and artificial intelligence requires ever-larger data centers that produce a large amount of heat, forming a key sustainability challenge of today. As such, there is an urgent need for methods that could enable data storage and transport in a more space- and energy-efficient way [Sci. Rep. 4, 6784, 2014].

A promising route for information technology is offered by magnon spintronics. In magnetic insulators, spin currents are carried by magnons (also named as spin-waves), which are the excitations of magnetically ordered systems [Nat. Phys. 11, 453, 2015]. Importantly, such spin currents are especially promising for information technology due to their low intrinsic damping, non-reciprocal transport, micrometer wavelengths at microwave frequencies, absence of Joule heating and strong interactions that enable signal transduction [Nat. Phys. 11, 1022, 2015].

Up to date, the main material employed in magnon spintronics is yttrium-iron-garnet (Y3Fe5O12, YIG), together with just a few other materials as different iron oxides like Gd3Fe5O12 or metallic magnets as permalloy or cobalt, among others [Rev. Mod. Phys. 96, 015005, 2924]. With the advent of two-dimensional (2D) materials, van der Waals magnets have brought new perspectives in the field of magnon spintronics since these materials exhibit higher tuneability. Indeed, magnons have been recently probed in van der Waals magnets as CrSBr or CrPS4 [Nature 609, 282, 2022; Nature 620, 533, 2023; Nat. Commun. 14, 2526, 2023; PRB 110, 174440, 2024]. Importantly, the know-how on the manipulation of 2D materials developed in the last 20 years brings new tuning knobs with easy experimental implementation that are absent in conventional magnonic materials such as, for instance, strain, stacking or twisting engineering.

In this project, we have performed a detail characterization of the magnetic properties of van der Waals magnets CrSBr and CrPS4 from bulk down to the single layer limit, including small angle neutron scattering, magnetic susceptibility, magneto-transport measurements or color-center magnetometry, among others. Our results establish van der Waals as promising candidates for magnon spintronics [Newton 1, 1, 100018, 2025].

Spin waves in a permalloy magnet imaged using a single-spin scanning microscope which I have develop