Electrical control of magnetism in multiferroic 2D materials

Informations projet

EMAGIN2D

N° de convention de subvention: 803092

DOI

10.3030/803092

Projet clôturé

Date de signature de la CE 28 Septembre 2018

Date de début 1 Decembre 2018

Date de fin 31 Mars 2025

Financé au titre de

• EXCELLENT SCIENCE - European Research Council (ERC)

Coût total

€ 1 500 000,00

Contribution de l’UE

€ 1 500 000,00

Coordonné par

UNIVERSITAT DE VALENCIA
Spain

Periodic Reporting for period 4 - EMAGIN2D (Electrical control of magnetism in multiferroic 2D materials)

Période du rapport: 2023-06-01 au 2025-03-31

Résumé du contexte et des objectifs généraux du projet

Layered magnetic materials have been central to our understanding of magnetism for more than a century, but only recently has spontaneous magnetization been observed in a free-standing single layer, ushering in the era of magnetic two-dimensional (2D) materials. This milestone opened exciting avenues for exploring topological and quantum magnetic phases within a 2D platform and integrating magnetism into precisely engineered layered structures and devices. A key frontier now lies in combining various types of ferroic orders within these flat structures, leading to the emergence of multiferroic 2D materials. These materials are expected to be key to two critical research and technological directions: firstly, the study of the physical interaction between ferromagnets and ferroelectrics at the atomic scale, enabling tailored magnetic properties for room temperature applications; secondly, the control of magnetism via electric fields rather than magnetic fields, a more energy-efficient approach. Such advancements are poised to shift the paradigm of current optoelectronic and spintronic devices by enabling alternative routes to exploit the magnetic states of a material.

EMAGIN2D has delved into the fundamental aspects of coexisting long-range orders —structural, electric, and magnetic— in magnetic 2D materials, examining their intricate interdependencies. Our investigations have unveiled a far more complex landscape than previously envisioned, characterized by a rich variety of magnetic phases and stacking configurations that are strongly interlinked and that are also sensitive to the number of layers, extending up to the mesoscale. We have identified strain and external electric fields as powerful fine-tuning mechanisms to control both the magnetic and structural states of these materials. To support future applications, we have demonstrated that physical vapor deposition can effectively scale up the growth of multiferroic 2D materials, laying the groundwork for the fabrication of multiferroic spintronic devices.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

1. Development of fabrication and high-throughput characterisation techniques for air-sensitive 2D materials and van der Waals heterostructures. We implemented soft protocols and non-destructive characterisation techniques —low-temperature deterministic transfer, wide-field hyperspectral, Raman and scanning probe microscopies— inside a glove box for the study of magnetic and multiferroic 2D materials in their pristine state.

2. Investigation of the interplay between mechanical and magnetic properties in magnetic 2D materials. We revealed that few-layer chromium trihalides exhibit a notably soft yet brittle mechanical behaviour, while their bulk counterparts display potential superplasticity.This is particularly relevant for the development of flexible and bendable magnetoelectric devices. As a proof of concept, we examined the behaviour of thin crystals of chromium sulfur bromide integrated into flexible transport devices, demonstrating that applied strain can effectively tune the magnetic anisotropy and switching fields.

3. Uncovering the optical, magnetic and structural phase complexity of the layered magnetic material chromium triiodide (CrI3). The detailed study of the magnetic properties with a complementary suite of techniques revealed at least three magnetic phases that are tightly linked to structural transitions, challenging prior assumptions. The system also exhibits intrinsic twisting of the layers along the stacking direction, coexistence of different crystalline phases and a phase transition that is remarkably quenched in the atomically thin crystals.

4. Optimised crystal growth of bulk and atomically-thin multiferroic 2D materials. The growth of challenging systems has been accessed by hybrid growth techniques that combine melt and vapour phase transport processes. The growth of atomically thin materials with intrinsic multiferroicity down to the monolayer has been successfully scaled up by the use of physical vapour deposition techniques.

5. Designer multiferroic van der Waals heterostructures. Artificial multiferroic materials were built one layer at a time using the cryogenic deterministic transfer technique. These structures interleave magnetic and ferroelectric states enabling a tailored magnetoelectric coupling.

6. Integration of multiferroic 2D materials into spin-optoelectronic devices for the electrical control of magnetism in 2D. These devices exhibit unusual asymmetric and hysteretic behaviours under electric and magnetic fields that hold promise for their exploitation in future spintronic diodes.

Dissemination: results were shared via invited seminars and talks at the meetings of the APS, IEEE, SPIE, and others, and promoted to the public through outreach events like 10alamenos9 Nanofest, Expociencia, and MedNight.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

Magnetic 2D materials are a young family of compounds with many unresolved fundamental questions, such as why bulk CrI3 has a ferromagnetic ground state whereas few-layer is antiferromagnetic. A lot of the research of EMAGIN2D has revolved around this profound conundrum with deep physical and technological implications, making progress well beyond the current understanding of magnetic 2D materials.

A holistic suite of techniques —magnetic force and transmission electron microscopies, muon spin rotation, synchrotron X-Ray diffraction, optical spectroscopy and computation— revealed the rich structural phase diagram of CrI3, including intrinsic twisted domains and a complex dependence of the structural, magnetic, electronic and optical properties with the layer number. Contrary to the general knowledge, where the evolution of the physical properties of layered materials with the thickness was thought to be only critical below the quantum confinement limit —when crystals are thinner than a few layers—, we identified a non-monotonic variation of the optical properties across different length scales: onsetting at the mesoscale, peaking at the nanoscale and decreasing again down to the single layer. These observations align remarkably with the thickness trends of the magnetic and structural properties of CrI3 and, remarkably, other van der Waals crystals, opening the door for new mechanisms for the modulation of the physical properties in 2D.

Building on these insights we tackled the control of magnetism in these 2D platforms through external stimuli, beyond the use of conventional gating. While stacking with ferroelectrics proved a challenging approach to modulate magnetism, devices integrating intrinsic multiferroic 2D materials showed a sizable electric-field-dependent magnetoresistance, paving the way for the control of the magnetic states with small electric fields. Finally, we demonstrated that strain can also effectively modulate magnetism in flexible devices made of magnetic 2D materials, such as CrSBr. These findings will herald new mechanisms for the energy-efficient control of magnetic states in next generation 2D technologies.

Integration of a van der Waals ferroelectric gate in a transport device

Schematic of the interrelated properties of magnetic and multiferroic 2D materials

Optical micrograph of a multiferroic 2D material grown by physical vapour deposition