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2D Chromium Tribromide Shows Path To Novel Electronics

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Studies expand understanding of the physics of 2D van der Waals materials

A team from Kazan Federal University has been investigating the 2D properties of the semiconductor CrBr3. They believe their work shows new possibilities for fabrication of magnetoelectronic devices based on 2D systems, which can be combined in heterostructures.

Their paper in the journal Quantum Materials focuses on the investigations of crystal, magnetic structures and vibrational properties of CrBr3 upon cooling. The methods of neutron and X-ray powder diffraction supplemented by Raman spectroscopy data allowed the authors to provide a consistent and complete microscopic picture for a temperature behaviour of microscopic properties of the model ferromagnetic van der Waals CrBr3 material.

Neutron diffraction made it possible to study in detail the role of structural parameters across the paramagnetic-ferromagnetic phase transition in coupling with the study of the magnetic structure of CrBr3. Mainly, anomalies in the temperature behaviour of the parameters of the crystal structure, interatomic distances, and angles below Curie temperature were revealed.

The picture above shows: a) Neutron diffraction patterns of CrBr3 .The ticks represent the calculated positions of the structural peaks of rhombohedral phase of CrBr3 for selected temperatures. The characteristic peaks with magnetic scattering contribution superimposed on nuclear scattering contribution are marked as "N+FM" and their (hkl) indexes are given. b) The rhombohedral structure of van der Waals crystal CrBr3 of R3¯ symmetry. c) The Cr3+-ordered magnetic moment as function of temperature.

The most important finding of the paper is the negative thermal volume expansion phenomenon observed in CrBr3 below the Curie temperature. The negative thermal expansion is the rare effect found for a limited number of materials.

In addition, the obtained Raman data provide new information on temperature variation of vibrational modes across the transition to ferromagnetic state, uncovering spin-phonon coupling effects. The information about thermal expansion behaviour is especially important for fabrication of heterostructures involving van der Waals materials and their practical applications.

The researchers say their results are important for current understanding in the physics of 2D van der Waals materials and may serve as a background for subsequent theoretical calculations and development of general models describing the structural, electronic and magnetic properties of 2D ferromagnetic van der Waals materials in bulk and atomically thin forms.

There is a number of still unexplored phenomena in this area. In the future, the researchers plan to investigate the structural and magnetic properties of CrBr3 under extreme conditions to improve the understanding of the observed physical phenomena. Van der Waals compounds with a different composition are also a promising topic.

'Spin-induced negative thermal expansion and spin-phonon coupling in van der Waals material CrBr3'; D. P. Kozlenko et al; npj Quantum Materials volume 6, Article number: 19 (2021)



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