Light Induced Electron Spin Resonance Properties of van der Waals CrX3 (X = Cl, I) Crystals

TL;DR

This study investigates how visible light affects the electron spin resonance properties of layered CrCl3 and CrI3 van der Waals magnets, revealing reversible and significant changes in their magnetic spectra upon illumination.

Contribution

It provides the first detailed ESR analysis of photo-induced effects in CrCl3 and CrI3, highlighting the role of photo-generated Cr2+ centers and their impact on magnetic properties.

Findings

CrCl3 ESR signal shifts and intensity decreases reversibly with light

CrI3 ESR signal intensity reduces and g-value increases under illumination

Photo-induced changes are explained by Cr2+ Jahn-Teller centers and slow relaxation mechanisms

Abstract

The research on layered van der Waals (vdW) magnets is rapidly progressing owing to exciting fundamental science and potential applications. In bulk crystal form, CrCl3 is a vdW antiferromagnet with in-plane ferromagnetic ordering below 17 K, and CrI3 is a vdW ferromagnet below 61 K. Here, we report on the electron spin resonance (ESR) properties of CrCl3 and CrI3 single crystals upon photo-excitation in the visible range. We noticed remarkable changes in the ESR spectra upon illumination. In the case of CrCl3, at 10 K, the ESR signal is shifted from g = 1.492 (dark) to 1.661 (light), line width increased from 376 to 506 Oe, and the signal intensity is reduced by 1.5 times. Most interestingly, the observed change in the signal intensity is reversible when the light is cycled on/off. We observed almost no change in the ESR spectral parameters in the paramagnetic phase (>20 K) upon illumination. Upon photo-excitation of CrI3, the ESR signal intensity is reduced by 1.9 times; the g-value increased from 1.956 to 1.990; the linewidth increased from 1170 to 1260 Oe at 60 K. These findings are discussed by taking into account the skin depth, the slow relaxation mechanism and the appearance of low-symmetry fields at the photo-generated Cr2+ Jahn-Teller centers. Such an increase in the g-value as a result of photo-generated Cr2+ ions is further supported by our many-body wavefunction calculations. This work has the potential to extend to monolayer vdWs magnets by combining ESR spectroscopy with optical excitation and detection.