Light induced electron spin resonance properties of van der Waals CrX3 (X = Cl, I) crystals

Michele Pizzochero, Ravi Yadav, Oleg Yazyev
AMER INST PHYSICS, 2020
Article

Résumé

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 17K, and CrI3 is a vdW ferromagnet below 61K. 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 10K, the ESR signal is shifted from g=1.492 (dark) to 1.661 (light), the linewidth increased from 376 to 506Oe, 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 (>20K) 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 1260Oe at 60K. 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.

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https://infoscience.epfl.ch/record/280202?ln=fr

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Michele Pizzochero, Ravi Yadav, Oleg Yazyev
AMER INST PHYSICS, 2020
Article

Résumé

Official source

https://infoscience.epfl.ch/record/280202?ln=fr

À propos de ce résultat

Concepts associés (14)

Un aimant permanent, ou simplement aimant dans le langage courant, est un objet fabriqué dans un matériau magnétique dur, c’est-à-dire dont l'aimantation rémanente et le champ coercitif sont grands

vignette|redresse=1.25|Spectromètre à résonance paramagnétique électronique La résonance paramagnétique électronique (RPE), résonance de spin électronique (RSE), ou en anglais electron spin resonance

Le paramagnétisme désigne en magnétisme le comportement d'un milieu matériel qui ne possède pas d'aimantation spontanée mais qui, sous l'effet d'un champ magnétique extérieur, acquiert une aimantat

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Methods and microfabrication techniques for subnanoliter magnetic resonance spectroscopy

Enrica Montinaro

Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR) and ferromagnetic resonance (FMR) spectroscopy can open up the possibility of studying many scientifically and biologically relevant samples at the µm and sub-µm scale. Examples of volume-limited systems include numerous species of microorganisms, mammalian zygotes, the majority of cells, proteins limited in growth, micro and nanostructured devices for the analysis of spin dynamics at the sub-µm scale. These volume-limited samples cannot be addressed by commercially available inductive spectrometers due to their constraint in sensitivity. It was previously proposed in our group that CMOS technology can be used to realize miniaturized high sensitivity inductive detection systems, having spin sensitivities at least two orders of magnitude greater than the commercially available spectrometers. During my PhD work, I developed methods and microfabrication techniques to perform NMR, EPR and FMR spectroscopy at the µm and sub-µm scale by using high sensitivity single chip CMOS detectors, previously proposed in our group. Microfluidic systems for the non-invasive handling of liquid samples and biological entities immersed in liquids are realized and integrated with the CMOS single chip NMR and EPR detectors. Microfluidic channels are fabricated via conventional microfabrication techniques and via two-photon polymerization, a 3D printing technique with a lateral resolution of 300 nm. The 3D printing technique is found to be an exceptional solution for NMR applications. Due to the flexibility in the design of the microfluidic systems, it is possible to reduce the magnetic field non-uniformieties with a consequent improval in spectral resolution. Spectral resolutions down to 0.007 ppm are reported for liquids having sample volumes of 100 pL. For the first time, NMR studies on intact biological entities submerged in liquid media of choice are performed, using 3D printed microfluidic systems. A spin sensitivity of 2.5·1013 spins/¿Hz is shown, sufficient to detect highly concentrated endogenous compounds in active volumes down to 100 pL with measurement times down to 3 h. EPR measurements on subnanoliter liquids and frozen solutions are reported, by the combination of commercially available capillaries and EPR single chip detectors. This is a first but important step towards the study of biological samples, whose paramagnetic ions have relaxation times too short to be measured at room temperature. Moreover, a novel method for the sensing of magnetic microbeads is presented, which is based on the detection of the change of susceptibility in FMR condition by the CMOS integrated detector. Due to the frequency and field dependence of the susceptibility, the detected variation is 20 times greater than the change in magnetization measured in static conditions by other approaches. The proposed detection scheme allows for single bead sensitivity over an active area of about 5·104 µm2. Lastly, sub-µm scale FMR detection capabilities of the single chip CMOS detector are shown by experiments on nanopatterned single permalloy (80% Ni - 20% Fe) and YIG (Yittrium Iron Garnet) dots. The combination of high sensitivity and large active area is seen as a considerable advantage with respect to other FMR detection methods, which suffer either from sensitivity or from reduced active area.

EPFL2017

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Two-dimensional (2D) materials, in particular graphene and transition metal dichalcogenides (TMDC), have attracted great scientific interest over the last decade, revealing exceptional mechanical, electrical and optical properties. Owing to their layered nature, subnanometer-thick single-layer forms of these crystals can be chemically grown or mechanically isolated, representing an ultimately scaled-down platform for further miniaturization of electronic devices. Indeed, the large family of 2D materials contains hundreds of members, including metals, semiconductors, insulators, and others, covering the whole spectrum of potential applications. Availability of all necessary building blocks for the construction of all-2D solid-state devices makes this platform ideal for fabrication of ultrathin, transparent and flexible devices. Extensively investigated graphene has demonstrated excellent electrical properties. However, the absence of a bandgap is an obstacle for its interaction with light and therefore limits applications in optics. In this aspect, atomically thin TMDC semiconductors appear to be an alternative, more promising platform, as they possess a direct band gap in the visible range in the monolayer form. Despite being only three atoms thick, these semiconductors demonstrate exceptionally large light absorption, efficient light emission, and strong light-matter interaction, attracting justified interest from the optics community. We will focus on their potential applications for optoelectronics in Chapter 4. Even more, broken inversion symmetry and strong spin-orbit coupling in single-layer TMDC crystals reveal a new quantum number, the so-called valley index. Indeed, this unique degree of freedom of charge carriers is known for some materials since the 1970s. However, in the case of 2D semiconductors, spin-valley locking mechanism and valley contrasting optical selection rules open new ways for addressing, manipulation and sensing of this pseudospin, opening the whole new field of valleytronics. Due to the large splitting of the valence band, spin and valley degree of freedom become locked in these atomically thin materials. We employ this unique feature in Chapter 5 for indirect injection of spins into graphene by pumping valley polarized carriers in an adjacent TMDC monolayer. Being gapless, graphene does not allow direct optical injection of spins. Another striking feature of 2D materials is their unique ability to be stacked in vertical heterostructures with strong electrical coupling between layers. The weak van der Waals force, which keeps components together, provides freedom in the assembly process, so that a lattice mismatch or a twist angle becomes unimportant in the first approximation. This provides a novel platform for harvesting complementary properties of the constituent materials, allowing the engineering of new artificial meta-materials as we demonstrate in Chapter 6. Furthermore, synergistic effects in van der Waals heterostructures enable completely new properties and phenomena, which do not exist in the single materials, with twist angle being an important knob for tuning these effects. We observe and exploit such novel phenomena arising in heterostructures of 2D semiconductors in the last chapter of this thesis. On the way to the realization of practical optoelectronic and valleytronic applications, this thesis studies fundamental aspects of rich spin-valley physics of atomically thin semiconductors

EPFL2019

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Methods and microfabrication techniques for subnanoliter magnetic resonance spectroscopy

Enrica Montinaro

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