Magnetic Order and Symmetry in the 2D Semiconductor CrSBr (2007.10715v1)

Abstract: The recent discovery of two-dimensional (2D) magnets offers unique opportunities for the experimental exploration of low-dimensional magnetism4 and the magnetic proximity effects, and for the development of novel magnetoelectric, magnetooptic and spintronic devices. These advancements call for 2D materials with diverse magnetic structures as well as effective probes for their magnetic symmetries, which is key to understanding intralayer magnetic order and interlayer magnetic coupling. Here we apply second harmonic generation (SHG), a technique acutely sensitive to symmetry breaking, to probe the magnetic structure of a new 2D magnetic semiconductor, CrSBr. We find that CrSBr monolayers are ferromagnetically ordered below 146 K, an observation enabled by the discovery of a giant magnetic dipole SHG effect in the centrosymmetric 2D structure. In multilayers, the ferromagnetic monolayers are coupled antiferromagnetically, with the N\'eel temperature notably increasing with decreasing layer number. The magnetic structure of CrSBr, comprising spins co-aligned in-plane with rectangular unit cell, differs markedly from the prototypical 2D hexagonal magnets CrI3 and Cr2Ge2Te6 with out-of-plane moments. Moreover, our SHG analysis suggests that the order parameters of the ferromagnetic monolayer and the antiferromagnetic bilayer are the magnetic dipole and the magnetic toroidal moments, respectively. These findings establish CrSBr as an exciting 2D magnetic semiconductor and SHG as a powerful tool to probe 2D magnetic symmetry, opening the door to the exploration of coupling between magnetic order and excitonic/electronic properties, as well as the magnetic toroidal moment, in a broad range of applications.

Overview of Magnetic Order and Symmetry in the 2D Semiconductor CrSBr

The research paper investigates the magnetic properties and symmetry characteristics of a novel two-dimensional (2D) magnetic semiconductor, chromium sulfide bromide (CrSBr). The paper employs second harmonic generation (SHG) to probe the magnetic order and symmetry of CrSBr, marking an effective departure from traditional methods such as magneto-optical and Raman spectroscopy, which are limited in their ability to directly assess magnetic symmetry.

The focus of this paper is the layer-dependent magnetic characteristics of CrSBr monolayers and multilayers. In monolayer form, CrSBr demonstrates ferromagnetic (FM) ordering below 146 K, while multilayers show antiferromagnetic (AFM) coupling between monolayers with an unexpectedly high Néel temperature that increases as the number of layers decreases. This finding interestingly contrasts with conventional understanding that expects decreased stability of magnetic orders as materials approach the 2D limit.

Key Numerical Results and Contradictory Claims

• CrSBr monolayers exhibit FM order below a Curie temperature (T_C) of 145.8 ± 1.8 K.
• AFM bilayers exhibit interlayer ordering with a Néel temperature (T_N) of 140.0 ± 0.1 K, increasing with fewer layers, reaching down as low as 132 K in the bulk.
• The SHG measurements revealed that CrSBr monolayers show a giant magnetic dipole SHG effect, which is considerably higher than anticipated according to the typical scaling of second-order susceptibilities.
• The paper also highlights how CrSBr presents a unique centrosymmetric structure that can nevertheless support SHG by breaking space inversion and time-reversal symmetries.

Methodological Insights

The SHG technique is particularly celebrated in this work for its sensitivity to symmetry breaking, crucial for understanding 2D magnetic materials. The use of SHG enables the detection of higher-order contributions such as magnetic dipoles and toroidal moments, which are unobtainable through conventional electric dipole SHG approaches in centrosymmetric materials.

• Magnetic dipole (MD) SHG: This mechanism identified in CrSBr monolayers offers a novel probe into magnetization because it remains significant despite the material's retention of centrosymmetry.
• Nonreciprocal electric dipole (ED) SHG: Observed in AFM CrSBr bilayers, indicates symmetry breaking between layers.

Theoretical and Practical Implications

The theoretical implications of this paper rest primarily on enhancing the understanding of interlayer and intralayer interactions in 2D magnetic semiconductors. By identifying magnetic dipole and toroidal moments as order parameters, this research opens new pathways for exploring coupling between electronic, excitonic, and magnetic properties.

Practically, such insights are highly relevant for the development of spintronic and magneto-opto-electronic devices, leveraging the material's air stability, semiconducting properties, and high magnetic ordering temperatures.

Future Directions

The anomalous increase in the Néel temperature with decreasing layer number suggests the existence of an intermediate ferromagnetic phase. Future research could aim to elucidate the microscopic mechanisms enabling such a phenomenon, which challenges existing theoretical models. Furthermore, applications could focus on exploiting the newly discovered magnetic behaviors and SHG characteristics of CrSBr for novel device architectures, particularly in the realms of 2D nanotechnology and quantum materials where control over magnetic properties is increasingly critical.

This paper thus underscores the potential inherent in combining 2D magnetics with semiconducting properties, heralding new frontiers in multiphase and multifunctional materials research. The findings posit SHG as a robust technique for eruditing magnetic order in other emerging 2D materials.