Giant enhancement of interlayer exchange in an ultrathin 2D magnet

Abstract: Following the recent isolation of monolayer CrI3, there has been a surge of new two-dimensional van der Waals magnetic materials, whose incorporation in van der Waals heterostructures offers a new platform for spintronics, proximity magnetism, and quantum spin liquids. A primary question in this burgeoning field is how exfoliating crystals to the few-layer limit influences their magnetism. Studies on CrI3 have shown a different magnetic ground state for ultrathin exfoliated films but the origin is not yet understood. Here, we use electron tunneling through few-layer crystals of the layered antiferromagnetic insulator CrCl3 to probe its magnetic order, finding a ten-fold enhancement in the interlayer exchange compared to bulk crystals. Moreover, temperature- and polarization-dependent Raman spectroscopy reveal that the crystallographic phase transition of bulk crystals does not occur in exfoliated films. This results in a different low temperature stacking order and, we hypothesize, increased interlayer exchange. Our study provides new insight into the connection between stacking order and interlayer interactions in novel two-dimensional magnets, which may be relevant for correlating stacking faults and mechanical deformations with the magnetic ground states of other more exotic layered magnets, such as RuCl3.

Giant Enhancement of Interlayer Exchange in Ultrathin 2D Magnets

In the study "Giant Enhancement of Interlayer Exchange in an Ultrathin 2D Magnet," Klein et al. investigate the magnetic properties of few-layer chromium trichloride (CrCl(_3)), focusing on interlayer exchange interactions and the role of crystallographic phase transformations in ultrathin exfoliated magnets. This work is set against the backdrop of the burgeoning field of two-dimensional van der Waals (vdW) magnetic materials, which hold promise for applications in spintronics, proximity magnetism, and quantum spin liquids.

Key Findings

At the heart of this investigation is the novel observation that the interlayer exchange interaction in ultrathin CrCl(_3) can be enhanced more than tenfold compared to its bulk counterpart. This finding is derived from electron tunneling measurements through few-layer CrCl(_3) crystals, which demonstrate an increased interlayer exchange coupling in contrast to the weak coupling observed in thicker, bulk structures. The transition from antiferromagnetic (AFM) interactions in bulk layers to strongly enhanced AFM interactions in ultrathin layers is attributed to altered stacking order in the crystal structure.

Raman spectroscopy further reveals that the monoclinic phase, present in the high-temperature bulk and known for favoring AFM interactions, remains stable even at low temperatures in the few-layer limit. This discovery is corroborated by density functional theory (DFT) calculations, which show that the monoclinic stacking favours AFM interlayer coupling while the rhombohedral phase can accommodate both FM and AFM interactions.

Experimental Methodology

The authors employed advanced tunneling junction methods, incorporating few-layer graphite electrodes and hexagonal boron nitride encapsulation, to probe the tunneling characteristics and magnetic alignment in CrCl(_3). Raman spectroscopy was utilized not only to confirm the absence of phase transitions in exfoliated films but also to pinpoint the persistence of monoclinic stacking at low temperatures—an assertion supported by marked differences in Raman peak shifts.

Implications and Future Directions

This study provides critical insight into the consequence of stacking order on magnetic properties in ultrathin vdW magnets. The enhanced interlayer exchange observed opens new possibilities for tuning magnetic interactions through mechanical modifications such as strain or twisting, offering a versatile avenue for the manipulation of spintronic properties in 2D materials.

As the field of 2D magnetic materials continues to evolve, the findings from this research could influence future theoretical models and experimental approaches to engineering magnetic properties for technological applications, including data storage and quantum computing. The exploitation of stacking-induced variations in exchange interactions across various layered magnets might pave the way for innovative device architectures with tailored magnetic properties.

Conclusion

Klein et al.'s work elucidates the profound impact of crystallographic stacking order on the magnetism of ultrathin CrCl(_3) and expands our understanding of the dynamics within vdW magnetic materials. The standout observation of a substantial enhancement in interlayer exchange implicates stacking order as a pivotal factor for magnetic property modulation, considerably enriching the toolkit available to researchers exploring 2D magnetic systems.