Session A24: 2D Magnets- vdW and Beyond

Room Temperature Topological Spin Textures in a van der Waals Ferromagnet Revealed by Multi-modal Lorentz Electron Microscopy

Two-dimensional van der Waals (2D vdW) magnets offer a promising platform for exploring magnetic and topological phases, owing to their unique layered structure and properties sensitive to stacking order [1,2]. Among the vdW materials for studying 2D magnetism, the Fe_NGeTe₂ (FGT, N=3-5) system is exceptional due to its tunability of magnetic properties with chemical doping and the existence of ferromagnetism above room temperature.

 

Here, we explore the chemically driven structural and magnetic phase transitions in (Fe_1-xCo_x)₅GeTe₂ (FCGT) using a combination of atomic resolution imaging and Lorentz four-dimensional scanning transmission electron microscopy (4D-STEM). Upon Co-doping, we find that the FCGT undergoes both structural and magnetic phase transitions from an antiferromagnetic, AA-stacking (x=0.46) to a ferromagnetic, polar AA’-stacking (x=0.50). Further, room temperature Néel-type skyrmions emerge in the AA’ phase as revealed by Lorentz 4D-STEM [3]. In summary, our work paves the way for studying structural and magnetic phase transition in vdW magnetic materials.

 

References:

[1] K. F. Mak, J. Shan, and D. Ralph, Nature Review Physics 1, 641 (2019).

[2] K. S. Burch, D. Mandrus, and J.-G. Park, Nature 563, 47 (2018).

[3] H. Zhang et al., Science Advances 8, eabm7103 (2022).   

ZrTe2/CrTe2: an epitaxial van der Waals platform for spintronics

The rapid discovery of two-dimensional (2D) van der Waals (vdW) quantum materials has led to heterostructures that integrate diverse quantum functionalities such as topological phases, magnetism, and superconductivity. In this context, the epitaxial synthesis of vdW heterostructures with well-controlled interfaces is an attractive route towards wafer-scale platforms for systematically exploring fundamental properties and fashioning proof-of-concept devices. We describe the synthesis by molecular beam epitaxy of a vdW heterostructure that interfaces two material systems of contemporary interest: a 2D ferromagnet (1T-CrTe₂) and a topological semimetal (ZrTe₂). These heterostructures are throughly characterized using a complete suite of in vacuo techniques such as angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy, as well as ex situ techniques, including high resolution transmission electron microscopy (HR-TEM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), polarized neutron reflectometry, magnetotransport, and spin torque ferromagnetic resonance. The HR-TEM, XPS, and XRD data show the stabilization of the 1T-CrTe₂ phase. The ARPES data from CrTe₂ and ZrTe₂ are consistent with first principles calculations. We find that one unit-cell (u.c.) thick 1T-CrTe₂ grown epitaxially on ZrTe₂ is a 2D ferromagnet with perpendicular magnetic anisotropy, indicated by a hysteretic anomalous Hall effect (AHE) loop. In thicker samples (12 u.c. thick CrTe₂), the AHE has characteristics that may arise from real-space Berry curvature. Finally, in bilayers with ultrathin CrTe₂ (3 u.c. thickness), we demonstrate current-driven magnetization switching in a full vdW topological semimetal/2D ferromagnet heterostructure device.

Work done in collaboration with Yongxi Ou, Wilson Yanez, Run Xiao, Max Stanley, Supriya Ghosh, Boyang Zheng, Wei Jiang, Yu-Sheng Huang, Timothy Pillsbury, Alex Grutter, Julie Borchers, Anthony Richardella, Chaoxing Liu, Tony Low, Vincent H. Crespi, and K. Andre Mkhoyan.

 

Dative epitaxy of covalent 2D magnets

    Moiré superlattices formed by stacking two-dimensional (2D) van der Waals (vdW) materials have become promising platforms for studying emergent phenomena. However, moiré superlattices obtained by exfoliation and restacking via aligning/twisting van der Waals layers are typically small in size and accompanied by gradual spatial modulation or local domain formation.

    In this work, we report hybrid covalent-vdW moiré superlattices of Cr₅Te₈/WSe₂ by epitaxial growth of the covalently bonded 2D magnet Cr₅Te₈ on top of a semiconducting vdW monolayer WSe₂ template, via a two-step chemical vapor deposition process. The thickness of Cr₅Te₈ is tunable down to a single unit cell, with a lateral dimension of up to 50 µm. The Cr self-intercalation within the vdW gap creates a Te-Cr-Se Janus interface with covalent-like bonding, which is absent in fully vdW heterostructures. The strong interlayer coupling stabilizes the metastable trigonal structure of Cr₅Te₈ to form a perfectly commensurate 3 × 3/7 × 7 Moiré superlattice via lattice strain. Magnetic domain structures of Cr₅Te₈ imaged by photoemission electron microscopy reveal distinct domain patterns in the 2D limit in which the shape and size are dramatically different from those of thicker samples. Magnetization reversal is further studied by reflective magnetic circular dichroism down to a thickness of two unit cells. Our work expands moiré superlattices into the regime of strong interlayer-coupling.   

Proximity Induced Chiral Quantum Light Generation in Strain-Engineered WSe2/NiPS3 Heterostructures

An ability to control the polarization of the single photons generated by the quantum light emitters holds the key to the realization of non-reciprocal single-photon devices and complex quantum networks. To date, such control is usually achieved via coupling of the quantum emitters to the complex photonic/meta-structures, injection of spin-polarized carriers/excitons, or application of high magnetic fields.   “Proximity effects” – the class of phenomena by which an atomically-thin material borrows properties of an adjacent material (such as magnetism) via quantum mechanical interactions – has recently been explored to achieve this highly desired polarization control.  By coupling transition metal dichalcogenides with various bulk and 2D magnetic materials, exciting effects such as strong enhancement of valley Zeeman splitting and spin-dependent charge transfer have been demonstrated.  However, chiral light emission without spin-polarized carrier/exciton injection at zero magnetic field remains elusive to date.  We recently demonstrate that chiral quantum light sources with a high degree of circular polarization (>0.9) and 80% single-photon purity can be realized by strain-engineering the WSe₂/NiPS_3­ heterostructure with nanoscale indentations.¹  Through state of art scanning diamond NV microscopy experiments and temperature-dependent magneto-photoluminescence studies, we show that the chiral quantum light emission arises from magnetic proximity interactions between localized excitons in the WSe₂ monolayer and out-of-plane magnetization of AFM defects in NiPS₃, both of which are co-localized by the strain field arising from the nanoscale indentations. Interestingly, a similar chiral localized excitonic emission is also observed in our more recent experiment performed on WSe₂/MnPS₃ and WSe₂/FePS₃ heterostructure with nano-indents.    

2D Spintronics: Skyrmions and beyond

The current electronics industry is facing challenges both from the fundamental physics limit of silicon on the small scale, and the new demand for big-data applications on the large scale. Spintronics, utilizing spin degree of freedom, is a promising for future beyond-CMOS devices and systems, thanks to their low power consumption, nonvolatility, and easy 3D integration. The emerging 2D magnets can preserve single-phase magnetism even in monolayer (~0.8 nm) limits, and thus they are promising to further scale down devices. They have a sharp interface and atomically thin nature, promising for designer quantum devices and more functionalities (e.g. stacking order, twist angle, thickness, and voltage control).

In this talk, I will discuss 2D spintronics with quantum devices on skyrmions, and their potential applications. I will begin by presenting my observations of real-space topological spin textures - magnetic skyrmions, in 2D devices. This work represents the first report of skyrmion lattice imaging in 2D layered magnets. Building on this, I will present my findings on the vertical imprinting of skyrmions onto neighboring layers in a 2D ferromagnet/2D ferromagnet system, demonstrating new functionality for skyrmion-based spintronics. In addition, future work on quantum devices is motivated that would focus on energy-efficient control in magnetism, for neuromorphic and quantum computing.