Session Y64: Characterizations of 2D Materials by Magnetometry and Spectroscopy

Spin interactions controlled by spin-orbit coupling in chromium trihalide 2D ferromagnets

Chromium trihalides (CrX₃, X = Cl, Br, I) provide an excellent material platform for exploring exotic two-dimensional (2D) spin orders. Three CrX₃ compounds have the same crystal structure and have the same Hamiltonian, but their different p-orbital ligand atoms X lead to different strength of spin-orbit coupling (SOC, λ), the key ingredient responsible for magnetic anisotropy. Here we present measured values of the spin interaction constants Heisenberg (J), Kitaev (K), and off-diagonal symmetric (Γ) exchange interactions in CrX₃ determined using field-angle dependent ferromagnetic resonance (FMR) spectroscopy and exact diagonalization. Furthermore, we investigate the correlation between those values and λ. The unique magnetic anisotropic feature originating from Kitaev interaction obtained from the FMR spectra directly shows the strong dependence on λ. K and Γ and their respective resulting two magnon band gaps ΔK and ΔΓ exhibit a strong λ² dependence, while J has a linear dependence on λ. This shows that the SOC strength λ of X plays a central role in the 2D magnetism of CrX₃.   

Observation of the antiferromagnetic diode effect

In a PN junction, the separation between positive and negative charges leads to diode transport, which is the foundation for nonlinear applications such as rectification and wave mixing. In the past few years, the intrinsic diode transport in noncentrosymmetric polar conductors has attracted great interest, because it suggests novel nonlinear applications and provides a symmetry-sensitive probe of Fermi surface electrons. More recently, such studies have been extended to noncentrosymmetric polar superconductors, realizing the superconducting diode effect. Here, we show that, even in a centrosymmetric crystal without directional charge separation, the spins of an antiferromagnet (AFM) can generate a clear spatial directionality, leading to an AFM diode effect. In the 2D even-layered MnBi2Te4, we observe a large nonlinear transport signal in the fully compensated AFM state. We demonstrate that, based on this effect, the AFM enables an in-plane field effect transistor and the harvesting of wireless electromagnetic energy. Our observation paves the way for AFM logic circuits, self-powered AFM spintronic devices, and other intriguing applications that bridge nonlinear electronics with AFM spintronics.   

Large exchange bias field in a fully van der Waals heterostructure with perpendicular magnetic anisotropy

New generation spintronics devices have attracted a lot of interest for their ultra-low power and high-speed applications. The discovery of 2D magnets boosted this field beyond the limitations drawn by standard materials in terms of scalability and engineering of multifunctional heterostructures.

Together with perpendicular magnetic anisotropy (PMA) that guarantees better performances in terms of scalability than in-plane (IP) systems, exchange bias (EB) plays a fundamental role in spin valves. It consists in the pinning of one of the two ferromagnetic (FM) layers by an adjacent antiferromagnetic (AFM) one, keeping the other one free. Exploiting 2D materials, while tunneling magnetic junctions with PMA were realized with Fe₃GeTe₂, EB was measured in IP systems only.

In this work, we fabricated heterostructures composed by an even number of layers of CrI₃ and few layers of CrBr₃. CrI₃ is an A-type AFM with spins aligned along the c axis with a metamagnetic transition around 750 mT, while CrBr₃ is a FM that shows PMA with a coercive field of 25 mT. Taking advantage of the different transitions that govern the dichroic spectra of the two materials, we disentangled the responses of the two flakes measuring a large EB of 15 mT that can be reached either by field cooling or initializing the AFM ground state of CrI₃ with a sufficiently large magnetic field. We succeeded to probe the effect down to the bilayer/bilayer limit.

These results open new possibilities to the realization of ultracompact and fully van der Waals spin valves with PMA and strong EB field, paving the way to next generation spintronics devices.   

Oral : Spectroscopic Study of the Electronic Structure of Two-dimensional Magnetic Materials

The recent discovery of ferromagnetism in 2D van der Waals chromium trihalides CrX₃ (X=Cl,Br and I) down to the monolayer has gained research attraction because of their interesting electronic and magnetic properties. The magnetic properties of CrX₃ can be manipulated by applying perturbations such as external magnetic field, strain, and pressure. This makes CrX₃ prime candidates for spintronics and magneto-resistive memory applications. It also highlights the importance of determining the key energy scales properly to understand the physics of CrX₃ and build a more reliable base Hamiltonian. We have measured Cr L-edge soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) spectroscopy for all CrX₃ in order to understand their electronic structure. Through a systematic study, with the use of atomic multiplet simulations, we show that our approach has yielded a set of more reliably determined energy scale parameters. Ultimately, our goal is to achieve a detailed understanding of the electronic structure of CrX₃ and determine how it is related to magnetic order and excitations in these fascinating systems.   

Extreme Nonlinear Opto-Magnonic Effects in a Layered Magnetic Semiconductor

The nonlinear dynamics of collective excitations offer both intriguing fundamental phenomena and significant practical applications. A prime illustration is the field of nonlinear optics, where diverse frequency mixing processes are central to advancing photonic technology. Demonstration of these frequency mixing processes in magnons holds considerable potential for practical applications in magnonics, an emerging frontier of spintronics and an important platform for developing quantum transducers and wave-based computing beyond traditional paradigms. Here we demonstrate the optical generation and detection of abundant magnonic frequency mixing processes in the antiferromagnetic semiconductor CrSBr. We employ above-gap pump pulses to launch coherent magnons, which we can then optically measure via strong magnon-exciton coupling. We observe a series of magnon sidebands arising from high-harmonic generation and, when breaking the system symmetry, the mixing of discrete magnon modes to produce sum and difference frequency generation (SFG & DFG). Further, we demonstrate control over the DFG in CrSBr by rotating an external magnetic field to tune its frequency over a broad range. This tuning allows us to push the DFG mode into resonance with one of the fundamental magnon modes, where we can controllably induce parametric amplification. These findings herald the opening of a new domain in nonlinear opto-magnonic coupling, offering innovative functionalities for hybrid quantum magnonics.   

Probing extrinsic 2D magnetism in a few-layer MoS2 having antidots

Inducing magnetism in non-magnetic two-dimensional (2D) materials provides a unique opportunity to realize magnetism at the 2D limit. Previous studies have shown that defects in some transition metal dichalcogenides (TMD) at the single- and few-layer thickness can induce long-range magnetic ordering in the material [1]. In this study, we create triangular defects (or antidots) in atomically thin molybdenum disulfide (MoS₂), one of the most popular semiconducting TMDs. Then, we characterize the defects with atomic force microscopy and fabricate field-effect transistor devices based on MoS₂ with antidots. When fabricating FET devices, we use graphites as contact electrodes to minimize the Schottky barrier [2] and encapsulate the device with multilayer hexagonal boron nitride for better device quality. We then perform magnetoresistance measurements on the MoS₂-FET devices with antidots inside a cryostat to probe the defect-induced 2D magnetism. This study aims to explore 2D magnetism induced by defects in otherwise non-magnetic 2D materials.

 

References:

Magnetic pair distribution function analysis of the 2D layered material CrPS4

Two-dimensional layered materials, characterized by weak van der Waals bonds between the layers in the bulk material, showcase the potential for distinct quantum behaviors and hold promise for applications in spintronics. Among these 2D materials, CrPS₄ is gaining considerable attention in the scientific community due to its magnetic and semiconducting properties. The 2D layers in CrPS₄ are weakly bound with van der Waals forces, suggesting possibilities for exfoliating it to monolayer or few-layer structures. CrPS₄ is an antiferromagnet with a Néel temperature of 36 K and an unusual two-stage magnetic transition. Within the 2D layers, the Cr spins couple ferromagnetically and these 2D layers are antiferromagnetically arranged along the c- direction. In this study, we performed detailed neutron total scattering measurements to access the magnetic and structural behavior. Specifically, we adopted the atomic and magnetic pair distribution function (mPDF) technique to undertake an analysis of the local structural and magnetic behavior of CrPS₄. We will discuss these results that follow the magnetism from the long-range ordered phase through to the short-range ordering above the Néel temperature.   

Electronic structure of above-room-temperature van der Waals ferromagnet Fe3GaTe2

A van der Waals (vdW) ferromagnet with high Curie temperature (T_C) has been highly pursued for better understanding robust long-range spin order in two-dimension (2D) and for developing its potential applications in spintronic devices. Fe₃GaTe₂, a newly discovered vdW ferromagnet with intrinsic ferromagnetism above room temperature, makes it necessary to have a comprehensive understanding of the microscopic origins of its higher T­_C. In this talk, we present the electronic structure of Fe₃GaTe₂ in its ferromagnetic ground state using angle-resolved photoemission spectroscopy (ARPES) and density-functional theory (DFT) calculations. Our results reveal a notable shift in chemical potential of Fe₃GaTe₂, compared to its sister material, Fe₃GeTe₂, while retaining similarities in overall electronic band structure. Based on DFT calculation, we elucidate the critical contribution of the Heisenberg exchange interaction (J_ex) and magnetic anisotropy energy (MAE) to the development of the high T­_C ferromagnetic ordering in Fe₃GaTe₂. Specifically, the J_ex parameter related to the out-of-plane interactions is significantly enhanced due to the decreased c-lattice constant in Fe₃GaTe₂. These findings provide valuable insights into the underlying electronic structure and its correlation with the emergence of high T­_C ferromagnetic ordering in Fe₃GaTe₂.   

Imaging equilibrium and non-equilibrium critical phenomena in mesoscopic magnets

We demonstrate a technique by which the dynamical magnetic susceptibility of a material can be measured via dephasing spectroscopy of nearby single-spin qubits. This probe measures a complementary regime of frequency and momentum compared to existing techniques, and can be applied to mesoscopic sample volumes. We use the technique to image and characterize the critical magnetic fluctuations of chromium trihalide nanoflakes near their magnetic melting temperatures. We further show how dephasing spectroscopy can be combined with external stimulus of the target material to study its critical dynamics out of equilibrium. This work can be extended to a variety of bulk materials, two-dimensional magnets, films, and devices.   

Tuning the magnetic properties of quasi-2D van der Waals material Fe2.7GeTe2 with pressure

It has been recently shown that the magnetic properties of the van der Waals (vdW) ferromagnet Fe_3-xGeTe₂ can be tuned via electric fields, strain, and more recently high pressure. In this work, we examine the pressure effect on the magnetism in Fe deficient Fe_2.7GeTe₂. Magnetic susceptibility results show a decrease in the magnetic ordering temperature from 154.78 K (1 bar) to 141.87 K (1.22 GPa). X-ray magnetic circular dichroism (XMCD) also saw a decrease in the magnetic ordering nearly vanishing at approximately 19.1 GPa. Pressure dependent Raman spectroscopy was performed at room temperature and showed a redshift in the Tellurium peaks (~120 and ~140 cm^-1) at 4.9 GPa. The spectrum abruptly changes at 7.9 GPa remaining the same until the highest pressure of 13.3 GPa is reached. Optical images show at 7.6 GPa, Fe_2.7GeTe₂ undergoes phase transition causing it to become dull grey in color. Upon decompression, the Te peaks returned and the shiny physical appearance of Fe_2.7GeTe₂ returned as well. This work demonstrates the significance of employing the external pressure in tuning the magnetic and optical properties of van der Waals magnets.   

Noise spectroscopy of unconventional spin transport phenomena in a two-dimensional Heisenberg ferromagnet

In recent studies, it has been demonstrated that spin transport coefficients in magnetic materials can be extracted through magnetic noise measurements of nearby spin qubits [1]. In this study, we placed nitrogen vacancy centers in close proximity to nanoflakes of CrCl3, a layered two-dimensional Heisenberg magnet, ranging from tens of layers to the monolayer limit. We are able to extract the spectral density of magnetic noise and imaginary susceptibility as functions of temperature and external magnetic field. Below the transition temperature, noise persists down to the lowest measurable temperature (5K), which can be explained by a model of a strongly interacting magnon gas where magnon-magnon collisions significantly influence the spin transport properties.

[1]Wang, Hailong, et al. "Noninvasive measurements of spin transport properties of an antiferromagnetic insulator." Science advances 8.1 (2022): eabg8562.   

Signature of Electromagnons and Multiferroic Domains in NiI2 through Raman Spectrum

Multiferroic van der Waals materials, in which coexistence of ferromagnetism (FM) and ferroelectricity (FE) within a single phase is observed, offer a promising avenue for achieving electric control of magnetism in the two-dimensional (2D) regime. This capability holds significant implications for the advancement of spintronic devices, including magnetic storage and magnetic random-access memory. To comprehend the intricate interplay between magnetism and ferroelectricity, it is imperative to scrutinize the response of their fundamental excitations to external stimuli, including magnetic fields, electrical doping, and temperature variations. Recently, type-II multiferroic Nickle Diiodide (NiI₂) was evident to exhibit an inversion symmetry breaking by spiral magnetism down to the atomic limit through the utilization of second harmonic generation (SHG) and birefringence measurement. In this talk, we present an investigation into the magneto-Raman spectrum of the type-II multiferroic order manifest in layered NiI₂, which is characterized by a proper-screw magnetic order with given handedness. By symmetry analysis, we use both linear and circular cross polarization to identify various multiferroic domains characterized by distinctive electromagnon modes. Our studies also uncover the significant magnetoelectric effect arising from the spin-phonon coupling in NiI₂. These observations establish NiI₂ as an intriguing platform for the exploration of emergent multiferroic effects and associated magnetic textures in the two-dimensional limit.   

Tuning magnetism through Cu and Cr substitutions in MPS3 van der Waals magnets

Magnetic van der Waals material MPX₃ (M= metal, X = S or Se) is an intensively studied platform to investigate 2D magnetism. Recent discoveries found that magnetism can persist down the atomic thin layers in some MPX3 compounds. So far, all discovered MPX3 materials display antiferromagnetic order in their ground state. Establishing long-range ferromagnetism has become one important task for the community. Cr-substitution has been found to show a magnetic field-driven polarized ferromagnetism. However, owing to the 3+ valence of Cr ions, substituting the di-valence metal ion in MPX3 is limited to only 10%, which makes it difficult to further explore the potential of this route. Here we report an alternative approach that balances the valence by co-substituting mono-valent metal ions together with the tri-valent Cr ions, through which we were able to replace all M²⁺ to more efficiently tune the magnetism.   

Quantum Weyl-Heisenberg antiferromagnet

The Heisenberg model is a fundamental model of quantum magnetism that has been used to study a wide variety of physical systems, from high-Tc superconductors to spin liquids. In this work, we show that the introduction of anisotropic couplings in the square-lattice nearest-neighbor antiferromagnetic Heisenberg model can lead to the emergence of exotic topological states, including Weyl magnons. We study the model with anisotropic couplings using the spin-wave approximation and compute the edge spectrum and Berry connection vector, which show clear evidence of localizedtopological charges. We discover phases that exhibit Weyl-type spin-wave dispersion, characterized by pairs of degenerate points and edge states, as well as phases supporting lines of degeneracy. We also identify a parameter regime in which there is an exotic state hosting gapless linear spin-wave dispersions with different longitudinal and transverse spin-wave velocities. Such states should be expected to occur naturally in most antiferromagnetic compounds which undergo structural transitions that reduce the point-group symmetry at lower temperature, and could also be realized in cold atom experiments.