Van Der Waals Ferromagnet Research Articles

Magnetic Skyrmions with Unconventional Helicity Polarization in a Van Der Waals Ferromagnet.

Skyrmion helicity, which defines the spin swirling direction, is a fundamental parameter that may be utilized to encode data bits in future memory devices. Generally, in centrosymmetric ferromagnets, dipole skyrmions with helicity of -π/2 and π/2 are degenerate in energy, leading to equal populations of both helicities. On the other hand, in chiral materials where the Dzyaloshinskii-Moriya interaction (DMI) prevails and the dipolar interaction is negligible, only a preferred helicity is selected by the type of DMI. However, whether there is a rigid boundary between these two regimes remains an open question. Herein, the observation of dipole skyrmions with unconventional helicity polarization in a van der Waals ferromagnet, Fe5- δ GeTe2 , is reported. Combining magnetometry, Lorentz transmission electron microscopy, electrical transport measurements, and micromagnetic simulations, the short-range superstructures in Fe5- δ GeTe2 resulting in a localized DMI contribution, which breaks the degeneracy of the opposite helicities and leads to the helicity polarization, is demonstrated. Therefore, the helicity feature in Fe5- δ GeTe2 is controlled by both the dipolar interaction and DMI that the former leads to Bloch-type skyrmions with helicity of ±π/2 whereas the latter breaks the helicity degeneracy. This work provides new insights into the skyrmion topology in van der Waals materials.

Wide field imaging of van der Waals ferromagnet Fe3GeTe2 by spin defects in hexagonal boron nitride

Emergent color centers with accessible spins hosted by van der Waals materials have attracted substantial interest in recent years due to their significant potential for implementing transformative quantum sensing technologies. Hexagonal boron nitride (hBN) is naturally relevant in this context due to its remarkable ease of integration into devices consisting of low-dimensional materials. Taking advantage of boron vacancy spin defects in hBN, we report nanoscale quantum imaging of low-dimensional ferromagnetism sustained in Fe3GeTe2/hBN van der Waals heterostructures. Exploiting spin relaxometry methods, we have further observed spatially varying magnetic fluctuations in the exfoliated Fe3GeTe2 flake, whose magnitude reaches a peak value around the Curie temperature. Our results demonstrate the capability of spin defects in hBN of investigating local magnetic properties of layered materials in an accessible and precise way, which can be extended readily to a broad range of miniaturized van der Waals heterostructure systems.

Critical properties of the Ising ferromagnet VI[formula omitted] monolayer

Motivated by the experimental observation of two-dimensional van der Waals ferromagnets, we investigated the critical properties of Ising ferromagnet VI3 monolayer using the real-space renormalization group method. It is worth noting that a block-spin construction we choose contains an even number of sites. Thereupon, we present a scheme that combines the majority rule and central rule to determine the block-spin values. The critical temperature TC is calculated to be 46.8 K under the first-order cumulant-expansion approximation, which is in excellent agreement with the recent numerical result of TC = 46 K using the Metropolis Monte Carlo simulations based on the classical Heisenberg model, and is slightly lower than the experimental value TC = 50 K of the corresponding bulk VI3. Furthermore, the critical exponents β and ν are estimated, and compared with the mean-field results and the exact solutions, respectively. Our numerical result β≅0.324 suggests that the magnetic phase transition of the VI3 monolayer appears to lie between the two-dimensional Ising universality class (β=0.125) and three-dimensional Heisenberg universality class (β=0.365). The theoretical results in this paper may stimulate further experimental studies of the VI3 monolayer.

Dipole-exchange spin waves in two-dimensional van der Waals ferromagnetic films and stripes

A spin-wave (SW) theory that includes the long-range dipole–dipole interactions is presented for monolayers of van der Waals (vdW) ferromagnets for which the magnetic ions lie on a two-dimensional honeycomb lattice. The dipolar interactions provide an additional anisotropy in these materials, along with the Ising exchange interaction and/or single-ion anisotropies that typically stabilize the two-dimensional magnetic ordering. Analytical results for the linearized SW energies are obtained for the ferromagnets in two geometries: complete films and finite-width stripes (or ribbons). In both cases it is found that the inclusion of the dipole–dipole interactions leads to a shift and sometimes a splitting of the magnetic modes in the vdW structure. Also, in the latter case, where the edges are assumed to be along the zigzag lattice directions, the dipole–dipole interactions are found to play a role, as well as the exchange interactions, in modifying the localized edge SWs. Numerical examples are given, including applications to the ferromagnet CrI3.

High-Spin Orbital Interactions Across van der Waals Gaps Controlling the Interlayer Ferromagnetism in van der Waals Ferromagnets.

We examined what interactions control the sign and strength of the interlayer coupling in van der Waals ferromagnets such as Fe3-xGeTe2, Cr2Ge2Te6, CrI3, and VI3 to find that high-spin orbital interactions across the van der Waals gaps are a key to understanding their ferromagnetism. Interlayer ferromagnetic coupling in Fe3-xGeTe2, Cr2Ge2Te6, and CrI3 is governed by the high-spin two-orbital two-electron destabilization, but that in VI3 by the high-spin four-orbital two-electron stabilization. These interactions explain a number of seemingly puzzling observations in van der Waals ferromagnets.

Relating spin-polarized STM imaging and inelastic neutron scattering in the van der Waals ferromagnetFe3GeTe2

Information about the magnetic properties of van der Waals ferromagnets comes so far largely from macroscopic techniques, with much less being known about the magnetism at the microscopic level. By comparing atomic-scale imaging of domain walls at the surface of the van der Waals ferromagnet Fe${}_{3\ensuremath{-}x}$GeTe${}_{2}$ with inelastic neutron scattering measurements of the magnon dispersion in the bulk, the authors elucidate here the interplay between the bulk and surface magnetism and image the low-energy electronic structure.

Optical anomalous Hall effect enhanced by flat bands in ferromagnetic van der Waals semimetal

Geometrical aspects of electronic states in condensed matter have led to the experimental realization of enhanced electromagnetic phenomena, as exemplified by the giant anomalous Hall effect (AHE) in topological semimetals. However, the guideline to the large AHE is still immature due to lack of profound understanding of the sources of the Berry curvature in actual electronic structures; the main focus has concentrated only on the band crossings near the Fermi level. Here, we show that the band crossings and flat bands cooperatively produce the large intrinsic AHE in ferromagnetic nodal line semimetal candidate Fe3GeTe2. The terahertz and infrared magneto-optical spectroscopy reveals that two explicit resonance structures in the optical Hall conductivity spectra σxy(ω) are closely related to the AHE. The first-principles calculation suggests that both the flat bands having large density of states (DOS) and the band crossings near the Fermi level are the main causes of these Hall resonances. Our findings unveil a mechanism to enhance the AHE based on the flat bands, which gives insights into the topological material design.

X-ray spectroscopy for the magnetic study of the van der Waals ferromagnet CrSiTe3 in the few- and monolayer limit

The study of magnetic order in few- and monolayer van der Waals materials poses a challenge to the most commonly employed magnetic characterization techniques as they normally lack magnetic sensitivity and/or lateral resolution enabling their thickness-dependent probing. Here we demonstrate the usefulness of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements, carried out at the Cr and Te M 5 edges, for the study of the ferromagnetic semiconductor CrSiTe3 (CST) in the form of single- and few-layer flakes. By scanning the sample under the incident x-ray beam, a map of the exfoliated system was obtained, which reproduced the optical micrographs showing the detailed distribution and thicknesses of the flakes. In this way, XAS/XMCD was performed at selected sample areas, revealing the thickness-resolved spectroscopic and magnetic properties of the flakes, such as the spin and orbital magnetic moments. The spin moment, in line with the saturation field, is decreasing with film thickness, revealing a single-domain and out-of-plane magnetization for the thinnest films. For CST, the electronic properties are governed by the strong covalent bond between the Cr 3d(e g ) and Te 5p states, giving rise to a superexchange scenario. We observed a gradually increasing ratio of orbital to spin moment for thinner flakes, which could be due to a further increase of the covalent mixing. Hysteresis loops were recorded at the Cr L 3 edge, showing an open loop for 10 down to ∼3 layers, while the bulk shows a wasp-waist shaped loop. With the transition temperature from the soft to the hard ferromagnetic state decreasing with thickness, the monolayer shows a narrowed, closed loop at 10 K, suggesting its transition temperature 10 K. Our study demonstrates the unique capabilities of XAS/XMCD for the study of few-layer van der Waals magnets, highlighting the interplay between electron correlation and ferromagnetism in CST.

Mechanical control of physical properties in the van der Waals ferromagnet Cr2Ge2Te6 via application of electric current

Cr2Ge2Te6 is a van der Waals ferromagnet with a Curie temperature at 66 K. Here we report a swift change in the magnetic ground state upon application of small DC electric current, a giant yet anisotropic magnetoelectric effect, and a sharp, lattice-driven quantum switching manifested in the I-V characteristic of the bulk single-crystal Cr2Ge2Te6. At the heart of these observed phenomena is a newly uncovered, strongly anisotropic magnetoelastic coupling that enables strongly anisotropic responses of the lattice to application of electric current and/or magnetic field, thus the exotic phenomena in Cr2Ge2Te6. Such a rare mechanical tunability in the magnetic semiconductors promises tantalizing prospects for unique functional materials and devices.

Construction of a vector-field cryogenic magnetic force microscope.

Owing to the high resolution of magnetic force microscopes (MFMs) operating at low temperatures and high-applied magnetic fields, they can be employed to study various phenomena observed in topological magnetic materials and superconductors. In this study, we constructed a low-temperature MFM equipped with a 2-2-9-T vector magnet and a three-axis fiber-optic alignment system. The three-axis alignment device enables in situ calibration of the scanner at low temperatures as well as optimizes the intensity and sensitivity of the interferometer signal. A massive homebuilt vibration isolation table lowers the resonance frequency of the system and minimizes mechanical noise. Consequently, the minimum detectable force gradient of our proposed model is close to the thermodynamic limit of the cantilever. To demonstrate the low-temperature capability of the MFM, we obtained magnetic domain images of the van der Waals ferromagnet Fe4GeTe2 and the Abrikosov superconducting vortices of an Nb film. Furthermore, we performed field angle-dependent MFM experiments in a van der Waals magnetic insulator Cr2Ge2Te6 to verify its vector-field functionality and observed a transition in the domains from the stripe to the bubble phase with respect to the magnetic field angle. The vector-field capability of our MFM can be useful for investigating various anisotropic magnetic phenomena in topological magnetic and superconducting materials.

Strong effects of uniaxial pressure and short-range correlations in Cr2Ge2Te6

Cr$_2$Ge$_2$Te$_6$ is a quasi-2D semiconducting van der Waals ferromagnet down to the bilayer with great potential for technological applications. Engineering the critical temperature to achieve room temperature applications is one of the critical next steps on this path. Here, we report high-resolution capacitance dilatometry studies on Cr$_2$Ge$_2$Te$_6$ single crystals which directly prove significant magnetoelastic coupling and provide quantitative values of the large uniaxial pressure effects on long-range magnetic order (${\partial}T_{\mathrm{C}}/{\partial}p_{\mathrm{c}}$ = 24.7 K/GPa and ${\partial}T_{\mathrm{C}}/{\partial}p_{\mathrm{ab}}$ = $-$15.6 K/GPa) derived from thermodynamic relations. Moderate in-plane strain is thus sufficient to strongly enhance ferromagnetism in Cr$_2$Ge$_2$Te$_6$ up to room temperature. Moreover, unambiguous signs for short-range magnetic order up to 200~K are found.

Non-Hermitian skin effect in magnetic systems

Far from being limited to a trivial generalization of their Hermitian counterparts, non-Hermitian topological phases have gained widespread interest due to their unique properties. One of the most striking non-Hermitian phenomena is the skin effect, i.e., the localization of a macroscopic fraction of bulk eigenstates at a boundary, which underlies the breakdown of the bulk-edge correspondence. Here we investigate the emergence of the skin effect in magnetic insulating systems by developing a phenomenological approach to describing magnetic dissipation within a lattice model. Focusing on a spin-orbit-coupled van der Waals (vdW) ferromagnet with spin-nonconserving magnon-phonon interactions, we find that the magnetic skin effect emerges in an appropriate temperature regime. Our results suggest that the interference between Dzyaloshinskii-Moriya interaction (DMI) and nonlocal magnetic dissipation plays a key role in the accumulation of bulk states at the boundaries.

Room-Temperature van der Waals Perpendicular Ferromagnet Through Interlayer Magnetic Coupling

High-density spintronic applications such as digital memory require strong perpendicular magnetic anisotropy (PMA), to maintain data stability above room temperature, which so far has been beyond the reach of stacked van der Waals ferromagnets. Here the authors demonstrate that the interlayer exchange coupling in a magnetic multilayer can effectively tune the Curie temperature and PMA of sandwiched two-dimensional (2D) Fe${}_{3}$GeTe${}_{2}$. The successful realization of room-temperature 2D ferromagnets bearing fully nonvolatile out-of-plane spin information could bring 2D spintronic applications to reality.

Measurement of electronic structure in van der Waals ferromagnet Fe5–x GeTe2

As a van der Waals ferromagnet with high Curie temperature, Fe5–x GeTe2 has attracted tremendous interests recently. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES), we systematically investigated the electronic structure of Fe5–x GeTe2 crystals and its temperature evolution. Our ARPES measurement reveals two types of band structures from two different terminations with slight kz evolution. Interestingly, across the ferromagnetic transition, we observed the merging of two split bands above the Curie temperature, suggesting the band splitting due to the exchange interaction within the itinerant Stoner model. Our results provide important insights into the electronic and magnetic properties of Fe5–x GeTe2 and the understanding of magnetism in a two-dimensional ferromagnetic system.

Josephson Spin-Valve Realization in the Magnetic Nodal-Line Topological Semimetal Fe3GeTe2

Three-dimensional van der Waals ferromagnet Fe3GeTe2 (FGT) is regarded as a candidate for the magnetic topological nodal line semimetal. We investigate lateral electron transport between two 3 μm spaced superconducting In leads beneath a thick three-dimensional FGT exfoliated flake. At a low temperature of 30 mK, we observe Josephson supercurrent that exhibits unusual critical current Ic suppression by the magnetic field B. The overall Ic(B) pattern is asymmetric in respect of the sign of the magnetic field B. We demonstrate, that the asymmetry is defined by the magnetic field sweep direction, so the Ic(B) pattern is strictly reversed (as magnetic field reversal) for the opposite sweeps. We also observe an interplay between maximum and minimum in Ic(B) in normal magnetic fields, while there are fast aperiodic Ic(B) fluctuations for the in-plane ones. These effects cannot be expected for homogeneous superconductor-ferromagnet-superconductor junctions, while they are known for Josephson spin valves. The mostly possible scenario for Josephson spin valve realization in FGT is the misalignment of spin polarizations of the Fermi arc surface states and ferromagnetic FGT bulk, but we also discuss possible influence of spin-dependent transport between magnetic domains.

Magnetic anisotropy of the van der Waals ferromagnet Cr2Ge2Te6 studied by angular-dependent x-ray magnetic circular dichroism

The van der Waals ferromagnet ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CGT) has a two-dimensional crystal structure where each layer is stacked through van der Waals force. We have investigated the nature of the ferromagnetism and the weak perpendicular magnetic anisotropy (PMA) of CGT by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) studies of CGT single crystals. The XMCD spectra at the Cr ${L}_{2,3}$ edge for different magnetic field directions were analyzed on the basis of the cluster-model multiplet calculation. The Cr valence is confirmed to be $3+$ and the orbital magnetic moment is found to be nearly quenched, as expected for the high-spin ${t}_{2g}^{3}$ configuration of the ${\mathrm{Cr}}^{3+}$ ion. A large ($\ensuremath{\sim}0.2$ eV) trigonal crystal-field splitting of the ${t}_{2g}$ level caused by the distortion of the $\mathrm{Cr}{\mathrm{Te}}_{6}$ octahedron has been revealed, while the single-ion anisotropy (SIA) of the Cr atom is found to have a sign opposite to the observed PMA and too weak compared to the reported anisotropy energy. The present result suggests that anisotropic exchange coupling between the Cr atoms through the ligand Te $5p$ orbitals having strong spin-orbit coupling has to be invoked to explain the weak PMA of CGT, as in the case of the strong PMA of ${\mathrm{CrI}}_{3}$.

All-Electrical Control of Compact SOT-MRAM: Toward Highly Efficient and Reliable Non-Volatile In-Memory Computing.

Two-dimensional van der Waals (2D vdW) ferromagnets possess outstanding scalability, controllable ferromagnetism, and out-of-plane anisotropy, enabling the compact spintronics-based non-volatile in-memory computing (nv-IMC) that promises to tackle the memory wall bottleneck issue. Here, by employing the intriguing room-temperature ferromagnetic characteristics of emerging 2D Fe3GeTe2 with the dissimilar electronic structure of the two spin-conducting channels, we report on a new type of non-volatile spin-orbit torque (SOT) magnetic tunnel junction (MTJ) device based on Fe3GeTe2/MgO/Fe3GeTe2 heterostructure, which demonstrates the uni-polar and high-speed field-free magnetization switching by adjusting the ratio of field-like torque to damping-like torque coefficient in the free layer. Compared to the conventional 2T1M structure, the developed 3-transistor-2-MTJ (3T2M) cell is implemented with the complementary data storage feature and the enhanced sensing margin of 201.4% (from 271.7 mV to 547.2 mV) and 276% (from 188.2 mV to 520 mV) for reading “1” and “0”, respectively. Moreover, superior to the traditional CoFeB-based MTJ memory cell counterpart, the 3T2M crossbar array architecture can be executed for AND/NAND, OR/NOR Boolean logic operation with a fast latency of 24 ps and ultra-low power consumption of 2.47 fJ/bit. Such device to architecture design with elaborated micro-magnetic and circuit-level simulation results shows great potential for realizing high-performance 2D material-based compact SOT magnetic random-access memory, facilitating new applications of highly reliable and energy-efficient nv-IMC.

Controlling magnetism through Ising superconductivity in magnetic van der Waals heterostructures

Van der Waals heterostructures have risen as a tunable platform to combine different electronic orders, due to the flexibility in stacking different materials with competing symmetry broken states. Among them, van der Waals ferromagnets such as CrI3 and superconductors as NbSe2 provide a natural platform to engineer novel phenomena at ferromagnet-superconductor interfaces. In particular, NbSe2 is well known for hosting strong spin-orbit coupling effects that influence the properties of the superconducting state. Here we put forward a ferromagnet/NbSe2/ferromagnet heterostructure where the interplay between Ising superconductivity in NbSe2 and magnetism controls the magnetic alignment of the heterostructure. In particular, we show that the interplay between spin-orbit coupling and superconductivity allows controlling magnetic states in van der Waals materials. Our results show how hybrid van der Waals ferromagnet/superconductor heterostructure can be used as a tunable materials platform for superconducting spin-orbitronics.

Manipulating Exchange Bias in a Van der Waals Ferromagnet.

Spintronics applications of thin-film magnets require control and design of specific magnetic properties. Exchange bias, originating from the pinning of spins in a ferromagnet by these of an antiferromagnet, is a part of the highly important elements for spintronics applications. Here, an exchange bias of ≈90 mT in a van der Waals ferromagnet encapsulated by two antiferromagnets at 5 K, the value of which is highly tunable by the field coolings, is reported. The non-antisymmetric dependence of exchange bias on field cooling is explained through considering an uncompensated interfacial magnetic layer of an antiferromagnet with a noncollinear spin texture, and a weak antiferromagnetic order in the oxidized layer, at two ferromagnet/antiferromagnet interfaces. This work opens up new routes toward designing and controlling 2D spintronic devices made of atomically thin van der Waals magnets.

Manipulation of topological spin configuration via tailoring thickness in van der Waals ferromagnetic Fe5−xGeTe2

Topological spin textures with versatile configurations exhibit fascinating physical behavior as high-efficiency information units. Recent observations of nontrivial spin textures in two-dimensional (2D) van der Waals (vdW) ferromagnets have shed light on their functionality as a paradigm for spintronic devices. Here, the spin configuration of single-crystal 2D vdW ${\mathrm{Fe}}_{5\text{\ensuremath{-}}x}\mathrm{Ge}{\mathrm{Te}}_{2}$ is investigated by Lorentz transmission electron microscopy, where the conversion from Bloch-type bubbles to N\'eel-type skyrmions can be observed by tuning sample thickness. Remarkably, high-density N\'eel-type skyrmions can be obtained via field-cooling manipulation. We further demonstrate the underlying mechanism by varying the sample thickness and Dzyaloshinskii-Moriya interaction (DMI) parameters in a series of micromagnetic simulations, manifesting the presence of DMI at the ${\mathrm{Fe}}_{5\text{\ensuremath{-}}x}\mathrm{Ge}{\mathrm{Te}}_{2}$ surface by first-principles calculation. Our findings disclose the rich variety of topological spin textures in 2D vdW ${\mathrm{Fe}}_{5\text{\ensuremath{-}}x}\mathrm{Ge}{\mathrm{Te}}_{2}$, possessing great promise for future nonvolatile memories based on spin topology.