Fe3GeTe2 Research Articles

Proximity-Induced Tunable Magnetic Order at the Interface of All-van der Waals-Layered Heterostructures.

Spin-orbit coupling (SOC) plays a crucial role in controlling the spin-charge conversion efficiency, spin torque, and complex magnetic spin structures. In this study, we investigate the interplay between SOC and ferromagnetism in heterostructures of large-SOC and magnetic materials. We highlight the importance of the SOC-proximity effect on magnetic ordering in all-van der Waals-layered heterostructures, specifically Fe3GeTe2(FGT)/monolayer W1-xVxSe2 (x = 0 and 0.05). By increasing the SOC strength, we demonstrate various magnetic orderings induced at the interface of the heterostructure, including spin-flop, spin-flip, and inverted magnetization. Moreover, we show a sharp magnetic switching from antiferromagnetic state to ferromagnetic state in FGT/W0.95V0.05Se2, which is characteristic of the synthetic antiferromagnetic structure. This proof-of-concept result offers the possibility of interface-tailoring spintronics, including two-dimensional magnetoresistive random access memory toggle switching. Our findings provide insight into the design and development of next-generation spintronic devices by exploiting the interplay between SOC and magnetic ordering in all-van der Waals-layered heterostructures.

Fe-Intercalation Dominated Ferromagnetism of van der Waals Fe3 GeTe2.

Fe3 GeTe2 have proven to be of greatly intrigue. However, the underlying mechanism behind the varying Curie temperature (Tc ) values remains a puzzle. This study explores the atomic structure of Fe3 GeTe2 crystals exhibiting Tc values of 160, 210, and 230 K. The elemental mapping reveals a Fe-intercalation on the interstitial sites within the van der Waals gap of the high-Tc (210 and 230 K) samples, which are observed to have an exchange bias effect by electrical transport measurements, while Fe intercalation or the bias effect is absent in the low-Tc (160 K) samples. First-principles calculations further suggest that the Fe-intercalation layer may be responsible for the local antiferromagnetic coupling that gives rise to the exchange bias effect, and that the interlayer exchange paths greatly contribute to the enhancement of Tc . This discovery of the Fe-intercalation layer elucidates the mechanism behind the hidden antiferromagnetic ordering that underlies the enhancement of Tc in Fe3 GeTe2 .

Enhanced coercivity and emergent spin-cluster-glass state in 2D ferromagnetic material, Fe[formula omitted]GeTe[formula omitted

Two-dimensional (2D) van der Waals (vdW) magnetic materials with high coercivity and high TC are desired for spintronics and memory storage applications. Fe3GeTe2 (F3GT) is one such 2D vdW ferromagnet with a reasonably high TC, but with a very low coercive field, HC (≲100 Oe). Here we present a phase-engineered sample of F3GT with enhanced coercivity (7–10 times), without compromising its fundamental magnetic properties (TC≃ 210 K). The parent F3GT phase is infused with a small percentage of randomly embedded clusters of a coplanar FeTe (FT) phase, which serves as both mosaic pinning centers between grains of F3GT above its antiferromagnetic transition temperature (TC1∼ 70 K) and also as anti-phase domains below TC1. As a result, the grain boundary disorder and metastable nature are greatly augmented, leading to highly enhanced coercivity, cluster spin glass, and meta-magnetic behavior. The enhanced coercivity (≃1 kOe) makes F3GT-2 much more useful for memory storage applications.

Fe-intercalated Fe3GeTe2: Potential quasi-van der Waals magnets

In recent years, with the fast development of magnetic devices for information technology, the demands of magnetic thin films with both high functional stability and integration flexibility rapidly increase. It is believed that building a magnet with complementary advantages of van der Waals (vdW) and non-vdW magnets, which can be described as a “quasi-vdW magnet,” will be highly appreciated. One may expect a quasi-vdW magnet to have chemical bonding between the neighboring sublayers for strong magnetic coupling, but to preserve clean surfaces with vdW feature for flexible interface engineering. For this purpose, an intercalation of magnetic atoms into the interlayer gaps of vdW magnets, as a powerful method for tuning the interlayer coupling, can be a practical approach. In this work, using the first-principles calculations, we study the potential to utilize the Fe-intercalation to transform the vdW magnets Fe3GeTe2 (FGT) into quasi-vdW magnets. As two extreme cases, it is revealed that: (i) the Fe-intercalated FGT bilayer Fe-[Fe3GeTe2]2 (Fe-[FGT]2) does have remarkable interlayer ferromagnetic coupling based on covalent bonding between the intercalated Fe atom and FGT monolayers and retains low exfoliation energy with vdW feature, suggesting that the Fe-[FGT]2 bilayer can be regarded as a quasi-vdW magnet; and (ii) the Fe-intercalation can transform the vdW FGT bulk into a non-vdW Fe-Fe3GeTe2 (Fe-FGT) bulk magnet. Accordingly, as for the intermediate cases, it is suggested that Fe-intercalated FGT multilayers (Fen−1-[FGT]n, n > 2) can also be potential quasi-vdW magnets, forming a family of magnetic thin films that provide alternative building blocks for microminiaturized magnetic devices.

Electric Field Induced Schottky to Ohmic Contact Transition in Fe3GeTe2/TMDs Contacts

Although the two-dimensional transition metal dichalcogenides (TMDs) present excellent electrical properties, the contact resistance at the interface of metal/TMDs limits the device performance. Herein, we use 2D metallic Fe3GeTe2 (FGT) as an electrode in contact with TMDs semiconductors MX2 (M = Mo, W; X = S, Se, Te) and investigate the contact properties of FGT/MX2 based on density functional theory calculations. We demonstrated that FGT/MX2 presents n-type Schottky contacts, and their n-type Schottky barrier heights are lower than that of the most common bulk metal contacts with MX2, suggesting that FGT can be used as an efficient metallic electrode for MX2. The transitions from n-type Schottky contact to p-type Schottky contact and from Schottky contact to Ohmic contact can be achieved in FGT/MX2 under the electric field. This work not only illustrates an effective method to modulate the contact types and Schottky barrier heights of FGT/MX2 contacts but also provides a route for designing the nanodevices based on FGT/MX2 electrical contacts.

Giant Magnetocaloric Effect in Magnets Down to the Monolayer Limit.

2D magnets can potentially revolutionize information technology, but their potential application to cooling technology and magnetocaloric effect (MCE) in a material down to the monolayer limit remain unexplored. Herein, it is revealed through multiscale calculations the existence of giant MCE and its strain tunability in monolayer magnets such as CrX3 (X=F,Cl,Br,I), CrAX (A=O,S,Se;X=F,Cl,Br,I), and Fe3 GeTe2 . The maximum adiabatic temperature change ( ), maximum isothermal magnetic entropy change, and specific cooling power in monolayer CrF3 are found as high as 11K, 35µJm-2 K-1 , and 3.5nWcm-2 under a magnetic field of 5T, respectively. A 2% biaxial and 5% a-axis uniaxial compressive strain can remarkably increase of CrCl3 and CrOF by 230% and 37% (up to 15.3 and 6.0K), respectively. It is found that large net magnetic moment per unit area favors improved MCE. These findings advocate the giant-MCE monolayer magnets, opening new opportunities for magnetic cooling atnanoscale.

Spin–Phonon Interactions and Anharmonic Lattice Dynamics in Fe3GeTe2

AbstractRaman scattering is performed on Fe3GeTe2 (FGT) at temperatures from 8 to 300 K and under pressures from the ambient pressure to 9.43 GPa. Temperature‐dependent and pressure‐dependent Raman spectra are reported. The results reveal respective anomalous softening and moderate stiffening of the two Raman active modes as a result of the increase of pressure. The anomalous softening suggests anharmonic phonon dynamics and strong spin–phonon coupling. Pressure‐dependent density functional theory and phonon calculations are conducted and used to study the magnetic properties of FGT and assign the observed Raman modes, and . The calculations proved the strong spin–phonon coupling for the mode. In addition, a synergistic interplay of pressure‐induced reduction of spin exchange interactions and spin–orbit coupling effect accounts for the softening of the mode as pressure increases.

Nonlocal Spin Valves Based on Graphene/Fe3GeTe2 van der Waals Heterostructures.

With recent advances in two-dimensional (2D) ferromagnets with enhanced Curie temperatures, it is possible to develop all-2D spintronic devices with high-quality interfaces using 2D ferromagnets. In this study, we have successfully fabricated nonlocal spin valves with Fe3GeTe2 (FGT) as the spin source and detector and multilayer graphene as the spin transport channel. The nonlocal spin transport signal was found to strongly depend on temperature and disappear at a temperature below the Curie temperature of the FGT flakes, which stemmed from the temperature-dependent ferromagnetism of FGT. The spin injection efficiency was estimated to be about 1%, close to that of conventional nonlocal spin valves with transparent contacts between ferromagnetic electrodes and the graphene channel. In addition, the spin transport signal was found to depend on the direction of the magnetic field and the magnitude of the current, which was due to the strong perpendicular magnetic anisotropy of FGT and the thermal effect, respectively. Our results provide opportunities to extend the applications of van der Waals heterostructures in spintronic devices.

Exchange bias controlled antisymmetric-symmetric magnetoresistances in Fe3GeTe2/graphite/Fe3GeTe2 trilayer

The magnetoresistance (MR) of spin values usually displays a symmetric dependence on the magnetic field. An antisymmetric MR phenomenon has been discovered recently that breaks the field symmetry and has the potential to realize multi-bit memory. In this work, we report a controllable switch between the antisymmetric and symmetric MRs and propose a multi-bit memory performance in Fe3GeTe2 (FGT)/graphite/FGT trilayer with modified vertical geometry. Via investigating the evolution of the antisymmetric MR depending on the spatial distribution, current direction, and magnetization configuration, we demonstrate that the antisymmetric MR results from the local nonequilibrium current through the trilayer. Furthermore, an exchange bias effect is induced which modifies the antisymmetric MR. A stable multi-bit memory is presented in the heterostructure. Such architecture for multi-state memory provides insights into other spin-valve structures to improve storage density.

High Pressure-Driven Magnetic Disorder and Structural Transformation in Fe3 GeTe2 : Emergence of a Magnetic Quantum Critical Point.

Among the recently discovered 2D intrinsic van der Waals (vdW) magnets, Fe3 GeTe2 (FGT) has emerged as a strong candidate for spintronics applications, due to its high Curie temperature (130 - 220 K) and magnetic tunability in response to external stimuli (electrical field, light, strain). Theory predicts that the magnetism of FGT can be significantly modulated by an external strain. However, experimental evidence is needed to validate this prediction and understand the underlying mechanism of strain-mediated vdW magnetism in this system. Here, the effects of pressure (0 - 20GPa) are elucidated on the magnetic and structural properties of Fe3 GeTe2 by means of synchrotron Mössbauer source spectroscopy, X-ray powder diffraction and Raman spectroscopy over a wide temperature range of 10 - 290 K. A strong suppression of ferromagnetic ordering is observed with increasing pressure, and a paramagnetic ground state emerges when pressure exceeds a critical value, PPM ≈ 15GPa. The anomalous pressure dependence of structural parameters and vibrational modes is observed at PC ≈ 7GPa and attributed to an isostructural phase transformation. Density functional theory calculations complement these experimental findings. This study highlights pressure as a driving force for magnetic quantum criticality in layered vdW magnetic systems.

Tunable asymmetric magnetoresistance in an Fe3GeTe2/graphite/Fe3GeTe2 lateral spin valve.

van der Waals (vdW) ferromagnetic heterojunctions, characterized by an ultraclean device interface and the absence of lattice matching, have emerged as indispensable and efficient building blocks for future spintronic devices. In this study, we present a seldom observed antisymmetric magnetoresistance (MR) behavior with three distinctive resistance states in a lateral van der Waals (vdW) structure comprising Fe3GeTe2 (FGT)/graphite/FGT. In contrast to traditional spin valves governed by the magnetization configurations of ferromagnetic electrodes (FEs), this distinct feature can be attributed to the interaction between FGT and the FGT/graphite interface, which is primarily influenced by the internal spin-momentum locking effect. Furthermore, modulation of the MR behavior is accomplished by employing the coupling between antiferromagnetic and ferromagnetic materials to adjust the coercive fields of two FEs subsequent to the in situ growth of an FGT oxide layer on FGT. This study elucidates the device physics and mechanism of property modulation in lateral spin valves and holds the potential for advancing the development of gate-tunable spintronic devices and next-generation integrated circuits.

Probing antiferromagnetism in exfoliated Fe3GeTe2 using magneto-transport measurements.

Among the material class of van der Waals magnets, Fe3GeTe2 (FGT) has emerged as one of the most studied owing to features such as its relatively high Curie temperature, metallic nature, and large spin polarization. Though most studies only investigate its explicitly ferromagnetic properties, FGT is also predicted to have an antiferromagnetic phase in the out-of-plane direction emerging at temperatures below 150 K, leading to a blend of ferromagnetic and antiferromagnetic ordering. Here, we explore the emergence of this phase and its effects in FGT/h-BN heterostructures using magneto-transport measurements. The devices' anomalous Hall and magnetoresistance responses exhibit a complex trend with temperature that is consistent with multiple magnetic phases. In addition to the usual out-of-plane sensing, we also rotate the applied field to the in-plane direction and observe behavior resembling the planar topological Hall effect. Intriguingly, this response follows a similar temperature trend to the out-of-plane response. We also use the out-of-plane anomalous Hall response to show that, at sufficiently low temperatures, both positive and negative field-cooling results in an increased saturation Hall resistance. Such a field-cooling divergence is consistent with antiferromagnetic ordering resulting in a spin-glass like state in the sample. In addition to providing insight into one of the most exciting candidate materials for 2D magnetic devices, our work demonstrates the power of magneto-transport measurements to probe complex behavior in vdW magnets where common magnetometry techniques used on bulk samples may not be viable.

Stacking and tuning effects on magneto-electronic and electric contact features for arsenene/Fe3GeTe2 van der Waals heterostructure

Recently, experimentally available Fe3GeTe2 (FGT) monolayer has attracted tremendous research interest due to its long-range ferromagnetic (FM) order. Here, we systematically study the magneto-electronic and electric contact properties of FGT-based van der Waals (vdW) heterostructures integrated by arsenene (As) with multiple stacking patterns. The low binding energy (−117.69 to −52.69 meV atom−1) proves their highly geometric stability, and the high magnetized energy (91.61–213.61 meV/unit cell) and magnetic exchange energy (64.53–84.43 meV/unit cell) provide a strong evidence for magnetism stability. Particularly, the magnetic proximity exchange effect as well as spin transfer and spin rearrangement can enhance the FM coupling in heterostructures, so that the highest Curie temperature T C = 173.93 K is achieved, being 19.73% higher than isolated FGT. Meanwhile, the arsenene is magnetized as well. In particular, metal-induced gap states appear in band gap of original As monolayer. Besides, metal-semiconductor contact performance is improved in constructed heterostructures by lowering electronic Schottky barrier height to nearly obtain Ohmic contact. Magnetic exchange energy rises further to improve magnetic stability with the interlayer spacing narrowed, and T C is increased up to 184.80 K, an increase of 27.21% compared to FGT monolayer. Being attributed to the proximity exchange effect strengthened with a shrunk interlayer spacing, and the electric contact behavior is also boosted further by this compressive strain to realize high-performance electric junction. Our findings provide a new route to extend the applications of FGT by constructing suitable vdW heterostructures.

Emergent, Non-Aging, Extendable, and Rechargeable Exchange Bias in 2D Fe3 GeTe2 Homostructures Induced by Moderate Pressuring.

As a crucial concept in magnetism and spintronics, exchange bias (ExB) measures the asymmetry in the hysteresis loop of a pinned ferromagnet (FM)/antiferromagnet (AFM) interface. Previous studies are mainly focused on FM/AFM heterostructures composed of conventional bulk materials, whose complex interfaces prohibit precise control and full understanding of the phenomenon. Here, the enabling power of 2D magnets is exploited to demonstrate the emergence, non-aging, extendability, and rechargeability of ExB in van der Waals Fe3 GeTe2 homostructures, upon moderate pressuring. The emergence of the ExB is attributed to a local stress-induced FM-to-AFM transition, as validated using first-principles calculations, and confirmed in magneto-optical Kerr effect and second harmonic generation measurements. It is also observed that, negligible ExB aging before the training effect suddenly takes place through avalanching, pronounced delay of the avalanche via timed pressure repetition (extendability), ExB recovery in the post-training sample upon refreshed pressuring (rechargeability), and demonstrate its versatile tunability. These striking findings offer unprecedented insights into the underlying principles of ExB and its training, with immense technological applications in sight.

Anisotropic laser-pulse-induced magnetization dynamics in van der Waals magnet Fe3GeTe2

Femtosecond laser-pulse excitation provides an energy efficient and fast way to control magnetization at the nanoscale, providing great potential for ultrafast next-generation data manipulation and nonvolatile storage devices. Ferromagnetic van der Waals materials have garnered much attention over the past few years due to their low dimensionality, excellent magnetic properties, and large response to external stimuli. Nonetheless, their behaviour upon fs laser-pulse excitation remains largely unexplored. Here, we investigate the ultrafast magnetization dynamics of a thin flake of Fe3GeTe2 (FGT) and extract its intrinsic magnetic properties using a microscopic framework. We find that our data is well described by our modeling, with FGT undergoing a slow two-step demagnetization, and we experimentally extract the spin-relaxation timescale as a function of temperature, magnetic field and excitation fluence. Our observations indicate a large spin-flip probability in agreement with a theoretically expected large spin–orbit coupling, as well as a weak interlayer exchange coupling. The spin-flip probability is found to increase when the magnetization is pulled away from its quantization axis, opening doors to an external control over the spins in this material. Our results provide a deeper understanding of the dynamics van der Waals materials upon fs laser-pulse excitation, paving the way towards two-dimensional materials-based ultrafast spintronics.

Spin filtering effect in all-van der Waals heterostructures with WSe2 barriers

Exploiting the spin degree of freedom to store and manipulate information provides a paradigm for future microelectronics. The development of van der Waals (vdW) heterostructures has created a fascinating platform for exploring spintronic properties in the two-dimensional (2D) limit. Transition-metal dichalcogenides such as tungsten diselenide (WSe2) have electronic band structures that are ideal for hosting many exotic spin–orbit phenomena. Here, we report the spin-filtering effect in all-vdW heterostructures with WSe2 barrier. Combining 2D-perpendicular magnetic anisotropy Fe3GeTe2 (FGT) with different thicknesses of WSe2, the FGT/WSe2/FGT spin valve shows distinct charge and spin transport behavior. Moreover, the negative magnetoresistance (−4.3%) could be inverted into positive magnetoresistance (up to +25.8%) with decreasing the WSe2 thickness. Furthermore, we proposed a spin-filtering model based on Δ-symmetry electrons tunneling to explain the crossover from negative to positive MR signal through ab initio calculation. These experimental and theoretical results illustrate the rich potential of the families of TMDC materials to control spin currents in 2D spintronic devices.

Tuning the density of zero-field skyrmions and imaging the spin configuration in a two-dimensional Fe3GeTe2 magnet

With the advent of ferromagnetism, two-dimensional (2D) van der Waals (vdW) magnets have attracted particular attention in exploring topological spin textures, such as skyrmions used for next-generation spintronic devices. The discovery of magnetic skyrmions in Fe3GeTe2 (FGT) has sparked interest in investigating the spin configurations of skyrmions in FGT. Here, we used an in situ Lorentz microscope to directly demonstrate the generation and sustainability of Bloch-type skyrmions in a zero magnetic field over a wide temperature range in 2D vdW FGT. By tuning the value of the external magnetic field, the highest-density hexagonal skyrmion lattice emerges after reducing the magnetic field to zero. Moreover, by tilting the FGT nanosheet, we found that the field-free Bloch-type skyrmions in FGT can also represent an invisible contrast when the tilt angle is zero, but a reversed magnetic contrast emerges at a high tilt angle. On the basis of our experiments, we discuss the possible mechanisms for such variable magnetic contrast. These findings offer valuable insights into the spin configurations of skyrmions in 2D vdW FGT and shed light on the identification of spin configurations via Lorentz microscopy.

Second-Harmonic Response in Magnetic Nodal-Line Semimetal Fe3GeTe2

We experimentally investigate second-harmonic transverse voltage response to ac electrical current for a magnetic nodal-line semimetal Fe3GeTe2 (FGT). For zero magnetic field, the observed second-harmonic voltage behaves as a square of the longitudinal current, as it should be expected for nonlinear Hall effect. The magnetic field behavior is found to be sophisticated: while the first-harmonic response shows the known anomalous Hall hysteresis in FGT, the second-harmonic Hall voltage is characterized by the pronounced high-field hysteresis and flat (B-independent) region with curves touching at low fields. The high-field hysteresis strongly depends on the magnetic field sweep rate, so it reflects some slow relaxation process. For the lowest rates, it is also accomplished by multiple crossing points. Similar shape of the second-harmonic hysteresis is known for skyrmion spin textures in nonlinear optics. Since skyrmions have been demonstrated for FGT by direct visualization techniques, we can connect the observed high-field relaxation with deformation of the skyrmion lattice. Thus, the second-harmonic Hall voltage response can be regarded as a tool to detect spin textures in transport experiments.

Nonlocal Manipulation of Magnetism in an Itinerant Two-Dimensional Ferromagnet.

Two-dimensional (2D) magnets are crucial in the construction of 2D magnetic and spintronic devices. Many devices, including spin valves and multiple tunneling junctions, have been developed by vertically stacking 2D magnets with other functional blocks. However, owing to limited local interactions at the interfaces, the device structures are typically extremely complex. To solve this problem, the nonlocal manipulation of magnetism may be a good solution. In this study, we use the magneto-optical Kerr effect technique to demonstrate the nonlocal manipulation of magnetism in an itinerant 2D ferromagnet, Fe3GeTe2 (FGT), whose magnetism can be manipulated via an antiferromagnet/ferromagnet interface or a current-induced spin-orbital torque placed distant from the local site. It is discovered that the coupling of a small piece of MnPS3 (∼40 μm2) with FGT can significantly enhance the coercive field and emergence of exchange bias in the entire FGT flake (∼2000 μm2). Moreover, FGT flakes with different thicknesses have the same coercive field at low temperatures if they are coupled together. Our study provides an understanding of the basic magnetism of 2D itinerant ferromagnets as well as opportunities for engineering magnetism with an additional degree of freedom.

Spin Valve Effect Based on Out‐Of‐Plane Magnetization Electrodes of Pt/Co/Ru with WSe2 and InSe Barriers

Spintronics research has been of emerging interest in the inorganic and organic electronics field, and magnetic materials play an important role in spintronics. Here, the spin valve (SV) devices are reported based on few‐layered van der Waals semiconducting WSe2 or InSe sandwiched between room‐temperature ferromagnet Pt/Co/Ru and exfoliated layered ferromagnetic metal Fe3GeTe2 (FGT). The metallic interface rather than the Schottky barrier is formed despite the semiconducting nature of WSe2 and InSe, which may be originated from the interface hybridization with Pt/Co/Ru and FGT. The magnetoresistance of both devices persists up to the Curie temperature of FGT, and the magnitude of the magnetoresistance does not vary with the applied current, which indicates these devices have stable device performance. The work reveals that easily fabricated Pt/Co/Ru multilayers could be stable 3D perpendicular magnetocrystalline anisotropy (PMA) spin injection electrodes applied in SV devices, facilitated by combining 2D magnets, which is expected to be an attractive candidate for 2D applications.