Magnetic van der Waals materials have mainly been investigated in their bulk form or as few-layer flakes. Because of the challenges in producing atomically thin films, only a few have been isolated as monolayers, which typically exhibit long-range magnetic order below 150 K. In this Letter, we use molecular beam epitaxy to synthesize Co-doped Fe_{5}GeTe_{2}, achieving precise control over both thickness and composition. We demonstrate ferromagnetism well above room temperature in multilayer samples and present clear evidence of ferromagnetic ordering in monolayers up to ∼200  K. The changes in Curie temperature and magnetic anisotropy as a function of composition exhibit similar trends in both monolayers and thicker films, indicating that the magnetic properties are primarily governed by intralayer magnetic interactions. Through element-specific x-ray magnetic circular dichroism and density-functional theory, we identify the substitution site of Co dopants and reveal the mechanism behind the Curie temperature enhancement induced by Co doping. Our findings suggest that, despite their weak magnetic moment, Co dopants strengthen the magnetic moments on neighboring Fe atoms and enhance the intralayer ferromagnetic exchange interactions.

Two-dimensional transition metal dichalcogenide (TMD) interfaces offer a versatile platform for studying quantum phenomena and developing device functionalities. When distinct TMD monolayers are stacked vertically or laterally stitched, their interfaces can exhibit unique electronic band alignments, giving rise to long-lived interlayer excitons, charge transfer effects, and moiré superlattices with correlated states. Here, we demonstrate that the interface between a large variety of two different epitaxially grown TMD monolayers controls the intensity and sign of the Rashba spin splitting, which is probed using THz spintronic emission. Optimized TMD heterobilayers, such as HfSe2/PtSe2, show enhanced THz emission that surpasses the spin-to-charge conversion efficiency of bulk TMDs, confirming the presence of Rashba states with large spin splitting at the interface. By combining spin- and angle-resolved photoemission spectroscopy with density functional theory, we reveal that the electronic hybridization between the two different TMD monolayers gives rise to extended in-gap states with strong Rashba spin-orbit coupling. The choice of TMD layers enables to engineer the sign and strength of spin-to-charge conversion in van der Waals heterobilayers, enabling to build efficient and tunable THz spintronic emitters.

Magnetic van der Waals materials have mainly been investigated in their bulk form or as few-layers flakes. Due to the challenges in producing atomically thin films, only a few have been isolated as monolayers, which typically exhibit long-range magnetic order below 150 K. In this work, we use molecular beam epitaxy to synthesize Co-doped Fe5GeTe2, achieving precise control over both thickness and composition. We demonstrate ferromagnetism well above room temperature in multilayer samples and present clear evidence of ferromagnetic ordering in monolayers up to ∼200 K. The changes in Curie temperature and magnetic anisotropy with composition exhibit similar trends in both monolayers and thicker films, indicating that the magnetic properties are primarily governed by intralayer magnetic interactions. Through element-specific X-ray magnetic circular dichroism and density functional theory, we identify the substitution site of Co dopants and reveal the mechanism behind the Curie temperature enhancement induced by Co doping. Our findings suggest that, despite their weak magnetic moment, Co dopants strengthen the magnetic moments on neighboring Fe atoms and enhance the intralayer ferromagnetic exchange interactions.

Van der Waals heterostructures have promised the realisation of artificial materials with multiple physical phenomena such as giant optical nonlinearities, spin-to-charge interconversion in spintronics and topological carrier protection, in a single layered device through an infinitely diverse set of quantum materials. However, most efforts have only focused on exfoliated material that inherently limits both the dimensions of the materials and the scalability for applications. Here, we show the epitaxial growth of large area heterostructures of topological insulators (Bi2Se3), transition metal dichalcogenides (TMDs, WSe2) and ferromagnets (Co), resulting in the combination of functionalities including tuneable optical nonlinearities, spin-to-charge conversion and magnetic proximity effects. This is demonstrated through coherent phase resolved terahertz currents, bringing novel functionalities beyond those achievable in simple homostructures. In particular, we show the role of different TMD polymorphs, with the simple change of one atomic monolayer of the artificial material stack entirely changing its optical, electrical and magnetic properties. This epitaxial integration of diverse two-dimensional materials offers foundational steps towards diverse perspectives in quantum material engineering, where the material polymorph can be controlled at technological relevant scales for coupling applications in, for example, van der Waals nonlinear optics, optoelectronics, spintronics, multiferroics and coherent current control.

We investigate the effect of confinement on the magnetic state of a 12 nm-thick Fe_5GeTe_2 layer grown by molecular beam epitaxy. We use quantitative scanning NV magnetometry to locally extract the magnetization in rectangular uniformly in-plane magnetized microstructures, showing no enhancement of the Curie temperature compared to magnetization measurements performed before patterning the film, in contrast to previous results obtained on thick Fe_3GeTe_2 flakes. Under the application of a weak out-of-plane magnetic field, we observe the stabilization of magnetic vortices at room temperature in micrometric squares. Finally, we highlight the effect of the size of the patterned micro-discs and micro-squares on the stabilization of the vortices using experiments and micromagnetic simulations. Our work thus proposes and demonstrates a way to stabilize non-collinear textures at room temperature in a van der Waals magnets using confinement, although we also show that this approach alone is not successful to enhance the Curie temperature of Fe_5GeTe_2 significantly above 300 K.

The diversity of 2D materials and their van der Waals (vdW) stacking presents fertile ground for engineering novel multifunctional materials and quantum states of matter. This permits unique opportunities to tailor the electronic properties of vdW heterostructures by the insertion of only a single 2D material layer. However, such vdW materials engineering at the atomic scale has yet to be investigated for spin-charge interconversion phenomena. Here, we report on the control of these effects at the monolayer level, where a drastic increase in intensity and change in sign of THz spintronic emission are demonstrated by inserting a single layer of MoSe2 between PtSe2 and graphene in a fully epitaxial, large area stacked structure. By using a combination of spin and angle resolved photoemission and density functional theory to reveal the electronic and spin structures, we illustrate two different mechanisms relying on charge transfer and electronic hybridization for the formation of Rashba states, which are responsible for spin-charge conversion and hence the THz spintronic emission. These findings open new pathways to design, at the atomic scale, efficient THz spintronic emitters made of 2D materials and other spintronic devices based on spin-charge interconversion phenomena.

van der Waals ferromagnets, such as Fe5GeTe2, offer a promising platform for spintronic devices based on chiral magnetic textures, provided a significant Dzyaloshinskii-Moriya interaction (DMI) can be induced to stabilize the textures. Here, we directly measure DMI in epitaxial Fe5GeTe2 thin films using Brillouin light scattering spectroscopy and observe a consistent DMI (D = 0.04 mJ/m2) across various thicknesses. Its weak thickness dependence, combined with the nominally symmetric film interfaces, suggests a bulk origin. Although we do not determine the microscopic mechanism, our findings are compatible with ab initio calculations linking DMI to partial ordering of Fe split sites. Additionally, we find a low magnetic dissipation (α < 0.02). The observed DMI, which could be further enhanced by optimizing the Fe site ordering, combined with low dissipation, makes Fe5GeTe2 a strong candidate for exploring the dynamics of chiral magnetic textures in two-dimensional materials.

Topological insulators (TIs) hold promise for manipulating the magnetization of a ferromagnet (FM) through the spin-orbit torque (SOT) mechanism. However, integrating TIs with conventional FMs often leads to significant device-to-device variations and a broad distribution of SOT magnitudes. In this work, we present a scalable approach to grow a full van der Waals FM/TI heterostructure by molecular beam epitaxy, combining the charge-compensated TI (Bi,Sb)(2)Te-3 with 2D FM Fe3GeTe2 (FGT). Harmonic magnetotransport measurements reveal that the SOT efficiency exhibits a non-monotonic temperature dependence and experiences a substantial enhancement with a reduction of the FGT thickness to 2 monolayers. Our study further demonstrates that the magnetization of ultrathin FGT films can be switched with a current density of J(c) similar to 10(10) A/m(2), with minimal device-to-device variations compared to previous investigations involving traditional FMs.

2D materials, such as transition metal dichalcogenides, are ideal platforms for spin‐to‐charge conversion (SCC) as they possess strong spin–orbit coupling (SOC), reduced dimensionality and crystal symmetries as well as tuneable band structure, compared to metallic structures. Moreover, SCC can be tuned with the number of layers, electric field, or strain. Here, SCC in epitaxially grown 2D PtSe 2 by THz spintronic emission is studied since its 1T crystal symmetry and strong SOC favor SCC. High quality of as‐grown PtSe 2 layers is demonstrated, followed by in situ ferromagnet deposition by sputtering that leaves the PtSe 2 unaffected, resulting in well‐defined clean interfaces as evidenced with extensive characterization. Through this atomic growth control and using THz spintronic emission, the unique thickness‐dependent electronic structure of PtSe 2 allows the control of SCC. Indeed, the transition from the inverse Rashba–Edelstein effect (IREE) in 1–3 monolayers (ML) to the inverse spin Hall effect (ISHE) in multilayers (>3 ML) of PtSe 2 enabling the extraction of the perpendicular spin diffusion length and relative strength of IREE and ISHE is demonstrated. This band structure flexibility makes PtSe 2 an ideal candidate to explore the underlying mechanisms and engineering of the SCC as well as for the development of tuneable THz spintronic emitters.

Two-dimensional materials-based ultrafast spintronics are expected to surpass conventional data storage and manipulation technologies, that are now reaching their fundamental limits. The newly discovered van der Waals (VdW) magnets provide a new platform for ultrafast spintronics since their magnetic and electrical properties can be tuned by many external factors, such as strain, voltage, magnetic field, or light absorption for instance. Here, we report on the direct relationship between magnetic order and Terahertz (THz) electrodynamics in FenGeTe2 (n = 3, 5) (FGT) films after being illuminated by a femtosecond optical pulse, studying their ultrafast THz response as a function of the optical pump-THz probe temporal delay. In Fe5GeTe2, we find clear evidence that light-induced electronic excitations directly influence THz electrodynamics similarly to a demagnetization process, contrasting with the effects observed in Fe3GeTe2, which are characterized by a thermal energy transfer among electrons, magnons, and phonons. We address these effects as a function of the pump fluence and pump-probe delay, and by tuning the temperature across the magnetic ordering Curie temperature, highlighting the microscopic mechanisms describing the out-of-equilibrium evolution of the THz conductivity. Finally, we find evidence for the incoherent-coherent crossover predicted by the Kondo-Ising scenario in Fe3GeTe2 and successfully simulate its light-driven electrodynamics through a three-temperature model. As indicated by these results, FGT surpasses conventional metals in terms of modulating their properties using an optical lever.

Two-dimensional and van der Waals ferromagnets are ideal platform to study low-dimensional magnetism and proximity effects in van der Waals heterostructures. Their ultimate two-dimensional character also offers the opportunity to easily adjust their magnetic properties using strain or electric fields. Among 2D ferromagnets, the Cr1+xTe2 compounds with x = 0–1 are very promising because their magnetic properties depend on the amount of self-intercalated Cr atoms between pure CrTe2 layers and the Curie temperature (TC) can reach room temperature for certain compositions. Here, we investigate the evolution of the composition as well as the structural and magnetic properties of thin Cr1.33Te2 (Cr2Te3) films epitaxially grown on graphene upon annealing. We observe a transition above 450 °C from the Cr1.33Te2 phase with perpendicular magnetic anisotropy and a TC of 180 K to a composition close to Cr1.39Te2 with in-plane magnetic anisotropy and a TC of 240–250 K. This phase remains stable up to 650 °C above which a pure Cr film starts to form. This work demonstrates the complex interplay between intercalated Cr, lattice parameters, and magnetic properties in Cr1+xTe2 compounds.

Terahertz (THz) Spintronic emitters based on ferromagnetic/metal junctions have become an important technology for the THz range, offering powerful and ultra-large spectral bandwidths. These developments have driven recent investigations of two-dimensional (2D) materials for new THz spintronic concepts. 2D materials, such as transition metal dichalcogenides (TMDs), are ideal platforms for SCC as they possess strong spin-orbit coupling (SOC) and reduced crystal symmetries. Moreover, SCC and the resulting THz emission can be tuned with the number of layers, electric field or strain. Here, epitaxially grown 1T-PtSe$_2$ and sputtered Ferromagnet (FM) heterostructures are presented as a novel THz emitter where the 1T crystal symmetry and strong SOC favor SCC. High quality of as-grown PtSe$_2$ layers is demonstrated and further FM deposition leaves the PtSe$_2$ unaffected, as evidenced with extensive characterization. Through this atomic growth control, the unique thickness dependent electronic structure of PtSe$_2$ allows the control of the THz emission by SCC. Indeed, we demonstrate the transition from the inverse Rashba-Edelstein effect in one monolayer to the inverse spin Hall effect in multilayers. This band structure flexibility makes PtSe$_2$ an ideal candidate as a THz spintronic 2D material and to explore the underlying mechanisms and engineering of the SCC for THz emission.

2D materials, such as transition metal dichalcogenides, are ideal platforms for spin-to-charge conversion (SCC) as they possess strong spin-orbit coupling (SOC), reduced dimensionality and crystal symmetries as well as tuneable band structure, compared to metallic structures. Moreover, SCC can be tuned with the number of layers, electric field, or strain. Here, SCC in epitaxially grown 2D PtSe2 by THz spintronic emission is studied since its 1T crystal symmetry and strong SOC favor SCC. High quality of as-grown PtSe2 layers is demonstrated, followed by in situ ferromagnet deposition by sputtering that leaves the PtSe2 unaffected, resulting in well-defined clean interfaces as evidenced with extensive characterization. Through this atomic growth control and using THz spintronic emission, the unique thickness-dependent electronic structure of PtSe2 allows the control of SCC. Indeed, the transition from the inverse Rashba-Edelstein effect (IREE) in 1-3 monolayers (ML) to the inverse spin Hall effect (ISHE) in multilayers (>3 ML) of PtSe2 enabling the extraction of the perpendicular spin diffusion length and relative strength of IREE and ISHE is demonstrated. This band structure flexibility makes PtSe2 an ideal candidate to explore the underlying mechanisms and engineering of the SCC as well as for the development of tuneable THz spintronic emitters.

L'introduction des systèmes 2D en spintronique a été à l'origine de plusieurs découvertes majeures et de phénomènes originaux. Ce chapitre passe en revue les avancées et applications dans ce domaine, comme l'effet Rashba-Edelstein inverse, le transport de spin dans les matériaux 2D et leur intégration dans les jonctions tunnel magnétiques, les mémoires magnétiques à accès aléatoire et les propriétés de spin des isolants topologiques.

Achieving the large-scale growth of 2D ferromagnetic materials with high Curie temperature and perpendicular magnetic anisotropy is highly desirable for the development of future ultra-compact magnetic sensors or magnetic memories. In this context, van der Waals (vdW) Cr 2 Te 3 appears as a promising candidate. It exhibits strong perpendicular magnetic anisotropy and a Curie temperature in bulk of 180 K. In this work, we will demonstrate the growth of quasi-freestanding few layers of Cr 2 Te 3 on various 2D, which exhibit strong variation in terms of magnetic anisotropy with the crystal structure and exotic magnetotransport properties with of charge transfer from the 2D substrates.

Topological insulators (TIs) are a promising class of materials for manipulating the magnetization of an adjacent ferromagnet (FM) through the spin-orbit torque (SOT) mechanism. However, current studies combining TIs with conventional FMs present large device-to-device variations, resulting in a broad distribution of SOT magnitudes. It has been identified that the interfacial quality between the TI and the FM is of utmost importance in determining the nature and efficiency of the SOT. To optimize the SOT magnitude and enable ultra-low-power magnetization switching, an atomically smooth interface is necessary. To this end, we have developed the growth of a full van der Waals FM/TI heterostructure by molecular beam epitaxy. The compensated TI (Bi0.4Sb0.6)2Te3 and ferromagnetic Fe3GeTe2 (FGT) were chosen because of their exceptional crystalline quality, low carrier concentration in BST and relatively large Curie temperature and perpendicular magnetic anisotropy in FGT. We characterized the magnitude of the SOTs by using thorough harmonic magnetotransport measurements and showed that the magnetization of an ultrathin FGT film could be switched with a current density Jc < 10^10 A/m^2. In comparison to previous studies utilizing traditional FMs, our findings are highly reliable, displaying little to no variation between devices.

Spintronic emitters have become an important THz source with gapless broadband THz emission and the ability to magnetically control the emitted polarization through ultrafast spin-to-charge conversion (SCC). This work has recently driven investigations of two-dimensional (2D) materials for new types of spintronic THz sources. Indeed 2D materials are ideal platforms for spin-to-charge conversion (SCC) as a result of their strong spin-orbit coupling and low crystal symmetries. One of such 2D materials is the transition-metal dichalcogenide PtSe 2 . In this work we present THz spintronic sources based on high quality epitaxially grown CoFeB/PtSe 2 /graphene heterostructures, with PtSe 2 thicknesses ranging from 1 to 15 monolayers. The unique thickness dependent electronic structure of PtSe 2 permits to demonstrate the different origins of the THz emission - from the inverse Rashba-Edelstein effect in monolayer PtSe 2 to the inverse spin Hall effect for multilayers through the strength of the THz emission. This unique bandstructure flexibility makes PtSe 2 an ideal candidate as a THz spintronic 2D material and to further study and explore the underlying mechanisms and engineering of the SCC for THz emission.

Achieving large-scale growth of two-dimensional (2D) ferromagnetic materials with high Curie temperature T-C and perpendicular magnetic anisotropy (PMA) is highly desirable for the development of ultracompact magnetic sensors and magnetic memories. In this context, van der Waals (vdW) Cr2Te3 appears to be a promising candidate. Bulk Cr2Te3 exhibits strong PMA and a T-C of 180 K. Moreover, both PMA and T-C might be adjusted in ultrathin films by engineering composition or strain or applying an electric field. In this work, we demonstrate the molecular beam epitaxy (MBE) growth of vdW heterostructures of five-monolayer quasifreestanding Cr2Te3 on three classes of 2D materials: graphene (semimetal), WSe2 (semiconductor), and Bi2Te3 (topological insulator). By combining structural and chemical analysis down to the atomic level with ab initio calculations, we confirm the single-crystalline character of Cr2Te3 films on the 2D materials with sharp vdW interfaces. They all exhibit PMA and T-C close to the bulk Cr2Te3 value of 180 K. Ab initio calculations confirm this PMA and show how its strength depends on strain. Finally, Hall measurements reveal a strong anomalous Hall effect, which changes sign at a given temperature. We theoretically explain this effect by a sign change of the Berry phase close to the Fermi level. This transition temperature depends on the 2D material in proximity, notably as a consequence of charge transfer. MBE-grown Cr2Te3/2D material bilayers constitute model systems for the further development of spintronic devices combining PMA, large spin-orbit coupling, and sharp vdW interface.

We present a study by Scanning Tunneling Microscopy, supported by ab initio calculations, of the interaction between graphene and monolayer (semiconducting) PtSe$_2$ as a function of the twist angle ${\theta}$ between the two layers. We analyze the PtSe$_2$ contribution to the hybrid interface states that develop within the bandgap of the semiconductor to probe the interaction. The experimental data indicate that the interlayer coupling increases markedly with the value of ${\theta}$, which is confirmed by ab initio calculations. The moir\'e patterns observed within the gap are consistent with a momentum conservation rule between hybridized states, and the strength of the hybridization can be qualitatively described by a perturbative model.

Achieving large-scale growth of two-dimensional (2D) ferromagnetic materials with high Curie temperature (TC) and perpendicular magnetic anisotropy (PMA) is highly desirable for the development of ultra-compact magnetic sensors and magnetic memories. In this context, van der Waals (vdW) Cr2Te3 appears as a promising candidate. Bulk Cr2Te3 exhibits strong PMA and a TC of 180 K. Moreover, both PMA and TC might be adjusted in ultrathin films by engineering composition, strain, or applying an electric field. In this work, we demonstrate the molecular beam epitaxy (MBE) growth of vdW heterostructures of five-monolayer quasi-freestanding Cr2Te3 on three classes of 2D materials: graphene (semimetal), WSe2 (semiconductor) and Bi2Te3 (topological insulator). By combining structural and chemical analysis down to the atomic level with ab initio calculations, we confirm the single crystalline character of Cr2Te3 films on the 2D materials with sharp vdW interfaces. They all exhibit PMA and TC close to the bulk Cr2Te3 value of 180 K. Ab initio calculations confirm this PMA and show how its strength depends on strain. Finally, Hall measurements reveal a strong anomalous Hall effect, which changes sign at a given temperature. We theoretically explain this effect by a sign change of the Berry phase close to the Fermi level. This transition temperature depends on the 2D material in proximity, notably as a consequence of charge transfer. MBE-grown Cr2Te3/2D material bilayers constitute model systems for the further development of spintronic devices combining PMA, large spin-orbit coupling and sharp vdW interface.