Ferromagnetism Near Room Temperature in the Cleavable van der Waals Crystal Fe5GeTe2

Emergent flat band and topological Kondo semimetal driven by orbital-selective correlations

Flat electronic bands are expected to show proportionally enhanced electron correlations, which may generate a plethora of novel quantum phases and unusual low-energy excitations. They are increasingly being pursued in d-electron-based systems with crystalline lattices that feature destructive electronic interference, where they are often topological. Such flat bands, though, are generically located far away from the Fermi energy, which limits their capacity to partake in the low-energy physics. Here we show that electron correlations produce emergent flat bands that are pinned to the Fermi energy. We demonstrate this effect within a Hubbard model, in the regime described by Wannier orbitals where an effective Kondo description arises through orbital-selective Mott correlations. Moreover, the correlation effect cooperates with symmetry constraints to produce a topological Kondo semimetal. Our results motivate a novel design principle for Weyl Kondo semimetals in a new setting, viz. d-electron-based materials on suitable crystal lattices, and uncover interconnections among seemingly disparate systems that may inspire fresh understandings and realizations of correlated topological effects in quantum materials and beyond.

On the Origin of the Above-Room-Temperature Magnetism in the 2D van der Waals Ferromagnet Fe3GaTe2

2D magnetic materials have attracted growing interest driven by their unique properties and potential applications. However, the scarcity of systems exhibiting magnetism at room temperature has limited their practical implementation into functional devices. Here we focus on the van der Waals ferromagnet Fe3GaTe2, which exhibits above-room-temperature magnetism (Tc = 350-380 K) and strong perpendicular anisotropy. Through first-principles calculations, we examine the magnetic properties of Fe3GaTe2 and compare them with those of Fe3GeTe2. Our calculations unveil the microscopic mechanisms governing their magnetic behavior, emphasizing the pivotal role of ferromagnetic in-plane couplings in the stabilization of the elevated Tc in Fe3GaTe2. Additionally, we predict the stability, substantial perpendicular anisotropy, and high Tc of the single-layer Fe3GaTe2. We also demonstrate the potential of strain engineering and electrostatic doping to modulate its magnetic properties. Our results incentivize the isolation of the monolayer and pave the way for the future optimization of Fe3GaTe2 in magnetic and spintronic nanodevices.

Highly Efficient Room-Temperature Spin-Orbit-Torque Switching in a Van der Waals Heterostructure of Topological Insulator and Ferromagnet

All-Van der Waals (vdW)-material-based heterostructures with atomically sharp interfaces offer a versatile platform for high-performing spintronic functionalities at room temperature. One of the key components is vdW topological insulators (TIs), which can produce a strong spin-orbit-torque (SOT) through the spin-momentum locking of their topological surface state (TSS). However, the relatively low conductance of the TSS introduces a current leakage problem through the bulk states of the TI or the adjacent ferromagnetic metal layers, reducing the interfacial charge-to-spin conversion efficiency (qICS). Here, a vdW heterostructure is used consisting of atomically-thin layers of a bulk-insulating TI Sn-doped Bi1.1Sb0.9Te2S1 and a room-temperature ferromagnet Fe3GaTe2, to enhance the relative current ratio on the TSS up to ≈20%. The resulting qICS reaches ≈1.65 nm-1 and the critical current density Jc ≈0.9 × 106 Acm-2 at 300 K, surpassing the performance of TI-based and heavy-metal-based SOT devices. These findings demonstrate that an all-vdW heterostructure with thickness optimization offers a promising platform for efficient current-controlled magnetization switching at room temperature.

Highly Efficient Room-Temperature Nonvolatile Magnetic Switching by Current in Fe3GaTe2 Thin Flakes

Effectively tuning magnetic state by using current is essential for novel spintronic devices. Magnetic van der Waals (vdW) materials have shown superior properties for the applications of magnetic information storage based on the efficient spin torque effect. However, for most of known vdW ferromagnets, the ferromagnetic transition temperatures lower than room temperature strongly impede their applications and the room-temperature vdW spintronic device with low energy consumption is still a long-sought goal. Here, the highly efficient room-temperature nonvolatile magnetic switching is realized by current in a single-material device based on vdW ferromagnet Fe3GaTe2. Moreover, the switching current density and power dissipation are about 300 and 60000 times smaller than conventional spin-orbit-torque devices of magnet/heavy-metal heterostructures. These findings make an important progress on the applications of magnetic vdW materials in the fields of spintronics and magnetic information storage.

Critical behaviors of van der Waals itinerant ferromagnet Fe3.8GaTe2

The critical behavior of the van der Waals ferromagnet Fe3.8GaTe2was systematically studied through measurements of isothermal magnetization, with the magnetic field applied along thec-axis. Fe3.8GaTe2undergoes a non-continuous paramagnetic to ferromagnetic phase transition at the Curie temperatureTc∼ 355 K. A comprehensive analysis of isotherms aroundTcutilizing the modified Arrott diagram, the Kouvel-Fisher method, the Widom scaling law, and the critical isotherm analysis yielded the critical exponent ofβ= 0.411,γ= 1.246, andδ= 3.99. These critical exponents are found to be self-consistent and align well with the scaling equation at high magnetic fields, underscoring the reliability and intrinsic nature of these parameters. However, the low-field data deviates from the scaling relation, exhibiting a vertical trend whenT Collapse

Key Words

• Fe3.8GaTe2
• critical behaviors
• critical exponent
• isothermal magnetization
• three-dimensional Heisenberg

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Affiliation(s)

Tianyang Yao School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People's Republic of China
W L Qubie School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People's Republic of China
Pushpendra Kumar School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India
Xu Bai School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People's Republic of China
Shixin Hu School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People's Republic of China
Desheng Xue School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People's Republic of China
Junli Zhang School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People's Republic of China

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Above-room-temperature chiral skyrmion lattice and Dzyaloshinskii-Moriya interaction in a van der Waals ferromagnet Fe3-xGaTe2

Skyrmions in existing 2D van der Waals (vdW) materials have primarily been limited to cryogenic temperatures, and the underlying physical mechanism of the Dzyaloshinskii-Moriya interaction (DMI), a crucial ingredient for stabilizing chiral skyrmions, remains inadequately explored. Here, we report the observation of Néel-type skyrmions in a vdW ferromagnet Fe3-xGaTe2 above room temperature. Contrary to previous assumptions of centrosymmetry in Fe3-xGaTe2, the atomic-resolution scanning transmission electron microscopy reveals that the off-centered FeΙΙ atoms break the spatial inversion symmetry, rendering it a polar metal. First-principles calculations further elucidate that the DMI primarily stems from the Te sublayers through the Fert-Lévy mechanism. Remarkably, the chiral skyrmion lattice in Fe3-xGaTe2 can persist up to 330 K at zero magnetic field, demonstrating superior thermal stability compared to other known skyrmion vdW magnets. This work provides valuable insights into skyrmionics and presents promising prospects for 2D material-based skyrmion devices operating beyond room temperature.

Bulk ferromagnetism in cleavable van der Waals telluride NbFeTe2

A van der Waals telluride, NbFeTe2, has been synthesized using chemical vapor transport reactions. The optimized synthetic conditions yield high-quality single crystals with a novel monoclinic crystal structure. Monoclinic NbFeTe2 demonstrates a (100) cleavage plane, bulk ferromagnetism below 87 K, and a metallic ground state-the necessary prerequisites for needed spintronics technologies.

Imaging the Magnetic Anisotropy in Ultrathin Fe4GeTe2 with a Nitrogen-Vacancy Magnetometer

Two-dimensional (2D) FenGeTe2, with n = 3, 4, and 5, has been realized in experiments, showing strong magnetic anisotropy with enhanced critical temperature (Tc). The understanding of its magnetic anisotropy is crucial for the exploration of more stable 2D magnets and its spintronic applications. Here, we report a quantitative reconstruction of the magnetization magnitude and its direction in ultrathin Fe4GeTe2 using nitrogen vacancy centers. Through imaging stray magnetic fields, we identified the spin-flop transition at approximately 80 K, resulting in a change of the easy axis from the out-of-plane direction to the in-plane direction. Moreover, by analyzing the thermally activated escape behavior of the magnetization near Tc in terms of the Ginzburg-Landau model, we observed the in-plane magnetic anisotropy effect and the formation capability of magnetic domains at ∼0.4 μm2 μT-1. Our findings contribute to the quantitative understanding of the magnetic anisotropy effect in a vast range of 2D van der Waals magnets.

Superstructure of Fe5-xGeTe2 Determined by Te K-Edge Extended X-ray Absorption Fine Structure and Te Kα X-ray Fluorescence Holography

The local structure of the two-dimensional van der Waals material, Fe5-xGeTe2, which exhibits unique structural/magnetic phase transitions, was investigated by Te K-edge extended X-ray absorption fine structure (EXAFS) and Te Kα X-ray fluorescence holography (XFH) over a wide temperature range. The formation of a trimer of Te atoms at low temperatures has been fully explored using these methods. An increase in the Te-Fe distance at approximately 150 K was suggested by EXAFS and presumably indicates the formation of a Te trimer. Moreover, XFH displayed clear atomic images of Te atoms. Additionally, the distance between the Te atoms shortened, as confirmed from the atomic images reconstructed from XFH, indicating the formation of a trimer of Te atoms, i.e., a charge-ordered superstructure. Furthermore, Te Kα XFH provided unambiguous atomic images of Fe atoms occupying the Fe1 site; the images were not clearly observed in the Ge Kα XFH that was previously reported because of the low occupancy of Fe and Ge atoms. In this study, EXAFS and XFH clearly showed the local structure around the Te atom; in particular, the formation of Te trimers caused by charge-ordered phase transitions was clearly confirmed. The charge-ordered phase transition is fully discussed based on the structural variation at low temperatures, as established from EXAFS and XFH.

Direct Observation of Room-Temperature Magnetic Skyrmion Motion Driven by Ultra-Low Current Density in Van Der Waals Ferromagnets

The recent discovery of room-temperature ferromagnetism in 2D van der Waals (vdW) materials, such as Fe3GaTe2 (FGaT), has garnered significant interest in offering a robust platform for 2D spintronic applications. Various fundamental operations essential for the realization of 2D spintronics devices are experimentally confirmed using these materials at room temperature, such as current-induced magnetization switching or tunneling magnetoresistance. Nevertheless, the potential applications of magnetic skyrmions in FGaT systems at room temperature remain unexplored. In this work, the current-induced generation of magnetic skyrmions in FGaT flakes employing high-resolution magnetic transmission soft X-ray microscopy is introduced, supported by a feasible mechanism based on thermal effects. Furthermore, direct observation of the current-induced magnetic skyrmion motion at room temperature in FGaT flakes is presented with ultra-low threshold current density. This work highlights the potential of FGaT as a foundation for room-temperature-operating 2D skyrmion device applications.

Room‐Temperature Antisymmetric Magnetoresistance in van der Waals Ferromagnet Fe3GaTe2 Nanosheets

Van der Waals (vdW) ferromagnetic materials have emerged as a promising platform for the development of 2D spintronic devices. However, studies to date are restricted to vdW ferromagnetic materials with low Curie temperature (Tc) and small magnetic anisotropy. Here, a chemical vapor transport method is developed to synthesize a high-quality room-temperature ferromagnet, Fe3GaTe2 (c-Fe3GaTe2), which boasts a high Tc = 356 K and large perpendicular magnetic anisotropy. Due to the planar symmetry breaking, an unconventional room-temperature antisymmetric magnetoresistance (MR) is first observed in c-Fe3GaTe2 devices with step features, manifesting as three distinctive states of high, intermediate, and low resistance with the sweeping magnetic field. Moreover, the modulation of the antisymmetric MR is demonstrated by controlling the height of the surface steps. This work provides new routes to achieve magnetic random storage and logic devices by utilizing the room-temperature thickness-controlled antisymmetric MR and further design room-temperature 2D spintronic devices based on the vdW ferromagnet c-Fe3GaTe2.

Layer-Number-Dependent Magnetism in the Co-Doped van der Waals Ferromagnet Fe3GaTe2

Recently, van der Waals (vdW) antiferromagnets have been proposed to be crucial for spintronics due to their favorable properties compared to ferromagnets, including robustness against magnetic perturbation and high frequencies of spin dynamics. High-performance and energy-efficient spin functionalities often depend on the current-driven manipulation and detection of spin states, highlighting the significance of two-dimensional metallic antiferromagnets, which have not been much explored due to the lack of suitable materials. Here, we report a new metallic vdW antiferromagnet obtained from the ferromagnet Fe3GaTe2 by cobalt (Co) doping. Through the layer-number-dependent Hall resistance and magnetoresistance measurements, an evident odd-even layer-number effect has been observed in its few-layered flakes, suggesting that it could host an A-type antiferromagnetic structure. This peculiar layer-number-dependent magnetism in Co-doped Fe3GaTe2 helps unravel the complex magnetic structures in such doped vdW magnets, and our finding will enrich material candidates and spin functionalities for spintronic applications.

Efficient Thermo-Spin Conversion in van der Waals Ferromagnet FeGaTe

Recent discovery of 2D van der Waals magnetic materials has spurred progress in developing advanced spintronic devices. A central challenge lies in enhancing the spin-conversion efficiency for building spin-logic or spin-memory devices. Here, the anomalous Hall and Nernst effects are systematically investigated to uncover significant spin-conversion effects in above-room-temperature van der Waals ferromagnet FeGaTe with perpendicular magnetic anisotropy. The anomalous Hall effect demonstrates an efficient electric spin-charge conversion with a notable spin Hall angle of over 6%. In addition, the anomalous Nernst effect produces a significant transverse voltage at room temperature without a magnetic field, displaying unique temperature dependence with a maximum transverse Seebeck coefficient of 440 nV K-1 and a Nernst angle of ≈62%. Such an innovative thermoelectric signal arises from the efficient thermo-spin conversion effect, where the up-spin and down-spin electrons move in opposite directions under a temperature gradient. The present study highlights the potential of FeGaTe to enhance thermoelectric devices through efficient thermo-spin conversion without the need for a magnetic field.

Reversible non-volatile electronic switching in a near-room-temperature van der Waals ferromagnet

Non-volatile phase-change memory devices utilize local heating to toggle between crystalline and amorphous states with distinct electrical properties. Expanding on this kind of switching to two topologically distinct phases requires controlled non-volatile switching between two crystalline phases with distinct symmetries. Here, we report the observation of reversible and non-volatile switching between two stable and closely related crystal structures, with remarkably distinct electronic structures, in the near-room-temperature van der Waals ferromagnet Fe5-δGeTe2. We show that the switching is enabled by the ordering and disordering of Fe site vacancies that results in distinct crystalline symmetries of the two phases, which can be controlled by a thermal annealing and quenching method. The two phases are distinguished by the presence of topological nodal lines due to the preserved global inversion symmetry in the site-disordered phase, flat bands resulting from quantum destructive interference on a bipartite lattice, and broken inversion symmetry in the site-ordered phase.

Prediction of intrinsic room-temperature ferromagnetism in two-dimensional CrInX2 (X = S, Se, Te) monolayers

Herein, using density functional theory, novel two-dimensional (2D) CrInX2 (X = S, Se, Te) structures are predicted to be practical ferromagnetic (FM) semiconductors. Phonon vibrations and molecular dynamics simulations verified their structural and thermodynamic stability. Sizable fully spin-polarized band gaps of 1.03 and 0.69 eV are found for CrInS2 and CrInSe2, while CrInTe2 exhibits half-metallic band nature (at 0 K with a perfect lattice). The high magnetic anisotropy energies are responsible for their long-range spin polarization. The Curie temperatures (Tc) are estimated to be 347, 397 and 447 K for CrInS2, CrInSe2 and CrInTe2, respectively, all well above the room-temperature. The high Tc originates from unusual FM direct exchange, the efficient super-exchange coupling between neighboring Cr eg-orbitals with zero virtual exchange gaps and the presence of dual Cr-X-Cr super-exchange channels. Our systematic study of the CrInX2 monolayer suggests that it could be a promising material for spintronics applications.

Room-Temperature, Current-Induced Magnetization Self-Switching in A Van Der Waals Ferromagnet

2D layered materials with broken inversion symmetry are being extensively pursued as  spin source layers to realize high-efficiency magnetic switching. Such low-symmetry layered systems are, however, scarce. In addition, most layered magnets with perpendicular magnetic anisotropy show a low Curie temperature. Here, the experimental observation of spin-orbit torque magnetization self-switching at room temperature in a layered polar ferromagnetic metal, Fe2.5 Co2.5 GeTe2 is reported. The spin-orbit torque is generated from the broken inversion symmetry along the c-axis of the crystal. These results provide a direct pathway toward applicable 2D spintronic devices.

Current-induced switching of a van der Waals ferromagnet at room temperature

Recent discovery of emergent magnetism in van der Waals magnetic materials (vdWMM) has broadened the material space for developing spintronic devices for energy-efficient computation. While there has been appreciable progress in vdWMM discovery, a solution for non-volatile, deterministic switching of vdWMMs at room temperature has been missing, limiting the prospects of their adoption into commercial spintronic devices. Here, we report the first demonstration of current-controlled non-volatile, deterministic magnetization switching in a vdW magnetic material at room temperature. We have achieved spin-orbit torque (SOT) switching of the PMA vdW ferromagnet Fe3GaTe2 using a Pt spin-Hall layer up to 320 K, with a threshold switching current density as low as [Formula: see text]1.69 [Formula: see text] 106 A cm-2 at room temperature. We have also quantitatively estimated the anti-damping-like SOT efficiency of our Fe3GaTe2/Pt bilayer system to be [Formula: see text], using the second harmonic Hall voltage measurement technique. These results mark a crucial step in making vdW magnetic materials a viable choice for the development of scalable, energy-efficient spintronic devices.

Strong In-Plane Magnetization and Spin Polarization in (Co0.15Fe0.85)5GeTe2/Graphene van der Waals Heterostructure Spin-Valve at Room Temperature

Van der Waals (vdW) magnets are promising, because of their tunable magnetic properties with doping or alloy composition, where the strength of magnetic interactions, their symmetry, and magnetic anisotropy can be tuned according to the desired application. However, so far, most of the vdW magnet-based spintronic devices have been limited to cryogenic temperatures with magnetic anisotropies favoring out-of-plane or canted orientation of the magnetization. Here, we report beyond room-temperature lateral spin-valve devices with strong in-plane magnetization and spin polarization of the vdW ferromagnet (Co0.15Fe0.85)5GeTe2 (CFGT) in heterostructures with graphene. Density functional theory (DFT) calculations show that the magnitude of the anisotropy depends on the Co concentration and is caused by the substitution of Co in the outermost Fe layer. Magnetization measurements reveal the above room-temperature ferromagnetism in CFGT and clear remanence at room temperature. Heterostructures consisting of CFGT nanolayers and graphene were used to experimentally realize basic building blocks for spin valve devices, such as efficient spin injection and detection. Further analysis of spin transport and Hanle spin precession measurements reveals a strong in-plane magnetization with negative spin polarization at the interface with graphene, which is supported by the calculated spin-polarized density of states of CFGT. The in-plane magnetization of CFGT at room temperature proves its usefulness in graphene lateral spin-valve devices, thus revealing its potential application in spintronic technologies.

Writing and Detecting Topological Charges in Exfoliated Fe5-xGeTe2

Fe5-xGeTe2 is a promising two-dimensional (2D) van der Waals (vdW) magnet for practical applications, given its magnetic properties. These include Curie temperatures above room temperature, and topological spin textures─TST (both merons and skyrmions), responsible for a pronounced anomalous Hall effect (AHE) and its topological counterpart (THE), which can be harvested for spintronics. Here, we show that both the AHE and THE can be amplified considerably by just adjusting the thickness of exfoliated Fe5-xGeTe2, with THE becoming observable even in zero magnetic field due to a field-induced unbalance in topological charges. Using a complementary suite of techniques, including electronic transport, Lorentz transmission electron microscopy, and micromagnetic simulations, we reveal the emergence of substantial coercive fields upon exfoliation, which are absent in the bulk, implying thickness-dependent magnetic interactions that affect the TST. We detected a "magic" thickness t ≈ 30 nm where the formation of TST is maximized, inducing large magnitudes for the topological charge density (∼6.45 × 1020 cm-2), and the concomitant anomalous (ρxyA,max ≃22.6 μΩ cm) and topological (ρxyu,T 1≃5 μΩ cm) Hall resistivities at T ≈ 120 K. These values for ρxyA,max and ρxyu,T are higher than those found in magnetic topological insulators and, so far, the largest reported for 2D magnets. The hitherto unobserved THE under zero magnetic field could provide a platform for the writing and electrical detection of TST aiming at energy-efficient devices based on vdW ferromagnets.

Thickness-Tunable Zoology of Magnetic Spin Textures Observed in Fe5GeTe2

The family of two-dimensional (2D) van der Waals (vdW) materials provides a playground for tuning structural and magnetic interactions to create a wide variety of spin textures. Of particular interest is the ferromagnetic compound Fe5GeTe2 that we show displays a range of complex spin textures as well as complex crystal structures. Here, using a high-brailliance laboratory X-ray source, we show that the majority (1 × 1) Fe5GeTe2 (FGT5) phase exhibits a structure that was previously considered as being centrosymmetric but rather lacks inversion symmetry. In addition, FGT5 exhibits a minority phase that exhibits a long-range ordered (√3 × √3)-R30° superstructure. This superstructure is highly interesting in that it is innately 2D without any lattice periodicity perpendicular to the vdW layers, and furthermore, the superstructure is a result of ordered Te vacancies in one of the topmost layers of the FGT5 sheets rather than being a result of vertical Fe ordering as earlier suggested. We show, from direct real-space magnetic imaging, evidence for three distinct magnetic ground states in lamellae of FGT5 that are stabilized with increasing lamella thickness, namely, a multidomain state, a stripe phase, and an unusual fractal state. In the stripe phase we also observe unconventional type-I and type-II bubbles where the spin texture in the central region of the bubbles is nonuniform, unlike conventional bubbles. In addition, we find a bobber or a cocoon-like spin texture in thick (∼170 μm) FGT5 that emerges from the fractal state in the presence of a magnetic field. Among all the 2D vdW magnets we have thus demonstrated that FGT5 hosts perhaps the richest variety of magnetic phases that, thereby, make it a highly interesting platform for the subtle tuning of magnetic interactions.

Spin-Orbit Torques and Magnetization Switching in (Bi,Sb)2Te3/Fe3GeTe2 Heterostructures Grown by Molecular Beam Epitaxy

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)2Te3 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 Jc ∼ 1010 A/m2, with minimal device-to-device variations compared to previous investigations involving traditional FMs.

Near-room temperature ferromagnetism and a tunable anomalous Hall effect in atomically thin Fe4CoGeTe2

Itinerant ferromagnetism at room temperature is a key factor for spin transport and manipulation. Here, we report the realization of near-room temperature itinerant ferromagnetism in Co doped Fe5GeTe2 thin flakes. The ferromagnetic transition temperature TC (∼323 K-337 K) is almost unchanged when the thickness is as low as 12 nm and is still about 284 K at 2 nm (bilayer thickness). Theoretical calculations further indicate that the ferromagnetism persists in monolayer Fe4CoGeTe2. In addition to the robust ferromagnetism down to the ultrathin limit, Fe4CoGeTe2 exhibits an unusual temperature- and thickness-dependent intrinsic anomalous Hall effect. We propose that it could be ascribed to the dependence of the band structure on thickness that changes the Berry curvature near the Fermi energy level subtly. The near-room temperature ferromagnetism and tunable anomalous Hall effect in atomically thin Fe4CoGeTe2 provide opportunities to understand the exotic transport properties of two-dimensional van der Waals magnetic materials and explore their potential applications in spintronics.

Observation of the Superstructure in Fe5-xGeTe2 by X-ray Fluorescence Holography

Fe5-xGeTe2 is a two-dimensional van der Waals material that exhibits ferromagnetic order with a high Curie temperature (TC) of around room temperature. In addition to TC, two magnetic transitions occur with decreasing temperature, and a charge-ordered state is observed at low temperatures. We employed Ge Kα X-ray fluorescence holography (XFH) for Fe5-xGeTe2 to directly investigate the local structure in the charge-ordered state, i.e., the 3 × 3 superstructure. The Ge Kα XFH results revealed local atomic structures around the Ge atom, thus clarifying the simultaneous locations and arrangements of the Te, Fe, and Ge atoms. The atomic positions relative to the Ge atom are useful for understanding the coexistence of the ideal 1 × 1 structure and 3 × 3 superstructure found in the charge-ordered state.

Electronic Structure of Above-Room-Temperature van der Waals Ferromagnet Fe3GaTe2

Fe3GaTe2, a recently discovered van der Waals ferromagnet, demonstrates intrinsic ferromagnetism above room temperature, necessitating a comprehensive investigation of the microscopic origins of its high Curie temperature (TC). In this study, we reveal the electronic structure of Fe3GaTe2 in its ferromagnetic ground state using angle-resolved photoemission spectroscopy and density functional theory calculations. Our results establish a consistent correspondence between the measured band structure and theoretical calculations, underscoring the significant contributions of the Heisenberg exchange interaction (Jex) and magnetic anisotropy energy to the development of the high-TC ferromagnetic ordering in Fe3GaTe2. Intriguingly, we observe substantial modifications to these crucial driving factors through doping, which we attribute to alterations in multiple spin-splitting bands near the Fermi level. These findings provide valuable insights into the underlying electronic structure and its correlation with the emergence of high-TC ferromagnetic ordering in Fe3GaTe2.

Nonreciprocal Antisymmetric Magnetoresistance and Unconventional Hall Effect in a Two-Dimensional Ferromagnet

Two-dimensional (2D) ferromagnets with high Curie temperatures provide a rich platform for exploring the exotic phenomena of 2D magnetism and the potential of spintronic devices. As a prototypical 2D ferromagnet, Fe5-xGeTe2 has recently been reported to possess a high Curie temperature with Tc ∼ 310 K, making it a promising candidate for advancing 2D nanoelectromechanical systems. However, due to its intricate magnetic ground state and magnetic domains, a thorough study of the transport behavior related to its lattice and domain structures is still lacking. Here, we report a nonreciprocal antisymmetric magnetoresistance in Fe5-xGeTe2 nanoflakes observed under an external magnetic field between 85-120 K. Through a detailed examination of its temperature, field orientation, and sample thickness dependence, we trace its origin to an additional electric field induced by the domain structure. This differs from the previously reported antisymmetric magnetoresistance due to thickness inhomogeneity. Notably, at lower temperatures, we observed an unconventional Hall effect (UHE), which can be attributed to the Dzyaloshinskii-Moriya interaction (DMI) resulting from the non-coplanar magnetic moment structure. The pronounced influence of sample thickness on magneto-transport properties underscores the competition between magnetic anisotropy and DMI in Fe5-xGeTe2 flakes with varying thicknesses. Our findings provide a deeper understanding of the magneto-transport behavior of the exotic magnetic structure in 2D ferromagnetic materials, which may benefit future spintronic device applications.

Direct Observation of Propagating Spin Waves in the 2D van der Waals Ferromagnet Fe5GeTe2

Magnetism in reduced dimensionalities is of great fundamental interest while also providing perspectives for applications of materials with novel functionalities. In particular, spin dynamics in two dimensions (2D) have become a focus of recent research. Here, we report the observation of coherent propagating spin-wave dynamics in a ∼30 nm thick flake of 2D van der Waals ferromagnet Fe5GeTe2 using X-ray microscopy. Both phase and amplitude information were obtained by direct imaging below TC for frequencies from 2.77 to 3.84 GHz, and the corresponding spin-wave wavelengths were measured to be between 1.5 and 0.5 μm. Thus, parts of the magnonic dispersion relation were determined despite a relatively high magnetic damping of the material. Numerically solving an analytic multilayer model allowed us to corroborate the experimental dispersion relation and predict the influence of changes in the saturation magnetization or interlayer coupling, which could be exploited in future applications by temperature control or stacking of 2D-heterostructures.

Lightning the Spin: Harnessing the Potential of 2D Magnets in Opto-Spintronics

Since the emergence of 2D magnets in 2017, the diversity of these materials has greatly expanded. Their 2D nature (atomic-scale thickness) endows these magnets with strong magnetic anisotropy, layer-dependent and switchable magnetic order, and quantum-confined quasiparticles, which distinguish them from conventional 3D magnetic materials. Moreover, the 2D geometry facilitates light incidence for opto-spintronic applications and potential on-chip integration. In analogy to optoelectronics based on optical-electronic interactions, opto-spintronics use light-spin interactions to process spin information stored in the solid state. In this review, opto-spintronics is divided into three types with respect to the wavelengths of radiation interacting with 2D magnets: 1) GHz (microwave) to THz (mid-infrared), 2) visible, and 3) UV to X-rays. It is focused on the recent research advancements on the newly discovered mechanisms of light-spin interactions in 2D magnets and introduces the potential design of novel opto-spintronic applications based on these interactions.

Progress and Prospects in Metallic FexGeTe2 (3 ≤ x ≤ 7) Ferromagnets

Thermal fluctuations in two-dimensional (2D) isotropy systems at non-zero finite temperatures can destroy the long-range (LR) magnetic order due to the mechanisms addressed in the Mermin-Wanger theory. However, the magnetic anisotropy related to spin-orbit coupling (SOC) may stabilize magnetic order in 2D systems. Very recently, 2D FexGeTe2 (3 ≤ x ≤ 7) with a high Curie temperature (TC) has not only undergone significant developments in terms of synthetic methods and the control of ferromagnetism (FM), but is also being actively explored for applications in various devices. In this review, we introduce six experimental methods, ten ferromagnetic modulation strategies, and four spintronic devices for 2D FexGeTe2 materials. In summary, we outline the challenges and potential research directions in this field.

Ferroic Phases in Two-Dimensional Materials

Two-dimensional (2D) ferroics, namely ferroelectric, ferromagnetic, and ferroelastic materials, are attracting rising interest due to their fascinating physical properties and promising functional applications. A variety of 2D ferroic phases, as well as 2D multiferroics and the novel 2D ferrovalleytronics/ferrotoroidics, have been recently predicted by theory, even down to the single atomic layers. Meanwhile, some of them have already been experimentally verified. In addition to the intrinsic 2D ferroics, appropriate stacking, doping, and defects can also artificially regulate the ferroic phases of 2D materials. Correspondingly, ferroic ordering in 2D materials exhibits enormous potential for future high density memory devices, energy conversion devices, and sensing devices, among other applications. In this paper, the recent research progresses on 2D ferroic phases are comprehensively reviewed, with emphasis on chemistry and structural origin of the ferroic properties. In addition, the promising applications of the 2D ferroics for information storage, optoelectronics, and sensing are also briefly discussed. Finally, we envisioned a few possible pathways for the future 2D ferroics research and development. This comprehensive overview on the 2D ferroic phases can provide an atlas for this field and facilitate further exploration of the intriguing new materials and physical phenomena, which will generate tremendous impact on future functional materials and devices.

Recent advances in 2D van der Waals magnets: Detection, modulation, and applications

The emergence of two-dimensional (2D) van der Waals magnets provides an exciting platform for exploring magnetism in the monolayer limit. Exotic quantum phenomena and significant potential for spintronic applications are demonstrated in 2D magnetic crystals and heterostructures, which offer unprecedented possibilities in advanced formation technology with low power and high efficiency. In this review, we summarize recent advances in 2D van der Waals magnetic crystals. We focus mainly on van der Waals materials of truly 2D nature with intrinsic magnetism. The detection methods of 2D magnetic materials are first introduced in detail. Subsequently, the effective strategies to modulate the magnetic behavior of 2D magnets (e.g., Curie temperature, magnetic anisotropy, magnetic exchange interaction) are presented. Then, we list the applications of 2D magnets in the spintronic devices. We also highlight current challenges and broad space for the development of 2D magnets in further studies.

Imaging Nucleation and Propagation of Pinned Domains in Few-Layer Fe5-xGeTe2

Engineering nontrivial spin textures in magnetic van der Waals materials is highly desirable for spintronic applications based on hybrid heterostructures. The recent observation of labyrinth and bubble domains in the near room-temperature ferromagnet Fe5-xGeTe2 down to a bilayer thickness was thus a significant advancement toward van der Waals-based many-body physics. However, the physical mechanism responsible for stabilizing these domains remains unclear and requires further investigation. Here, we combine cryogenic scanning diamond quantum magnetometry and field reversal techniques to elucidate the high-field propagation and nucleation of bubble domains in trilayer Fe5-xGeTe2. We provide evidence of pinning-induced nucleation of magnetic bubbles and further show an unexpectedly high layer-dependent coercive field. These measurements can be easily extended to a wide range of magnetic materials to provide valuable nanoscale insight into domain processes critical for spintronic applications.

Large-Area Synthesis of Ferromagnetic Fe5- x GeTe2 /Graphene van der Waals Heterostructures with Curie Temperature above Room Temperature

Van der Waals (vdW) heterostructures combining layered ferromagnets and other 2D crystals are promising building blocks for the realization of ultracompact devices with integrated magnetic, electronic, and optical functionalities. Their implementation in various technologies depends strongly on the development of a bottom-up scalable synthesis approach allowing for realizing highly uniform heterostructures with well-defined interfaces between different 2D-layered materials. It is also required that each material component of the heterostructure remains functional, which ideally includes ferromagnetic order above room temperature for 2D ferromagnets. Here, it is demonstrated that the large-area growth of Fe5- x GeTe2 /graphene heterostructures is achieved by vdW epitaxy of Fe5- x GeTe2 on epitaxial graphene. Structural characterization confirms the realization of a continuous vdW heterostructure film with a sharp interface between Fe5- x GeTe2 and graphene. Magnetic and transport studies reveal that the ferromagnetic order persists well above 300 K with a perpendicular magnetic anisotropy. In addition, epitaxial graphene on SiC(0001) continues to exhibit a high electronic quality. These results represent an important advance beyond nonscalable flake exfoliation and stacking methods, thus marking a crucial step toward the implementation of ferromagnetic 2D materials in practical applications.

Strain Engineering of Intrinsic Ferromagnetism in 2D van der Waals Materials

Since the discovery of the low-temperature, long-range ferromagnetic order in monolayers Cr2Ge2Te6 and CrI3, many efforts have been made to achieve a room temperature (RT) ferromagnet. The outstanding deformation ability of two-dimensional (2D) materials provides an exciting way to mediate their intrinsic ferromagnetism (FM) with strain engineering. Here, we summarize the recent progress of strain engineering of intrinsic FM in 2D van der Waals materials. First, we introduce how to explain the strain-mediated intrinsic FM on Cr-based and Fe-based 2D van der Waals materials through ab initio Density functional theory (DFT), and how to calculate magnetic anisotropy energy (MAE) and Curie temperature (TC) from the interlayer exchange coupling J. Subsequently, we focus on numerous attempts to apply strain to 2D materials in experiments, including wrinkle-induced strain, flexible substrate bending or stretching, lattice mismatch, electrostatic force and field-cooling. Last, we emphasize that this field is still in early stages, and there are many challenges that need to be overcome. More importantly, strengthening the guideline of strain-mediated FM in 2D van der Waals materials will promote the development of spintronics and straintronics.

2D Ferroic Materials for Nonvolatile Memory Applications

The emerging nonvolatile memory technologies based on ferroic materials are promising for producing high-speed, low-power, and high-density memory in the field of integrated circuits. Long-range ferroic orders observed in 2D materials have triggered extensive research interest in 2D magnets, 2D ferroelectrics, 2D multiferroics, and their device applications. Devices based on 2D ferroic materials and heterostructures with an atomically smooth interface and ultrathin thickness have exhibited impressive properties and significant potential for developing advanced nonvolatile memory. In this context, a systematic review of emergent 2D ferroic materials is conducted here, emphasizing their recent research on nonvolatile memory applications, with a view to proposing brighter prospects for 2D magnetic materials, 2D ferroelectric materials, 2D multiferroic materials, and their relevant devices.

Giant coercivity enhancement in a room-temperature van der Waals magnet through substitutional metal-doping

Fe_xGeTe₂ (x = 3, 4, and 5) systems, two-dimensional (2D) van der Waals (vdW) ferromagnetic (FM) metals with high Curie temperatures (T_C), have been intensively studied to realize all-2D spintronic devices. Recently, an intrinsic FM material Fe₃GaTe₂ with high T_C (350-380 K) has been reported. As substitutional doping changes the magnetic properties of vdW magnets, it can be a powerful means for engineering the properties of magnetic materials. Here, the coercive field (H_c) is substantially enhanced by substituting Ni for Fe in (Fe_1-xNi_x)₃GaTe₂ crystals. The introduction of a Ni dopant with x = 0.03 can enhance the value of H_c up to ∼200% while maintaining the FM state at room temperature. As the doping level increases, T_C decreases, whereas H_c increases up to 7 kOe at x = 0.12, which is the highest H_c reported so far. The FM characteristic is almost suppressed at x = 0.68 and a spin glass state appears. The enhancement of H_c resulting from Ni doping can be attributed to domain pinning induced by substitutional Ni atoms, as evidenced by the decrease in magnetic anisotropy energy in the crystals upon Ni doping. Our findings provide a highly effective way to control the H_c of the 2D vdW FM metal Fe₃GaTe₂ for the realization of Fe₃GaTe₂ based room-temperature operating spintronic devices.

Ordering of room-temperature magnetic skyrmions in a polar van der Waals magnet

Control and understanding of ensembles of skyrmions is important for realization of future technologies. In particular, the order-disorder transition associated with the 2D lattice of magnetic skyrmions can have significant implications for transport and other dynamic functionalities. To date, skyrmion ensembles have been primarily studied in bulk crystals, or as isolated skyrmions in thin film devices. Here, we investigate the condensation of the skyrmion phase at room temperature and zero field in a polar, van der Waals magnet. We demonstrate that we can engineer an ordered skyrmion crystal through structural confinement on the μm scale, showing control over this order-disorder transition on scales relevant for device applications.

Manipulation and Optical Detection of Artificial Topological Phenomena in 2D Van der Waals Fe5 GeTe2 /MnPS3 Heterostructures

2D ferromagnet is a good platform to investigate topological effects and spintronic devices owing to its rich spin structures and excellent external-field tunability. The appearance of the topological Hall Effect (THE) is often regarded as an important sign of the generation of chiral spin textures, like magnetic vortexes or skyrmions. Here, interface engineering and an in-plane current are used to modulate the magnetic properties of the nearly room-temperature 2D ferromagnet Fe₅ GeTe₂ . An artificial topology phenomenon is observed in the Fe₅ GeTe₂ /MnPS₃ heterostructure by using both anomalous Hall Effect and reflective magnetic circular dichroism (RMCD) measurements. Through tuning the applied current and the RMCD laser wavelength, the amplitude of the humps and dips observed in the hysteresis loops can be modulated accordingly. Magnetic field-dependent hysteresis loops demonstrate that the observed artificial topological phenomena are induced by the generation and annihilation of the magnetic domains. This work provides an optical method for investigating the topological-like effects in magnetic structures and proposes an effective way to modulate the magnetic properties of magnetic materials, which is important for developing magnetic and spintronic devices in van der Waals magnetic materials.

Interfacial engineering of ferromagnetism in wafer-scale van der Waals Fe4GeTe2 far above room temperature

Despite recent advances in exfoliated vdW ferromagnets, the widespread application of 2D magnetism requires a Curie temperature (T_c) above room temperature as well as a stable and controllable magnetic anisotropy. Here we demonstrate a large-scale iron-based vdW material Fe₄GeTe₂ with the T_c reaching ~530 K. We confirmed the high-temperature ferromagnetism by multiple characterizations. Theoretical calculations suggested that the interface-induced right shift of the localized states for unpaired Fe d electrons is the reason for the enhanced T_c, which was confirmed by ultraviolet photoelectron spectroscopy. Moreover, by precisely tailoring Fe concentration we achieved arbitrary control of magnetic anisotropy between out-of-plane and in-plane without inducing any phase disorders. Our finding sheds light on the high potential of Fe₄GeTe₂ in spintronics, which may open opportunities for room-temperature application of all-vdW spintronic devices.

Coexistence of Merons with Skyrmions in the Centrosymmetric Van Der Waals Ferromagnet Fe5- x GeTe2

Fe_{5- x} GeTe₂ is a centrosymmetric, layered van der Waals (vdW) ferromagnet that displays Curie temperatures T_c (270-330 K) that are within the useful range for spintronic applications. However, little is known about the interplay between its topological spin textures (e.g., merons, skyrmions) with technologically relevant transport properties such as the topological Hall effect (THE) or topological thermal transport. Here, via high-resolution Lorentz transmission electron microscopy, it is shown that merons and anti-meron pairs coexist with Néel skyrmions in Fe_{5- x} GeTe₂ over a wide range of temperatures and probe their effects on thermal and electrical transport. A THE is detected, even at room T, that senses merons at higher T's, as well as their coexistence with skyrmions as T is lowered, indicating an on-demand thermally driven formation of either type of spin texture. Remarkably, an unconventional THE is also observed in absence of Lorentz force, and it is attributed to the interaction between charge carriers and magnetic field-induced chiral spin textures. These results expose Fe_5-x GeTe₂ as a promising candidate for the development of applications in skyrmionics/meronics due to the interplay between distinct but coexisting topological magnetic textures and unconventional transport of charge/heat carriers.

A Room-Temperature Spin-Valve with van der Waals Ferromagnet Fe5 GeTe2 /Graphene Heterostructure

The discovery of van der Waals (vdW) magnets opened a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW ferromagnets are limited to cryogenic temperatures, inhibiting their broader practical applications. Here, the robust room-temperature operation of lateral spin-valve devices using the vdW itinerant ferromagnet Fe₅ GeTe₂ in heterostructures with graphene is demonstrated. The room-temperature spintronic properties of Fe₅ GeTe₂ are measured at the interface with graphene with a negative spin polarization. Lateral spin-valve and spin-precession measurements provide unique insights by probing the Fe₅ GeTe₂ /graphene interface spintronic properties via spin-dynamics measurements, revealing multidirectional spin polarization. Density functional theory calculations in conjunction with Monte Carlo simulations reveal significantly canted Fe magnetic moments in Fe₅ GeTe₂ along with the presence of negative spin polarization at the Fe₅ GeTe₂ /graphene interface. These findings open opportunities for vdW interface design and applications of vdW-magnet-based spintronic devices at ambient temperatures.

Theoretical studies on electronic, magnetic and optical properties of two dimensional transition metal trihalides

Two dimensional transition metal trihalides have drawn attention over the years due to their intrinsic ferromagnetism and associated large anisotropy at nanoscale. The interactions involved in these layered structures are of van der Waals types which are important for exfoliation to different thin samples. This enables one to compare the journey of physical properties from bulk structures to monolayer counterpart. In this topical review, the modulation of electronic, magnetic and optical properties by strain engineering, alloying, doping, defect engineering etc have been discussed extensively. The results obtained by first principle density functional theory calculations are verified by recent experimental observations. The relevant experimental synthesis of different morphological transition metal trihalides are highlighted. The feasibility of such routes may indicate other possible heterostructures. Apart from spintronics based applications, transition metal trihalides are potential candidates in sensing and data storage. Moreover, high thermoelectric figure of merit of chromium trihalides at higher temperatures leads to the possibility of multi-purpose applications. We hope this review will give important directions to further research in transition metal trihalide systems having tunable band gap with reduced dimensionalities.

A general thermodynamics-triggered competitive growth model to guide the synthesis of two-dimensional nonlayered materials

Two-dimensional (2D) nonlayered materials have recently provoked a surge of interest due to their abundant species and attractive properties with promising applications in catalysis, nanoelectronics, and spintronics. However, their 2D anisotropic growth still faces considerable challenges and lacks systematic theoretical guidance. Here, we propose a general thermodynamics-triggered competitive growth (TTCG) model providing a multivariate quantitative criterion to predict and guide 2D nonlayered materials growth. Based on this model, we design a universal hydrate-assisted chemical vapor deposition strategy for the controllable synthesis of various 2D nonlayered transition metal oxides. Four unique phases of iron oxides with distinct topological structures have also been selectively grown. More importantly, ultra-thin oxides display high-temperature magnetic ordering and large coercivity. Mn_xFe_yCo_3-x-yO₄ alloy is also demonstrated to be a promising room-temperature magnetic semiconductor. Our work sheds light on the synthesis of 2D nonlayered materials and promotes their application for room-temperature spintronic devices.

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.

Large tunneling magnetoresistance in van der Waals magnetic tunnel junctions based on FeCl2 films with interlayer antiferromagnetic couplings

Antiferromagnets (AFMs) are some of the most promising candidates for next-generation magnetic memory technology owing to their advantages over conventional ferromagnets (FMs), such as zero stray field and THz-range magnetic resonance frequency. Motivated by the recent synthesis of FeCl₂ films with interlayer AFM and intralayer FM couplings, we investigated the magnetic properties of few-layer FeCl₂ and the spin-dependent transmissions of graphite/bilayer FeCl₂/graphite and Au/n-layer FeCl₂/Au magnetic tunnel junctions (MTJs) using first-principles calculations combined with the nonequilibrium Green's function. The interlayer AFM coupling of FeCl₂ is certified to be stable and independent of the stacking orders and relative displacement between layers. Furthermore, based on the Au electrode with better conductive performance than the graphite electrode and monolayer 1T-FeCl₂ with complete spin polarization, high Curie temperature and large magnetic anisotropic energy, a high tunnel magnetoresistance (TMR) ratio of 2.7 × 10³% is achieved in Au/bilayer FeCl₂/Au MTJs at zero bias and it increases with different layers of FeCl₂ (n = 2-10). These excellent spin transport properties of Au/n-layer FeCl₂/Au MTJs based on two-dimensional (2D) AFM barriers with out-of-plane magnetization directions suggest their great potential for application in high-reliability, high-speed and high-density spintronic devices.

Significant enhancement of ferromagnetism above room temperature in epitaxial 2D van der Waals ferromagnet Fe5-δGeTe2/Bi2Te3 heterostructures

Two-dimensional (2D) van der Waals (vdW) ferromagnetic metals Fe_xGeTe₂ with x = 3-5 have raised significant interest in the scientific community. Fe₅GeTe₂ shows prospects for spintronic applications since the Curie temperature T_c has been reported near or higher than 300 K. In the present work, epitaxial Fe_5-δGeTe₂ (FGT) heterostructures were grown by Molecular Beam Epitaxy (MBE) on insulating crystalline substrates. The FGT films were combined with Bi₂Te₃ topological insulator (TI) aiming to investigate the possible beneficial effect of the TI on the magnetic properties of FGT. FGT/Bi₂Te₃ films were compared to FGT capped only with AlO_x to prevent oxidation. SQUID and MOKE measurements revealed that the growth of Bi₂Te₃ TI on FGT films significantly enhances the saturation magnetization of FGT as well as the T_c well above room temperature (RT) reaching record values of 570 K. First-principles calculations predict a shift of the Fermi level and an associated enhancement of the majority spin (primarily) as well as the total density of states at the Fermi level suggesting that effective doping of FGT from Bi₂Te₃ could explain the enhancement of ferromagnetism in FGT. It is also predicted that strain induced stabilization of a high magnetic moment phase in FGT/Bi₂Te₃ could be an alternative explanation of magnetization and T_c enhancement. Ferromagnetic resonance measurements evidence an enhanced broadening in the FGT/Bi₂Te₃ heterostructure when compared to FGT. We obtain a large spin mixing conductance of g↑↓eff = 4.4 × 10²⁰ m^-2, which demonstrates the great potential of FGT/Bi₂Te₃ systems for spin-charge conversion applications at room temperature.

TMB12: a newly designed 2D transition-metal boride for spintronics and electrochemical catalyst applications

Exploring two-dimensional (2D) ferromagnetic materials with a high transition temperature and large magnetic anisotropy is extremely essential for highly efficient spintronic applications. With the density functional theory method, we predicted planar hypercoordinate transition-metal borides, TMB₁₂ (TM = Ti, V, Cr, Mn, Fe; B = boron), by the condensation of TM@B₈ and B₄ units. The results showed that these transition-metal borides possess superior thermal, dynamic and mechanical stabilities. Interestingly, the TMB₁₂ monolayer with TM = (V, Cr) is confirmed as a robust ferromagnetic metal with a high Curie temperature of ∼335 K and ∼221 K, respectively. In addition, the system with TM = (Mn, Fe) is found to be an antiferromagnetic metal with a Néel temperature of ∼173 K and ∼91 K, respectively. In particular, large perpendicular magnetic anisotropy is identified for CrB₁₂, MnB₁₂, and FeB₁₂ monolayers, around 198-623 μeV. Furthermore, four TMB₁₂ (TM = Ti, V, Cr, Mn) systems are determined to be candidate catalysts for the hydrogen evolution reaction, with nearly zero free energy of hydrogen adsorption (ΔG_H = -0.0003 to -0.03 eV). Our study highlighted potential 2D metal borides for spintronic devices and high efficiency electrochemical catalysts.

Above-Room-Temperature Ferromagnetism in Thin van der Waals Flakes of Cobalt-Substituted Fe5GeTe2

Two-dimensional (2D) magnetic van der Waals materials provide a powerful platform for studying the fundamental physics of low-dimensional magnetism, engineering novel magnetic phases, and enabling thin and highly tunable spintronic devices. To realize high-quality and practical devices for such applications, there is a critical need for robust 2D magnets with ordering temperatures above room temperature that can be created via exfoliation. Here, the study of exfoliated flakes of cobalt-substituted Fe₅GeTe₂ (CFGT) exhibiting magnetism above room temperature is reported. Via quantum magnetic imaging with nitrogen-vacancy centers in diamond, ferromagnetism at room temperature was observed in CFGT flakes as thin as 16 nm corresponding to 16 layers. This result expands the portfolio of thin room-temperature 2D magnet flakes exfoliated from robust single crystals that reach a thickness regime relevant to practical spintronic applications. The Curie temperature T_c of CFGT ranges from 310 K in the thinnest flake studied to 328 K in the bulk. To investigate the prospect of high-temperature monolayer ferromagnetism, Monte Carlo calculations were performed, which predicted a high value of T_c of ∼270 K in CFGT monolayers. Pathways toward further enhancing monolayer T_c are discussed. These results support CFGT as a promising platform for realizing high-quality room-temperature 2D magnet devices.

Layer-Number-Dependent Magnetism and Anomalous Hall Effect in van der Waals Ferromagnet Fe5GeTe2

Realization of ferromagnetism in the two-dimensional (2D) van der Waals (vdW) crystals opens up a vital route to understand the magnetic ordering in the 2D limit and to design novel spintronics. Here, we report enriched layer-number-dependent magnetotransport properties in the vdW ferromagnet Fe₅GeTe₂. By studying the magnetoresistance and anomalous Hall effect (AHE) in nanoflakes with thicknesses down to monolayer, we demonstrate that while the bulk crystals exhibit soft ferromagnetism with an in-plane magnetic anisotropy, hard ferromagnetism develops upon thinning, and a perpendicular easy-axis anisotropy is realized in bilayer flakes, which is accompanied by a pronounced enhancement of AHE because of extrinsic mechanisms. For the monolayer flakes, the hard ferromagnetism is replaced by spin-glass-like behavior, in accordance with the localization effect in the 2D limit. Our results highlight the thickness-based tunability of the magnetotransport properties in the atomically thin vdW magnets that promises engineering of high-performance spintronic devices.

Thickness-Tunable Growth of Composition-Controllable Two-Dimensional FexGeTe2

Two-dimensional (2D) magnetic materials provide an ideal platform for investigating novel magnetism and spin behavior in low-dimensional systems while being restricted by the deficiency of accurate bottom-up synthesis. To overcome this difficulty, a facile and universal flux-assisted growth (FAG) method is proposed to synthesize the multicomponent Fe_xGeTe₂ (x = 3-5) with different Fe contents and even alloyed with hetero metal atoms. This one-to-one method ensures the stoichiometry consistency from the Fe_xGeTe₂ and M_yFe_5-yGeTe₂ (M = Co, Ni) bulk crystal precursors to the 2D nanosheets, with controllable composition. Tuning the growth temperatures can provide thickness-tunable products. Changeable magnetic properties of Fe_xGeTe₂ and alloyed Co_yFe_5-yGeTe₂ are substantiated by the superconducting quantum interference device and reflective magnetic circular dichroism. This method generates thickness-tunable high-crystallinity Fe_xGeTe₂ samples without phase separation and exhibits a high tolerance to different substrates and a large temperature window, providing a new avenue to synthesize and explore such multicomponent 2D magnets and even the alloyed ones.

Controllable Topological Magnetic Transformations in the Thickness-Tunable van der Waals Ferromagnet Fe5GeTe2

Recent observations of topological meron textures in two-dimensional (2D) van der Waals (vdW) magnetic materials have attracted considerable research interest for both fundamental physics and spintronic applications. However, manipulating the meron textures and realizing the topological transformations, which allow for exploring emergent electromagnetic behaviors, remain largely unexplored in 2D magnets. In this work, utilizing real-space imaging and micromagnetic simulations, we reveal temperature- and thickness-dependent topological magnetic transformations among domain walls, meron textures, and stripe domain in Fe₅GeTe₂ (FGT) lamellae. The key mechanism of the magnetic transformations can be attributed to the temperature-induced change of exchange stiffness constant within layers and uniaxial magnetic anisotropy, while the magnetic dipole interaction as governed by sample thickness is crucial to affect the critical transformation temperature and stripe period. Our findings provide reliable insights into the origin and manipulation of topological spin textures in 2D vdW ferromagnets.