Metallic FexGeTe2 (3 ≤ x ≤ 7) Ferromagnets: Comparison
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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. 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. 

  • Mermin-Wanger theory
  • Fe stoichiometry
  • strain
  • light control
  • electrical control

1. Introduction

The Mermin-Wanger theory [1][2] asserts that thermal fluctuations occur in 2D isotropy systems at non-zero finite temperatures, which destroy the long-range magnetic order (LRMO). Specifically, exchange interactions alone should not generate magnetic order in 2D systems, and magnetic anisotropy [3][4][5] is also needed to maintain the LRMO. Surprisingly, it was found experimentally that low-temperature long-range ferromagnetic order (LRFO) can exist in the Cr2Ge2Te6 monolayer [4] and CrI3 monolayer [5][6]. Soon after, a vast range of 2D magnetic systems, including metallic (Fe3GeTe2 (FGT) [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]), semiconductors (Cr2Ge2Te6 [4][24][25][26][27][28][29][30][31][32], CrI3 [5][33]), and topological insulators (MnBi2Te4 [34]), were successively implemented to promote the development of spintronics.
Recently, FexGeTe2 (3 ≤ x ≤ 7) has received intense attention as a metallic and high Curie temperature (TC) ferromagnet. Six synthesis methods, including solid-state reaction (SSR) [35][36], chemical vapor transport (CVT) [8][13][37], the flux method [11][21][38][39][40][41][42][43][44][45], exfoliation [14][15][34][46][47][48][49][50], chemical vapor deposition (CVD) [51][52], and molecular beam epitaxy (MBE) [7][53][54][55][56][57][58], have been used to attempt to obtain wafer-scale FexGeTe2 (3 ≤ x ≤ 7) materials with room-temperature ferromagnetism (RTFM). However, the TC of the MBE-prepared FexGeTe2 (3 ≤ x ≤ 7) samples (see Figure 1) ranges from 390 to 530 K, with FGT (TC ≈ 400 K) [56], Fe4GeTe2 (F4GT; TC ≈ 530 K) [59], and Fe5GeTe2 (F5GT; TC ≈ 390 K) [60]. Furthermore, RTFM has also been tuned with ten strategies: Fe stoichiometry [9][39][51][59][61][62][63][64][65], strain engineering [46][48][66][67][68][69][70][71][72][73][74][75], hydrostatic pressure [76][77][78][79][80][81], light control [53][82], electrical control [83][84], proximity effects [56][57][85][86][87][88][89], doping engineering [14][20][38][43][44][62][90][91][92][93][94][95][96][97][98][99][100][101][102][103][104][105][106], intercalation [107][108] or irradiation [109], twisting [110][111], and patterning [16]. So far, twisting (see Figure 1) has only regulated the magnetic order theoretically and has not been achieved experimentally. Moreover, four typical devices have also been fabricated based on FGT, magnetic tunnel junctions (MTJ) [112][113][114][115], tunneling spin valves [18][99][116][117], nonlocal spin valves [118], and spin–orbit torque devices [20][119], in order to enrich their physical properties and develop their spintronic applications.
Molecules 28 07244 g001
Figure 1. Overview of six synthesis methods and ten strategies for ferromagnetic FexGeTe2 (3 ≤ x ≤ 7) materials. Black font represents the synthesis methods and tunable strategies for obtaining the RTFM; white font represents the synthesis methods and strategies that can tune magnetism in experiments; green font represents only theoretically achievable tunable strategies.

2. Crystal Structure of Ferromagnetic FexGeTe2

The Fe3GeTe2 monolayer [46] comprises five atomic layers. Specifically, Te atoms are located in the bottom and top layers, while Fe (I) atoms are located in the second and fourth layers. Notably, the intermediate layer is composed of Fe (II) atoms and Ge atoms. The local magnetic moments of Fe atoms for the FGT monolayer were determined by DFT-LDA (density functional theory–local density approximation) to be 1.723 µB and 1.005 µB, with the out-of-plane direction being its easy axis. They may be related to the several partially occupied d-bands passing through the Fermi level. In addition, the number ratio of Fe3+ to Fe2+ in 2D FexGeTe2 [64] is related to the x value. When the value of x is 3, the ratio of Fe3+/Fe2+ is 2:1. However, when the x value is 5, only Fe3+ is present.

3. Synthesis of Metallic FexGeTe2 with FM

3.1. Solid-State Reaction (SSR)

The solid-state reaction (SSR) is an experimental method for preparing bulk FGT crystals. As early as 2006, Deiseroth et al. [35] successfully prepared FGT crystals with hexagonal plates using SSR, which exhibited novel air stability and black metallic properties. Through magnetic testing, it was found that below 230 K, the crystals exhibited FM. Meanwhile, above 230 K, they exhibited Curie–Weiss paramagnetic behavior. After increased annealing, black Fe3−δGeTe2 (0 < δ < 0.3) polycrystalline powders could be easily obtained with SSR. The lattice parameters increase monotonically with decreasing δ (it represents the degree of iron deficiency in FGT), but, when δ exceeds 0.3, FeTe2 will appear as an impurity phase. Its magnetic phase transition temperature is about 240 K. Furthermore, its saturation behavior slows down in high magnetic fields, which is different from ordinary ferromagnets. In order to obtain large quantities of high-quality FGT single crystals, Li et al. [120] designed a new experimental method of solid-phase sintering followed by recrystallization. The as-grown plate-like sample (~10 g) is a layered single crystal with a smooth and complete surface, and its size can reach up to 8.5 mm. By intercalating sodium into as-grown FGT, Weber et al. [107] raised its TC to 350 K. After intercalation, the sample retained obvious layered features, with edge lengths of a grain size ranging from 10–50 μm.

3.2. Chemical Vapor Transport (CVT)

One main difference from SSR is that CVT often uses iodine [8][13][14][15][17][37][39][61][119] or TeCl4 [12] as the transport agent. However, the samples obtained via SSR and CVT were both bulk single crystals. Previous studies have mainly focused on the magnetic microstructures of quasi-2D FGT. Based on the prediction that FGT monolayer could be mechanically exfoliated [46], soon after, Chu et al. [15] and Zhang et al. [14], respectively, obtained FGT monolayer samples with the assistance of Au film and Al2O3, respectively. Actually, Zhang et al. [121] exfoliated FGT monolayer from the most possible cleaving planes (001), with a thickness of 1.75 nm and a nearest neighbor atomic spacing of 0.338 nm, which was highly consistent with the lattice constant (a = 0.399 nm; c = 1.63 nm) of the FGT crystal. However, thin layer FGT was highly prone to deteriorate in air, and the device fabrication processes needed to be carried out in a glove box [49]. Notably, many novel physics-related effects, such as patterning-induced RFTM [16], gate-tunable FM [14], and layer-dependent FM [15], have been discovered.

3.3. Flux Growth

The flux method [122][123][124] is commonly used to prepare single crystals. For example, Canfield et al. [122][123] grew a wide variety of single-crystal binary or ternary intermetallic compounds from molten flux solutions. However, the thickness and lateral size of the samples could not be accurately controlled, with mechanical exfoliation still required to obtain thinner samples when fabricating FGT devices. Recently, Gong et al. [44][45] proposed a universal flux-assisted growth (FAG) method to synthesize FexGeTe2 and MyFe5−yGeTe2 (M = Co, Ni) nanosheets on various substrates. In addition, the sample thickness and lateral size of FGT could be precisely controlled by the growth temperature or cosolvents. Although the FGT samples with a thickness of 5–10 nm were prepared on various substrates, in order to obtain atomically thin materials (ATMs), a confinement environment must be provided through two substrates. Up to 80 layered and non-layered ATMs [45] have also been successfully synthesized using FAG, which provides a new strategy for preparing wafer-scale 2D materials.

3.4. Exfoliation

3.4.1. Mechanical Exfoliation

Conventional mechanical exfoliation [125][126] can cleave thin FGT flakes onto SiO2/Si substrates, but its thinnest thickness is around 4.8 nm. After depositing Au onto SiO2/Si substrate, thinner samples can be obtained, and the Au substrate improves the yield to grow various thin layers of materials, including graphene [127][128], MoS2 [47][129][130][131][132], WSe2 [47][129][133], Bi2Te3 [129], and FGT [15][47]. Nevertheless, only a small amount of material can be exfoliated to a monolayer, which hinders the development of 2D magnetic materials. Notably, an Al2O3-assisted exfoliation method was also designed to produce monolayer FGT [14] and MnBi2Te4 [34] single crystals. When the sample was thinned from bulk to a monolayer, its TC decreased from 180 K to 20 K.

3.4.2. Liquid-Phase Exfoliation

Although many methods including SSR [107], CVT [14][15][17][19][61][119][121][134], flux [39][43][44][45], and MBE [7][53][54][55][56][57][58][59][60] have been used to prepare 2D FGT, an economical method for the large-scale preparation of few- or single-layer FGT nanoflakes is still lacking. As a typical example, Ma et al. [50] developed three-stage sonication-assisted liquid-phase exfoliation (TS-LPE) to produce large semiconductive FGT nanoflakes. After ball milling, the sample size and thickness are reduced by the milling time), exposing more boundaries. Stirring causes the interlayer spacing to expand, weakening the interlayer force to facilitate detachment and obtain high-integrity nanoflakes. In addition, XRD analysis [135] reflects the evolution of interlayer spacing. The expansion of interlayer spacing causes the FGT unit cell to move away from the equilibrium state in the c-direction, making them unstable and prone to spall. In practice, the oxidation on the surface layer altered the electronic structure of the FGT system, making the FGT sample semiconductive and different from the metallic FGT prepared using other methods.

3.5. Chemical Vapor Deposition (CVD)

So far, researchers have mainly used CVT to prepare 2D magnetic bulk single crystals, which are then exfoliated into atomic layers to prepare devices. However, poor control of the number of layers and a limited sample size have hindered the development of 2D magnets. As a typical example, Liu et al. [51] designed a confined space chemical vapor deposition (CS-CVD) method for preparing 2D FGT or F5GT ferromagnets. They found that the optimal growth temperature was 570–580 °C, with an optimal distance of 10 cm between the Fe/Ge precursor and the Te precursor. When the thickness of the F5GT flakes changed from 4 nm to 1 nm, the TC value decreased by 100K. Very recently, Liu et al. [52] also introduced a general competitive-chemical-reaction-controlled CVD method for producing FGT crystals. The sample was a single layer with a grain size of ~50 μm.

3.6. Molecular Beam Epitaxy (MBE)

Wafer-scale single crystalline FGT thin films were grown on various substrates using the molecular beam epitaxial (MBE) [7][54][55][56][57][136] technique. After heterointegration with the topological insulator Bi2Te3 (Figure 7A), the TC of FGT can be increased to 400 K. This enhancement may be related to the interface exchange coupling. Remarkably, when the thickness of F4GT decreases, its TC is increased from 270 K to 530 K (Figure 7B). For F4GT thin films with a thickness of 10 nm, increasing the dosage of Fe can enhance their TC. Although MBE can alone prepare wafer-scale FexGeTe2 (3 ≤ x ≤ 7) materials with RTFM, this method requires a high vacuum environment, which makes it expensive and limits its industrial applications.

4. Controlling FM in Metallic FexGeTe2

4.1. Fe Stoichiometry

In this research area, the earliest discovery was that the FM in polycrystalline FGT bulk structures [9] was related to the Fe content. The higher the Fe content, the larger the lattice constant of the a-axis and the smaller the lattice constant of the c-axis. Single crystal samples show similar results to the polycrystalline samples. Moreover, the TCand MS decreased with the decrease in Fe content. Subsequently, ferromagnetic F4GT [44][59][61] and F5GT [39][42][44][51][62][137][138][139][140][141][142][143][144] materials were also obtained in experiments. However, most previous reports have focused on FGT materials with a single Fe stoichiometry, and there have been few studies on FexGeTe2 materials using the same experimental method. In addition, theoretical calculations [63] revealed that as the Fe content increased, the interlayer gap gradually increased, and the magnetic anisotropy of its monolayer changed from out-of-plane (FGT) to in-plane (F4GT and F5GT).

4.2. Strain Engineering

Strain engineering is an efficient strategy for modulating the FM of 2D materials [67][68][145]. However, previous theoretical works have focused on applying strain to FGT supercells by changing the lattice constants [46][70][71][73][146] and calculating the exchange coupling, magnetic anisotropy, and magnetic moment of strain through ab initio DFT. Furthermore, the TC could be estimated according to mean field theory (MFT) [10][61][65][147][148][149], random phase approximation (PRA) [147][149], or Monte Carlo (MC) [148][149][150][151] simulation. Recently, Miao et al. [48] and Yan et al. [72] loaded FGT nanoflakes into a three-point-bending experimental setup and applied uniaxial tensile strain to the sample on a polyimide (PI) or polyvinyl alcohol (PVA)/polyethylene terephthalate (PET) flexible polymer substrate by moving the needle.

4.3. Hydrostatic Pressure

Tuning the exchange coupling and magneto-crystalline anisotropy by applying hydrostatic pressure is another commonly used method for regulating 2D magnetism, which has been achieved in Cr2Gr2Te6 [152][153], CrI3 [33][154], and FGT [77][78][79] systems. The ferromagnetic evolution of FGT nanosheets under different pressures can be revealed through in situ magnetic circular dichroism (MCD) spectroscopy. Furthermore, the magnetic hysteresis loop at 30 K exhibited a rectangular shape below 7 GPa, while its loop presented an eight-shaped skewed shape above 7.3 GPa. Moreover, TC increases as the pressure further decreases, which may be related to the strengthening of the exchange interactions.

4.4. Light Control

The continuous modulation of monolayer transition-metal dichalcogenides (TMDs) without intrinsic magnetism, including MoS2 [155], WS2 [155], and WSe2 [156], has been achieved using the optical approach. Recently, Tengdin et al. [82] demonstrated that spin polarization was transferred from Mn sublattices to Co on the Heusler compound Co2MnGe via femtosecond laser pulse, which is closely related to the wave function of electrons before and after being excited by light. The ultrafast spin transfer caused by the instantaneous incident light on the material does not only occur in Co2MnGe, but is also a common feature of many materials. Notably, Xu et al. [53] reported that the magnetic anisotropy energy (MAE) and TC were mediated with a femtosecond laser pulse. The optical doping effect alters the electronic structure of FGT, thereby affecting exchange interactions, TC, and MAE. The TC of FGT was estimated to be ~200 K. Under the excitation of a femtosecond laser, electrons transitioned from an occupied state to an unoccupied state, causing the Fermi level EF to shift downwards and crossing the enhanced density of states (DOS). Furthermore, some clear magnetic hysteresis loops at room temperature (RT) can be observed in FGT samples with different thicknesses, according to Polar-MOKE measurements. The TC of FGT can be increased to above RT through light control, providing many opportunities for the development of spintronic applications for 2D magnets.

4.5. Electrical Control

Previous studies have shown that electric fields modify the magnetism of metal films [157][158][159] and Fe/MgO junctions [160] by influencing the behavior of the electrons. Recently, Wang et al. [83] calculated the effect of the electric field on the magnetic anisotropy of the FGT monolayer. The effect of orbital splitting caused by electron doping on magnetic anisotropy was more pronounced; meanwhile, the influence of hole doping related to orbital occupation was relatively weak. In addition, the change in magnetic anisotropy was more obvious in the single-gate configuration. Additionally, the generation of negative differential conductance (NDC) [84] can also be driven by a local electric field in FGT. Furthermore, the three peaks in the Fe d orbits underwent significant shifts under the electric field. As the electric field was enhanced, the off-plane FM of FGT weakened, resulting in a decrease in MAE. Remarkably, in single-layer FGT, the electric field induces charge transfer in the FGT monolayer in the field direction. Therefore, applying an electric field has become an effective way to mediate 2D FM.

4.6. Proximity Effects

Proximity effects [85][161][162][163][164][165] are another dominant area of the research into 2D materials. For example, by using 2D magnetic materials adjacent to a bulk semiconductor substrate [166] or 2D materials with strong spin–orbit coupling [167], their magnetism can be enhanced. Intriguingly, Zhang et al. [85] fabricated antiferromagnetic FePS3(FPS)/ferromagnetic Fe3GeTe2(FGT) heterostructures and detected the enhancement of TC and HC through proximity coupling effects. Furthermore, FPS/FGT/FPS exhibits a slightly different modulation of HC compared to FPS/FGT, which is related to AFM-FM coupling. Moreover, the long-range magnetic order induced by topology triggered by femtosecond laser pulses [57] could also be maintained at room temperature.

4.7. Doping Engineering

4.7.1. Doping with 3d Transition-Metals

Doping 3d-transition metal atoms is an effective strategy for controlling magnetism [66][69][90][168][169][170][171][172]. Theoretical calculations have shown that almost all 3d-transition metal atoms (except for Co atoms) [97] are more inclined to replace Fe1 atoms. The charge transfer generated by doping atoms weakens the magnetic moment of Fe atoms, while the weakening effect of Fe1 atomic magnetic moment is more significant. However, the magnetism increases after doping with Co atoms, which may be related to the shrinking of the a-axis lattice constant. In experiments, doping 3d-transition metal atoms in bulk single-crystal samples were usually achieved via CVT [93] or self-flux [38]. Doping Ni atoms suppressed the ferromagnetic order, which rapidly decreased with the increase in the doping amount. The TC decreased from 212 K to 50 K, and after reaching 0.44, the magnetic moment remained almost constant. Furthermore, the long-range magnetic order was suppressed and subsequently transformed into a glassy magnetic phase. However, doping Co atoms may cause an increase in HC and the appearance of hard magnetic phases; this is related to the movement of pinned domain walls [8]. Bulk F5GT single crystals were also doped with Co atoms via CVT [95][96]. As the amount of Co used for doping increases, it can drive the evolution of the lattice and of magnetism. However, the nominal doping concentration was slightly different from the measured one, with only a specific concentration being more consistent. Afterward, Co atoms were doped into the lattice, resulting in a slight increase in their interlayer spacing. However, Tian et al. [96] found that doping with 20% Co could increase its TC to 337 K and induce complex magnetic phase transitions at higher Co doping levels. Furthermore, hexagonal 2D CoyFe5−yGeTe2 and NiyFe5−yGeTe2 nanoflakes were prepared via flux-assisted growth [44]. 

4.7.2. Doping with Non-Metallic Atoms

Not only can Fe atoms be substituted with Co or Ni atoms [38][44][93][95][96][97], but doping can also be achieved by replacing Ge atoms with As atoms [92][98]. The doping of As atoms caused a decrease in the a-axis lattice constant and an increase in the c-axis lattice constant, thereby reducing the density of spin states below the Fermi level, resulting in a decrease in TC [92]. Furthermore, its MS decreased linearly with the increase in the doping amount in polycrystalline Fe3−yGe1−xAsxTe2 (0 ≤ x ≤ 0.85). Similarly, the expansion of the F5GT unit cell [98] in the c-axis direction and the contraction in the ab plane was also observed after doping with the As atom. In addition, its TC and MS decreased in polycrystalline Fe5Ge1−yAsyTe2 (0 ≤ y ≤ 1), a phenomenon similar to that observed in the Fe3-yGe1−xAsxTe2 (0 ≤ x ≤ 0.85) samples. Moreover, the stacking disorder caused by the local AFM coupling can reduce its MS.

4.7.3. Electron Doping

Remarkably, Deng et al. [14] found that FGT devices could be operating in ionic gates, which provides a new approach for mediating 2D FM. Although they did not fully explain the relationship between it’s ferromagnetism and electron doping, the importance of this strategy was acknowledged. Soon after, gate-control was implemented to regulate magnetic resistance [99], magnetic phases [62][101], and interlayer coupling [100][102][173]. Furthermore, the TC and HC in FGT flakes [101] were decreased after Li+ doping from lithium-ion-conducting glass-ceramics (LICGC). In addition, electron doping influenced the Fe–Ge plane in the middle of the FGT monolayer, weakening it’s resistance and enhancing it’s TC [105].

4.7.4. Hole Doping

Inspired by the gate-mediated RTFM in FGT thin flakes [14], many attempts have been made to control its ferromagnetism through hole [43][94][106] or electron [43][94][105] doping. In particular, the magnetic anisotropy in exfoliated Fe2.75GeTe2 flakes was inhibited by hole doping, resulting in a decrease in HC. The magnetic anisotropy could undergo a 93% attenuation, but the change in the magnetic moment was very small. Furthermore, the electronic structure of Fe2.75GeTe2 single crystals changes due to hole doping, causing significant changes in magnetic anisotropy. In addition, another report [94] suggested that hole doping was beneficial for maintaining the long-range ferromagnetic order.

4.8. Intercalation or Irradiation

Recently, inserting sodium into Fe2.78GeTe2 powders [107] can raise its TC to ~300 K. After intercalating Na, more exposed edges appeared, and their layered features remained unchanged in a single crystal structure. More specifically, the Fe, Ge, and Te elements were evenly distributed in the sample, while the inserted Na was concentrated at the edge. A phase transition occurred from PM (Fe2.78GeTe2) to FM (NaFe2.78GeTe2) at 200 K. Furthermore, the magnetic hysteresis loops are also measured at 350 K. Notably, impurity phases, such as Fe or Fe2−xGe, dominated the RTFM in the NaFe2.78GeTe2 samples. Alternatively, the TC and exchange bias could be mediated with Fe-intercalation [108], which induces magnetic order by reinforcing magnetic coupling. However, the detected TeGe antisite defects had no modulation effect on the TC of different samples. Thus, Na intercalation provides a novel strategy for enhancing TC, which is related to the tensile strain.

4.9. Twisting

Twisting 2D materials can introduce some novel properties, such as magnetism [174][175] and superconductivity [176], which trigger the interaction topology with magnetism in 2D ferromagnets, resulting in the formation of skyrmions [177][178] or magnons [179][180] in the twisting system. In fact, the stacking order directly affects the magnetism of bilayer CrI3 by changing the crystal structure [174] or interlayer magnetic coupling [174]. Surprisingly, the magnetism was obtained in double bilayer CrI3 [181] at small twist angles. Although the phase transition from AFM to FM has been theoretically achieved in twist-stacking bilayer FGT [110][175], it has not yet been experimentally achieved [182].

4.10. Patterning

Magnetic domain patterns on FGT surfaces can be modulated with various mechanisms [8][13][183][184], one of which is the phase transition from FM to AFM related to interlayer coupling [13]. The photoemission electron microscopy (PEEM) image clearly shows the magnetic domain structure of FGT nanosheets, and the stripe domain structure disappears after reaching the TC of 230 K. After patterning the FGT sample into diamond and rectangular shapes using a focused ion beam (FIB), striped magnetic domain structures, similar to those in the unpatterned FGT, were also observed. However, the striped domain structure did not completely disappear and was significantly weakened at 230 K.

5. Band Structure of Ferromagnetic FexGeTe2

Like its bulk form, the FGT monolayer band structures near the Fermi level can mainly be attributed to the contribution of the Fe 3d orbitals. Moreover, it was confirmed that the FGT monolayer has the itinerant FM order according to Stoner’s criterion [46][185]. Remarkably, the Stoner model related to itinerant electrons can be used to better elucidate the spontaneous magnetization in most 2D metallic ferromagnets. In addition, the electronic band structures of all the FexGeTe2 systems are metallic, similar to the FGT monolayer. Furthermore, the F4GT and F5GT bilayer [186] have band structures similar to the FGT bulk and FGT monolayer, exhibiting metallic magnetic properties. However, there is a significant difference in the polarizability of F4GT and F5GT near the Fermi level, which leads to their different transport characteristics. The unique nature of FGT gives its related devices many advantages, including nonvolatility, low reversal magnetic field, and the magnetic field reversal FexGeTe2 electrodes through the spin-polarized current.

6. FexGeTe2-based Devices

To the best of our knowledge, four typical devices have been constructed based on metallic FexGeTe2 ferromagnets, including magnetic tunnel junctions (MTJ) [112][113][114][115][136], tunneling spin valves [18][99][116][117], nonlocal spin valves [118] and spin–orbit torque (SOT) devices [20][119][187]. Anonlinear behavior originating from tunneling characteristics [115] was exhibited in the I–V curve. Furthermore, a typical spin-valve behavior was also identified in the hysteresis loops. After applying a specific voltage, the spin-transfer torque (STT) generated by the current caused the bottom FGT electrode to switch, which was closely related to MAE. After applying a voltage to FGT/Pt hybrid devices [20], a current was generated between FGT and Pt, forming spin–orbit torques in Pt. Furthermore, a hard magnetic loop similar to an FGT device has also been observed in FGT/Pt devices. As the applied in-plane magnetic field Hx was increased, its transition current decreased, regardless of the direction of Hx. This switch was related to the magnetic domain and domain walls. Moreover, the low switching current of the FGT monolayer was beneficial for exploring more effective devices. Very recently, Wang et al. [187] increased the TC of FGT to RT by the topological insulator Bi2Te3 and achieved the SOT-driven magnetization switching at RT.

7. Magnetic Skyrmions in Metallic FexGeTe2

As topological magnetic materials, the spin textures in FexGeTe2 are also regulated by the Fe stoichiometry. For instance, Bloch-type skyrmion bubbles [188] were observed in FGT by using Lorentz transmission electron microscopy (LTEM). However, in another study [189], Néel-type skyrmions were reported, which indicated the existence of Dzyaloshinskii-Moriya interaction (DMI) in FGT. Generally, FGT was considered as a centrosymmetric material in the space group P63/mmc [35], which should not possess the asymmetric DMI. A possible interpretation is that the asymmetric interface between the pristine and oxidized FGT may induce an interfacial DMI [189]. More recently, a comprehensive study was implemented to answer this question more clearly [190]. It is found that the crystal structure of Fe3−xGeTe2 can be tuned by the Fe stoichiometry, that is, Fe3−xGeTe2 changes from centrosymmetric P63/mmc space group to non-centrosymmetric P63mc space group when x is larger than ~0.2 [190]. Notably, the P63mc space group allows an in-plane DMI in Fe2.74GeTe2, leading to the formation of Néel-type skyrmions. Whereas in Fe3.00GeTe2, the centrosymmetric P63/mmc structure forbids the DMI. Thus, Bloch-type skyrmion bubbles were observed when the dipole–dipole interaction and magnetic anisotropy obtained a delicate balance.

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