Epitaxial Growth of Large‐Scale 2D CrTe2 Films on Amorphous Silicon Wafers With Low Thermal Budget

2D van der Waals (vdW) magnets open landmark horizons in the development of innovative spintronic device architectures. However, their fabrication with large scale poses challenges due to high synthesis temperatures (>500 °C) and difficulties in integrating them with standard complementary metal‐oxide semiconductor (CMOS) technology on amorphous substrates such as silicon oxide (SiO2) and silicon nitride (SiNx). Here, a seeded growth technique for crystallizing CrTe2 films on amorphous SiNx/Si and SiO2/Si substrates with a low thermal budget is presented. This fabrication process optimizes large‐scale, granular atomic layers on amorphous substrates, yielding a substantial coercivity of 11.5 kilo‐oersted, attributed to weak intergranular exchange coupling. Field‐driven Néel‐type stripe domain dynamics explain the amplified coercivity. Moreover, the granular CrTe2 devices on Si wafers display significantly enhanced magnetoresistance, more than doubling that of single‐crystalline counterparts. Current‐assisted magnetization switching, enabled by a substantial spin–orbit torque with a large spin Hall angle (85) and spin Hall conductivity (1.02 ×  107 ℏ/2e  Ω⁻¹  m⁻¹), is also demonstrated. These observations underscore the proficiency in manipulating crystallinity within integrated 2D magnetic films on Si wafers, paving the way for large‐scale batch manufacturing of practical magnetoelectronic and spintronic devices, heralding a new era of technological innovation.


Introduction
3][4][5][6][7] Such layered materials have the advantages of polarizing spin/valley degrees of freedom and breaking the time-reversal symmetry of adjacent materials through the proximity effect. [8][21] Significant progress has been reported on the epitaxy of CrTe 2 films on single-crystalline substrates, such as graphene/SiC and sapphire.In this scenario, the crystalline quality of epitaxial CrTe 2 films is intricately tied to the substrate and demands a substantial lattice match for optimal results.Besides, through chemical bonding and structural symmetry at the interface, the specific epitaxial substrate influences the magnetic behavior of epitaxial layers, which inevitably brings about strong substrate selectivity.Nonetheless, when aiming to integrate 2D magnets into Si-based technology, their growth should ideally take place on Si wafers featuring amorphous dielectric layers positioned atop them.The challenge persists due to the lack of an epitaxial relationship between the amorphous substrates and the materials intended for deposition using such a growth technique.Up to now, effective synthesis protocols of 2D magnetic films with desired morphology and large dimensions on amorphous substrates have yet to be demonstrated. [26,27]Moreover, achieving the epitaxial growth of 2D magnets typically demands high-temperature treatment (exceeding 500 °C), a condition that conflicts with the permissible thermal budget of complementary metal-oxide semiconductor (CMOS) devices. [28]Temperatures surpassing 400 °C pose risks to the dopant distribution within silicon, compromise transistor interconnection, and jeopardize the integrity of the dielectric layer. [29]Hence, there is an imminent need for novel methodologies to enable the highvolume production of extensive 2D magnetic films that align seamlessly with Si process technology.
In our experimental setup, we devised a low-thermal budget technique to produce millimeter-scale CrTe 2 thin films, incorporating a Bi 2 Te 3 seed layer, on amorphous silicon nitride (SiN x )/Si substrates.[32][33] Presently, SiN x films find utility in various applications, including non-linear optical applications and 2D field-effect transistors (FETs). [30,31]Consequently, the potential application of growing 2D magnetic films on SiN x holds promise within the realm of microelectronics, presenting a streamlined avenue for the heterogeneous integration of diverse functional materials for practical scalability. [26]wing to the absence of lattice order, cultivating singlecrystalline, epitaxial films on an amorphous substrate proves to be a challenging endeavor.Conversely, the amorphous substrate provides a platform for deliberate crystallinity design and engineering within the impending layers.To achieve structured arrangements on amorphous substrates, the choice of a Bi buffer layer holds significance for several reasons.Primarily, relatively modest bonding between bilayer Bi contributes to alleviating the impact of lattice mismatch on film crystallinity over amorphous substrates. [34]Second, the atomic configuration of the Bi termination layer manifests a hexagonal arrangement; thus, inducing a preferential lattice orientation in the subsequent epitaxial layer.In fact, the successful growth of polycrystalline Bi films on amorphous glass substrates has been demonstrated, [35] thereby confirming the advantageous role of the Bi buffer in facilitating the epitaxial layering process on amorphous substrates.

Results and Discussion
An overview of the Bi 2 Te 3 -assisted MBE growth is schematically given in Figure 1a.Initially, starting with an amorphous SiN x /Si substrate, we grow a Bi buffer layer, which serves a dual purpose: it levels the amorphous substrate while also establishing an atomically precise surface for subsequent growth.It should be noted that Bi atoms form polycrystalline grains with different in-plane rotation angles, as evidenced by atomic-resolution scanning tunneling microscopy (STM) characterizations (Figure 1b).Subsequently, the synthesis involves the creation of a Bi 2 Te 3 layer.During this phase, Bi and Te atoms initiate nucleation and subsequently form islands, maintaining the hexagonal symmetry inherited from the underlying Bi layer.Due to the moderate deposition temperature (200 °C), Te atoms can easily sublimate and diffuse into the Bi precursor film and form the Bi 2 Te 3 layer. [36]The Bi 2 Te 3 layer functions as a buffer, facilitating the establishment of an atomically smooth surface for the subsequent layer's growth.Moving forward, our process involves the Cr element is uniformly distributed in the CrTe 2 film, without spreading to Bi 2 Te 3 and SiN x layers.e) TEM selected area diffraction pattern of the heterostructure region along [0001] zone axis.f) Thermal budget benchmarking for 2D magnetic thin films (covering Fe 5+x GeTe 2 , [68] Fe 4 GeTe 2 , [69] CrTe 2 , [19][20][21]23] VSe 2 , [70] Cr 5 Te 8 , [18,41] CrTe, [42] CrSe 2 , [43] and CrSe [44] ) using MBE and CVD synthetic techniques. Fora better comparison, we define the annealing temperatures of CVD methods the same as the growth temperatures.
growth of the 2D vdW CrTe 2 layer.This step is characterized by a one-to-one grain growth on the Bi 2 Te 3 layer, resulting in the CrTe 2 layer adopting the identical hexagonal symmetry, as observed in Bi 2 Te 3 , and complete with in-plane grain rotations.Finally, we apply a Te capping layer onto the stack to prevent oxidation.
To get a glimpse of the growth mechanisms of epi-layers on amorphous substrates, microscopic characterization (i.e., STM and transmission electron microscopy [TEM]) of epitaxial films during the epitaxial growth is carried out.Figure 1b presents a topographic image taken after growing a Bi film on the amorphous substrate.Several terraces and edges are observed.Atomically resolved images of Bi grains show an ordered hexagonal structure.The line profile (Figure S4, Supporting Information) shows that the height of each edge is ≈3.6 Å, which is corresponding to the interlayer distance of Bi bilayers.These results indicate that the geometric structure of the Bi layer on the SiN x /Si substrate is identical to that of the (111) surface of Bi single crystals reported previously. [37]Further, upon closer scrutiny of the atomicresolution STM images, it becomes apparent that the atomic configuration within two neighboring grains assumes distinct inplane angles within the as-grown hexagonal Bi terraces.This observation strongly implies the notable presence of grain boundaries.In fact, within individual grains, the surface presents a re-markable atomic flatness, characterized by the absence or scarcity of surface steps.However, it's worth noting that higher step accumulations occasionally manifest near these grain boundary regions.
After the growth of 20 nm Bi 2 Te 3 , the epi-layer becomes flat with a uniform step height of 10 Å (Figure S4, Supporting Information), related to the standard 1 quintuple layer (QL). [38]ith the assistance of the buffer layer, the deposited 21 nm CrTe 2 film shows terraces with a uniform step height of ≈6 Å (Figure 1c), demonstrating the atomically flat CrTe 2 epi-layer with a layer-by-layer growth mode. [20]The topographic STM image of CrTe 2 verifies the presence of phase boundaries.Further compelling evidence can be found by resolving the grains.The atomic arrangement in adjacent CrTe 2 grains presents a hexagonal structure with different in-plane angles, suggesting the polycrystalline CrTe 2 epitaxial films with grain boundaries.The cross-sectional views of the scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) images for the heterostructure are exhibited in Figure 1d.Combining with the energy dispersive X-ray spectroscopy (EDX) mapping, the STEM result further substantiates the presence of distinct, well-defined interfaces between different layers, confirming minimal interlayer diffusion.On the other hand, the plane-view crystal structure of the Bi 2 Te 3 /CrTe 2 heterostructure prominently exhibits a discernible Moiré superlattice, providing a clear indication of the [0001] orientation in the heterostructure (Figure 1e).Except for the low-frequency diffraction patterns from hexagonal crystal structure, the additional high-order diffraction spots at hexagonal lattice sites may be attributed to two sets of satellite peaks from Bi 2 Te 3 and CrTe 2 lattices (more details in Section S2, Supporting Information). [39]Moiré superlattice, which is formed due to lattice mismatch or interlayer twist angle, gives rise to different interfacial couplings; and therefore, can generate new electronic, optical, and magnetic properties in heterostructures. [40]Hence, through the combined use of STM, TEM, and EDX characterizations, it becomes evident that the CrTe 2 films experience epitaxial growth, forming grains exhibiting varying in-plane rotations.This growth mechanism is facilitated by the presence of the Bi 2 Te 3 seeding layer on amorphous substrates.Subsequent sections will delve into further details, revealing that this selective synthesis of crystallinity leads to the creation of 2D CrTe 2 films on amorphous substrates, showcasing distinctive magnetic properties and novel emergent functionalities.
As shown in Figure 1f, a plot illustrates the relationship between the annealing temperature of substrates and the growth temperature for the production of synthetic 2D magnetic thin films.][43][44] In contrast, our study successfully achieves a significantly lower growth temperature of 270 °C through the implementation of a seeded growth technique.This achievement serves to differentiate our research as it not only capitalizes on the advantages of a low growth temperature that seamlessly integrates with CMOS tech-nology but also culminates in the attainment of superior film crystallinity.
Having identified the granular feature of CrTe 2 with various in-plane rotations, we then characterize its magnetic properties.The ferromagnetic (FM) nature of the grown CrTe 2 film is well demonstrated by zero-field-cooled (ZFC) and field-cooled (FC) out-of-plane (OOP) magnetization curves (Figure S7, Supporting Information).Isothermal magnetization curves obtained under a magnetic field applied parallel to the c axis (H//c) and the ab plane (H//ab) are illustrated in Figure 2a,b.The polycrystalline CrTe 2 displays broad FM hysteresis loops along the c axis.Conversely, the in-plane (IP) curves are characteristic of a hard-axis signature, underscoring the robust FM order and the pronounced PMA of CrTe 2 .The quantification of PMA can be achieved through the anisotropy field equation  0 H A =  0 H s + 4M s , in which  0 is the permeability of the vacuum, H s is the saturation field, and M s is the saturation magnetization.Extraction of H s and M s is facilitated via hysteresis loops.In Figure 2c, the graph illustrates the comparison of  0 H A between polycrystalline and single-crystalline CrTe 2 .The single-crystalline CrTe 2 thin films are grown on bilayer graphene/SiC substrates by the same method as reported before. [20]It is noteworthy that as the temperature rises, both samples demonstrate a corresponding decrease in  0 H A , showcasing a parallel decrease in PMA magnitude.
Apart from PMA, it is crucial to gain insight into the coercivity ( 0 H C ) as one of the key properties of functional magnetic materials.Figure 2d presents the OOP hysteresis loop of a polycrystalline CrTe 2 thin film, and the hysteresis loop of CrTe 2 on a single-crystalline SiC substrate is plotted for comparison.Evidently, the polycrystalline CrTe 2 thin film showcases a hysteresis loop that is both broad and skewed, a characteristic attributed to the gradual evolution of domain structures.In contrast, the single-crystalline counterpart presents a square-shaped hysteresis loop featuring a nearly vertical jump, which is suggestive of a singular, robust magnetic phase.Further, their hysteresis loops exhibit quite different  0 H C s, specifically 1.03 T for polycrystalline CrTe 2 and 0.076 T for single-crystalline CrTe 2 at 50 K.A close view of the  0 H C of these two samples with respect to the temperature is displayed in Figure 2e.Though both  0 H C s decrease with increasing temperature, the  0 H C of polycrystalline CrTe 2 is ten times larger than that of the single-crystalline counterpart. [20]The notable  0 H C value observed in polycrystalline CrTe 2 can be attributed to the presence of weak intergranular exchange coupling, [45] a phenomenon arising from the distinct, isolated grain structure and domain-wall pinning occurring at grain boundaries. [46,47][50][51][52][53][54][55] This emphasizes how the synthesis approach, highlighting crystallinity selectivity while incorporating isolated grains and weak exchange coupling, can indeed serve as a potent pathway to achieving significant  0 H C values and enhancing storage density in recording media.
To further check the suitability of growing 2D magnets on Si, we extend our investigation to grow CrTe 2 amorphous SiO 2 /Si substrates.Intriguingly, our findings unveil an enormous coercivity of ten-kilo-oersted within the epitaxial polycrystalline CrTe 2 thin film, as elucidated in Figure S10, Supporting Information.This revelation not only underscores the efficacy of our approach but also holds the promise of a universal method potentially transferrable to various 2D vdW materials.Moreover, this methodology appears to exhibit impressive versatility as it demonstrates applicability irrespective of the size and type of substrates employed.
In addition to the prior bulk magnetic measurements, a comprehensive assessment of the magnetic states within CrTe 2 is undertaken.This involves the implementation of X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) analyses, focusing on the Cr L 2,3 edge, to gain insights into the local electronic structure and magnetic ground state (Figure 2g).Examination of the spin-orbit split core levels via XAS reveals a distinctive multiple-peak pattern, indicative of a heterogeneous mixture of Cr exhibiting diverse valence states.Interestingly, the spectral line shape bears similarities to that observed in Bi 2−x Cr x Se 3 , [56] implying that the majority of Cr atoms is largely in a near-trivalent state, with a minor contribution of divalent Cr arising from the terminating surface.Notably, the pronounced XMCD spectrum, deduced through the subtraction of XAS data, provides clear evidence of the inherent FM ground state within the synthesized CrTe 2 layer on the amorphous substrate.Further, a mapping of the XAS spectra of the as-grown CrTe 2 film confirms the consistent presence of identical features, thereby reinforcing the notion of extensive uniformity within the synthesized material.
Subsequent to the spectroscopy investigation, our study extends to Lorentz transmission electron microscopy (L-TEM) imaging.This technique is harnessed to provide direct visualization of magnetic domains, offering insights into the intricate magnetic spin texture within the CrTe 2 material.The bright and dark contrasts are formed in the L-TEM images due to the process that the electron beam converges or diverges (Figure 3a).When an electron beam passes through an orthogonal magnetic induction, it deflects due to the Lorentz force and generates magnetic contrasts.The perpendicular magnetic domains are invisible on the imaging plane, resulting from the parallel alignment between the perpendicular magnetization and the electron beam.Therefore, it is necessary to tilt the sample to introduce a projected component that is perpendicular to the beam.The comparison of L-TEM images at different tilting angles is a well-established approach for detecting the spin configuration of the Néel-type magnetic texture. [57]As schematically shown in Figure 3a, tilting the sample to the opposite  angles ( = ±20°) relative to the electron beam gives rise to a reversal of magnetic contrast corresponding to IP magnetization.
As the temperature is lowered to 100 K, left-bright and rightdark stripe contrasts show up at  = −20°(Figure 3b).In contrast to the labyrinthine domain structure commonly observed in single-crystalline materials exhibiting PMA, the granular CrTe 2 showcases a distinctive worm-like domain pattern.Intriguingly, when the L-TEM image is captured at an angle of  = 0°, the stripe contrasts vanish, only to reappear with inverted contrasts (leftdark and right-bright) at  = +20°.This phenomenon strongly implies the existence of Néel-type domain walls.To decipher the domain evolution across varying sample tilting angles, we present line profiles corresponding to the observed worm-like domains in Figure 3c.Notably, a characteristic valley-and-peak pattern materializes within the domain wall at −20°, aligning with the left-bright and right-dark regions mentioned earlier.
Remarkably, this pattern transforms into an inverse peak-andvalley arrangement upon tilting to an angle of +20°.Meanwhile, at  = 0°, the previously observed valley-and-peak pattern becomes indiscernible.Collectively, these findings unveil the presence of Néel-type magnetic domains within the granular CrTe 2 structure.
To comprehend the magnetic field-driven dynamics of the domains, a detailed investigation employing L-TEM imaging at varying temperatures and magnetic fields is conducted.At room temperature, the L-TEM image does not exhibit magnetic contrasts, reflective of the paramagnetic (PM) state (Figure 3d).As the temperature drops to 100 K, CrTe 2 transitions into an FM state characterized by Néel-type domains.The evolution of the worm-like domains in response to perpendicular magnetic fields is illustrated in Figure 3e-i.With the magnetic field ranging from 0 to 1000 Oe, the width of the worm-like domains contracts while their density diminishes.This behavior is captured in the bottom inset of Figure 3g, presenting an in-plane magnetic induction map of a specific worm-like domain, reconstructed using the transport intensity equation. [58]These findings offer further confirmation of the magnetic nature of the worm-like structures observed in L-TEM images.At a magnetic field of 1400 Oe, the magnetic contrast vanishes due to the establishment of a uniformly field-polarized state.
For deeper insight into the field-induced modulation of magnetic domains, micromagnetic simulations are performed to replicate the process (Refer to Experimental Section for detailed information).The substantial effective interfacial Dzyaloshinskii-Moriya interaction (DMI) observed in 2D CrTe 2 /Bi 2 Te 3 heterostructures is a direct outcome of the synergistic interplay between the robust spin-orbit coupling in Bi 2 Te 3 and an atomically sharp interface with asymmetric exchange interactions.This noteworthy combination serves to substantiate and validate the formation of Néel-type magnetic domains within the heterostructures.Figure 3j illustrates the line-scanning intensity profiles of the magnetic domain marked by the green arrow (Figure 3e) at different magnetic fields.The bright and dark contrasts correspond to the peaks and valleys in the line scanning curves.As indicated by the arrows, the width of the worm-like domain, within which the magnetization orientation is antiparallel to the external field, contracts and eventually disappears with an increasing magnetic field.In addition, the field-driven evolution of Néel-type worm-like domains has been studied in other regions (marked by colorful arrows), revealing field-dependent characteristics of an FM ground state.Figure 3k succinctly presents the width of various worm-like domains as a function of the magnetic field.It is evident that the domain width reduces with escalating field strength; however, these magnetic domains within CrTe 2 switch incoherently at distinct switching fields.This behavior, as expounded below, can be attributed to the weak exchange coupling between different grains.
To assess the potential applicability of polycrystalline CrTe 2 in the development of spintronic devices, we fabricate Hall bar devices and conduct an in-depth study of their electrical properties across various temperatures.Illustrated in Figure 4a, a distinct anomalous Hall effect (AHE) characterized by squareshaped hysteresis loops becomes apparent below 180 K, a hallmark of robust ferromagnetism coupled with strong PMA.The anomalous Hall resistance (R AHE ) exhibits a nonmonotonic trend with temperature, peaking around the Curie temperature (T C ) (Figure 4c).This phenomenon, a suppressed R AHE below T C , has been previously reported in the CrTe 2 /Bi 2 Te 3 system, [59] a result anticipated from the alteration in momentum-space Berry curvature and the diminished electron-phonon or magnon scattering.Moreover, the reduced R AHE above T C signifies the transition of CrTe 2 into the PM phase.
Turning to the magnetoresistance (MR) measured at varying temperatures with an OOP magnetic field, Figure 4b reveals butterfly-shaped curves with two peaks corresponding to the  0 H C values.These peaks coincide with the regions where MR is most pronounced near the switching fields.Remarkably, the MR magnitude at 10 K reaches ≈0.8% at 3 T, comparable to that of single-crystalline CrTe 2 thin flakes. [24]The negative MR, commonly observed in ferromagnetic materials, is attributed to the suppression of spin scattering by the applied magnetic field. [60]Notably, an intriguing observation lies in the nonlinear segment of the MR curves at low fields.As depicted in Figure 4b, a prominent feature is the rapid drop in resistance at low fields, followed by a gradual linear negative MR at higher fields (fitted by orange dashed lines).Extracted MR values at 3 T are plotted in Figure 4d.The temperature-dependent MR ratio displays a distinct rise as T C is approached, followed by a decline from the peak value of 12.55% at 180 K to 9.34% at 200 K within the PM state.Remarkably, the MR ratio is nearly three times that of single-crystalline CrTe 2 , [23] suggesting a distinctive MR mechanism within polycrystalline CrTe 2 .
Considering the granular nature of polycrystalline CrTe 2 , our findings align well with the granular giant magnetoresistance (GMR) effect, [61,62] where strong spin-dependent electron scattering occurs at the interfaces of granular films.To further explore this relationship, the temperature dependence of R AHE and the GMR ratio is examined (Figure 4c,d).Both R AHE and GMR exhibit peaks near T C , implying the potential influence of thermal spin fluctuations on the magneto-transport of CrTe 2 .Unlike the low-temperature regime where low-frequency magnons play a predominant role in transport characteristics, spin fluctuations take center stage near T C and reach their maximum.In the context of granular GMR, these spin fluctuations are suppressed by an external magnetic field, resulting in diminished spin-dependent scattering and substantial negative MR.This quantitative correspondence between R AHE and the GMR ratio unveils the granular GMR effect in polycrystalline CrTe 2 , wherein spin-dependent scattering of polarized electrons becomes dominant at grain boundaries. [63]n a continuation of our investigation, we provide evidence for the spin-orbit torque (SOT) effect within the Hall bar device.This is achieved by applying a charge current through Bi 2 Te 3 , thereby inducing an out-of-equilibrium spin-orbit torque that effectively modifies the magnetization when an oblique magnetic field of up to 2 kOe is applied. [64,65]Depending on the current's polarity, the SOT can lead to either a decrease or an increase in the  0 H C .The effects of SOT are demonstrated in Figure 4e,f through field switching experiments.In the SOT experiment, a direct current (I dc ) is applied to the Hall bar for a duration of 0.2 ms.During the entire measurement, a magnetic field is introduced at an angle of 80°away from the film's normal direction, tilting the magnetization away from its perpendicular easy axis and facilitating deterministic switching.When a positive I dc is applied, the SOT generates an effective field, denoted as H SO+ , which facilitates the rotation of magnetization toward the anisotropy field.This prevents the reversal of magnetization by the external field, as illustrated in the insets.Consequently, a relatively broad magnetic hysteresis loop and an amplified  0 H C are attained.Conversely, with a negative I dc , the spin polarization on the Bi 2 Te 3 surface changes direction, leading to an opposing field denoted as H SO− .This counteracting field assists the magnetization reversal process.This scenario results in a narrower loop and a reduced  0 H C .
The remarkable ability to effectively manipulate  0 H C by adjusting I dc , spanning from 0.5 to 1 mA, highlights the stability of current-assisted magnetization reversal within the CrTe 2 /Bi 2 Te 3 system on amorphous substrates.Figure S18, Supporting Information provides detailed insights into the SOT efficiency (), which is measured to be 683 Oe⋅MA − ¹⋅cm −2 .This efficiency corresponds to an effective spin Hall angle ( SH ) of 85 and a spin Hall conductivity of 1.02 × 10 7 ℏ/2e Ω − ¹ m − ¹.These values are truly significant and underscore the potential of CrTe 2 as a highly promising material system for integration with Si in magnetic random-access memory (MRAM) devices.

Summary and Outlook
Our research has demonstrated the profound impact of grain boundaries on the magnetic properties of CrTe 2 magnets, revealing essential insights into their behavior.Specifically, grain boundaries play a pivotal role by pinning and obstructing the switching of magnetic domains, leading to incoherent domain switching in contrast to coherent magnetization reversal modes.This incoherent switching is facilitated by weakened exchange coupling between neighboring grains.The resulting enhanced  0 H C of CrTe 2 thin films on amorphous substrates arises from a combination of factors, including the pinning of magnetic moments at grain boundaries, the presence of large PMA, and weak intergranular exchange coupling.
Notably, the large magnetic anisotropy and  0 H C are vital attributes of ultrahigh-density functional magnetic materials, offering excellent thermal stability.Moreover, the influence of grain boundaries manifests in the evolution of Néel-type magnetic domains at a microscopic level.Due to weak intergranular exchange coupling, the worm-like domains switch incoherently with magnetic fields.Further, our findings reveal intriguing aspects of CrTe 2 devices, such as spin-dependent electron scattering occurring at the interface, leading to the granular GMR effect.These unique characteristics, combined with the Néel-type stripe domains and topological surface states (TSS)-driven SOT switching, position CrTe 2 as a highly promising material for engineering spin-valves and beyond-CMOS devices.The controllability of grain boundaries, intrinsic ferromagnetism, and magneticelectrical behavior of 2D CrTe 2 magnets through crystallinity engineering open up a versatile and reliable approach for heterointegrating highly mismatched material systems.In addition, our research provides a protocol for the integration of high-quality, large-scale 2D magnets on silicon wafers, paving the way for diverse applications such as high-density MRAM and spintronic logic device arrays.
While our work presents a crucial foundation for 2D magnet research, challenges lie ahead in extending these methods to a broad range of layered magnets, adapting to various temperatures, and meeting specific on-demand applications, such as quantum computers and memories.Nevertheless, the guidelines established in our study offer a pathway to address these challenges and further advance the field of 2D magnetism, unleashing the potential of Si-based 2D magnets for a plethora of technological advancements.

Experimental Section
Sample Preparation: The CrTe 2 films in this study were grown in ultrahigh vacuum (UHV) using an MBE system on SiN x /Si (001) TEM substrates and wafer chips with Bi 2 Te 3 buffer layers.The SiN x membrane for the TEM sample was ≈50 nm thick to ensure electron transparency, and the etched TEM window size was ≈100 μm × 100 μm.Bi (99.999%) atoms were first evaporated with substrate maintained at room temperature and annealed at 160 °C for 20 min.After that, Bi and Te (99.999%) were evaporated from crucibles at 490 °C and 300 °C, respectively, with the substrate maintained at≈200 °C.Finally, Cr (99.999%) and Te were evaporated from an E-beam evaporator and crucible, respectively.The temperature of sub-strates was first set to 240 °C for the growth of the initial 3 nm CrTe 2 at the interface, and then, the temperature was raised slowly to 270 °C for the rest of the growth.Before taking samples out of the MBE chamber, 12 nm Te was deposited on the surface at room temperature to avoid contamination.The crystalline structure and film thickness were measured with a Rigaku Smartlab X-ray diffraction/X-ray reflectometry system.
Scanning Tunneling Microscopy Characterization: During the MBE growth, STM characterization was in situ performed on each epi-layer.The topography of the sample surface was mapped in situ by an Aarhus STM housed in the growth chamber.The topographic images were taken with the tunneling junction set-up: V = 1500 mV, I = 0.01 nA.The atomic resolution images were taken with the tunneling junction set-up: V = 10 mV, I = 0.1 nA.
Energy Dispersive X-Ray Spectroscopy Measurement: The cross-section TEM samples were prepared using a Zeiss Nvision focus ion beam (FIB).EDX experiment was operated at an acceleration voltage of 200 kV on an FEI Talos F200X TEM/STEM equipped with a Super X energy-dispersive spectrometer of Bruker.
Magnetic Characterization: Magnetization measurements of the CrTe 2 thin films were carried out with a superconducting quantum interference device (SQUID)-vibrating sample magnetometer (VSM) system (MPMS3, Quantum design).This system is capable of cooling samples down to 1.9 K and can generate a variable magnetic field up to ±7 T along both IP and OOP directions.FC and ZFC curves were measured by increasing the temperature from 10 to 250 K with an OOP magnetic field of 1000 Oe.
X-Ray Absorption Spectroscopy and X-Ray Magnetic Circular Dichroism Characterization: XAS and XMCD measurements were performed on beamline 07U of the Shanghai Synchrotron Radiation Facility (SSRF) in total-electron yield (TEY) mode at 8 K and a magnetic field of 3 T. Oppositely circular polarized X-rays with 100% polarization was used to resolve XMCD signals in normal incidence.The light helicity was applied perpendicular to the film plane and in parallel with the incident beam.The dichroic spectrum was obtained by taking the difference of XAS spectra, that is,  + −  − .

Lorentz Transmission Electron Microscopy Measurement:
The cryo L-TEM measurements were performed in an FEI Tecnai F20ST TEM instrument operating at 200 kV in an out-of-focus Fresnel model using a Gatan liquid nitrogen cryo holder.The L-TEM system was able to reach a minimum temperature of 100 K.A small amount of current was applied to the main objective lens to generate a magnetic field, which was parallel to the electron beam.The out-of-focus L-TEM images were taken at a defocus length of a nominal 5 mm.The L-TEM images presented in this work were obtained by subtracting the L-TEM image acquired at the highest field of 2000 Oe, in which the magnetization was fully saturated.The details of the image process are in Section S10, Supporting Information.The integrated magnetic induced map was retrieved from a single processed under-focus L-TEM image by using a single-image-TIE function in the open-source Py-Lorentz package, [58] which was feasible only if the source of contrast was magnetic Fresnel contrast.

Micro Magnetic Simulations:
The open source micromagnetic code Mumax3 [66] was used for the simulations, and the object-oriented Micro-Magnetic framework (OOMMF) [67] was used to plot the magnetization profiles.A stripe domain was relaxed under the different magnetic fields to compare with the experimentally observed domain patterns.The material parameters were chosen according to the SQUID measurement: M S = 1.Electrical Transport Option: Transport measurements of the CrTe 2 sample were performed using a Quantum Design DynaCool Physical Property Measurement System (PPMS) equipped with an ETO (electrical transport option) module.The ETO module was employed to measure the anomalous Hall resistance (in Ohms) of the CrTe 2 sample; while varying the magnetic field at different temperatures.Both IP and OOP magnetic field directions were considered during the measurements.For the SOT experiment, a Keithley 6221 current source was utilized to generate alternating currents and pulsed direct currents.The Hall voltage of the Hall bars was measured using two SR830 lock-in amplifiers.The SOT measurement was carried out by analyzing the AHE in response to the application of pulsed direct currents.These switching currents had an amplitude ranging from 0.5 to 1 mA and a time duration of 0.2 ms.The measurements were conducted at a temperature of 180 K, with the magnetic field oriented at an angle of 80°away from the film's normal direction.To minimize the influence of Joule heating, each data point was recorded after allowing a pause of 90-210 s following the application of a direct current pulse.Further, to measure the Hall responses, an alternating current with a root mean square amplitude of 0.1 mA was utilized.These carefully designed experimental procedures ensured the collection of accurate and reliable data during the measurements.

Figure 1 .
Figure 1.Synthesis of polycrystalline CrTe 2 on SiN x /Si via crystallinity engineering.a) Schematics of the MBE growth procedure.Bi 2 Te 3 works as a buffer layer to promote epitaxial growth of CrTe 2 .The final structure from top to bottom is Te/CrTe 2 /Bi 2 Te 3 /SiN x /Si (001).b) Topographic and atomically resolved STM images of the as-grown hexagonal Bi layer with grain boundaries on SiN x /Si substrate.Bi grains show various atomic orientations.c) Surface morphology and atomically resolved Te surfaces of epitaxial CrTe 2 with a highly ordered hexagonal structure.The atomic arrangement of CrTe 2 grains shows different in-plane orientations.d) Left: typical HAADF-STEM images of the prepared heterostructure from cross-sectional views.The dashed lines indicate the boundaries of CrTe 2 /Bi 2 Te 3 with Te cap and SiN x substrate.Scale bar: 20 nm.Right: EDX color maps of Si, N, Cr, Bi, and Te, respectively.Cr element is uniformly distributed in the CrTe 2 film, without spreading to Bi 2 Te 3 and SiN x layers.e) TEM selected area diffraction pattern of the heterostructure region along [0001] zone axis.f) Thermal budget benchmarking for 2D magnetic thin films (covering Fe 5+x GeTe 2 ,[68] Fe 4 GeTe 2 ,[69] CrTe 2 ,[19][20][21]23] VSe 2 ,[70] Cr 5 Te 8 ,[18,41] CrTe,[42] CrSe 2 ,[43] and CrSe[44] ) using MBE and CVD synthetic techniques. Fora better comparison, we define the annealing temperatures of CVD methods the same as the growth temperatures.

Figure 2 .
Figure 2. Significant enhancement of coercivity through weak intergranular exchange coupling.a,b) Isothermal field-dependent magnetization at indicated temperatures with H//c axis (a) and H//ab-plane (b), respectively.c) Extracted μ 0 H A as a function of temperature.d) Enlarged view of normalized hysteresis loops of polycrystalline and single-crystalline CrTe 2 on SiN x /Si and SiC substrates, respectively.The insets are the corresponding magnetizing schematics.e) Variation of  0 H C , with respect to the temperature.f) Comparison of  0 H C of CrTe 2 in this work with other FM PMA systems.g) Top: mapping of the XAS spectra over CrTe 2 film, where the colored rectangles mark the measured positions.Bottom: a typical pair of XAS with opposite circular polarizations and XMCD spectra obtained with a magnetic field of 3 T at 8 K.

Figure 3 .
Figure 3. Capturing Néel-type magnetic domain dynamics by L-TEM imaging.a) Schematics of L-TEM imaging of Néel-type magnetic stripe domains.On the sample surface, blue and red arrows represent the OOP magnetization.The yellow and purple area represents the position of Néel domain walls.On the imaging plane, blue and red arrows represent the projected IP component of the magnetic moments.Bright and dark contrasts are formed in the domain wall region.b) Experimental L-TEM images of CrTe 2 /Bi 2 Te 3 at 100 K with tilted sample angles of −20°, 0°, and 20°in a 600 Oe perpendicular magnetic field.The light blue, green, and orange arrows represent the direction of line scanning.Scale bar: 500 nm.c) Line profiles of L-TEM contrast intensity across the magnetic domains at different tilting angles.d) L-TEM images measured at 300 K, with sample tilted by −20°.Scale bar: 1 μm.e) Formation of magnetic worm-like domains at 100 K following a ZFC protocol.The light blue dashed lines mark the same sample position in (d,e).f-i) Evolution of magnetic domains as a function of the perpendicular magnetic field.The green, yellow, orange, purple, red, and olive arrows indicate the Néel-type domains at the same positions.The top right corner inset shows the simulated magnetic domains.The red stripe represents the central domain, which is surrounded by adjacent domain walls (marked by white lines).The outer blue regions are corresponding to the opposite magnetic domains.In (g), a reconstructed magnetic induction map of the selected region is highlighted by a purple dashed box.The color wheel represents the IP magnetization orientation.j) Intensity profiles of magnetic contrasts across a worm-like domain (marked by the green arrow) under various magnetic fields.k) Relation between the Néel-type worm-like domain width and applied field within regions marked by colorful arrows.

Figure 4 .
Figure 4. Granular GMR and SOT-induced switching in CrTe 2 Hall bar devices.a) Field-dependent anomalous Hall loops at various temperatures.The magnetic field is applied along the OOP direction.b) Temperature evolution of the MR curves of CrTe 2 devices.Orange dashed lines are linear fittings of MR at high fields.c,d) Temperature dependence of R AHE (c) and GMR ratio derived from transport measurements (d).The FM and PM phases are color-coded by light red and blue, respectively.e,f) Effects of I dc on anomalous Hall loops in CrTe 2 /Bi 2 Te 3 bilayer at 180 K. Inset is the experimental setup, with |H| = 0.2 T.