Exploring Interfacial Effects in Transition Metal Dichalcogenide/Ferrimagnetic Alloy Heterostructures

Abstract

Ultrathin ferrimagnetic heterostructures have emerged as promising platforms for next-generation spintronic devices, yet the role of two-dimensional substrates in modulating their magnetic properties remains underexplored. Here, we report a comprehensive study of the thickness- and temperature-dependent magnetic behavior of amorphous Fe₇₃Co₈Gd₁₉ films (4–32 nm) deposited on Si, WSe₂ bilayer, and WSe₂ monolayer substrates. Structural integrity and stoichiometry were confirmed via X-Ray Diffraction (XRD), X-Ray Reflectivity (XRR), Raman spectroscopy, and Energy-Dispersive Spectroscopy (EDS) analysis. In-plane magnetometry from 10–300 K reveals that monolayer WSe₂ promotes stronger interfacial spin alignment, with the 4 nm film exhibiting a sharp increase in coercivity below 50 K, where $H_c$ exceeds 23 mT and even surpasses thicker counterparts, alongside enhanced saturation magnetization (∼790 kA/m at 100 K). This dramatic enhancement of coercivity is the most significant result of this work, underscoring the dominant role of interfacial coupling in governing low-temperature magnetic hardness. Conversely, films on bilayer exhibit suppressed magnetization and soft magnetic behavior ($H_c$ < 10 mT) across all temperatures, making them attractive for ultralow-power and high-speed spintronic applications. These findings demonstrate that atomically thin WSe₂ interfaces can modulate coercivity, magnetization, and squareness through proximity effects, establishing a tunable and thermally stable platform for spintronic device applications.

1. Introduction

The development of ultrathin magnetic heterostructures with tunable magnetic properties has driven major advances in spintronics, particularly in emerging applications such as spin-transfer torque magnetic random-access memory (STT-MRAM), spin–orbit torque (SOT) devices, and terahertz-range spin-torque oscillators [1,2,3,4]. Among the most promising material systems for these applications are amorphous rare earth–transition metal (RE–TM) ferrimagnetic alloys, which combine high thermal stability, low Gilbert damping, and fast magnetization dynamics [2]. These alloys consist of two antiferromagnetically coupled magnetic sublattices, one associated with the transition metal (TM) and the other with the rare earth (RE) elements, resulting in a magnetic compensation temperature at which the opposing sublattice moments cancel. A key feature of amorphous RE–TM ferrimagnets such as FeCoGd is that this compensation point, along with properties such as saturation magnetization ($M_s$), anisotropy ($H_A$), coercivity ($H_c$), and magnetic squareness ($M_r/M_s$), can be systematically tuned through stoichiometry, thickness, temperature and interface engineering [2,5,6,7,8,9,10,11]. These tunable magnetic characteristics make ultrathin FeCoGd films ideal platforms for exploring high-frequency ferrimagnetic dynamics, including sub-terahertz precessional modes driven by spin-transfer torque [2,3,4].

While prior work has extensively explored the temperature and composition dependence of magnetic compensation and spin dynamics in FeCoGd and similar amorphous alloys [5,8,9,12], fewer studies have addressed how atomically sharp interfaces in ultrathin films can be used to tailor these magnetic properties via proximity-driven modulation. Particularly promising in this context are heterostructures that incorporate two-dimensional transition metal dichalcogenides (TMDs) such as WSe₂, which are known to exhibit strong spin–orbit coupling, broken inversion symmetry, and gate-tunable spin and valley degrees of freedom [13,14,15,16,17]. When placed in contact with magnetic films, TMDs can act as spin–orbit proximity layers, enabling interfacial tuning of spin texture, anisotropy, and magnetization dynamics [13,14,15,16,18]. Recent studies have demonstrated that TMD/ferromagnet and TMD/ferrimagnet interfaces can support nontrivial interfacial spin–orbit fields, interfacial modifications to spin–momentum locking, such as spin canting or tilting induced by proximity coupling to the TMD layer, and anisotropic magnetoresistance signatures, all pointing to strong interface-induced modulation of magnetic and spin transport phenomena [13,14,15,16,18,19]. These effects have also gained interest in the context of antiferromagnetic and compensated spin systems [20], where interface and symmetry effects can influence spin transport without net magnetization. In addition, edge- and defect-induced magnetic moments have been reported in certain TMD systems, particularly in nanostructured geometries where edge states constitute a significant fraction of the material. In macroscopic thin-film heterostructures, however, such edge contributions are expected to be minimal compared to the interfacial area between the magnetic layer and the 2D substrate [21].

Despite the growing interest in proximity-driven phenomena in TMD-based heterostructures, the temperature-dependent magnetic behavior of amorphous RE–TM ferrimagnets such as FeCoGd interfaced with WSe₂ remains largely unexplored. We therefore focus on WSe₂, a tungsten-based TMD with strong intrinsic spin–orbit coupling and broken inversion symmetry, making it an attractive platform for enhancing interfacial spin–orbit proximity effects in adjacent magnetic layers. Moreover, comparing monolayer and bilayer WSe₂ provides a controlled means to vary inversion symmetry and interlayer screening while maintaining nearly identical interfacial chemistry. Precise thin-film synthesis helps preserve interface integrity and minimize unintended intermixing, which is critical when investigating proximity-driven effects in ultrathin heterostructures. Similar considerations regarding controlled magnetic thin-film growth and interface-sensitive thickness effects have been emphasized in prior studies [22].

We investigate the thickness and temperature-dependent magnetic properties of ultrathin Fe₇₃Co₈Gd₁₉ films grown on Si, WSe₂ bilayer, and WSe₂ monolayer substrates. Using structural analyses such as X-Ray Diffraction (XRD), X-Ray Reflectivity (XRR) and Raman Spectroscopy, and compositional analysis such as Energy-Dispersive Spectroscopy (EDS), along with detailed magnetic characterization through Vibrating Sample Magnetometer (VSM), we extracted the evolution of $H_c$, $M_s$ and $M_r/M_s$ in the temperature range of 10–300 K. We show that both film thickness and the atomic-scale structure of the WSe₂ substrate play critical roles in the systematic modulation of coercivity, saturation magnetization, and reversal coherence, establishing TMD/FiM systems as a promising platform for thermally tunable and interface-engineered spintronic device applications [2,6,13,16,19,23].

2. Experimental Details

WSe₂ was synthesized on Si/SiO₂ substrates using a water-assisted CVD method under atmospheric pressure [24,25]. High-purity WSe₂ powder (99.9%, (Thermo Fisher (Ward Hill, MA, USA)) was loaded into a ceramic boat and uniformly distributed over approximately 1 inch of the boat’s length. The ceramic boat containing the WSe₂ powder was placed at the center of a 1-inch-diameter quartz tube, which was then inserted into a high-temperature furnace. Si substrates with 285 nm SiO₂ were pre-cleaned with acetone, isopropanol, and deionized water. During growth, substrates were positioned downstream at 700–750 °C, while the precursor zone was maintained at 1025 °C. The furnace temperature was ramped to 1025 °C under a constant N₂ flow (200 sccm). Upon reaching temperature, water vapor was introduced by diverting N₂ through a bubbler containing 2 mL of deionized water at 23 °C to promote W-rich domain growth. After several minutes, the gas was switched to Ar + 5% H₂ (200 sccm) to enhance crystalline quality. By varying the growth time, monolayer and bilayer WSe₂ samples were synthesized. The furnace was then allowed to cool naturally to room temperature. The spatial coverage and uniformity of the WSe₂ samples were verified by stitching optical microscope images into a mosaic map of the sample surface. For “monolayer” substrates, wafers were selected in which monolayer regions covered the majority of the observed area and bilayer/multilayer regions were minimal. For “bilayer” substrates, wafers were selected where bilayer regions dominated and higher-order layers were sparse. Multilayer-rich wafers were excluded to avoid additional interfacial complexity. Each selected substrate type (monolayer, bilayer, or bare Si) was then used for Fe₇₃Co₈Gd₁₉ deposition to enable a systematic comparison of magnetic properties as a function of WSe₂ layer number. The measured TMD coverage for the WSe₂ monolayer and bilayer substrates was 84.9% and 84.2%, respectively.

Prior to alloy deposition, the DC magnetron sputtering rates for Fe, Co, and Gd targets were calibrated at a working Ar pressure of 7.3 mTorr. Single-element Fe, Co, and Gd films were sputtered at room temperature onto Si substrates over a 20–100 W power range at a fixed deposition time. Film thicknesses, measured by XRR, were divided by the deposition time to obtain deposition rates for each power setting, and linear fits to these data yielded power–rate curves for each source. With Fe power fixed at 100 W, the calibration curves indicated optimal Co and Gd powers of 8 W and 24 W, respectively, to deliver an average Fe:Co:Gd flux ratio of 73:8:19. The resulting alloy composition was verified by scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS). Using the calibrated Fe₇₃Co₈Gd₁₉ deposition rate, alloy thin films were deposited at room temperature on Si, Si/WSe₂ bilayer, and Si/WSe₂ monolayer substrates by DC magnetron sputtering. The base pressure prior to deposition was maintained below 1.9 × ${10}^{-7}$ Torr, and the working pressure during sputtering was 7.3 mTorr. Film thickness was controlled by varying the sputtering time, yielding nominal values of 4, 8, 16, and 32 nm. All films were capped with a 5 nm Ta layer to minimize surface oxidation and contamination.

Chemical composition was confirmed by SEM-EDS, verifying the intended Fe₇₃Co₈Gd₁₉ atomic composition on the 32 nm set of samples. Structural characterizations were carried out by XRD to verify the amorphous nature of the FeCoGd films, and by XRR to determine thickness, density, and interface roughness. Raman spectroscopy was used to assess the integrity of the WSe₂ substrates after deposition, performed with a confocal Raman spectrometer (LabRAM HR Evolution, Horiba Scientific (Irvine, CA, USA)) in backscattering geometry using a 532 nm excitation laser. Magnetic characterization was performed using a Quantum Design VSM. In-plane magnetization hysteresis (M–H) loops were measured at 10, 20, 25, 30, 40, 50, 100, 150, 200, 250, and 300 K. From each loop, coercivity ($H_c$), saturation magnetization ($M_s$), and magnetic squareness ($M_r/M_s$) were extracted to characterize the magnetic response as a function of temperature, thickness, and substrate.

3. Results

3.1. Structural and Compositional Confirmation

To evaluate how the crystalline quality of the WSe₂ layer is preserved after depositing the magnetic films, and confirming the amorphous nature of the magnetic films, Raman spectroscopy and X-ray diffraction (XRD) were performed on representative samples. Figure 1a,b shows optical images of monolayer WSe₂ regions before and after the deposition of a 4 nm Fe₇₃Co₈Gd₁₉ (FCG) film. The images reveal that the morphology of the WSe₂ flakes are preserved after the deposition process. The Raman spectra of monolayer and bilayer WSe₂, before and after deposition of the FCG film are presented in Figure 1c,d. Raman spectra were taken in seven different spots of each sample to check for homogeneity in the sample. The in-plane $E_{2g}^1$ and out-of-plane $A_{1g}$ vibrational modes, are located near 250 cm⁻¹ and 260 cm⁻¹, respectively [17,26]. The peaks remain relatively unaltered, in terms of peak position and full width at half maximum, after FCG deposition, suggesting that the crystalline quality of the WSe₂ layers is preserved. Small shifts of less that a wavenumber were observed occasionally at some specific points in the surface that could be associated with random and negligible local strain, but this is not representative of the entire sample [17]. These Raman analyses confirm that monolayer and bilayer WSe₂ provide a stable platform for investigating proximity-induced magnetic effects in ultrathin FCG films [6,7,13].

XRD patterns of Fe₇₃Co₈Gd₁₉ thin films grown on Si, WSe₂ bilayer and WSe₂ monolayer substrates (Figure 2a–c) show only Si substrate-related peaks, with no observable diffraction peaks from the magnetic alloy. This absence of long-range order confirms the amorphous nature of the FeCoGd phase [2,5]. XRR measurements demonstrated that the intended thicknesses (32 ± 3, 16 ± 0.7, 8 ± 0.9, and 4 ± 0.2 nm) were achieved. The films exhibited smooth interfaces, with roughness values below 1.7 nm and densities ranging from 7.8 to 8.8 g/cm³ across all samples.

3.2. Temperature Dependent Magnetic Behavior and Effect of Substrate and Thickness

In-plane M–H hysteresis loops were measured from 10 K to 300 K for Fe₇₃Co₈Gd₁₉ films on Si, WSe₂ bilayer (84.2% coverage), and WSe₂ monolayer (84.9% coverage) substrates. Upon cooling to 10 K (Figure 3a–c), all films exhibit increased coercivity and more square loop shapes compared to their 300 K counterparts, consistent with reduced thermal activation. The 32 nm samples display well-defined square loops and significantly higher coercivity, indicative of stronger domain wall pinning [27]. Among the 4 nm films, the WSe₂ monolayer sample exhibits the largest coercive field and the lowest squareness, suggesting enhanced magnetic hardening accompanied by reduced reversal coherence at low temperature. In contrast, the WSe₂ bilayer exhibits a wasp-waisted loop shape, indicative of spatially inhomogeneous magnetization reversal, which may arise from interfacial spin disorder or local anisotropy variations.

Figure 4a–c show the temperature dependence of saturation magnetization ($M_s$) for 4 nm and 32 nm Fe₇₃Co₈Gd₁₉ films. Across all substrates, $M_s$ exhibits a broad maximum between 100 K and 200 K, a common feature of RE–TM ferrimagnets where the unequal thermal evolution of the rare-earth and transition-metal sublattices produces a non-monotonic variation in the net moment [2,5,9,10]. At temperatures below 50 K, $M_s$ decreases again, consistent with the system approaching compensation. The $M_s$ values discussed here are extracted from fully saturated hysteresis loops at each temperature, ensuring that the reported magnetization reflects true saturation within the 10–300 K measurement range. On Si, both thicknesses follow similar trends, with the 4 nm film maintaining slightly higher $M_s$ across the entire range. For WSe₂ bilayer, however, the 4 nm film consistently shows lower $M_s$ than the 32 nm film, displaying a suppression of net moment at reduced thickness [7]. In contrast, the WSe₂ monolayer system displays a distinct enhancement, where the 4 nm film reaches the highest $M_s$ among all samples (∼790 kA/m at 100 K).

Figure 4d–f show the temperature evolution of magnetic squareness ($M_r/M_s$) for 4 nm and 32 nm Fe₇₃Co₈Gd₁₉ films. For all substrates, 32 nm films retain high and stable squareness (>0.80), indicative of coherent magnetization reversal across the full temperature range. In contrast, 4 nm films exhibit marked thermal reduction in squareness, with $M_r/M_s$ steadily decreasing as temperature rises. This effect is most pronounced for the WSe₂ monolayer, where squareness drops by over 50%—from ∼0.72 at 10 K to ∼0.35 at 300 K. The Si and bilayer samples follow similar trends, though with more moderate reduction.

Figure 5a–c show the temperature dependence of coercive field ($H_c$) for 4 nm and 32 nm Fe₇₃Co₈Gd₁₉ films on Si, WSe₂ bilayer, and WSe₂ monolayer substrates. In all cases, $H_c$ increases sharply below ∼50 K, consistent with suppressed thermal activation and enhanced domain wall pinning [27]. For the 32 nm films, $H_c$ reaches ∼22 mT at 10 K on both Si and bilayer substrates. In contrast, the 4 nm WSe₂ monolayer sample exhibits the steepest increase, with $H_c$ rising from ∼2.5 mT at 300 K to ∼24 mT at 10 K, surpassing its thicker counterpart. Meanwhile, 4 nm samples on Si and bilayer remain below 10 mT and consistently below their 32 nm counterparts across the temperature range.

4. Discussion

The Raman spectroscopy measurements confirm that the WSe₂ substrates retain their morphology and vibrational integrity after FeCoGd deposition, validating the compatibility of the sputtering process with 2D substrates. The persistence of the Raman-active modes near 250 cm⁻¹, along with the distinct monolayer and bilayer WSe₂ regions observed optically, demonstrates that the interfacial structure remains well-defined, a critical requirement for achieving reproducible proximity effects [7]. The absence of diffraction peaks in the XRD data corroborates the amorphous nature of the FeCoGd films, consistent with prior reports on RE–TM alloys, and rules out crystallinity as a factor influencing magnetic response or tunability [2,5]. Together with XRR confirming smooth interfaces (roughness below 1.7 nm) and EDS verifying consistent elemental composition across all samples, these findings establish a structurally and chemically controlled platform for exploring proximity-driven magnetic behavior [6]. The elemental composition showed no substrate dependence, reinforcing that the magnetic variations discussed in later sections arise from intrinsic thickness effects [6,7,8,19].

We note, however, that while XRD, XRR, Raman spectroscopy, and EDS confirm amorphous growth, interface continuity, and overall compositional consistency, these techniques do not directly resolve atomic-scale chemical sharpness or nanoscale intermixing at the interface. Substrate-dependent magnetic variations could, in principle, also arise from conventional mechanisms such as interfacial strain, local density variations, or the formation of magnetically reduced interfacial layers. Nevertheless, prior studies have demonstrated robust interface-driven spin–orbit phenomena in metal/TMD heterostructures, including graphene, WSe₂ and WS₂-based systems [13,15,16,18], and these studies strongly suggest modification of magnetic properties arising from the interface with such two-dimensional layers. The preservation of WSe₂ post-deposition, confirmed by Raman scattering, indicates that the observed magnetic modulation is associated with the interface rather than structural degradation. Additional nanoscale structural characterization (e.g., cross-sectional TEM/STEM or depth-sensitive spectroscopy) that would further confirm the interfacial origin of the observed effects will be the subject of future reports.

Edge- and defect-induced magnetic moments in TMD materials have been discussed in the literature, particularly for nanoribbons and highly confined geometries [21]. In the present samples, magnetic measurements probe macroscopic areas, and any edge-localized magnetism confined to atomically narrow flake perimeters represents a negligible fraction of the total interfacial area. Given typical lateral flake dimensions on the order of tens of microns, the areal contribution of edge regions is several orders of magnitude smaller than that of the continuous Fe₇₃Co₈Gd₁₉ layer. Therefore, edge-related magnetism is unlikely to dominate the observed substrate-dependent trends.

At room temperature, the magnetic response of FCG films shows a clear dependence on both film thickness and substrate. Coercivity ($H_c$) increases modestly with thickness across all substrates, consistent with enhanced magnetic hardness from larger switching volumes and domain-wall pinning [27]. The WSe₂ monolayer system stands out in the ultrathin limit, where the 4 nm film exhibits reduced squareness (∼0.35). By contrast, 4 nm ultrathin films grown on WSe₂ bilayer substrates consistently exhibit lower $M_s$. Because the measured WSe₂ coverage is comparable for the monolayer and bilayer substrates (84.9% and 84.2%, respectively), the systematic differences in magnetic behavior between these two cases are unlikely to arise from the uncovered Si/SiO₂ regions. The combination of suppressed $M_s$ and reduced squareness at 4 nm may arise from interfacial spin disorder that disrupts moment alignment near the TMD interface. Additionally, enhanced surface or interfacial anisotropy in the ultrathin limit may cant spins out of plane or promote non-uniform reversal, both of which suppress in-plane magnetization and squareness [5,7,8]. These variations suggest that proximity-induced spin suppression is most pronounced in ultrathin films, but weakens as the FCG layer becomes thicker, reflecting the reduced reach of interfacial effects [7,8,13]. Overall, the room-temperature trends indicate that while thickness governs magnetic hardness, TMD-based substrates provide an additional degree of freedom for tailoring net moment and reversal sharpness [6,13,19].

Building on this baseline, cooling the films reveals how reduced thermal activation further amplifies interfacial effects and magnetic hardness. Temperature-dependent trends in saturation magnetization ($M_s$) further emphasize the role of interfacial modulation. All samples exhibit a broad $M_s$ maximum between 100 K and 200 K. This maximum reflects the temperature-dependent imbalance between the FeCo (TM) and Gd (RE) magnetic sublattices. Within the measured 10–300 K range, the FeCo sublattice moment increases and then remains relatively high, while the Gd sublattice is strongly thermally quenched and continues decreasing up to ∼100 K. Between 100–200 K, FeCo remains large and Gd remains minimal, producing the largest difference between their magnetizations and thus a broad maximum in the net $M_s$. Beyond ∼200 K, the FeCo moment begins to thermally decrease while the Gd moment partially recovers, reducing the TM–RE sublattice difference and causing the observed decline in $M_s$ (T) [5,9,10]. The 4 nm WSe₂ monolayer sample displays the highest $M_s$ (∼790 kA/m at 100 K). This enhancement may result from improved spin alignment or local ordering at the monolayer interface, which promotes enhanced saturation magnetization in the ultrathin limit [8]. The increased simultaneous observation of $H_c$ and $M_s$ in 4 nm films on monolayer suggests that interfacial coupling with WSe₂ monolayer effectively suppresses thermal spin disorder, thereby enhancing magnetic rigidity and stabilizing the magnetic moment against thermal fluctuations at low temperatures. This behavior may offer advantages for systems requiring robust magnetic response at reduced dimensions, such as thermally stable switching layers in ferrimagnetic spintronic architectures [6,7,13]. These findings demonstrate that ultrathin films on monolayer can preserve or even enhance magnetic performance under conditions where saturation magnetization is typically suppressed in amorphous ferrimagnets [6].

In contrast to the 4 nm films on monolayer WSe₂, the 4 nm film on bilayer WSe₂ shows a pronounced suppression of $M_s$ across all temperatures. In the ultrathin limit (∼4 nm), the saturation magnetization becomes highly sensitive to small changes in effective magnetic thickness and interfacial magnetic structure. XRR measurements indicate roughness values of approximately 0.47 nm on Si and ∼0.18–0.19 nm on monolayer and bilayer WSe₂ substrates, corresponding to a non-negligible fraction of the nominal thickness. Since the roughness values for monolayer and bilayer WSe₂ are comparable, roughness alone cannot explain the reduced $M_s$ on bilayer WSe₂. The suppression is therefore more consistent with a substrate-dependent magnetically modified interfacial region whose relative contribution is maximized in the ultrathin limit and diminishes as thickness increases. By comparison, the monolayer WSe₂ substrate is associated with higher magnetization in ultrathin films, suggesting that the nature of the interface plays an important role in determining the saturation magnetization. Possible explanations reported in prior studies include reduced interlayer screening and stronger spin–orbit proximity effects in monolayer TMD interfaces, which can influence magnetic ordering in adjacent layers [7,8,13,15]. The $M_s$ suppression of films on bilayer may be advantageous for ultra dense, low-power spintronic platforms. Lower $M_s$ reduces stray (dipolar) fields and inter-element cross-talk in closely spaced devices [7,28]. In spin–orbit torque (SOT) switched structures, the critical switching current roughly scales with $M_s$t, where t is the magnetic layer thickness, hence reducing $M_s$ may help lower the write current without changing geometry [7,28]. For spin–torque oscillators (STOs), a lower $M_s$ can also reduce the demagnetizing field and is compatible with high frequency ferrimagnetic dynamics [4]. Notably, the WSe₂ bilayer/Fe₇₃Co₈Gd₁₉ heterostructures exhibit the lowest saturation magnetization across the entire temperature range among all samples, with the 4 nm film maintaining a soft magnetic character ($H_c$ < 10 mT) from 10 K to 300 K. Whereas ultrathin films on monolayer, with high M_s and large H_c, are well-suited for thermally stable switching-layer applications, films on bilayer provide complementary advantages for dynamic spintronic devices, where their lower M_s and softer magnetic response support low-power operation and high-frequency ferrimagnetic dynamics [3,4,7,28].

The 32 nm films maintain a high squareness of hysteresis loops with $M_r/M_s$ > 0.80 across all substrates, whereas the 4 nm monolayer sample drops from ∼0.72 at 10 K to ∼0.35 at 300 K, indicating less coherent reversal at elevated temperature in the ultrathin limit. This behavior is consistent with thermally activated multi-domain reversal and interface-sensitive switching reported for ultrathin RE–TM films [27,29]. This decline may also reflect partial thermal unpinning of spins at the FiM/TMD interface, which reduces reversal coherence as domain-wall motion becomes less constrained [27,29]. Similar, but less pronounced, trends in films on WSe₂ bilayer and on Si show that the thickness of the magnetic layer mainly determines how uniformly the magnetization switches direction, while the TMD interface further adjusts this behavior. These interface effects become more noticeable as thickness decreases [6,7]. The robustness of the 32 nm loops underscores the stabilizing effect of thickness, while the differences between substrates emphasize the critical role of interface engineering in tailoring thermal stability and reversal dynamics in ferrimagnetic heterostructures [6,13,19]. Taken together, the increase in $H_c$ at low T and the drop in $M_r/M_s$ for 4 nm films indicate a pinning-dominated but less coherent reversal in the ultrathin limit [27]. While domain-wall pinning and disorder effects contribute to coercivity in amorphous films, the systematic thickness- and substrate-dependent scaling observed here suggests an additional interfacial contribution in the ultrathin limit. This is consistent with a stronger interface/proximity influence at small thickness and partial decoupling as thickness increases [6,7,8,15,19].

The temperature-dependent coercivity ($H_c$) of TMD/Fe₇₃Co₈Gd₁₉ heterostructures increases sharply below 50 K across all substrates, reflecting a substantial enhancement of magnetic hardness at low temperatures. This trend indicates a transition from thermally assisted to pinned magnetization reversal, consistent with prior reports in ultrathin ferrimagnets where enhanced interfacial anisotropy and spin pinning, driven by suppressed thermal activation, contribute to domain-wall immobilization [6,7,27,29]. The effect is especially pronounced in 4 nm films on WSe₂ monolayer, where $H_c$ exceeds 23 mT, surpassing even the 32 nm counterpart, suggesting that interfacial effects may become increasingly significant in the ultrathin limit when thermal fluctuations are reduced [7,8,15]. The pronounced contrast between films grown on monolayer- and bilayer-WSe₂ substrates underscores the sensitivity of proximity-driven effects to the atomic-scale structure of the 2D TMD substrate [8].

Notably, in the FeCoGd on WSe₂ monolayer system, a clear crossover occurs near 75 K, where the coercivity of the 4 nm film surpasses that of the 32 nm counterpart. Above this temperature, coercivity increases with thickness, consistent with bulk-like domain-wall pinning [27], whereas below 75 K, the rapid hardening of the ultrathin film indicates that interfacial anisotropy and exchange coupling become the dominant contributions to magnetization reversal [6,7,8]. This crossover therefore marks a transition from volume- to interface-dominated behavior, consistent with a substrate-dependent interfacial magnetic contribution at the WSe₂/FeCoGd boundary [13,15,16].

Collectively, these findings demonstrate that the magnetic response of TMD/Fe₇₃Co₈Gd₁₉ heterostructures arises from the coupled effects of temperature, thickness, and interfacial proximity. WSe₂ substrates provide an effective means to tune coercivity, saturation magnetization, and reversal coherence, underscoring the central role of interface-driven modulation in ultrathin ferrimagnets. This tunability highlights the importance of precise interface engineering in TMD/ferrimagnetic alloys and points to their broader potential in next-generation spintronic technologies. Additionally, the observed temperature-dependent degradation in reversal coherence may hinder reliable switching in spintronic devices such as SOT–MRAM and FiM oscillators, where high squareness is essential for thermal stability, energy-efficient operation, and robust data retention [3,4,13,19].

5. Conclusions

We systematically investigated thickness- and temperature-dependent magnetic properties in ultrathin Fe₇₃Co₈Gd₁₉ films grown on Si, WSe₂ bilayer, and WSe₂ monolayer substrates. The most notable finding is the sharp increase in coercivity ($H_c$) at low temperatures, particularly in ultrathin films on WSe₂-monolayer where $H_c$ exceeds 23 mT below 50 K and surpassing thicker counterparts. This dramatic enhancement of magnetic hardness highlights the dominant role of interfacial coupling when thermal fluctuations are suppressed. Beyond coercivity, structural and magnetic characterization reveal that both thickness and the atomic-scale configuration of the TMD substrate critically influence saturation magnetization and reversal coherence. While WSe₂ monolayers promote enhanced spin alignment and strong interfacial coupling, bilayer substrates suppress magnetization and maintain a soft magnetic character. These contrasting behaviors establish TMD/FiM heterostructures as versatile platforms for interface-driven control of ferrimagnetic properties with relevance to tunable spintronic device applications. Looking forward, extending this work to quantify out-of-plane anisotropy and damping, and to achieve device-level integration, will be essential for realizing ultrathin ferrimagnets in next-generation spintronic technologies.