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Takahiro Yamazaki, Takahiro Kawasaki, Alexandre Lira Foggiatto, [Ryo Toyama](https://orcid.org/0000-0002-7398-5803), Kentaro Fuku, Varun Kumar Kushwaha, Yoshinori Kotani, Takuo Ohkochi, Kotaro Higashi, Naomi Kawamura, [Yuya Sakuraba](https://orcid.org/0000-0003-4618-9550), [Yuma Iwasaki](https://orcid.org/0000-0002-7117-277X), Masato Kotsugi

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[Uncovering the origin of magnetic moment enhancement in Fe–Co–Ir alloys via high-throughput XMCD](https://mdr.nims.go.jp/datasets/6d57c900-2754-413e-97e4-ac7dcaffd2df)

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1  Uncovering the origin of Magnetic Moment Enhancement in Fe–Co–Ir 1 Alloys via High-Throughput XMCD 2  3 Takahiro Yamazaki1,*, Takahiro Kawasaki1, Alexandre Lira Foggiatto1, Ryo Toyama2, Kentaro Fuku1, 4 Varun Kumar Kushwaha2, Yoshinori Kotani3, Takuo Ohkochi3,4, Kotaro Higashi3, Naomi Kawamura3, 5 Yuya Sakuraba2, Yuma Iwasaki5, Masato Kotsugi1 6  7 1 Department of Material Science and Technology, Tokyo University of Science, 6-3-1 Niijuku, 8 Katsushika, Tokyo 125-8585, Japan 9 2 Research Center for Magnetic and Spintronic Materials (CMSM), National Institute for Materials 10 Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan 11 3 Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan 12 4 Laboratory of Advanced Science and Technology for Industry, University of Hyogo, Hyogo 678-13 1205, Japan 14 5 Center for Basic Research on Materials (CBRM), National Institute for Materials Science (NIMS), 15 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan 16 *Corresponding author: takahiro.yamazaki@rs.tus.ac.jp  17  18 Abstract 19 High-magnetization ferromagnets play a crucial role in advancing spintronics, magnetic sensing, and 20 high-density data storage technologies. Fe–Co–Ir alloys, previously identified through machine 21 learning to potentially surpass the Slater-Pauling limits for transition-metal magnets, present a 22 valuable platform for uncovering the mechanisms behind their enhanced magnetic properties. This 23 study investigates the influence of Ir doping on the element-specific magnetic moments by high-24 throughput X-ray Magnetic Circular Dichroism (XMCD) techniques using both soft and hard X-rays 25 on Fe–Co–Ir single-crystal composition-spread thin films. A single-crystal composition-spread thin 26 film of (Fe75Co25)100-xIrx (x = 0–11 at%) was fabricated on a MgO(100) substrate using a combinatorial 27 sputtering technique, enabling a systematic analysis of compositional variations. XMCD 28 measurements revealed that Ir doping enhances the magnetic moments of Fe, Co, and Ir, with a 29 particular emphasis on the orbital contribution of Fe and Ir. These experimental findings are supported 30 by ab-initio calculations, which highlight increased electron localization and stronger spin-orbit 31 coupling between Ir 5d electrons and Fe/Co 3d electrons as the primary mechanisms behind the 32 enhanced magnetization. This study provides a deeper understanding of the electronic and magnetic 33 moments in Fe–Co–Ir alloys, offering valuable insights for the development of next-generation 34 ferromagnetic materials optimized for advanced technological applications. 35  36 2  Keywords: ferromagnetic materials, soft and hard X-ray magnetic circular dichroism (XMCD), 1 element-specific analysis, magnetic moments, Fe–Co–Ir alloys, combinatorial sputtering, ab-initio 2 calculations, high-throughput measurements 3  4 I. INTRODUCTION 5 High-magnetization ferromagnets are indispensable in the evolution of spintronics, magnetic sensing, 6 and high-density data storage technologies, where their precise control over spin dynamics remains a 7 fundamental component of next-generation device development [1–4]. Among these materials, the 8 binary Fe75Co25 alloy stands out due to the strongest magnetic moments among the 3d transition metal 9 magnets based on the Slater-Pauling rule [5], combined with its high Curie temperature and robust 10 phase stability [6–10]. To further enhance magnetization, epitaxially grown films with their controlled 11 crystal structures and lattice parameters have demonstrated remarkably high magnetization and 12 offering optimal conditions for magnetic enhancement [11–14]. For instance, ultrathin epitaxial films 13 of Fe–Co–Mn systems exhibit large magnetization [13,14], though this has proven difficult to achieve 14 in non-epitaxial films or bulk materials [10]. Accurately predicting, validating, and fully understanding 15 the mechanisms driving the magnetic properties of Fe–Co based alloys, especially given the 16 complexities of multi-component systems, is a significant scientific challenge. 17 Recent advances in computational techniques, such as integration of machine learning and 18 ab-initio calculations, have greatly accelerated the discovery of new materials from vast compositional 19 spaces [15–23]. For example, Iwasaki et al. predicted magnetic moments in Fe–Co–Ir–Pt alloys that 20 exceed those of conventional binary Fe–Co alloys through an autonomous exploration framework, 21 which was successfully validated by fabricating magnetron-sputtered (Fe75Co25)97.5Ir2.5 polycrystalline 22 films [21]. However, the use of polycrystalline films and macroscopic magnetization measurement via 23 vibrating-sample magnetometer (VSM) limited the detailed investigation of intrinsic magnetic 24 properties. To overcome these limitations, advanced experimental approaches for quantitative 25 determination of microscopic magnetic moments using single-crystal films are needed to validate 26 these predictions and provide deeper insights into the fundamental magnetic mechanisms. 27 The addition of 5d transition metals plays a critical role in tuning the magnetic properties of 28 transition-metal based alloys [21, 24–35]. Toyama et al. demonstrated intriguing phenomena in single-29 crystal Fe–Co–Ir thin films, such as anisotropic magnetoresistance (AMR) effect, anomalous Hall 30 effect (AHE), and anomalous Nernst effect (ANE) [24, 25]. Similarly, other alloy systems like Fe–Pt–31 Ir [30] and Fe–Au [31], Ni–Pd [32] have shown unique magnetic behaviors, such as perpendicular 32 magnetic anisotropy, antisymmetric exchange interaction and enhanced magnetic moments, 33 respectively. These effects, largely driven by spin-orbit coupling, result in significant changes to both 34 electronic structure and magnetism. Despite these advances, the interplay between the electronic 35 structure modifications induced by 5d elements and the magnetic states arising from 3d electron 36 3  behavior remains insufficiently understood. 1 To overcome the challenges in accurately analyzing magnetic properties, we have 2 implemented a high-throughput approach utilizing X-ray Magnetic Circular Dichroism (XMCD) on 3 compositionally graded thin films. Combinatorial synthesis is an efficient method for rapidly 4 discovering new materials, enabling the systematic investigation of extensive material libraries [36–5 39]. Combined with XMCD technique, it provides a detailed, element-specific understanding of 6 magnetic moments by distinguishing between orbital and spin contributions to overall magnetization 7 [40–43]. By using soft X-rays for 3d transition metals such as Fe and Co, and hard X-rays for heavy 8 metal like Ir, we can achieve a detailed understanding of the individual elements’ contributions to 9 magnetic behavior. Unlike earlier studies on non-single-crystal films [23], our work focuses on single-10 crystal Fe–Co–Ir thin films, offering a more controlled environment for probing the mechanisms 11 underlying magnetic moment enhancements. This approach provides clearer insights into the intricate 12 atomic interactions responsible for the observed magnetic improvements. 13 This study employs cutting-edge X-ray Magnetic Circular Dichroism (XMCD) techniques 14 to investigate the role of Ir doping in enhancing the magnetic properties of single-crystal composition-15 spread thin films of (Fe75Co25)100-xIrx (x = 0–11 at%). Using a combinatorial sputtering system, we 16 systematically explored composition-dependent magnetic behaviors. Through the high-throughput 17 XMCD measurements utilizing both soft and hard X-rays, we captured element-specific magnetic 18 moments, uncovering how Ir doping intricately influences the spin and orbital contributions to 19 magnetization. Complemented by ab-initio calculations, our analysis provides detailed insights into 20 the electronic structure and magnetic moments responsible for the observed enhancements in 21 magnetization. These experimental and theoretical approaches advance the understanding of 22 electronic and magnetic interactions in Fe–Co–Ir alloys, contributing to the development of next-23 generation ferromagnetic materials optimized for technological applications. 24  25 II. METHODS 26 A. Sample preparation 27 Single-crystal composition-spread thin film of (Fe75Co25)100-xIrx (x = 0–11 at%) was prepared on 28 MgO(100) substrate using a combinatorial sputtering system (CMS-A6250X2, Comet Co., Ltd.), 29 following methodologies reported in prior studies [24,25,44]. This approach allows the acquisition of 30 a composition gradient within a single film (Fig. 1(a)), enabling high-throughput analysis by numerous 31 measurement points. For the composition-spread film, Fe and Co targets were co-sputtered, with 32 power setting adjusted to yield a 0.44 nm-thick film corresponding to the Fe75Co25 composition. To 33 achieve a wedge-shaped deposition, a linear moving mask was used, varying the thickness gradient 34 from 0.00 to 0.06 nm over a 7 mm width. The substrate was then rotated 180°, and a similar wedge-35 shaped Ir layer was deposited using the same mask, ensuring a complementary gradient. The process 36 4  was repeated 60 times, resulting in a 30 nm thick (Fe75Co25)100-xIrx film. The sputtering process was 1 conducted at room temperature in an argon atmosphere at a pressure of 0.8 Pa. Post-deposition, the 2 film was subjected to vacuum annealing at 653 K for 30 minutes to promote uniform composition 3 distribution. To prevent oxidation, a 2-nm thick Ru capping layer was subsequently deposited. The 4 compositional analysis using fluorescence X-ray analysis (ZSX Primus Ⅱ, Rigaku) confirmed a 5 gradient from Fe75.4Co24.6 at one end of the film to (Fe76.1Co23.9)89.0Ir11.0 at the opposite end, 6 establishing a linearly continuous compositional gradient across a span of 7 nm. The phase transition 7 from A2-disordered to B2-ordered Fe75Co25–Ir structure was confirmed with Ir concentration at least 8 from 2.1 at% [24], with Ir atoms occupying specific lattice sites, shown in blue sites as shown in Fig. 9 1(b). 10  11  FIG. 1. (a) Schematic illustration of the fabricated compositionally graded Fe75Co25–Ir film, exhibiting a gradient in composition from Fe75.4Co24.6 to (Fe76.1Co23.9)89.0Ir11.0 across a 7 mm span. The film is capped with a 2.0 nm Ru layer and is deposited on an MgO(100) substrate. (b) Schematic of the B2-ordered crystal structure in the Fe–Co–Ir alloy, highlighting the atomic arrangement within the alloy. (c) Experimental setup for X-ray Magnetic Circular Dichroism (XMCD) measurements using a synchrotron X-ray source generated by a twin helical undulator, with measured positions indicated along the compositional gradient of the film.  12 B. Soft and hard XMCD measurements 13 XMCD measurements were performed to investigate the element-specific electronic structure and 14 magnetic moments of the Fe, Co, and Ir elements in the (Fe75Co25)100-xIrx (x = 0–11 at%) alloy. These 15 measurements were conducted at the SPring-8 synchrotron facility in Japan. For the L2,3 absorption 16 edges of Fe and Co, XMCD spectra were collected at BL25SU [33,45], while XMCD spectra for the 17 L2,3 absorption edges of Ir were obtained at BL39XU [46]. Circularly polarized soft X-rays, generated 18 by a twin helical undulator [47], were directed parallel to the applied magnetic field (Fig. 1(c)). XMCD 19 measurements were performed at 77 K under magnetic fields of 1.9 T at BL25SU and 2.4 T at 20 BL39XU, respectively. Notably, the magnetic field was confirmed to be nearly saturated during the 21 5  measurements (see Fig. S1 in the supplementary materials). 1  X-ray Absorption Spectroscopy (XAS) spectra were acquired using the total electron yield 2 method, with XMCD spectra derived from the difference between XAS spectra obtained under left 3 and right circularly polarized X-rays. At BL25SU, the XAS spectra for Fe and Co were measured over 4 the energy ranges of 700–751 eV and 772–834 eV, respectively. To reduce noise in the XAS signals, 5 a five-point smoothing process was applied to the spectra. At BL39XU, the spectral ranges for Ir were 6 11.18–11.27 keV and 12.79–12.87 keV. The compositional gradient within the Fe–Co–Ir film allowed 7 for the collection of 32 distinct XAS spectra at BL25SU and 14 at BL39XU, effectively capturing 8 across different compositions. The sub-millimeter beam spot diameter ensured precise correspondence 9 of each data point to a specific compositional range, thereby enhancing the consistency and reliability 10 of the data. 11  12 C. Ab-initio calculations 13 Ab-initio calculations were carried out using the Korringa-Kohn-Rostoker Coherent Potential 14 Approximation (KKR-CPA) method, with the computations executed using the AkaiKKR software 15 [32]. The calculations focused on both the A2-disordered and B2-ordered structures. For each 1 at% 16 increment in Ir content, spin and orbital magnetic moments, as well as the density of state (DOS) were 17 computed for the B2-ordered (Fig. 1(b)) and A2-disordered structure to investigate the electronic and 18 magnetic states of the alloy. 19  20 III. RESULTS AND DISCUSSION 21 A. XMCD spectra analysis 22 Fig. 2 illustrates the examples of XAS and XMCD spectra at the L3 and L2 absorption edges of Fe, Co, 23 and Ir in the (Fe75Co25)95Ir5 thin film. In the XAS spectra, μ+ and μ−, correspond to the absorption for 24 left- and right-circularly polarized X-rays, respectively, with normalization applied using the pre-edge 25 and post-edge regions. The XMCD spectra can be derived from the differences between these spectra. 26 The spectral characteristics align with those reported in previous studies on similar alloy systems 27 [26,41]. The XMCD signal from Ir, as shown in Fig. 2(c), suggests that Ir, despite typically known for 28 its paramagnetic nature, can exhibit measurable magnetization under an external magnetic field. This 29 can be seen by the noticeable XMCD signal. Note that the secondary peak at the L2 absorption edge 30 of Ir, marked by an asterisk in the figure, is likely not intrinsic to Ir and may instead result from 31 diffraction effects related to the MgO(100) substrate or slight variations in lattice constants. 32 6   FIG. 2. The XAS (upper panels) and XMCD (lower panels) spectra at the L3 and L2 absorption edges for (a) Fe, (b) Co, and (c) Ir in Fe–Co–Ir alloys, specifically for the composition (Fe75Co25)95Ir5 in a compositionally graded film.  1 B. Sum-rule analysis for spin and orbital magnetic moments 2 The analysis of XMCD data reveals contributions from Fe, Co, and Ir to the magnetic properties of 3 the Fe–Co–Ir alloy system. These contributions were quantified using sum-rule analysis, which 4 determines the effective spin magnetic moment ( mspineff = mspin+7〈Tz〉) and the orbital magnetic 5 moment (morb ) for each element [40,41]. The 〈Tz〉  term, also known as the magnetic dipole term, 6 quantifies the anisotropy in the spin density distribution. It arises from non-spherical contributions to 7 the spin magnetic moment due to spin-orbit interaction and the crystal field. The calculations 8 expressed as follows: 9 𝑚𝑠𝑝𝑖𝑛𝑒𝑓𝑓= −6𝑝 − 4𝑞𝑟𝑛ℎ (1) 10 𝑚𝑜𝑟𝑏 = −4𝑞3𝑟𝑛ℎ (2) 11 where p represents the integrated intensity of the XMCD spectrum at the L3 edge, and q is the 12 integrated intensity over both the L3 and L2 edges. The parameter r denotes the integrated intensity of 13 the white line region in the XAS spectra, adjusted by subtracting a step function. The nh indicates the 14 number of holes in the 3d and 5d bands, which were determined using ab-initio KKR calculations for 15 each element in the Fe-Co-Ir alloy system. Detailed results, including the dependence of nh on Ir 16 composition, are provided in Fig. S2 of the supplementary materials. 17 Prior studies have demonstrated the efficacy of XMCD measurements and the sum-rule 18 analysis for various magnetic materials, such as L10-FeNi alloys [48] and Fe–Co–Cr atomic-layered 19 films [49]. While 〈Tz〉  is generally included in the calculation of mspineff , its contribution is often 20 negligible in thick films. In this study, the 30 nm-thick Fe–Co–Ir films exhibit negligible 〈Tz〉 21 contributions, consistent with previous findings for similar systems [41,50]. Furthermore, it is 22 important to consider that the saturation effect may influence the quantitative accuracy of the sum-23 7  rule analysis, particularly in bulk-like films such as the present 30 nm-thick Fe–Co–Ir samples [51]. 1 While the Ir addition is minimal and the perpendicular incidence angle is consistent across all 2 measurement points, saturation effects could still vary depending on the intrinsic X-ray penetration 3 depth and electron escape depth, which may differ with composition. Although angular-dependent 4 measurements are required to address these effects quantitatively, the present study is limited to a 5 single perpendicular geometry. As such, while the saturation effect correction does not affect the 6 relative comparison of elemental magnetic moment variations, we note that this should be considered 7 for a precise discussion of absolute magnetic moment values. 8 Fig. 3 shows the experimental results for the spin and orbital magnetic moments of Fe, Co, 9 and Ir, plotted as functions of Ir concentration. The sum-rule analysis reveals a positive correlation 10 between Ir content and the magnetic moments of all three elements. The slopes of the spin and orbital 11 moments demonstrate minimal differences, highlighting the significant contribution of each element 12 to the overall magnetization of the alloy. The linear fits of our experimental data provide further insight. 13 Fe, the predominant element, shows the most significant increase in both spin and orbital moments—14 1.07 and 1.41 times those of Fe75Co25, respectively. Co exhibits a moderate increase, with its spin and 15 orbital moments rising to 1.12 and 1.08 times their initial values. Notably, Ir shows a pronounced 16 enhancement, with its spin moment increasing 1.07 times and its orbital moment to 1.53 times their 17 original values, emphasizing its crucial role in enhancing the overall magnetic properties. It should be 18 noted that the data point for 1 at% Ir was excluded from the linear fitting for the Ir element because 19 the signal intensity of the Ir spectrum at such low concentrations was weak, resulting in low reliability. 20 The scatter in the measurement data can be attributed to challenges in thin-film experiments, such as 21 the use of 30 nm-thick films and short signal accumulation times for XMCD measurements, 22 particularly for low-signal elements. Nevertheless, the trends observed from the linear fits allow us to 23 draw meaningful conclusions regarding Ir effect on magnetic behavior.  24 Interestingly, Ir, which is typically paramagnetic, exhibits measurable magnetic moments 25 when alloyed with Fe and Co, supporting prior reports of Ir-induced magnetization in similar systems 26 [21,26–29]. For example, Iwasaki et al. [21] demonstrated that adding up to 8 at% of Ir concentration 27 to Fe–Co alloys can enhance saturation magnetization, a result supported by ab-initio calculations. In 28 contrast, Krishnamurthy et al. [27] observed a decrease in overall magnetic moments in Fe–Ir alloys 29 with Ir additions up to 17 at%. and Jiao et al. [29] similarly reported a reduction of the total 30 magnetization for Co–Ir alloys when the Ir concentration was increased from 8.9 to 18.9 at%. Our 31 results suggest that small amount of Ir can enhance the magnetic moments of individual elements in 32 Fe–Co–Ir alloys. However, as Ir content increases, the overall magnetization tends to decrease, likely 33 due to the weak magnetic contribution of Ir. Additionally, previous studies may show varying trends 34 because of factors such as non-single-crystal samples or differences in the ranges of Ir concentrations 35 investigated. It is also important to note that the scatter observed in the measurement plots is likely 36 8  due to challenges inherent in thin-film experiments, including the use of 30 nm-thick films and shorter 1 signal accumulation times required to capture multiple data points to analysis the trend of Ir addition. 2 These limitations affected the precision of XMCD measurements, particularly for elements with low 3 signal strength. Nevertheless, the linear fits still reveal clear trends, enabling us to draw meaningful 4 conclusions regarding the effect of Ir on the magnetic properties. 5  6  FIG. 3. Experimental results of spin and orbital magnetic moments for each element of (a, d) Fe, (b, e) Co, and (c, f) Ir, showing their variation with Ir concentration. (a), (b), and (c) display the spin magnetic moments. Also, (d), (e), and (f) illustrate the orbital magnetic moments. To illustrate the trends, linear approximations obtained using the least squares method have been included for each element.  7 Fig. 4 presents the theoretical spin and orbital magnetic moments of Fe, Co, and Ir for both A2-8 disordered and B2-ordered structures as functions of Ir concentration. Since prior studies [24], have 9 confirmed the presence of B2-ordered structure in Fe–Co–Ir alloys through XRD measurements, we 10 extended our theoretical calculations to include both A2 and B2 structures to fully capture the effects 11 of Ir addition. The results show that the magnetic moments increase with Ir concentration in both 12 structures, but the B2 structure yields consistently higher magnetic moments than the A2 structure. 13 In the B2-ordered structure, Fe demonstrates a larger increase, with the spin moment 14 increasing by 1.12 times and the orbital moment by 1.39 times as Ir content increases from 0% to 11 15 at%. Co exhibits a more modest increase, with the spin and orbital moments rising by 1.07 and 1.25 16 times, respectively. Ir shows the most notable enhancement, with its spin moment increasing by 1.11 17 times and its orbital moment showing a remarkable 8.28-fold increase. These results highlight the 18 9  critical role of Ir in enhancing the magnetic properties, especially in the B2 structure. 1 Both experimental and theoretical data consistently indicate that Ir addition enhances the 2 magnetic moments of Fe, Co, and Ir, with the largest effects seen in Fe and Ir. Although we performed 3 XMCD measurements on samples with an expected structural transition from A2 to B2 with at least 4 2.1% Ir addition, the changes were too subtle to definitively confirm the transition in this experiment. 5 This limitation should be carefully considered when interpreting the results, as it may affect the ability 6 to fully verify the influence of the B2 structure on the magnetic moments. Nonetheless, the theoretical 7 findings highlight the potential impact of the B2 ordering on the magnetic properties. 8  9  FIG. 4. Theoretical calculations of spin and orbital magnetic moments for each element: (a, d) Fe, (b, e) Co, and (c, f) Ir, with variation in Ir composition. (a), (b), and (c) display the spin magnetic moments, while (d), (e), and (f) illustrate the orbital magnetic moments. The symbols △ and 〇 represent the data for the A2-disordered phase and B2-ordered phase, respectively.  10 C. Total magnetic moments 11 Fig. 5 compares the experimental and theoretical results for the spin and orbital magnetic moments of 12 Fe, Co, and Ir, derived from the trends from Fig. 3. The addition of Ir enhances the magnetic moments 13 of all three elements in both experimental and theoretical datasets. However, significant differences 14 are observed in the absolute values, particularly in the orbital magnetic moments, which are higher in 15 the experimental results than in theoretical calculations.  16 This discrepancy can be partly explained by the fact that the theoretical calculations assume 17 a perfectly B2-ordered structure, while the experimental films exhibit a B2 ordering degree less than 18 10  1 [24]. Additionally, the theoretical models tend to underestimate spin-orbit coupling and do not fully 1 incorporate surface and interface effects, particularly in thin films or nanoscale structures, potentially 2 leading to an underestimation of orbital magnetic moments, as noted in previous studies [43,43]. The 3 delocalization of electrons in 5d elements in Ir and the inadequate consideration of electron-electron 4 interactions may further contribute to these differences [28]. Importantly, the results show that the 5 increase in magnetic moments with Ir addition is primarily driven by the orbital moments rather than 6 the spin moments. This observation aligns with findings by Imada et al., who demonstrated that strong 7 spin-orbit interactions in 5d elements, such as Pt, can localize 3d electrons and significantly enhance 8 the orbital angular momentum contribution [52].  9  10  FIG. 5. Comparison of experimental (upper panel) and theoretical (lower panel) results of spin and orbital magnetic moments for each element of (a) Fe, (b) Co, and (c) Ir.  11 The analysis of the density of states (DOS) for the d-orbitals of Fe, Co, and Ir atoms in Fe–12 Co–Ir alloys, performed using ab-initio calculations, offers critical insights into the influence of Ir 13 incorporation on the electronic and magnetic properties within the B2-ordered structure. As shown in 14 Fig. 6, the DOS for each element at 3, 7, and 10 at% Ir concentrations reveals distinct shifts in 15 particularly at the Fermi level. These shifts are accompanied by a sharpening of DOS peaks for Fe and 16 Co, indicating increased electron localization, with Fe showing a stronger interaction with Ir due to 17 enhanced spin-orbit coupling (SOC) from the Ir 5d electrons [53,54]. This aligns with the observed 18 increase in Fe orbital magnetic moment, underscoring the role of Ir in tuning the magnetic behavior. 19 In  contrast, Ir DOS shifts with increasing concentration, with majority spin states moving to lower 20 energies and a rise in DOS intensity, suggesting enhanced spin polarization [28,55]. XMCD results 21 confirm these theoretical changes, showing induced magnetism in Ir due to 3d-5d exchange 22 interactions with ferromagnetic Fe and Co. This interaction amplifies the overall magnetic properties, 23 demonstrating the synergistic effect of Ir with Fe and Co [27, 52]. 24 Our study reveals a consistent increase in the magnetic moment within the 0–11 at% Ir 25 11  concentration range for Fe75Co25 alloys. Ir doping enhances electron localization, which plays a key 1 role in increasing the orbital magnetic moments of Fe, Co, and Ir. This complex interplay between 2 SOC and electron localization advances our understanding of magnetic behavior in these alloys and 3 sets the stage for future quantitative analyses. While higher Ir concentrations in other systems have 4 been associated with reduced magnetic moments due to antiferromagnetic exchange interactions 5 [27,50], further investigations beyond the 11% Ir concentration could shed light on the limits of this 6 effect. Moreover, exploring the correlation between magnetic moments, electronic states, and other 7 physical properties could provide deeper insights into the effects of Ir doping and SOC. These efforts 8 will help further elucidate the underlying mechanisms driving the magnetic behavior of these alloys. 9  10  FIG. 6 Density of states (DOS) and element-resolved d-orbital and total DOS per atom in Fe–Co–Ir alloys at varying Ir concentrations: (a) 3 at%, (b) 7 at%, and (c) 10 at%. Panels (d), (e), and (f) illustrate the d-orbital contributions to the DOS for Fe, Co, and Ir, respectively.  11 IV. CONCLUSIONS 12 This study quantitively investigated the influence of Ir doping on element-specific magnetic moments 13 of Fe–Co–Ir alloys using soft and hard X-ray Magnetic Circular Dichroism (XMCD) technique on 14 composition-spread films, combined with ab-initio calculations. A compositionally graded thin film 15 of (Fe75Co25)100-xIrx (x = 0–11 at%) was fabricated using a combinatorial sputtering technique, enabling 16 systematic compositional analysis.  17 High-throughput XMCD measurements revealed significant enhancements in the orbital 18 magnetic moments of Fe and Ir due to Ir doping, with Fe increasing to 1.41 times and Ir to 1.53 times 19 their original values. Theoretical calculations supported these findings, attributing the enhancements 20 to increased electron localization and augmented spin-orbit coupling. The interaction between the 5d 21 electrons of Ir and the 3d electrons of Fe and Co was identified as a key factor, facilitating a shift in 22 12  electronic states to lower energy levels, thereby amplifying the orbital contributions to magnetic 1 moments. 2 Theoretical calculations of the density of states (DOS) revealed that Ir addition shifts the Fe 3 and Co electronic states to lower energy levels, resulting in enhanced magnetic moments, primarily 4 through increased orbital contributions. The calculations also showed that the magnetic moments in 5 the B2-ordered crystal structure are larger than those in the A2-disordered structure, strongly 6 supporting the experimental findings.  This validates that the B2-ordered phase contributes to the 7 observed magnetization enhancements. The pronounced increase in orbital magnetic moments with Ir 8 doping further underscores the importance of spin-orbit coupling in this system. These trends suggest 9 that Ir serves as a catalyst for boosting magnetic properties, particularly through its interaction with 10 Fe, which exhibited the largest increases in magnetic moments in both experimental and theoretical 11 data. This reinforced the critical role of Ir in improving the magnetic behavior of the alloy. 12 Overall, Ir doping effectively enhances the magnetic characteristics of Fe–Co–Ir alloys, 13 leading to significant increases in both spin and orbital contributions to the overall magnetization. The 14 strong agreement between experimental results and theoretical models reinforces the reliability of 15 these findings, offering valuable insights into the electronic and magnetic interactions within these 16 alloys. This study not only provides essential insights into the interplay of spin-orbit coupling and 17 magnetic moments in 5d-doped ferromagnetic alloys but also establishes a clear foundation for 18 designing high-performance ferromagnetic materials for advanced technological applications. 19  20 ACKNOWLEDGEMENTS 21 The XMCD experiments at BL25SU and BL39XU were carried out under the approval of SPring-8 22 (Proposals: 2022A1027, 2022B1004, and 2023A1179). Also, this study was supported in part by JST-23 CREST (Grant No. JPMJCR21O1) and JST/ACT-X (Grant No. JPMJAX22AL), and Japan Society 24 for the Promotion of Science, JSPS (KAKENHI: Grants No. 21H04656, 22K14590, 23K13636). 25   26 13  References 1 [1] O. Gutfleisch, M. A. Willard, E. Brück, C. H. Chen, S. G. Sankar, and J. P. Liu, Adv. Mater. 2 23, 821 (2011). 3 [2] J. M. Silveyra, E. Ferrara, D. L. Huber, and T. C. Monson, Science 362, eaao0195 (2018). 4 [3] W. D. Callister Jr. and D. G. 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