# Manipulation of energy-resolved magneto-optical effect in yttrium iron garnet films achieved by covering with nonmagnetic metals

Chun-Kai Hu, Jun-Xiao Lin, Hsiu-An Liu, Wan-Hsiu Chang Chien, Wen-Bin Wu, Jiann-Shing Lee, Chun-Rong Lin, Stéphane Mangin, Jiunn Chen, Hua-Shu Hsu

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## ► To cite this version:

Chun-Kai Hu, Jun-Xiao Lin, Hsiu-An Liu, Wan-Hsiu Chang Chien, Wen-Bin Wu, et al., Manipulation of energy-resolved magneto-optical effect in yttrium iron garnet films achieved by covering with nonmagnetic metals. Chinese Journal of Physics, 2024, 90 (1), pp.717-725. 10.1016/j.cjph.2024.04.027. hal-05006426

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# Manipulation of energy-resolved magneto-optical effect in yttrium iron garnet films achieved by covering with nonmagnetic metals

Chun-Kai Hu,¹ Jun-Xiao Lin,² Hsiu-An Liu,¹ Wan-Hsiu Chang Chien,¹ Wen-Bin Wu³, Jiann-Shing Lee,¹ Chun-Rong Lin,¹ Stéphane Mangin², Jiunn Chen¹, * and Hua-Shu Hsu¹, *

¹ Department of Applied Physics, National Pingtung University, 4-18, Minsheng Road, Pingtung, 90044, Taiwan

² Université de Lorraine, Institut Jean Lamour, UMR CNRS, Nancy 7198, France

³National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

*Authors to whom correspondence should be addressed: asesrrc@gmail.com
(J. Chen); hshsu@mail.nptu.edu.tw (H.-S. Hsu)

## ABSTRACT

The effect of the interface-driven magnetic optical (MO) on Pt/Yttrium iron garnet (YIG) and Cu/YIG has been investigated using energy resolved magnetic circular dichroism (ER-MCD) system. By mapping the spatial MCD at different excitation energies, the results distinct signatures indicate manipulation of the spin-polarized electronic structure of YIG through interface charge transfer. Specifically, compared to the Cu/YIG region, we found that the Pt/YIG region exhibited a higher level of MCD enhancement and even a pronounced MCD sign inversion at certain excitation energies. In contrast, the saturation moment of the Pt-covered YIG film decreased significantly, while the enhancement was only slight for the Cu-covered YIG film. Based on the first-principles calculations, the presence of distinct polarized interface states of tetrahedral Fe ions in metal/YIG systems results in conflicting observations regarding optical and magnetic proximity effects. We present evidence that ER-MCD can effectively reveal dominant spin-polarized optical transitions in ferrimagnetic (FIM) materials, providing direct access to the polarized interface states in the nonmagnetic metal (NM)/FIM
heterosystem. This capability opens opportunities to manipulate and customize the magneto-optical effect with energy resolution for various applications in magneto-optics.
# I. INTRODUCTION

Since the discovery of giant magnetoresistance, which revolutionized the field of spintronics, extensive research has focused on nonmagnetic (NM) and ferromagnetic (FM) heterostructures [1,2]. Understanding the physical properties of NM/FM systems is crucial for advancing spintronics research [3–5]. Magneto-optical (MO) techniques have proven to be invaluable tools for analyzing magnetic properties in NM/FM systems [3–6]. These techniques rely on detecting changes in the polarization or intensity of light resulting from interactions with magnetic samples [7–10]. Typically, a fixed-energy laser system is employed as the optical source in MO experiments. However, in some cases, variations in the MO signal at specific optical excitation energy level may not be readily apparent due to the energy dependence of the spin-polarized electronic structure of the material.

Numerous materials with multiple sublattices, such as ferrimagnetic (FIM) iron garnets, that exhibit intriguing physical properties have been reported [11–13]. Novel interface-driven properties of FIM materials in NM/FIM heterostructures have been demonstrated [11–15], and research involving NM/FM heterostructure and current-driven spin-orbital torque, the spin Seebeck effect, and the magnetic proximity effect (MPE) has been reported [5,16–19]. Nevertheless, the use of a single-energy light source to study the MO response of FIM materials is challenging because the complex crystal structure of such materials exhibits an intricate band pattern [16,17]. This motivated by these findings, we investigated the interface-driven MO effect using multiple-energy light sources, which enables to directly access dominant spin-polarized optical transitions. The results of this study may be useful for both fundamental research and practical applications.

In this study, we investigate the interface-driven MO polarization changes using
yttrium iron garnet (YIG), a widely studied MO material with excellent spintronic
properties and characteristics of FIM insulator. To explore this phenomenon, we
employed energy-resolved magnetic circular dichroism (ER-MCD) and focused on
Pt/YIG and Cu/YIG systems due to their distinct high and low spin-orbit interactions,
respectively [12,20–23]. Although the ferromagnetic ordering of Pt near the Pt/YIG
interface has been extensively studied, it is still a topic of debate [12, 19, 24–27].
However, the MPE and its influence on the interfacial YIG magneto-transport have
received less attention. By using a multiple-energy light source, we revealed that
interfacial charge transfer has a distinct impact on the optical proximity effect and MPE
on Pt- and Cu-covered YIG films. Notably, when Pt was deposited on YIG, we observe
enhancements in the MCD signal at specific excitation energies. Furthermore, the sign
of the MCD signal, corresponding to the elliptically polarized direction, could be
reversed by introducing a Pt layer on the YIG film. Interestingly, this reversal was not
observed in the case of Cu coverage. The saturation moment of the Pt-covered YIG film
was significantly reduced, while a slight enhancement was observed for Cu-covered
YIG film, as confirmed by superconducting quantum interference device (SQUID)
measurements. These observations can be attributed to changes in the ER spin-
polarized structures hybridized among Pt, Cu, and YIG in the interface region. The
results demonstrate the potential of interface engineering to design and control MO
effect in FIM material. Moreover, these findings emphasize the importance of using a
multiple-energy light source in the MO technique, as the choice of excitation energy
can significantly impact the observed experimental phenomena.

## II. EXPERIMENTAL DETAILS

YIG thin films (~30 nm) were grown on a quartz substrate at room temperature
through radio frequency magnetron sputtering by using a YIG target in an Ar
atmosphere at a working pressure of $2 \times 10^{-2}$ Torr. To ensure a clean deposition environment, the base pressure of the sputtering chamber was reduced to $1 \times 10^{-6}$ Torr using a turbomolecular pump. The sputtering power of 100 W was applied during the deposition. Initially, the as-sputtered YIG films were amorphous and did not exhibit any ferromagnetic signal. To enhance their crystallinity and magnetic properties, the YIG thin films underwent post-deposition annealing. The annealing process took place in air at a temperature of 750 °C for 5 hours [28].

Polycrystalline magnetic oxides occasionally possess uncontrollable defects, which can result in non-uniform magnetic distributions that may influence experimental results and their interpretation. To address this, T-shaped Pt (~15 nm) and Cu (~17 nm) layers were deposited on the YIG thin films using shadow masks under the same sputtering conditions. The geometry arrangement of the YIG films with the Pt and Cu layers is presented in Fig. 1(a). The T-shaped design of the Pt and Cu layers facilitated the observation of an overlay effect in the x-y plane of the YIG films. The experimental setup for ER-MCD mapping is illustrated in Figure 1(b). It consisted of a spectrometer (JASCO J-815) and an electromagnet, and a 450-W water-cooled xenon lamp as the light source. The polarization of light was controlled by a photoelastic modulator operating at a frequency of 50 kHz. The sample was positioned at the center of the electromagnet, allowing circular polarized light penetrated the sample after passing through a hole in the electromagnet. The MCD signal, which represents the absorption between the right circular polarized light and the left circular polarized light, was detected using a photomultiplier tube. A two-dimensional (2D) moving platform was incorporated into the system to enable programmable scanning, which was necessary for ER-MCD and absorption mapping. This mapping approach prevented any misinterpretations caused by single-point measurements. The absorption mapping at
3.05 eV is displayed in the right panel of Fig. 1(c), showing consistency with the optical image in Fig. 1(a).

<figure><img src="image_3.png" /><figcaption>**FIG. 1.** (a) T-shaped Pt and Cu films were coated on YIG thin films. The figure displays the optical image obtained using a conventional camera. (b) Schematic of the ER-MCD mapping system. Broadband-energy circular polarized light passes through a sample placed at the center of an electromagnet, and absorption data are collected using a photomultiplier tube. Programmable scanning, which is required for ER-MCD mapping and absorption mapping, is realized using a 2D moving platform. (c) Absorption image obtained using our mapping system.</figcaption></figure>

### III. RESULTS AND DISCUSSION

#### A. MCD RESULTS IN DIFFERENT EXCITATION ENERGIES

MCD mapping was performed at a magnetic field of 0.8 T using three distinct excitation energies: 1.96 eV, 3.05 eV, and 2.76 eV. Among these energies, 1.96 eV (red light) and 3.05 eV (blue light) are commonly used in MO applications [29], while 2.76 eV corresponds to the energy at which the MCD sign is reversed in Pt/YIG systems
[16,17,28,30]. At 1.96 eV, which is commonly employed for magnetic characterization of magnetic metals in MO techniques, precise identification of the T-shaped coverage area formed by either Cu or Pt within the scanning region was not achievable due to measurement limitations. This observation suggests that neither Cu nor Pt coverage on YIG resulted in significant changes in the MCD signal. However, at 3.05 eV, which has been used in previous studies for MO measurements of YIG, the MCD mapping exhibited no substantial increase in the MCD signal compared to the signal from the Cu-covered YIG region. In contrast, a noticeable enhancement in the MCD signal was observed for the YIG film covered with the T-shaped Pt layer, indicating a significant increase in the MCD signal due to Pt coverage. This result indicates that different metal layers deposited on YIG can induce varying interactions between the metal layers and YIG, leading to different intensities of the MO effect. This finding suggests that Pt, being a heavier metal compared to Cu, exhibited a stronger interaction with YIG under 3.05 eV excitation.

Interestingly, at the excitation energy of 2.76 eV, the MCD signal observed for the Cu-covered region of the YIG film was significantly weakened to a small but vivid value and change its sign compared to the signal observed for YIG alone. This suggests that Cu also interacted with YIG, leading to a decrease in the intensity and a switching of polarization of the MCD signal under 2.76 eV excitation. This result contrasts with the observed made at 1.96 eV and 3.05 eV. Furthermore, when using the 2.76 eV light source, the MCD signal corresponding to YIG clearly exhibited an inverted and changed its sign in the Pt-covered region. Magnetic-field-dependent MCD (MCD-H) loops in bare YIG, Cu/YIG, and Pt/YIG regions at 1.96 eV, 3.05 eV, and 2.76 eV, respectively, are presented in Figs. 2(d), 2(e), and 2(f). These MCD-H loops were consistent with the findings our MCD mapping.

The previous MCD results obtained for the Cu- and Pt-covered YIG films at
various excitation energies highlight the significant energy selectivity exhibited by MCD imaging. The observed changes in ER-MCD resulting from different NM coatings on the YIG films suggest the potential for designing MO effects through interfacial engineering. However, it is crucial to note that using MCD measurements with a single-energy light source to characterize the magnetic properties of samples can lead to erroneous interpretations, as demonstrated by the following examples:

(1) Based on the observations [Fig. 2(a) and (d)] made at the excitation energy of 1.96 eV, we might have concluded that neither Pt nor Cu coating affects the magnetic properties of YIG.

(2) Based on the observations [Fig. 2(b) and (e)] made at the excitation energy of 3.05 eV, we might have concluded that only Pt increases the magnetism of YIG and that Cu coverage has no effect on the magnetism of YIG.

(3) Based on the observations [Fig. 2(c) and (f)] made at the excitation energy of 2.76 eV, we might have concluded that Cu affects the magnetism of YIG by weakening and slightly inverting YIG magnetic moment. In addition, we might have concluded that Pt can significantly reverse the YIG magnetic moment.

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>
**FIG. 2.** MCD images obtained using our mapping system at (a) 1.96 eV, (b) 3.05 eV, and (c) 2.76 eV. Panels (d), (e), and (f) show the MCD-H loops corresponding to the bare YIG, Cu/YIG, and Pt/YIG regions at 1.96, 3.05, and 2.76 eV, respectively.

## B. ER-MCD SPECTRUM AND CORRESPONDING MAGNETIC PROPERTIES

The MO signal of a magnetic sample is primarily governed by the spin-polarized band structure of the material. Our results [Fig. (2)] demonstrate that using a single-energy light source to obtain an MO signal and determine the magnetic properties of a material introduces a certain degree of uncertainty, particularly when dealing with materials possessing complex band structures. In contrast, ER-MCD, which manifests the dominant spin-dependent optical transition, offers a powerful method for scanning the signals arising from polarized charge transfer near the band edge [8,9,26,28,30–35]. This technique is sensitive to spin-dependent chemical potential variation, such as interfacial spin-polarized charge transfer leading to magnetic enhancement in an amorphous-carbon encapsulated magnetite or spin-polarized charge transferring states at the host band edge leading to magnetism in a dilute magnetic semiconductor system [36–38]. In these systems, the absorption and ER-MCD spectrum exhibit a similar amount of blue shift, indicative of spin-polarized charge transfer. Our previous study reported the successful application of the ER-MCD technique to probe the electronic structure of YIG [30,39]. Additional information on the detailed electronic structure of YIG, based on GGA+U calculations, as well as the characteristic features observed in ER-MCD associated with channels of spin-dependent optical transition, are summarized in the Supplemental Information. Fig. 3(a) illustrates the features T (2.54 eV) and E (2.91 eV), which exhibit a blue shift in energy for both metal-covered YIG samples.
In contrast to the 3.5% magnetic enhancement for Cu/YIG shown in Fig. 3(b), the saturation magnetization obtained from a SQUID measurement exhibits a significant magnetic reduction of 12.2% for Pt/YIG. The influences of Pt coverage on the magnetic properties of YIG thin films were investigated by Wang *et al* [40]. Based on their soft X-ray MCD experimental results suggested that selective charge transfer from Pt to Fe ions at tetrahedral sites of YIG was responsible for the observation magnetic reduction in Pt/YIG. This finding appears to align with our observation of the blue shift of the ER-MCD feature (E), indicating minority charge transfers from Pt to the unoccupied states of the A-site: $A(e^{2\uparrow}t_2^{3\uparrow}) \rightarrow A(e^{2\uparrow}t_2^{3\uparrow}e^{\downarrow})$. Here, the superscript characters ↑ and ↓ represent electronic state with spin direction parallel (majority) or antiparallel (minority) to the total magnetization, respectively. However, the interfacial charge transfer from metal to YIG alone does not seem to fully explain the contrasting trends observed in the optical and MPE for Cu/YIG and Pt/YIG. Several contradictions between the optical and MPEs remain unaddressed for Cu/YIG and Pt/YIG systems.
<figure><img src="image_10.png" /><figcaption>FIG. 3. (a) ER-MCD spectrum and (b) normalized magnetic hysteresis loop for YIG and metal-covered YIG thin film measured using SQUID. T (2.54 eV) and E (2.91 eV) represent the ligand-to-metal transition in YIG of O(2p↑) → B(t<sub>2g</sub>↑) and O(2p↓) → A(e↓) and, respectively. (See Supplementary Material for details).</figcaption></figure>

C. FIRST PRINCIPLES CALCULATIONS

To investigate the underlying electronic origins that account for the puzzling
behavior observed in the Pt/YIG and Cu/YIG heterosystems, we conducted electronic
and magnetic structure calculations using first-principles methods. Specifically, these
calculations were based on density functional theory (DFT) and carried out using the
Vienna Ab initio Simulation Package (VASP) [41]. In our study, we constructed a
superlattice structure comprising a YIG (100) slab with a thickness of approximately
12.2 Å. This was combined with a four-layer sample of either Pt or Cu, where each
layer had a thickness of 6.6 Å. To minimize interactions between periodic images, a
vacuum layer of approximately 16 Å was included. In our calculations, we utilized the
generalized-gradient approximation by using Perdew-Burke-Ernzerhof (PBE-GGA)
[42] to treat the exchange-correlation function. This approximation is commonly
employed in density functional theory (DFT) calculations to describe the electronic structure of materials. To account for electron-electron interactions in the *d* orbitals of the transition metals, we applied the Hubbard-*U* correction. In our study, we employed a simplified Dudarev scheme [43] to incorporate this correction. The cut-off energy of 500 eV for the plane-wave basis set was used in the context of the projector augmented wave (PAW) method [44]. Monkhorst-Pack 3×3×1 k-mash sampling was adopted for the static calculations. Electronic relaxation criteria were 10⁻⁶ eV, and the internal coordinate was relaxed until the largest force became less than 0.01 eV/Å. The in-plane dimension of 12.376×12.376 Å² was fixed at the experimental value of the bulk YIG. Spin-orbit couplings and noncollinear magnetism are not considered in our calculations. The unit cell formula Fe<sub>B</sub><sup>↓</sup>(20)Fe<sub>A</sub><sup>↓</sup>(28)Y(28)O(96)Metal(72) contains Fe ions in the A-site of the spin-majority which anti-ferromagnetically coupled with Fe ions in the B-site of the spin-minority, to form a ferrimagnetic configuration.

<table><caption>**TABLE I.** GGA+U ($U_{eff}^A$ = 4.7 eV and $U_{eff}^B$ = 3.75 eV) computed magnetic moment (μ<sub>B</sub>) based on Fe<sub>B</sub>(20)Fe<sub>A</sub>(28)Y(28)O(96)Metal(72). The value in the parentheses gives the magnetic moment averaged over Pt or Cu atoms (μ<sub>B</sub>/atom).</caption><thead><tr><th></th><th>Total</th><th><i>B</i>-site Fe(3d)</th><th><i>A</i>-site Fe(3d)</th><th>Metal (<i>d</i>)</th><th><i>d</i>-total</th></tr></thead><tbody><tr><td>Pt/YIG</td><td>30.29</td><td>-79.66</td><td>108.17</td><td>-1.65<br>(0.023)</td><td>27.00</td></tr><tr><td>Cu/YIG</td><td>42.32</td><td>50.23</td><td>-13.62</td><td>0.019<br>(0.0002)</td><td>36.7</td></tr></tbody></table>

From a simplified ionic perspective, each Fe ions appear to be in *d*⁵ high-spin state configurations, $A(e^{2↑}t_2^{3↑})$ and $B(t_{2g}^{3↓}e_g^{2↓})$, at each crystallographic site in YIG. In Table I, the computed average magnetic moment for Pt (0.023 μ<sub>B</sub>/atom) is consistent with the order of magnitude obtained from the XMCD experiment [22] and the *ab initio*
calculation [27]. Notably, the magnetization direction of Pt is opposite to that of spin-majority in YIG. In the case of Cu/YIG, the total magnetization direction for Fe ions in each crystallographic site is apparently reserved. The calculations have predicted distinctly different magnetic structures for the Cu/YIG and the Pt/YIG heterosystem. The presence of the Cu overlayer alters the magnetic structure of YIG, while an induced magnetic moment is observed in the Pt overlayer. Furthermore, the calculations predict a higher total magnetization for the Cu/YIG system compared to the Pt/YIG system, which is consistent with the results obtained from SQUID measurements.

<figure><img src="image_11.png" /><figcaption>**FIG. 4.** Spin-polarized PDOS of Fe<sub>B</sub>(20)Fe<sub>A</sub>(28)Y(28)O(96)Metal(72) obtained through GGA+U calculations for (a) Cu/YIG, (b) Pt/YIG. Lower panels: the corresponding enlarged view of the interface states near the Fermi level. In all graphs, the right and</figcaption></figure>
the left panel represent the PDOS of the spin-majority and the spin-minority, respectively. Optical charge transitions of spin-majority (solid arrow) and spin-minority (dashed arrow) for each heterosystem are also indicated. The Fermi level is indicated by horizontal zero lines.

Fig. 4 shows the computed spin polarized projected density of states (SPDOS) for Cu/YIG and Pt/YIG, providing insights into the electronic structure of the NM/YIG system. Additionally, in Fig. 5 presents the corresponding spin density (SPDEN) and charge density difference (CDD) plots. In the case of Cu/YIG, the SPDOS reveals a spin inversion phenomenon in YIG, where the majority spin switches to the Fe ions at the octahedral site. Most of the Fe ions at the tetrahedral site in Cu/YIG exhibit a minority spin. Furthermore, the presence of the $A(e^\uparrow)$ localized state in the energy range 8-9 eV below the Fermi level ($E_F$) suggests that only a few Fe ions at the tetrahedral site ($Fe_A$) retain a majority spin direction. Unlike Pt/YIG, which exhibits a typical spinel-ferrite-like anti-symmetry in spin components, the SPDOS of Cu/YIG indicates a penetration into the opposite spin component in both occupied and unoccupied Fe 3d states. The spin inversion in Cu/YIG can be clearly illustrated by the computed SPDEN, in which the sign of spin density of some Fe ions at tetrahedral (red circle) and octahedral site (blue circle) are the same. Moreover, the computed SPDEN have indicate a short-range FM ordering that parallel to the magnetization direction in the YIG. In contrast, the computed SPDEN for Pt/YIG suggests a FM ordering that is anti-parallel to the magnetization direction in the Pt overlayer near the heterointerface. This indicates that the spin moment of the Pt overlayer is opposite to that of YIG, resulting in a reduction in the saturation magnetization. Nevertheless, it is worth to address here the major contribution for magnetic reduction in Pt/YIG is the interface charge transfer (ICT) of spin-minority from Pt to $Fe_A$ [33]. This charge transfer results
in the transformation of the spin configuration like $Fe_A(e^{2\uparrow}t_2^{3\uparrow}) \rightarrow Fe_A(e^{2\uparrow}t_2^{3\uparrow}e^{\downarrow})$, and that causes the polarized interface state $Fe_A(e^{\downarrow})$ as depicted in Fig. 4 (b) (lower panel). In contrast, due to the spin inversion in Cu/YIG. Fig. 4 (a) (lower panel) clearly shows interface states comprising both $Fe_A(e^{\downarrow})$ and $Fe_A(e^{\uparrow})$, indicating ICT from Pt to $Fe_A$ in Cu/YIG contains both spin channels of $Fe_A(e^{2\uparrow}t_2^{3\uparrow}) \rightarrow Fe_A(e^{2\uparrow}t_2^{3\uparrow}e^{\downarrow})$ and $Fe_A(e^{2\downarrow}t_2^{3\downarrow}) \rightarrow Fe_A(e^{2\downarrow}t_2^{3\downarrow}e^{\uparrow})$. The existence of $Fe_A(e^{\uparrow})$ explains the observation of a slightly enhancement of saturation moment in Cu/YIG. Furthermore, the CDD plots in Fig. 5 also confirm the existence of these interface states as the charge transferred from the metal overlayer to the YIG substrate. These results highlight the role of interface charge transfer and the associated spin configurations in determining the magnetic properties of the Pt/YIG and Cu/YIG heterosystems.

MCD signal resolve dominate optical charge transition (OCT) of spin channels across the optical gaps in YIG host system joining the occupied $O(2p)$ valence states and the unoccupied $Fe(3d)$ states. The absorptive differences between spin channels parallel ($I^+$) and anti-parallel ($I^-$) to the net magnetization are recorded simultaneously as a function of incident wavelength. Dominate spin channel can be resolved from the sign of the ER-MCD signal as defined by $\frac{I^+ - I^-}{I^+ + I^-}$. In the cases of bare YIG, the leading edge of feature E in the ER-MCD spectrum is characterized by dominant OCT of $O(2p^\downarrow) \rightarrow Fe_A(e^\downarrow)$. This indicates the involvement of spin-down electrons in the optical transitions. In Pt/YIG, the presence of spin-polarized interface state $A(e^\downarrow)$ results in the blue shift of feature E. This is because the polarized ICT from Pt to $Fe_A$ enlarges the optical gap of spin minority channel, altering the electronic transitions involved. On the other hand, the predicted spin inversion in Cu/YIG leads to a reduction in the size of the minority gap size because that changed from $O(2p^\downarrow) \rightarrow Fe_A(e^\downarrow)$ to $O(2p^\downarrow) \rightarrow Fe_B(t_{2g}^\downarrow)$. This spin inversion is absent for Pt/YIG heterosystem and can
account for the observations of the overall suppression of overall MCD enhancement
as well as the retardation of the blueshift of characteristic features in Cu/YIG. Therefore,
the results of the calculations lead to conclude the probing of ER-MCD enables the
sensitivity on polarized interface states manipulated by metal coverage. And provides
valuable insights into the electronic structure and optical properties of the Pt/YIG and
Cu/YIG heterosystem.

<figure><img src="image_12.png" /></figure>
**FIG. 5.** Isosurface of the spin density (at ±0.0015 *e*/*a*<sub>0</sub><sup>3</sup>) of Cu/YIG and Pt/YIG. Lower panels are the corresponding isosurface (at ±0.004 *e*/*a*<sub>0</sub><sup>3</sup>) of charge difference density calculated from Δρ(M/YIG) = ρ(M/YIG) − ρ(YIG) − ρ(M). Fe ions at octahedral and tetrahedral are represented by blue and red spheres. Yttrium, Pt and Cu are represented by dark green, grey, and gold spheres.

IV. CONCLUSIONS

In this study, YIG thin films covered with Pt or Cu layers were investigated
through MCD, which revealed changes in ER-MO effects. An ER approach was
employed, and the MCD signals that were obtained provided insight into the spin-
polarized electronic structure of the materials. We could observe a higher level of MCD
enhancement and even significant MCD sign reversal at certain excitation energies
compared to the Cu/YIG region. This result is related to the change of the ER spin-
polarized structure due to the orbital hybridization of the charge transferring states
among Pt, Cu, and YIG. In this proof-of-concept study, we demonstrated that an ER-
MCD spectrum may provide an effective protocol for studying and designing MO
profiles for both fundamental research and practical applications. Our findings also
provide information on ER spin-polarized DOS that further clarifies the spin and orbital
current transport behavior at the NM/YIG interface.
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Supplementary Material

Figure S1 (a) shows the calculated GGA+U ($U_{eff}^A$ = 4.7 eV and $U_{eff}^B$ = 3.75 eV) Fe<sub>A</sub>(16)Fe<sub>B</sub>(24)Y(24)O(96) (1a3d) electronic structure, where A and B denote the tetrahedral and octahedral sublattices, respectively. The valence is a broad band of mainly O(2p) that touches $E_F$ and is well separated from the localized Fe(3d) states below ~6 eV. The crystal field splits the Fe(3d)-O(2p) hybridized states into the $t_2$ and *e* states for Fe ions at the A-site and the $t_{2g}$ and $e_g$ state for Fe ions at the B-site. The antibonding $t_2$ orbitals are energetically superior to the off-axial *e* orbitals. Fe ions at the B-site cause opposite signs of crystal field splitting to occur. The electronic occupation in the computed Fe(3d) bands at 7-8 below the $E_F$ are close to high-spin Fe<sup>3+</sup> ions in the octahedral (A-site) and tetrahedral (B-site) sublattices, respectively, which are denoted by $A(e^{2\uparrow}t_2^{3\uparrow})$ and $B(t_{2g}^{3\downarrow}e_g^{2\downarrow})$. Here we define ↑ as the spin majority and, thus, the direction of net magnetization. The spin direction of the unoccupied states of Fe ions at each site are opposite, forming an antiferromagnetic configuration.

This shows a small gap in the *B*-site unoccupied states of spin majority. It is worth noting that the unneglectable *s*-band, *A*(4s<sup>↓</sup>) and *B*(4s<sup>↓</sup>) reconcile the *B*(e<sup>↓</sup>) and the *A*(t<sub>2</sub><sup>↓</sup>), respectively, and show localized and delocalized in energy [Fig. S1(b) (left panel)].
<figure><img src="image_13.png" /></figure>

**FIG. S1.** (a) PDOS of Fe<sub>B</sub>(16)Fe<sub>A</sub>(24)Y(24)O(96) obtained from GGA+U ($U_{eff}^A$ = 4.7 eV and $U_{eff}^B$ = 3.75 eV) with Gaussian broadening of 0.05 eV. The dashed line at zero is the set Fermi energy (*E*<sub>F</sub>). For details regarding the calculations, refer to Ref [2]. (b) (Left panel) The subtractive difference of the computed total density of states (TDOS) within energy interval 2–5 eV above *E*<sub>F</sub>. The PDOS of Fe(4s) is also shown. (Right panel) ER-MCD of YIG. The characteristic optical transitions manifested in ER-MCD are labeled, and the details are listed in **Table I**.

As presented in Fig. S1(b), the subtractive difference of TDOS (left panel) fit well to the peak positions in ER-MCD (right panel), including Fe(3d) and Y(4d). The result indicates that ER-MCD probes $B(t_{2g}^\uparrow)$ and $A(e^\downarrow)$ at energies of 2.54 eV (feature labeled by T) and 2.91 eV (feature labeled by E). For the energy interval above $A(e^\downarrow)$, 2.91–3.5 eV (feature labeled by T<sub>2</sub>), manifest the $A(4s^\downarrow)$ of extending nature with which hybridized to $A(3d^\downarrow)$. These 4s bands are essential for the uncompensation between the calculation and the ER-MCD. Because Laporte-forbidden were relaxed to a larger extent as the involving polarized Fe(4s<sup>\downarrow</sup>) and Y(4d<sup>\downarrow</sup>), and that explains the underestimation on TDOS peak ratio in ER-MCD energy interval designate to $A(e^\downarrow)/B(t_{2g}^\uparrow)$, and on the spectrum weight of higher energy interval (D) designate to Y(4d<sup>\downarrow</sup>)
†).

<table><caption>**TABLE SI.** Characteristic main optical transitions of YIG manifest in ER-MCD.</caption><thead><tr><th>Label</th><th>Energy (eV)</th><th>Main transitions</th><th>Initial(I)/final(F)<br>state characters</th></tr></thead><tbody><tr><td>T</td><td>2.54-2.74</td><td>O(2p<sup>†</sup>) → B(t<sub>2g</sub><sup>†</sup>)</td><td>F</td></tr><tr><td>E</td><td>2.74-3.0</td><td>O(2p<sup>↓</sup>) → A(e<sup>↓</sup>)</td><td>F</td></tr><tr><td>T<sub>2</sub></td><td>3.0-3.5</td><td>O(2p<sup>↓</sup>) → A(t<sub>2g</sub><sup>↓</sup>) ⊗ A(4s<sup>↓</sup>)</td><td>F</td></tr><tr><td>O</td><td>3.5-4.0</td><td>O(2p<sup>†</sup>) → A(3d<sup>↓</sup>)</td><td>I</td></tr><tr><td>D</td><td>4.0-</td><td>O(2p<sup>↑</sup>) → Y(4d<sup>↑</sup>)</td><td>F</td></tr></tbody></table>

The ER-MCD at the energy interval of 3.5-4.0 eV (feature labeled O) exhibits no correspondence to the TDOS of the unoccupied band energy interval assigned to the initial O(2p) contribution. The characteristic features identified by the ER-MCD system that indicate key changes are summarized in Table SI.