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[Shigenori Ueda](https://orcid.org/0000-0001-9425-0614), [Yuichi Fujita](https://orcid.org/0000-0002-1798-1066), [Yuya Sakuraba](https://orcid.org/0000-0003-4618-9550)

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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

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[Near-interface electronic and magnetic states of insulator/Co2MnSi structures probed by hard x-ray photoemission combined with x-ray total reflection](https://mdr.nims.go.jp/datasets/d5976893-5243-4414-ab7c-e0f87e0deff5)

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Near-interface electronic and magnetic states of insulator/${\rm Co}_2{\rm Mn}{\rm Si}$ heterostructures probed by hard x-ray photoemission combined with x-ray total reflectionPHYSICAL REVIEW B 109, 085109 (2024)Near-interface electronic and magnetic states of insulator/Co2MnSi heterostructures probedby hard x-ray photoemission combined with x-ray total reflectionShigenori Ueda ,1,2,* Yuichi Fujita ,3,4,† and Yuya Sakuraba 31Research Center for Electronic and Optical Materials, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305–0044, Japan2Synchotron X-ray Station at SPring-8, NIMS, Sayo, Hyogo 679–5148, Japan3Research Center for Magnetic and Spintronic Materials, NIMS, Tsukuba, Ibaraki 305–0047, Japan4International Center for Young Scientists, NIMS, Tsukuba, Ibaraki 305–0047, Japan(Received 14 September 2023; revised 22 December 2023; accepted 17 January 2024; published 7 February 2024)Depth-dependent electronic and magnetic states of AlOx and MgO capped Co2MnSi thin films were mea-sured by using hard x-ray photoemission spectroscopy (HAXPES) combined with x-ray total reflection (TR).TR-HAXPES revealed that the near-interface electronic and magnetic states of Co2MnSi films differed fromthose of bulk measured in non-TR condition. The decrease of the Co and Mn magnetizations near the interfacealong the easy magnetization axis in the bulk region relative to those in the bulk region and the changes inthe valence band profiles were experimentally detected by nondestructive HAXPES utilizing TR. The possibleorigin of the reduction of the Co and Mn magnetizations and the changes in the valence band profile near theAlOx/Co2MnSi interface was due to an enhanced spin-wave excitation originating from the weakened exchangeinteraction between the local magnetic moments compared to the bulk region of Co2MnSi, which can slightlymodify the valence band electronic states, near the interface at a finite temperature. These results suggest thatthe combination of HAXPES with TR is useful to experimentally detect the electronic and magnetic states ofnear-interface and buried bulk regions in nondestructive way for insulator/ferromagnet heterojunctions.DOI: 10.1103/PhysRevB.109.085109I. INTRODUCTIONHalf-metallic ferromagnets, in which one spin state ex-hibits a metallic behavior and the other state exhibits aninsulating behavior, are useful for spintronic device applica-tions [1], because the perfect spin polarization (SP, 100%)at the Fermi-level (EF) is realized. One of potential spin-tronic applications is magnetoresistance (MR) devices usingtunnel and giant MR (TMR and GMR) junctions, in whichthe half-metals are used for magnetic electrodes, since hugeMR ratios are expected due to the perfect SP at EF inthe half-metals. However, the performances of TMR andGMR junctions show a strong reduction with increasingtemperature, even though the predicted half-metals such asL21-ordered Co-based Heusler alloys (Co2Y Z) with highCurie temperature (TC) sufficiently above room tempera-ture (RT) are used as a magnetic electrode. For example,the Co2MnSi(CMS)/MgO/CMS TMR junction [2] showedthe TMR ratios of 2010% at 4.2 K and 335% at RT, andthe Co2FeGa0.5Ge0.5/Ag/Co2FeGa0.5Ge0.5 GMR junction [3]*Corresponding author: UEDA.Shigenori@nims.go.jp†Present address: National Institute for Advanced Industrial Sci-ence and Technology, Tsukuba, Ibaraki 305–8568, Japan.Published by the American Physical Society under the terms of theCreative Commons Attribution 4.0 International license. Furtherdistribution of this work must maintain attribution to the author(s)and the published article’s title, journal citation, and DOI.showed the current-perpendicular-to-plane GMR ratios of285% at 10 K and 82% at RT. The strong reduction inTMR and GMR ratios with increasing temperature is a long-standing issue to be solved.Jourdan et al. [4] reported that an epitaxial CMS(001) thinfilm showed a high SP of ∼93% at RT by means of a surface-sensitive spin-resolved photoelectron spectroscopy (PES).However, the temperature-dependent SP has not been reportedin their work. Recently, a bulk-sensitive spin-resolved hardx-ray photoemission spectroscopy (HAXPES) technique withan ultracompact built-in Mott-type spin-filter on a sampleholder has been developed [5] and employed for the epitaxialCMS thin film, which revealed the high SP (∼90%) at EFand its almost temperature-independent behavior up to RT, inthe bulk region [6]. This result implies that the performancesof TMR and GMR would be governed by the electronicand magnetic states near insulator/CMS and metal/CMS in-terfaces, respectively. In the theoretical investigations basedon first-principles calculations on the MgO/CMS interfaces,Sakuma et al. [7] reported the decrease of the Co magneticmoment at the interface compared to that in bulk caused by thereduction of the exchange constant near the interface. Miuraet al. [8] also reported the reduction of the exchange con-stant in Co near the MgO/CMS interface and a noncollinearmagnetic structure near the interface. In the experimentalstudy for the MgO/CMS structure, Tsunegi et al. [9] reportedthat the decrease of the Co magnetic moment in the inter-face region with respect to that in the inner CMS film bymeans of the depth-resolved x-ray magnetic circular dichro-ism (XMCD) in the Co L2,3 absorption measurements. Since2469-9950/2024/109(8)/085109(10) 085109-1 Published by the American Physical Societyhttps://orcid.org/0000-0001-9425-0614https://orcid.org/0000-0002-1798-1066https://orcid.org/0000-0003-4618-9550https://crossmark.crossref.org/dialog/?doi=10.1103/PhysRevB.109.085109&domain=pdf&date_stamp=2024-02-07https://doi.org/10.1103/PhysRevB.109.085109https://creativecommons.org/licenses/by/4.0/UEDA, FUJITA, AND SAKURABA PHYSICAL REVIEW B 109, 085109 (2024)the depth-resolved XMCD in Ref. [9] is obtained from theCo LMM Auger electrons with the kinetic energy of ∼800eV, the inelastic mean-free-path (IMFP) of only ∼1.4 nminside CMS estimated from the TPP-2M (Tanuma, Powell,Penn) equation [10] for such electrons seems to be too shortto probe both the near-interface and the inner CMS film. Inaddition, XMCD is useful for detecting the spin and orbitalmagnetic moments via the magneto-optical sum-rule [11,12],but it does not directly give the density of states (DOS) ofmaterials. Therefore, experimental methods, which can detectdepth-resolved electronic and magnetic states, are requiredfor the insulator/ferromagnet heterojunctions to clarify thedifference in the electronic and magnetic states between thenear-interface region and the ferromagnet film inside.Fecher et al. [13] have compared the electronic structuresof the CMS(50 nm) films underneath MgO(2 and 20 nm)layers by HAXPES at RT and have demonstrated that theelectronic structure around EF for the CMS film underneaththe 20-nm-thick MgO layer is detectable and is almost iden-tical to that underneath the 2-nm-thick MgO layer, which areperformed in non-x-ray total reflection (TR) condition to gaina bulk sensitivity. This bulk sensitivity of HAXPES is owingto large IMFP of photoelectrons with the kinetic energy ofseveral keV in solids [14–17]. Thus, an insulator/ferromagnetstructure is suitable for exploring its near-interface electronicstate around EF by HAXPES, when IMFP of photoelectronscan be shortened.There are three possible ways to effectively control IMFPof photoelectrons in solids. The first one is the photonenergy-dependent PES measurements. This is because thekinetic energy of the photoelectrons can be tuned by theexcitation photon energy and lower kinetic energy of pho-toelectrons gives shorter IMFP according to the TPP-2Mequation. [10] However, the photon energy-dependent valenceband PES measurements have a problem in direct comparisonof the depth-resolved electronic states, since the photoion-ization cross-sections of atomic orbitals strongly depend onthe photon energy [18–21]. Therefore, direct comparison ofthe valence band spectra measured with different photonenergy needs a careful consideration of the photoionizationcross-sections. The second one is the take-off-angle (TOA)dependent HAXPES measurements with a fixed photon en-ergy, since an effective-IMFP (λ) is given by IMFP ×sin(TOA), where TOA is referred to a sample surface. Here,λ corresponds to a depth (d) from surface (or interface), atwhich the photoelectron intensity given by exp(−d/λ) re-duces to 1/e. In this case, the matrix element effects [22],which depend on experimental geometry and give the angulardistribution of photoelectrons for single crystalline samples,affect the valence band spectral shapes in the TOA-dependentmeasurements [23]. This leads to the difficulty in the di-rect comparison of the valence band spectra measured withdifferent TOAs. The last one is the use of x-ray TR inHAXPES. The shorter x-ray attenuation length (λp) in theTR condition compared to IMFP of several-keV electrons insolids drastically reduces λ in HAXPES at a fixed photonenergy as described elsewhere [23]. When the incidence an-gle (θ ) of x-ray with respect to a sample surface in the TRcondition is increased by ∼1.5◦, one can change TR- to non-TR-HAXPES experiments. The small angle change of 1.5◦in HAXPES minimizes the modification of the valence bandspectra caused by the matrix element effects, which allowsthe direct comparison of the valence band spectra for differentλ. Therefore, we employed non-TR- and TR-HAXPES tocompare buried bulk with near-interface electronic states, re-spectively, for the insulator/ferromagnet structures. To probenear-interface band dispersion of CMS underneath an AlOxlayer, we employed soft x-ray angle-resolved photoemissionspectroscopy (SX-ARPES) in the non-TR condition, becauseIMFP of photoelectrons detected in SX-ARPES is shorter thanλ in TR-HAXPES. In this work, we investigate the electronicand magnetic states of near-interface and buried bulk CMSunderneath AlOx and MgO layers by nondestructive HAXPESin the TR and non-TR conditions. The magnetic states of CMSwere measured by magnetic circular dichroism (MCD) in theCo and Mn 2p core-level HAXPES. The magnetizations ofCo and Mn in the near-interface region were reduced to ∼0.77times compared to those in the bulk region. The difference inthe valence band of CMS between the near-interface and bulkregions was also detected. SX-ARPES for AlOx/CMS filmexhibited that even in near interface region, the observed banddispersion was well explained by the bulk band dispersion ofCMS.II. EXPERIMENTFor the HAXPES experiments, two samples of epitaxialCMS films (30 nm) grown on MgO(001) substrates with dif-ferent capping layers consisting of AlOx(3 nm) and MgO(2nm) were prepared by DC or RF sputtering at RT. Details ofthe preparation and characterization of the L21-ordered CMSfilms were described elsewhere [6]. On the top of the CMSfilms, Al(3 nm) and MgO(2 nm) capping layers were de-posited individually. The Al and MgO layers were depositedby DC and RF sputtering, respectively. On the top of the MgOlayer, an additional Al(1 nm) capping layer was deposited.Then, AlOx layers were formed by ex situ air oxidation ineach sample. The coercivity, saturation magnetic moment,and remanent to saturation magnetization (MR/MS) ratioof AlOx/CMS/MgO(001) and AlOx/MgO/CMS/MgO(001)structures were deduced to be ∼16.5 Oe, ∼ 4.3 μB, and∼0.97, and ∼20.8 Oe, ∼ 4.2 μB, and ∼0.97, respectively, atRT. These values are consistent with those for the epitaxialCMS films reported in the previous work [24]. For the softx-ray ARPES measurements, an epitaxial CMS film (30 nm)was grown on a Ag(001)/Cr(001)/MgO(001) substrate withthe thickness of Ag (Cr) film of 80 (30) nm. Then, an Al(1nm) capping layer was deposited on the CMS film by DCsputtering. The Al(1 nm) layer was ex situ oxidized in air toform an AlOx layer. The coercivity, saturation magnetization,and MR/MS for the CMS film with the Ag/Cr buffer layer atRT were ∼10.0 Oe, ∼ 4.0 μB, and ∼0.99, respectively. All thefabricated samples were cut to the in-plane size of 5 mm × 10mm with the long side parallel to the [100] direction of theCMS layer.The HAXPES measurements were conducted at the un-dulator beamline BL15XU of SPring-8 [17]. The left- orright-handed circularly polarized (LCP or RCP) x-ray (5.95keV) was used for HAXPES. The degree of LCP and RCPx-rays (PC) was ∼0.95 as described in Ref. [25]. A grazing in-085109-2NEAR-INTERFACE ELECTRONIC AND MAGNETIC … PHYSICAL REVIEW B 109, 085109 (2024)(a)(b)(c)(d)FIG. 1. Co 2p core-level MCD-HAXPES spectra of the AlOx(3 nm) and AlOx(1 nm)/MgO(2 nm) capped CMS(30 nm)/MgO(001)structures for the non-TR and TR conditions. (a) Near-interface and (b) bulk regions of the CMS film with the AlOx(3 nm) capping layer.(c) and (d) Same as (a) and (b), respectively, but for the AlOx(1 nm)/MgO(2 nm) capping layer.cidence of x-rays with nearly normal emission geometry wasemployed [17]. Photoelectrons were analyzed and detected bya high-resolution hemispherical electron analyzer (VG Sci-enta R4000). Total energy resolution was set to 150 meV. Thebinding energy (EB) was calibrated by EF of a Au film. Thesample temperature was set to RT (300 K). A magnetic fieldof 3 kOe was in situ applied along the easy magnetizationaxis ([100] direction of CMS) by a permanent magnet in ananalysis chamber, and then the remanent states were probedby HAXPES. The magnetization direction is nearly parallelto the incoming x-rays. The electronic and magnetic statesof the near-interface (bulk) region of the CMS thin filmswere obtained in the TR (non-TR) condition. The incidenceangle of x-rays with respect to the sample surfaces was setto 2.0◦ (0.368◦) for the non-TR (TR) condition as referredto the calculated TR critical angle, θC = 0.505◦, according toRef. [26]. The footprint of x-rays on the sample was 25 µm ×1.0(5.4) mm in full-width at half-maximum (FWHM) for thenon-TR (TR) condition. The IMFPs of photoelectrons in CMSwere calculated by the TPP-2M equation [10] and λ wasobtained by λp IMFP/(λp + IMFP), which depended on theincidence angle [23,26].The SX-ARPES measurements were performed at thetwin-helical undulator beamline BL25SU of SPring-8 [27].Both the LCP and RCP soft x-rays were used simultaneouslyin SX-ARPES. Therefore, we did not measure the MCD inSX-ARPES. PC was reported to be ∼0.96 [28]. The incidenceangle of soft x-rays was set to 5◦ with respect to the samplesurface. The resultant footprint of x-ray on the sample was ap-proximately 10 µm × 10 µm in FWHM. Photoelectrons wereanalyzed and detected by a high-resolution hemisphericalelectron analyzer (VG Scienta DA30). Total energy resolutionand angular resolution of SX-ARPES were set to ∼80 meVand ∼ 0.2◦, respectively. The sample temperature was set to250 K.III. RESULTSFigure 1 shows the Co 2p core-level HAXPES spectra ofthe AlOx and AlOx/MgO capped CMS thin films obtainedusing LCP and RCP x-rays. In the TR (non-TR) condition, λwas calculated to be ∼2.0 (∼6.1) nm in the Co 2p core-levelregion. The intensity difference between the LCP and RCPspectra was defined as MCD. The Co 2p HAXPES spectrashowed main peaks in the 2p3/2 and 2p1/2 regions at EB of∼778.4 and ∼793.5 eV, respectively. A small hump at EB of∼782.6 eV was found in the spectra and was also commonlyseen in the Heusler alloys containing Co atoms [29–32]. Thehump structure was slightly larger in the MgO/CMS film forλ ∼ 2.0 nm than the others. Moreover, an additional humpstructure was detected in the 2p1/2 region. These hump struc-tures indicated by the black arrows in Fig. 1(c) were dueto oxidation of Co atoms near the MgO/CMS interface. TheMCD profiles were similar each other regardless of the cap-085109-3UEDA, FUJITA, AND SAKURABA PHYSICAL REVIEW B 109, 085109 (2024)(b)(a) (c)(d)FIG. 2. Mn 2p core-level MCD-HAXPES spectra of the AlOx(3 nm) and AlOx(1 nm)/MgO(2 nm) capped CMS(30 nm)/MgO(001)structures for the non-TR and TR conditions. (a) Near-interface and (b) bulk regions of the CMS film with the AlOx(3 nm) capping layer.(c) and (d) Same as (a) and (b), respectively, but for the AlOx(1 nm)/MgO(2 nm) capping layer.ping layer or λ, even though oxidation of Co atoms was foundin the MgO/CMS interface. A remarkable MCD signal withthe negative-to-positive sign change from lower to higher EBside was found in the Co 2p3/2 main peak, and the oppositesign change was found in the 2p1/2 main peak.Since the magnitude of MCD is proportional to the mag-netization projected onto the x-ray propagation direction, wecan analyze the element specific magnetization by MCD-HAXPES. When we focused on the negative MCD signal atEB of ∼778.3 eV in the Co 2p3/2 region in Fig. 1, the MCDgiven in asymmetry of 13.5–12.8% for λ ∼ 6.1 nm (bulk)reduced to 10.5–10.3% for λ ∼ 2.0 nm (near-interface) in thecapped CMS films. Here, the MCD in asymmetry correspondsto the raw MCD divided by the sum of the RCP and LCP spec-tra after subtracting an integrated-type background. Thus, themagnetization of Co atoms along the [100] direction of CMSnear the AlOx/CMS and MgO/CMS interfaces is reduced to∼0.77 times compared to that of the bulk region of CMS. Asimilar reduction of the Co magnetization for the MgO/CMSinterface was reported in the depth-resolved XMCD in the CoL2,3 absorption measurements [9].Figure 2 shows the Mn 2p core-level HAXPES and MCDspectra of the AlOx and MgO capped CMS films with thecalculated λ in the TR (non-TR) condition of ∼2.0 (∼6.3)nm. The Mn 2p HAXPES spectra of the AlOx/CMS film forλ ∼ 2.0 and ∼6.3 nm were similar each other as shown inFigs. 2(a) and 2(b). The spectra showed two sharp peaks inthe 2p3/2 region at EB of ∼638.4 and ∼639.6 eV and twobroad peaks in the 2p1/2 region at EB of ∼649.8 and ∼650.4eV. For the MgO/CMS film, the Mn 2p spectra showed addi-tional broad peaks indicated by the black arrows in Figs. 2(c)and 2(d). Moreover, the intensity of the broad peaks stronglyenhanced in the spectrum for λ ∼ 2.0 nm compared to thatfor λ ∼ 6.3 nm. These broad peaks attribute to oxidation ofMn atoms near the MgO/CMS interface. This result suggeststhat the Mn atoms near the MgO/CMS interface are stronglyoxidized, which would be induced by a degraded interface ofMgO/CMS prepared by a sputtering deposition of MgO onCMS [33]. By assuming an exponential decay of the pho-toemission intensity as a function of depth and the oxidizedMn atoms concentrated below the MgO layer, the thickness ofoxidized CMS film is estimated to be ∼1.5 nm. Note that theSi 2s core-level HAXPES spectra for the MgO/CMS film alsoshowed significant oxidation near the interface (not shown).As can be seen in Fig. 2, the MCD profiles for theAlOx/CMS and MgO/CMS films were similar each other re-gardless of oxidation or λ. A remarkable MCD signal withthe negative-to-positive sign change from lower to higher EBside was found in the Mn 2p3/2 region, and the opposite signchange was found in the 2p1/2 region. Since the componentsof oxidized Mn atoms in the Mn 2p HAXPES spectra didnot show an additional MCD feature, the oxidized Mn atoms085109-4NEAR-INTERFACE ELECTRONIC AND MAGNETIC … PHYSICAL REVIEW B 109, 085109 (2024)(a)(b)FIG. 3. Enlarged MCD spectra of (a) the Co 2p3/2 and (b) Mn2p3/2 regions for the AlOx(3 nm)/CMS(30 nm)/MgO(001) structure.The spectra measured in the TR (non-TR) condition are indicated byblue (red) lines. In both (a) and (b), the spectra were normalized atthe negative MCD signals for comparison.would be in a paramagnetic or antiferromagnetic state, that is,the magnetization of the oxidized Mn atoms along the [100]direction was vanished. The Mn 2p MCD at EB of ∼638.4eV of 33.0–33.2% for λ ∼ 6.3 nm (bulk) reduced to 24.7–26.2% for λ ∼ 2.0 nm (near-interface). Here, the MCD signalin the AlOx/MgO/CMS structure came from the CMS filmunderneath the oxidized CMS layer. Thus, the magnetizationof Mn atoms along the [100] direction near the AlOx/CMS andoxidized CMS/CMS interfaces was reduced to ∼0.76 timescompared to that of CMS in the bulk region. This reductionfactor was comparable to the case of the Co atoms.Figure 3 compares the MCD profiles in the Co 2p3/2 andMn 2p3/2 core levels for the near-interface and bulk regionsof the AlOx/CMS film. Here, the comparison of MCD profilesfor the MgO/CMS film is ignored, since the oxidized CMSlayer exists between the MgO and CMS layers. The MCDprofiles are normalized at the huge negative MCD in eachregion. One sees that the MCD profiles for the near-interfaceand bulk regions are identical each other in both the Co andFIG. 4. Valence band HAXPES spectra of the AlOx(3 nm) andAlOx(1 nm)/MgO(2 nm) capped CMS(30 nm)/MgO(001) structuresfor the non-TR and TR conditions. The dotted curves are identical tothe valence band spectrum of bulk region of CMS with the AlOx(3nm) capping layer and are superimposed on the other spectra forcomparison.Mn 2p3/2 regions within the statistical errors in the MCD in-tensity. The energy splitting between the negative and positiveMCD signals is caused by the spin exchange interaction be-tween the 2p core-hole and 3d electrons in the photoemissionfinal states and is proportional to the magnitude of magneticmoment [34], that is, a larger local magnetic moment leads toa larger energy splitting in MCD-HAXPES [35]. Therefore,the magnetizations of Co and Mn along the [110] directionwas reduced in near-interface region, although the magnitudesof magnetic moments of Co and Mn in near-interface regionare comparable to those in the bulk region, respectively, forthe AlOx/CMS film.Figure 4 shows the valence band spectra of the AlOx/CMSand MgO/CMS films with the calculated λ of ∼2.0 and ∼6.9nm. Each spectrum was obtained by the sum of LCP andRCP spectra after subtracting an integrated-type background.085109-5UEDA, FUJITA, AND SAKURABA PHYSICAL REVIEW B 109, 085109 (2024)(a) (b)o oFIG. 5. SX-ARPES intensity maps along the �-K-X direction for the AlOx(1 nm) capped CMS(001) film with the excitation photon energy(hν ) of (a) 455 and (b) 552 eV at 250 K with IMFP of ∼1.0 nm. The horizontal dotted lines indicate EF. The vertical dashed lines indicate thehigh symmetry points (� or X).The spectra were normalized at the hump structure locatedat EB of 2.6 eV, for comparison. The spectra consisted ofthree structures below 2 eV; the main peak (labeled by α) andtwo shoulders (labeled by β and γ ). These structures werealso found in the previous HAXPES spectra for CMS [13].The valence band spectra for the AlOx/CMS and MgO/CMSfilms in the bulk region (λ ∼ 6.9 nm) were similar each other,while the peak α and shoulder β slightly reduced in theMgO/CMS film. The slight reduction in the MgO/CMS filmwas mainly caused by the presence of Mn-oxides, whichreduced the metallic Mn DOS near the interface. In fact,the valence band spectrum of the MgO/CMS film near theinterface (λ ∼ 2.0 nm) showed a significant reduction of αand β and a slight reduction of γ by the increase of the Mnoxides as seen in the Mn 2p spectra in Fig. 2(c). For thenear-interface valence band spectrum of the AlOx/CMS film(λ ∼ 2.0 nm), a slight decrease (increase) of α (γ ) comparedto the spectrum for the AlOx/CMS film (λ ∼ 6.9 nm) wasfound, while Co and Mn oxides were obviously invisiblein the Co and Mn 2p spectra in Figs. 1(a) and 2(a). Thisresult indicates that in the AlOx/CMS film, the near-interfaceelectronic structure is different from the bulk one as well asthe difference in the magnetization between the near-interfaceand bulk regions. Furthermore, the effects of defects or off-stoichiometry to the electronic structure near the interface inthe AlOx/CMS film is negligibly weak, if any. It is obviousthat the energy shift of α, β, and γ structures is very small.If defects or off-stoichiometry exists near the interface, thetotal electron number in the unit cell changes and the chemicalpotential shift (the energy shift of α, β and γ structures)occurs.Figure 5 shows the SX-ARPES results for the AlOx(1nm)/CMS(001) sample with the excitation photon energies of455 and 552 eV measured at 250 K. At these photon energies,IMFP was calculated to be ∼1.0 nm. This value is half of λin TR-HAXPES. The normal emission from the (001) surfacecorresponds to the �-X direction in the reciprocal space. Onesees a clear photon energy dependence in the ARPES resultsin the figures. In Fig. 5(a), the observed band dispersion mea-sured at the excitation energy of 455 eV is symmetrical tok//[110] and a convex band dispersion with the apex locatedat 0.24 eV and � point originated from the minority spinstates, which are expected from the band dispersion calcula-tions for bulk CMS [36–38]. The energy of 0.24 eV is veryclose to the minority spin valence band maximum reported inspin-resolved HAXPES for CMS at 300 K [6]. In addition,the weak intensity across EF is seen at the X point due tothe majority spin states. In contrast to Fig. 5(a), the banddispersion measurement at the excitation energy of 552 eVin Fig. 5(b) exhibits a clear “X”-shaped band dispersion at theX point near EF and the weakened intensity at the apex of theconvex band dispersion due to the minority spin states. Theseband features are very similar to the results in SX-ARPESfor bulk Co2MnGe [39] as a predicted half-metal [36]. Aclear photon-energy-dependent band dispersion suggests thatthree-dimensional bulk band structure is remained in the near-interface region (IMFP∼1.0 nm) for the AlOx/CMS sample.Note that we cannot find surface resonances located near EFat around the � point, which has been reported in SX-ARPESfor an AlOx/CMS structure by Lidig et al. [40].IV. DISCUSSIONSince the MgO/CMS interface consists of the oxi-dized CMS layer underneath MgO layer in contrast to theAlOx/CMS interface, in the following, we discuss the near-interface electronic and magnetic states of the AlOx/CMSfilm. When the single magnetic domain structure near theinterface of the AlOx/CMS film is assumed owing to the highMR/MS ratio of 0.97, we consider a possible mechanism of thereduction of the magnetization near the interface according tothe theoretical work on the surface magnetism [41]. For bareferromagnetic films, it is expected that an exchange interac-tion at a surface (JS) differs from that in a bulk region (JB),085109-6NEAR-INTERFACE ELECTRONIC AND MAGNETIC … PHYSICAL REVIEW B 109, 085109 (2024)since the environment of the magnetic moments at the surfacediffers from that in the bulk region. According to Ref. [41],in the case of that JS/JB = 0.1 on the path perpendicular tothe surface and JS/JB = 1.0 on the path parallel to the surfaceat the temperature of 0.3TC, the magnetization at the surfacereduces to 0.8 times compared to the bulk magnetization ow-ing to thermal spin-wave excitations near the surface. Whenthe interactions at the interface between the AlOx and CMSfilms to the magnetic properties are negligible, the temper-ature of 0.3TC and the reduction of magnetization (0.8) inthe calculation [41] are very close to RT/TC of 0.304 for theCMS films and the observed reduction (∼ 0.77) in the Coand Mn MCD signals near the interface in our experiments,respectively. Therefore, the reduced magnetization near theinterface of AlOx/CMS film is due to possible enhanced spin-wave excitations. The enhanced spin-wave excitations nearthe interface of the CMS film is virtually comparable to theincrease of temperature in the bulk region to satisfy Bloch’sT 3/2 law in terms of spin-wave excitations. It is expected thatthe fraction of the Co eg and t2g partial DOSs can change [6],when the temperature of CMS increases. This fraction changecan reduce the peak α in the valence band spectrum of CMSin Fig. 4 for the CMS film near the interface compared to thatin the bulk region.The spin-wave excitations (or precessions) of local mag-netic moments only change the magnetizations along the easymagnetization axis. That is, the magnitude of local magneticmoment is unchanged by the spin-wave excitations. As seenin Fig. 3, the magnitude of Co (Mn) local magnetic momentis almost identical for the near-interface and bulk regionsin the AlOx/CMS structure. Nawa et al. [42] reported thetemperature-dependent spin-dependent DOS for bulk CMSby means of the density functional theory calculations withthe disordered local moment method, which treats the fluc-tuation of local magnetic moments at finite temperature. Inother words, they treated the fluctuation as a substitution ofthe precession (or spin-wave excitation) of the local magneticmoments. As seen in Fig. 3 in Ref. [42] for L21-ordered CMS,the majority spin DOS peak at 1 eV corresponding to thepeak α in Fig. 4 decreases with increasing temperature. Inaddition, the majority spin DOS subpeak at 0.5 eV in Ref. [42]corresponding to shoulder structure β in Fig. 4 slightly shiftstoward to EF with increasing temperature. In the minority spinDOS near EF, the DOS gradually increases with increasingtemperature, while changes of the majority spin DOS nearEF are insensitive to temperature. This causes the increase ofDOS near EF with increasing temperature. These tendenciesare consistent with the difference in the valence band spectrabetween the near-interface and bulk region for AlOx/CMSstructure. That is, in the AlOx/CMS structure, the reducedintensity of peak α, slight shift of shoulder structure β, andslight increase of shoulder structure γ for the near-interfacevalence band spectrum compared to the valence band spec-trum for the bulk region as shown in Fig. 4. Therefore, itis a plausible origin for the spin-wave excitations that areenhanced due to the weakened exchange interaction betweenthe local magnetic moments near the interface region of theAlOx/CMS structure, which leads the difference in the elec-tronic and magnetic states between the near-interface and bulkregions.In the above-discussion, we did not consider aninterface-induced magnetic anisotropy (IMA) at an insula-tor/ferromagnet interface as is similar to a surface-inducedmagnetic anisotropy in a ferromagnetic thin film [41]. TheIMA can cause that the magnetization inclines toward tothe direction perpendicular to the film plane near the in-terface, even if the in-plane easy magnetization axis of thefilm is realized (in other words, noncollinear magnetization)[34,43]. In addition, at the insulator/ferromagnetic interfaces,where AlOx or MgO is used as an insulator, the perpendicu-lar magnetization is introduced by a strong IMA, when theferromagnetic film thickness is less than 1 nm [44–47]. Inour experiments, the inclined magnetization near the interfaceis also expected, if the reduced net magnetization near theinterface is due to the noncollinear magnetization caused bythe IMA, since the CMS film is sufficiently thicker than 1 nm.As mentioned above, the magnitude of MCD is proportionalto the magnetization projected onto the propagation directionof x-rays, the inclined magnetization can reduce the MCDsignal near the interface. Therefore, the PMA can be anotherpossible mechanism in the reduction of magnetization near theinterface in the AlOx/CMS structure.When the noncollinear magnetization near interface due tostrong PMA induced by the IMA is realized, not the mag-netization but the magnitude of local magnetic moment isexpected to be slightly enhanced as reported in the perpen-dicularly magnetized MgO/Fe [34,48] and MgO/CMS films[49]. In both cases, ultrathin Fe and CMS magnetic filmsunderneath the MgO layer are used, and the increase of thelocal magnetic moment induced by the interfacial effects isfound. In contrast, as can be seen in Fig. 3, the magnitudeof Co (Mn) local magnetic moment near-interface and bulkregions is almost identical each other in the AlOx/CMS struc-ture. Therefore, we may conclude that the reduction of themagnetizations of Co and Mn near the interface is not due tothe IMA.Finally, we consider the highly spin-polarized surface res-onances in the bare and AlOx-capped CMS(001) reported inRefs. [4] and [40], respectively. Jourdan et al. [4] have re-ported the high SP of ∼93% at EF for the bare CMS(001) filmby means of surface-sensitive spin-resolved PES at RT withthe excitation energy of 21.2 eV. This high SP is inconsistentwith the reduction of the magnetization (∼0.77) near interfaceof the AlOx/CMS structure in this work, when we assumethat SP of 100% in the bulk region is realized at RT and thatSP is proportional to the magnetization. In this assumption,the expected SP near the interface is ∼77%. In contrast, thesimilar SP for bare CMS(001) has been found in surface-sensitive spin-resolved PES using the s-polarized light withthe energy of 37 eV at RT [50]. In Ref. [50], spin-resolved PESwith the photon energy dependence (30–70 eV) has also beenperformed, and the signature of surface resonance in CMSis not clearly observed. The discrepancy between Refs. [4]and [50] might be caused by the difference in the probedmomentum space, because k//[001] depends on the excitationphoton energy, in principle. At the excitation energy of 21.2eV, k//[001] is out of the � point, which misleads to high SP asdescribed in Ref. [50].Although the SX-ARPES results on the AlOx/CMS filmreported in Ref. [40] show the surface resonance near EF, the085109-7UEDA, FUJITA, AND SAKURABA PHYSICAL REVIEW B 109, 085109 (2024)SX-ARPES results in this work (Fig. 5) showed no obvioussurface resonance. We are aware of that the results agreewell with the corresponding band dispersion for Co2MnGe(Fig. 4 in Ref. [39]) and also with the dispersion for Co2MnSireported in Ref. [40], but only if the assignment of the � and Xpoints is reversed, such that the � point in k//[001] at an integermultiple of the reciprocal lattice vector length confused theidentification of the � and X points. When the assigned Xpoint is corrected to the � point in the bottom panel of Fig. 3in Ref. [40], the observed band dispersion is very similar toour SX-ARPES result shown in Fig. 5(b), that is, a metallicband dispersion at the X point and no band dispersion near EFat the � point. The confusion of the � and X points in Ref. [40]is also confirmed by the k//[001]-dependent SX-ARPES resultfor bulk Co2MnGe [39], which has the L21 structure with asimilar lattice parameter and half-metallicity. Therefore, wecan conclude that the surface resonance is not an origin of thedifference of the electronic and magnetic states between thenear-interface and bulk regions of the AlOx/CMS structure.In Fig. 4, the shoulder structure γ in the valence bandspectrum for the near-interface region of the AlOx/CMS struc-ture is slightly larger than that for the bulk region, but theSX-ARPES results shown in Fig. 5 do not show any addi-tional band states near EF due to the slight increase of γ . Ifthe origin of slight increase of γ arises from the enhancedspin-wave excitations (or enhanced fluctuation of local mag-netic moments), s-orbital-like minority spin states around EFare introduced by the fluctuation of local magnetic momentsaccording to Ref. [8]. Since the photoionization cross-sectionratio of the s orbital to d orbital in the valence region forthe 3d transition metal is very large in HAXPES [25] butsmall in SX-PES [21], the higher sensitivity to the s orbitalin HAXPES can detect the s orbital states induced by the fluc-tuation, even if the states are quite small. In contrast, the lowersensitivity to the s orbital in SX-PES cannot detect the quitesmall s orbital states. Or there is a possibility that such s statesare missing in the SX-ARPES results in Fig. 5 due to thatthe probed momentum space is limited in the �-X direction,whereas HAXPES gives the Brillouin zone averaged spectra.From the above discussion, we can conclude that thereasonable origin of the differences in the electronic and mag-netic states between the near-interface and bulk region of theAlOx/CMS structure is the enhanced spin-wave excitationsnear the interface compared to the bulk region. When the TMRratios in many TMR junctions [1] are governed by the magne-tization near the interface, the strong reduction of TMR ratiowith increasing temperature can be understood by a fasterreduction of the magnetization along the easy magnetizationdirection near the interface with increasing temperature orig-inated from the enhanced spin-wave excitations as comparedto the bulk magnetization.V. SUMMARYIn summary, we conducted the depth-dependent HAX-PES measurements for the AlOx/CMS and AlOx/MgO/CMSstructures to clarify the differences in the electronic andmagnetic states between the near-interface and bulk regionsfor the CMS films utilizing the TR and non-TR conditions.TR-HAXPES combined with MCD can clearly detect thenear-interface electronic and magnetic states of the CMSfilms. At the MgO/CMS interface, a strong oxidation of theMn and Si atoms underneath the MgO layer and no oxidationunderneath the AlOx layer were experimentally detected bynondestructive HAXPES combined with TR. The Co and Mn2p core-level MCD-HAXPES measurements for the near-interface and bulk regions of the AlOx/CMS film clearlyrevealed that the Co and Mn magnetizations along the [100]direction near the interface reduce to ∼0.77 times with respectto those in the bulk region. We can conclude that the possibleorigin of the reduction of the Co and Mn magnetizations andthe changes in the valence band profile near the AlOx/CMSinterface compared to the CMS film in the bulk region isthe enhanced spin-wave excitations due to the weakened ex-change interaction, which can slightly modify the valenceband electronic states, near the interface, by considering pre-cession of local magnetic moments at a finite temperature.Thus, the combination of HAXPES with TR is very useful forexploring the electronic and magnetic states of near-interfaceand bulk regions for the insulator/ferromagnet heterojunctionsin the nondestructive way.ACKNOWLEDGMENTSThe HAXPES measurements were performed with theapproval of NIMS Synchrotron X-ray Station at SPring-8 (Proposal No. 2020A4604 and No. 2020A4701). TheSX-ARPES experiments were performed with the approvalof JASRI/SPring-8 (Proposal No. 2021A1147). S.U. wouldlike to thank T. Muro, T, Ohsawa, K. Yamagami, and K.Kuroda for technical support in SX-ARPES. This workwas partially supported by Tokodai Institute for ElementalStrategy (TIES) and Data Creation and Utilization Type Ma-terial Research and Development Project from MEXT, Japan[Grants No. JPMXP0112101001, No. JPMXP1122683430,and No. JPMXP1122715503] and JSPS KAKENHI [GrantsNo. 20K05336 and No. 17H06152].[1] K. Elphick, W. Frost, M. Samiepour, T. Kubota, K. Takanashi,H. Sukegawa, S. Mitani, and A. Hirosawa, Heusler alloys forspintronic devices: Review on recent development and futureperspectives, Sci. Technol. Adv. Mater. 22, 235 (2021).[2] B. Hu, K. Moges, Y. Honda, H.-x. Liu, T. Uemura, M.Yamamoto, J.-i. Inoue, and M. Shirai, Temperature dependenceof spin-dependent tunneling conductance of magnetic tunneljunctions with half-metallic Co2MnSi electrodes, Phys. Rev. B94, 094428 (2016).[3] J. W. Jung, Y. Sakuraba, T. Sasaki, Y. Miura, and H. Hono,Enhancement of magnetoresistance by inserting thin NiAl lay-ers at the interfaces in Co2FeGa0.5Ge0.5/Ag/Co2FeGa0.5Ge0.5current-perpendicular-to-plane pseudo spin valves, Appl. Phys.Lett. 108, 102408 (2016).085109-8https://doi.org/10.1080/14686996.2020.1812364https://doi.org/10.1103/PhysRevB.94.094428https://doi.org/10.1063/1.4943640NEAR-INTERFACE ELECTRONIC AND MAGNETIC … PHYSICAL REVIEW B 109, 085109 (2024)[4] M. Jourdan J. Minar, J. Braun, A. Kronenberg, S. Chadov, B.Balke, A. Gloskovskii, M. Kolbe, H. J. Elmers, G. Schönhense,H. Ebert, C. Felser, and M. Klaui, Direct observation ofhalf-metallicity in the Heusler compound Co2MnSi, Nat.Commun. 5, 3974 (2014).[5] S. Ueda and Y. Sakuraba, Direct observation of spin-resolvedvalence band electronic states from a buried ferromagnetic layerwith hard x-ray photoemission, Sci. Technol. Adv. Mater. 22,317 (2021).[6] S. Ueda, Y. Miura, Y. Fujita, and Y. Sakuraba, Direct probingof temperature-independent bulk-halfmetallicity in Co2MnSiby spin-resolved hard x-ray photoemission, Phys. Rev. B 106,075101 (2022).[7] A. Sakuma, Y. Toga, and H. Tsuchiura, Theoretical study onthe stability of magnetic structures in the surface and interfacesof Heusler alloys, Co2MnAl and Co2MnSi, J. Appl. Phys. 105,07C910 (2009).[8] Y. Miura, K. Abe, and M. Shirai, Effects of interfacial non-collinear magnetic structures on spin-dependent conductancein Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions: A first-principles study, Phys. Rev. B 83, 214411 (2011).[9] S. Tsunegi, Y. Sakuraba, K. Amemiya, M. Sakamaki, E. Ozawa,A. Sakuma, K. Takanashi, and Y. Ando, Observation of mag-netic moments at the interface region in magnetic tunneljunctions using depth-resolved x-ray magnetic circular dichro-ism, Phys. Rev. B 85, 180408(R) (2012).[10] S. Tanuma, C. J. Powell, and D. R. Penn, Calculations ofelectron inelastic mean free paths. V. Data for 14 organic com-pounds over the 50–2000 eV range, Surf. Interf. Anal. 21, 165(1994).[11] B. T. Thole, P. Carra, F. Sette, and G. van der Laan, x-raycircular dichroism as a probe of orbital magnetization, Phys.Rev. Lett. 68, 1943 (1992).[12] P. Carra, B. T. Thole, M. Altarelli, and X. Wang, X-ray circulardichroism and local magnetic fields, Phys. Rev. Lett. 70, 694(1993).[13] G. H. Fecher, B. Balke, A. Gloskowskii, S. Ouardi, C. Felser, T.Ishikawa, M. Yamamoto, Y. Yamashita, H. Yoshikawa, S. Ueda,and K. Kobayashi, Detection of the valence band in buriedCo2MnSi-MgO tunnel junctions by means of photoemissionspectroscopy, Appl. Phys. Lett. 92, 193513 (2008).[14] Y. Takata, M. Yabashi, K. Tamasaku, Y. Nishino, D. Miwa,T. Ishikawa, E. Ikenaga, K. Horiba, S. Shin, M. Arita, K.Shimada, H. Namatame, M. Taniguchi, H. Nohira, T. Hattori,S. Sodergren, B. Wannberg, and K. Kobayashi, Development ofhard x-ray photoelectron spectroscopy at BL29XU in SPring-8,Nucl. Instrum. Methods Phys. Res. Sect. A. 547, 50 (2005).[15] K. Kobayashi, M. Yabashi, Y. Takata, T. Tokushima, S. Shin,K. Tamasaku, D. Miwa, T. Ishikawa, H. Nohira, T. Hattori,Y. Sugita, O. Nakatsuka, S. Sakai, and S. Zaima, Highresolution-high energy x-ray photoelectron spectroscopy usingthird-generation synchrotron radiation source, and its applica-tion to Si-high k insulator systems, Appl. Phys. Lett. 83, 1005(2003).[16] C. S. Fadley, Hard x-ray photoemission with angular resolu-tion and standing-wave excitation, J. Electron Spectrosc. Rel.Phenom. 190, 165 (2013).[17] S. Ueda, Application of hard x-ray photoelectron spectroscopyto electronic structure measurements for various functional ma-terials, J. Electron Spectrosc. Rel. Phenom. 190, 235 (2013).[18] J. H. Scofield, Theoretical photoionization cross sections from1 to 1500 keV, Tech. Rep. UCRL-51326 (Lawrence LivermoreLaboratory, 1973).[19] M. B. Trzhaskovskaya, V. I. Nefedov, and V. G. Yarzhemsky,Photoelectron angular distribution parameters for elements Z =1 to Z = 54 in the photoelectron energy range 100–5000 eV,At. Data Nucl Data Tables 77, 97 (2001).[20] M. B. Trzhaskovskaya, V. K. Nikulin, V. I. Nefedov, and V.G. Yarzhemsky, Non-dipole second order parameters of thephotoelectron angular distribution for elements Z = 1−100 inthe photoelectron energy range 1–10 keV, At. Data Nucl. DataTables 92, 245 (2006).[21] J. J. Yeh and I. Lindau, Atomic subshell photoionization crosssections and asymmetry parameters: 1 � Z � 103, At. DataNucl, Data Tables 32, 1 (1985).[22] S. M. Goldberg, C. S. Fadley, and S. Kono, Photoionizationcross-sections for atomic orbitals with random and fixed spatialorientation, J. Electron Spectrosc. Rel. Phenom. 21, 285 (1981).[23] S. Ueda, Depth-resolved electronic structure measurements byhard x-ray photoemission combined with x-ray total reflection:Direct probing of surface band bending of polar GaN, Appl.Phys. Express 11, 105701 (2018).[24] H. Kijima, T. Ishikawa, T. Marukame, H. Koyama, K.-i.Matsuda, T. Uemura, and M. Yamamoto, Epitaxial growth offull-Heusler alloy Co2MnSi thin films on MgO-buffered MgOsubstrates, IEEE Trans. Magn. 42, 2688 (2006).[25] S. Ueda and I. Hamada, Polarization dependent bulk-sensitivevalence band photoemission spectroscopy and density func-tional theory calculations: Part I. 3d transition metals, J. Phys.Soc. Jpn. 86, 124706 (2017).[26] B. L. Henke, E. M. Gullikson, and J. C. Davis, X-ray interac-tions: Photoabsorption, scattering, transmission, and reflectionat E = 50–30,000 eV, Z = 1–92, At. Data Nucl. Data Tables54, 181 (1993).[27] T. Muro, Y. Senba, H. Ohashi, T. Ohkochi, T. Matsushita, T.Kinoshita, and S. Shin, Soft x-ray ARPES for three-dimensionalcrystals in the micrometre region, J. Synchrotron Rad. 28, 1631(2021).[28] T. Hirono, H. Kimura, T. Muro, Y. Saitoh, and T. Ishikawa,Full polarization measurement of SR emitted from twin helicalundulators with use of Sc/Cr multilayers at near 400 eV, J.Electron Spectrosc. Rel. Phenom. 144 1097 (2005).[29] J. Barth, G. H. Fecher, B. Balke, S. Ouardi, T. Graf, C. Felser, A.Shkabko, A. Weidenkaff, P. Klaer, H. J. Elmers, H. Yoshikawa,S. Ueda, and K. Kobayashi, Itinerant half-metallic ferromagnetsCo2TiZ (Z = Si, Ge, Sn): Ab initio calculations and measure-ment of the electronic structure and transport properties, Phys.Rev. B 81, 064404 (2010).[30] S. Ouardi, G. H. Fecher, B. Balke, A. Beleanu, X. Kozina, G.Stryganyuk, C. Felser, W. Klöß, H. Schrader, F. Bernardi, J.Morais, E. Ikenaga, Y. Yamashita, S. Ueda, and K. Kobayashi,Electronic and crystallographic structure, hard x-ray pho-toemission, and mechanical and transport properties of thehalf-metallic Heusler compound Co2MnGe, Phys. Rev. B 84,155122 (2011).[31] J. Winterlik, G. H. Fecher, B. Balke, T. Graf, V. Alijani, V.Ksenofontov, C. A. Jenkins, O. Meshcheriakova, C. Felser, G.Liu, S. Ueda, K. Kobayashi, T. Nakamura, and M. Wójcik, Elec-tronic, magnetic, and structural properties of the ferrimagnetMn2CoSn, Phys. Rev. B 83, 174448 (2011).085109-9https://doi.org/10.1038/ncomms4974https://doi.org/10.1080/14686996.2021.1912576https://doi.org/10.1103/PhysRevB.106.075101https://doi.org/10.1063/1.3058622https://doi.org/10.1103/PhysRevB.83.214411https://doi.org/10.1103/PhysRevB.85.180408https://doi.org/10.1002/sia.740210302https://doi.org/10.1103/PhysRevLett.68.1943https://doi.org/10.1103/PhysRevLett.70.694https://doi.org/10.1063/1.2931089https://doi.org/10.1016/j.nima.2005.05.011https://doi.org/10.1063/1.1595714https://doi.org/10.1016/j.elspec.2013.06.008https://doi.org/10.1016/j.elspec.2013.01.009https://doi.org/10.1006/adnd.2000.0849https://doi.org/10.1016/j.adt.2005.12.002https://doi.org/10.1016/0092-640X(85)90016-6https://doi.org/10.1016/0368-2048(81)85067-0https://doi.org/10.7567/APEX.11.105701https://doi.org/10.1109/TMAG.2006.878850https://doi.org/10.7566/JPSJ.86.124706https://doi.org/10.1006/adnd.1993.1013https://doi.org/10.1107/S1600577521007487https://doi.org/10.1016/j.elspec.2005.01.188https://doi.org/10.1103/PhysRevB.81.064404https://doi.org/10.1103/PhysRevB.84.155122https://doi.org/10.1103/PhysRevB.83.174448UEDA, FUJITA, AND SAKURABA PHYSICAL REVIEW B 109, 085109 (2024)[32] R. Fetzer, S. Ouardi, Y. Honda, H.-x. Liu, S. Chadov, B.Balke, S. Ueda, M. Suzuki, T. Uemura, M. Yamamoto, M.Aeschlimann, M. Cinchetti, G. H. Fecher, and C. Felser,Spin-resolved low-energy and hard x-ray photoelectron spec-troscopy of off-stoichiometric Co2MnSi Heusler thin filmsexhibiting a record TMR, J. Phys. D 48, 164002 (2015).[33] S. Tsunegi, Y. Sakuraba, M. Oogane, N. D. Telling, L. R.Shelford, E. Arenholz, G. van der Laan, R. J. Hicken, K.Takanashi, and Y. Ando, Tunnel magnetoresistance in epi-taxially grown magnetic tunnel junctions using Heusler alloyelectrode and MgO barrier, J. Phys. D 42, 195004 (2009).[34] S. Ueda, M. Mizuguchi, M. Tsujikawa, and M. Shirai, Elec-tronic structures of MgO/Fe interfaces with perpendicularmagnetization revealed by hard x-ray photoemission with an ap-plied magnetic field, Sci. Technol. Adv. Mater. 20, 796 (2019).[35] X. Kozina, G. H. Fecher, G. Stryganyuk, S. Ouardi, B. Balke, C.Felser, G. Schönhense, E. Ikenaga, T. Sugiyama, N. Kawamura,M. Suzuki, T. Taira, T. Uemura, M. Yamamoto, H. Sukegawa,W. Wang, K. Inomata, and K. Kobayashi, Magnetic dichroismin angle-resolved hard x-ray photoemission from buried layer,Phys. Rev. B 84, 054449 (2011).[36] S. Picozzi, A. Continenza, and A. J. Freeman, Co2MnX (X =Si, Ge, Sn) Heusler compounds: An ab initio study of theirstructural, electronic, and magnetic properties at zero and el-evated pressure, Phys. Rev. B 66, 094421 (2002).[37] G. Qion, W. Ren, and D. J. Singh, Interplay of local momentand itinerant magnetism in cobalt-based Heusler ferromagnets:Co2TiSi, Co2MnSi, and Co2FeSi, Phys. Rev. B 101, 014427(2020).[38] S. Chernov, C. Lidig, O. Fedchenko, K. Medjianik, S.Babenkov, D. Vasilyev, M. Jourdan, G. Schönhense, and H.J. Elmers, Band structure tuning of Heusler compounds: Spin-and momentum-resolved electronic structure analysis of com-pounds with different band filling, Phys. Rev. B 103, 054407(2021).[39] T. Kono, M. Kakoki, T. Yoshikawa, X. Wang, K. Goto, T. Muro,R. Umetsu, and A. Kimura, Visualizing half-metallic bulk bandstructure with multiple Weyl cones of the Heusler ferromagnet,Phys. Rev. Lett. 125, 216403 (2020).[40] C. Lidig, J. Minár, J. Braun, H. Ebert, A. Gloskovskii, J. A.Krieger, V. Strocov, M. Kläui, and M. Jourdan, Surface reso-nance of thin films of the Heusler half-metal Co2MnSi probedby soft x-ray angular resolved photoemission spectroscopy,Phys. Rev. B 99, 174432 (2019).[41] H. C. Siegmann, Surface and 2D magnetism, J. Phys.: Condens.Mater. 4, 8395 (1992).[42] K. Nawa, I. Kurniawan, K. Masuda, Y. Miura, C. E. Patrick,and J. B. Staunton, Temperature-dependent spin polarization ofHeusler Co2MnSi from the disordered local-moment approach:Effects of atomic disorder and nonstoichiometry, Phys. Rev. B102, 054424 (2020).[43] T. Kawauchi, Y. Miura, X. Zhang, and K. Fukutani, Interface-driven noncolliner magnetic structure and phase transition of Fethin films, Phys. Rev. B 95, 014432 (2017).[44] S. Monso, B. Rodmacq, S. Aiffret, G. Casali, F. Fettar, B. Gilles,B. Dieny, and P. Boyer, Crossover from in-plane to perpendic-ular anisotropy in Pt/CoFe/AlOx sandwiches as a function ofAl oxidation: A very accurate control of the oxidation of tunnelbarriers, Appl. Phys. Lett. 80, 4157 (2002).[45] B. Rodmacq, A. Manchon, C. Ducruet, S. Aiffret, and B. Dieny,Influence of thermal annealing on the perpendicular magneticanisotropy of Pt/Co/AlOx trilayers, Phys. Rev. B 79, 024423(2009).[46] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H. D. Gan,M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, and H. Ohno,A perpendicular-anisotropy CoFeB-MgO magnetic tunnel junc-tion, Nat. Mater. 9, 721 (2010).[47] C.-H. Lambert, A. Rajanikanth, T. Hauet, S. Mangin, E. E.Fullerton, and S. Andrieu, Quantifying perpendicular magneticanisotropy at the Fe-MgO(001) interface, Appl. Phys. Lett. 102,122410 (2013).[48] H. X. Yang, M. Chshiev, B. Dieny, J. H. Lee, A. Manchon,and K. H. Shin, First-principles investigation of the very largeperpendicular magnetic anisotropy at Fe|MgO and Co|MgOinterfaces, Phys. Rev. B 84, 054401 (2011).[49] T. Saito, T. Katayama, T. Ishikawa, M. Yamamoto, D. Asakura,T. Koide, Y. Miura, and M. Shirai, Interface structure of half-metallic Heusler alloy Co2MnSi thin films facing an MgOtunnel barrier determined by x-ray magnetic circular dichroism,Phys. Rev. B 81, 144417 (2010).[50] S. Andrieu, A. Neggache, T. Hauet, T. Devolder, A. Hallal, M.Chshiev, A. M. Bataille, P. Le Fèrve, and F. Bertran, Direct evi-dence for minority spin gap in the Co2MnSi Heusler compound,Phys. Rev. B 93, 094417 (2016).085109-10https://doi.org/10.1088/0022-3727/48/16/164002https://doi.org/10.1088/0022-3727/42/19/195004https://doi.org/10.1080/14686996.2019.1633687https://doi.org/10.1103/PhysRevB.84.054449https://doi.org/10.1103/PhysRevB.66.094421https://doi.org/10.1103/PhysRevB.101.014427https://doi.org/10.1103/PhysRevB.103.054407https://doi.org/10.1103/PhysRevLett.125.216403https://doi.org/10.1103/PhysRevB.99.174432https://doi.org/10.1088/0953-8984/4/44/004https://doi.org/10.1103/PhysRevB.102.054424https://doi.org/10.1103/PhysRevB.95.014432https://doi.org/10.1063/1.1483122https://doi.org/10.1103/PhysRevB.79.024423https://doi.org/10.1038/nmat2804https://doi.org/10.1063/1.4798291https://doi.org/10.1103/PhysRevB.84.054401https://doi.org/10.1103/PhysRevB.81.144417https://doi.org/10.1103/PhysRevB.93.094417