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Shingo Takezawa, Michele Riva, Florian Dörr, Micheal Schmid, [Taisuke Sasaki](https://orcid.org/0000-0002-5952-7638), [Takanobu Hiroto](https://orcid.org/0000-0002-6176-5782), Masato Kotsugi, [Koichiro Yaji](https://orcid.org/0000-0002-0721-1316), [Yuya Sakuraba](https://orcid.org/0000-0003-4618-9550), [Naoka Nagamura](https://orcid.org/0000-0002-7697-8983)

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[Initial Growth Process of Cu Thin Films on bcc-FeCo/MgO(100) via Molecular Beam Epitaxy](https://mdr.nims.go.jp/datasets/f186bc6f-afc7-414f-95bc-7d82fe496194)

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e-Journal of Surface Science and Nanotechnology 24, 21–25 (2026)Initial Growth Process of Cu Thin Films onbcc-FeCo/MgO(100) via Molecular Beam EpitaxyShingo Takezawa,a, b Michele Riva,c Florian Dörr,c Micheal Schmid,c Taisuke Sasaki,b Takanobu Hiroto,bMasato Kotsugi,a Koichiro Yaji,b, d Yuya Sakuraba,b Naoka Nagamura a, b, e, †a Department of Materials Science and Technology, Tokyo University of Science, Katsushika-ku, Tokyo 125-8585, Japanb National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japanc Institute of Applied Physics, TU Wien, Wiedner Hauptstraße, Vienna 1040, Austriad Unprecedented-scale Data Analytics Center, Tohoku University, Sendai, Miyagi 980-8577, Japane Research Institute of Electrical Communication, Tohoku University, Sendai, Miyagi 980-8577, Japan† Corresponding author: NAGAMURA.Naoka@nims.go.jpReceived: 10 October, 2025; Accepted: 1 February, 2026; J-STAGE Advance Publication: 26 February, 2026; Published: 26 February, 2026We investigated the thickness-dependent structural evolution of Cu/Fe-Co(100) thin films grown by molecular beam epitaxy (MBE). At the initialstage of growth, Cu adopted a body-centered cubic (bcc) structure templatedby the underlying FeCo substrate. Once the Cu thickness exceeded approx-imately 3 nm, a structural transformation into the face-centered cubic (fcc)phase was observed. The MBE-grown films exhibited interfacial disloca-tions and developed a disordered surface in the thickness range of 3–5 nm.Although metastable ultrathin bcc-Cu films on FeCo(100) grown by MBEare less stable than sputtered multilayers, MBE offers a unique advantage: itenables in-situ advanced characterization such as angle-resolved photoem-ission spectroscopy (ARPES) in the ultrathin regime around 1 nm. Thiscapability is essential for designing high-performance devices and probing interface-driven electronic phenomena.Keywords Thin film; Copper; Nanostructures; Molecular beam epitaxyI. INTRODUCTIONThe growth process of thin films is an important topic insurface science, as it is strongly influenced by interactions atthe film/substrate interface [1]. Bulk copper typically crys-tallizes in a face-centered cubic (fcc) structure, but it canadopt a body-centered cubic (bcc) structure when templatedby an appropriate substrate. However, such bcc-Cu filmsoften contain numerous defects, steps, and disordered regions[2–4]. Recently, single-phase, low-defect bcc-Cu thin filmshave been reported in sputtered FeCo/Cu/FeCo(100) multi-layers [5–8]. These multilayers exhibit a high magnetoresist-ance ratio (40.5%) when used as trilayer current-in-planegiant magnetoresistance (GMR) devices [5], underscoringtheir potential for spintronic applications. The exceptionallyhigh GMR performance has been attributed to the formationof continuous Cu/FeCo(100) interfaces, directly linking thecrystallography of the Cu layer to device functionality. Sput-tered bcc-Cu films on TiN(100) have been reported to under-go a structural transformation into the fcc phase once the Cuthickness exceeds approximately 4–5 nm [9]. These findingshighlight the importance of a systematic study for the thick-ness-dependent structural changes in Cu/FeCo(100) at theinitial stages of growth, especially in view of potential futureapplications.As noted above, many previous studies on thin film het-erostructures have employed sputter deposition, owing tostrong film adhesion and applicability to a wide range ofmaterials. In contrast, molecular beam epitaxy (MBE) pro-vides advantages as it enables precise control of film thick-ness in the atomic level in ultra-high vacuum (UHV) con-ditions. Moreover, establishing a reliable MBE growthprocess makes it possible to prepare atomically clean sur-faces and to employ advanced in-situ characterization tech-niques owing to the portability of molecular beam cells. Inthis study, we conducted a systematic investigation of thethickness-dependent structural evolution of Cu ultrathin filmsgrown on FeCo(100) by MBE.Regular Papere-J. Surf. Sci. Nanotechnol. 24, 21–25 (2026) | DOI: 10.1380/ejssnt.2026-004 21mailto:NAGAMURA.Naoka@nims.go.jphttps://doi.org/10.1380/ejssnt.2026-004II. EXPERIMENTALPrevious reports have demonstrated that high-quality bcc-Fe0.5Co0.5(100) thin films can be grown on MgO(100) sub-strates by both sputtering [5] and MBE [10]. Based on this,we employed FeCo films on MgO(100) as the underlyingtemplate for Cu growth. Cu/FeCo(100)/MgO(100) thin filmswere fabricated by MBE in a UHV chamber with a basepressure of 5 × 10−8 Pa [Figure 1(a)]. To examine thickness-dependent structural behavior, five samples with Cu thick-nesses of 1, 3, 4, 5, and 30 nm were prepared. MgO(100)substrates (Crystal Base, 10mm × 10mm × 0.5mm) wereannealed at 500 °C for 10 h to obtain clean surfaces. FeColayers thicker than 15 nm were deposited by electron-beam(EB) evaporation at a substrate temperature of 300 °C with adeposition rate of 0.17Åmin−1. Subsequently, Cu layerswere deposited by EB evaporation at room temperature witha deposition rate of 1.3Åmin−1. Deposition rates and filmthicknesses were calibrated by a quartz crystal microbalance.The evaporation sources were a high-purity Fe0.5Co0.5 rod(TOSHIMA Manufacturing, 2mm × 2mm × 20mm, 4N)and a Cu wire (Nilaco, 0.25mm in diameter, 5N).In-situ surface crystallinity was evaluated immediatelyafter sample preparation by low-energy electron diffraction(LEED: OCI BDL600) with the ViPErLEED system [11–13].The R-factors, which describes the agreement between twoLEED-I(V) spectra, were calculated using Pendry’s method[14]. To prevent oxidation during subsequent ex-situ meas-urements, a 1 nm-thick Al capping layer was deposited on the1, 3, 4, and 5 nm-Cu samples. Ex-situ structural character-ization was performed using X-ray diffraction (XRD: RigakuSmartLab) with Cu Kα radiation (45 kV, 200mA) for allsamples. Following XRD measurements, transmission elec-tron microscopy (TEM: FEI Titan G2 80-200) was carriedout on the 4 nm-Cu sample. The accelerating voltage was200 kV. Cross-sectional TEM specimens were prepared usingthe lift-out technique with a dual-beam focused ion-beam/scanning electron microscope (FIB/SEM: FEI Helios G4).Prior to the specimen preparation, a Pt protection layer wasdeposited on the surface.III. RESULTS AND DISCUSSIONSFigure 1(b, f ) shows the LEED images of the samples.Well-defined 1 × 1 diffraction patterns were observed fromthe bare MgO(100) substrate surface [Figure 1(b)] and fromthe bcc-FeCo(100) film surface [Figure 1(c)], confirming theformation of clean surfaces. A similarly sharp diffractionpattern was also obtained for the 1 nm-Cu ultrathin film[Figure 1(d)], identical to that of the bcc-FeCo(100) surface[Figure 1(c)]. When the 1 nm-Cu ultrathin film adopts a bcc-Cu(100) structure templated by the underlying bcc-Fe-Co(100), as reported in previous studies on sputtered films,the diffraction patterns of FeCo(100) and Cu(100) becomeindistinguishable. In contrast, no diffraction spots were visi-ble for the 3, 4, and 5 nm-Cu films, indicating a loss ofstructural order at the surface in this intermediate thicknessregime. For the 30 nm-Cu film surface [Figure 1(f )], diffrac-tion spots reappeared, signifying the recovery of structuralorder at the surface. At the same time, the high backgroundintensity suggests the presence of numerous surface defectsand steps. Notably, the appearance of the diffraction spots inthe 30 nm-Cu film [Figure 1(f )] differs significantly fromthose of the 1 nm-Cu film [Figure 1(d)].To further clarify the difference between the LEED dif-fraction spots of the 1 and 30 nm-Cu films, we measured theI(V) spectra of the diffraction spots. Figure 2 shows theLEED-I(V) spectra of the 01 spots from Figure 1(c, d, f ),indexed on the basis of previous reports. The I(V) spectrafrom all comparable LEED spots, which illustrated in Fig-ure S1 in Supplementary Material, of 1 nm-Cu(100) andFeCo(100) matched well (R-factor = 0.19), indicating thatthe Cu layer inherits the FeCo structure.The I(V) spectra from all comparable LEED spots (Fig-ure S1) of 30 nm-Cu(100) deviated strongly from those of1 nm-Cu(100) (R-factor ≧ 0.95), highlighting a thickness-driven structural change. These results indicate that Cu ini-tially adopts a bcc structure templated by the FeCo under-layer, undergoes surface disorder at intermediate thicknesses,then transforms into a different structure at larger thicknesses.To further clarify this thickness-driven structural evolutionin the Cu(100)/FeCo(100) structure, out-of-plane XRD meas-Figure 1: (a) Schematic of sample preparation. (b–f ) LEED pat-terns of each surface at an incident electron energy of 210 eV.Regular Papere-J. Surf. Sci. Nanotechnol. 24, 21–25 (2026) | DOI: 10.1380/ejssnt.2026-004 22https://doi.org/10.1380/ejssnt.2026-004urements were performed [Figure 3(a)]. A 002bcc diffractionpeak originating from the bcc-FeCo layer was observed in allsamples. For the 30 nm-Cu film, an additional 002fcc-Cu peakappeared, confirming a transformation into the fcc phase withincreasing Cu thickness. Notably, a weak 002fcc-Cu peak wasalready present at 3 nm, earlier than the 4–5 nm reported forsputtered Cu/TiN(001) films [9], suggesting that the differ-ences in growth method and wettability play a role. Fig-ure 3(b) presents the results of in-plane XRD measurements.In-plane scans of the 30 nm-Cu sample revealed 220MgO,200bcc, and 220fcc-Cu peaks, establishing the in-plane epitax-ial relationship of MgO[110] || FeCo[100] || fcc-Cu[110]. Incontrast, the 1 nm-Cu sample exhibited only 220MgO and200bcc peaks, with no detectable fcc-related peaks.To examine the structure of the Cu layer in the intermedi-ate thickness regime where LEED spots were not observed,cross-sectional high-angle annular dark-field scanning TEM(HAADF-STEM) and energy-dispersive X-ray spectroscopy(EDS) analysis were performed on the 4 nm-Cu sample[Figure 4(a)]. Variations in contrast within the Cu layerindicated nanoscale steps or island-like structures. EDSmapping further revealed only minor interdiffusion at theCu/FeCo interface.The high-magnification image [Figure 4(b)] shows theatomic arrangements of each layer, with the electron beamincident along the bcc[1�10] direction. The arrangement ofatomic columns clearly differs between the FeCo and Culayers. The FeCo columns exhibited a rectangular periodicitycorresponding to bcc[110], whereas the Cu exhibited asquare periodicity, which corresponds to fcc[001]. The meas-ured interplanar spacings were 2.0Å for (110)bcc-FeCo, 1.3Åfor (001)bcc-FeCo, and 1.8Å for both (100)Cu and (001)Cu, asindicated in Figure 4(b). These values are consistent with thecalculated spacings from the lattice constants of bcc-FeCo(a = 2.86Å) and fcc-Cu (a = 3.61Å) [5, 7]. Together with thepresence of the 002fcc-Cu peak in the XRD profile [Fig-ure 3(a)], these findings confirm that the Cu layer is orientedas fcc-Cu[001]. It has been reported that both bcc and fccphases coexist within the Cu layer in FeCo/4 nm-Cu/FeComultilayer structures fabricated by sputtering [7]. In contrast,the Cu layer was uniformly in the fcc phase and no bcc phasewas detected in our MBE-grown 4 nm-Cu/FeCo sample. Thisdifference is likely attributable not only to the growth methodbut also to the fact that, in the previous study, both upper andlower interfaces serve as bcc templates, which influencesstructural stability.Furthermore, dislocations were observed at the Cu/FeCointerface, including regions where extra half planes of atomswere inserted as marked in Figure 4(b). However, thesedislocations were not strictly periodic, instead, irregularlydistributed segments were also identified. This indicates that,in addition to lattice mismatch, preexisting defects androughness at the FeCo surface likely contribute to theirformation.Regarding the surface, while the lattice in the Cu film wasclearly observed in the TEM image, no diffraction spots werevisible in the LEED image of the 4 nm-Cu sample [Fig-ure 1(e)]. This implies that surface flatness is not well main-Figure 2: LEED-I(V) spectra from 01 spots, averaged over the foursymmetry-equivalent spots for FeCo(100), 1 nm-Cu(100), and30 nm-Cu(100). Figure 3: XRD results. (a) Thickness dependence of out-of-planescan. Each spectrum was normalized with the intensity of 002MgOpeaks. (b) In-plane scan.Regular Papere-J. Surf. Sci. Nanotechnol. 24, 21–25 (2026) | DOI: 10.1380/ejssnt.2026-004 23https://doi.org/10.1380/ejssnt.2026-004tained at the intermediate thickness regime, as also confirmedin Figure S2 in Supplementary Material. The surface flatnessrecovered as the film thickness increases. Such a behaviorhas also been reported for sputter-deposited films [9].In contrast, for the 1 nm-Cu sample, the presence of sharp1 × 1 LEED spots [Figure 1(d)] indicates that the surface iswell ordered in the ultrathin thickness regime. Furthermore,previous studies have reported that Cu grows in a layer-by-layer on both Fe(100) and Co(100) substrates in this thick-ness range [15–17]. The surface energy of Cu is lower thanthat of either Fe or Co. This suggests that, in the initial stagesof Cu growth on FeCo(100), the Cu layer covers the entiresurface of FeCo. In addition, the low R-factor obtainedbetween the I(V) curves of 1 nm-Cu(100) and FeCo(100)indicates that, these two surfaces share the same underlyingstructure. These results confirm that the 1 nm-Cu layer isnon-amorphous and retains a bcc structure inherited fromthe FeCo substrate.In GMR devices, the magnetoresistance ratio is mainlygoverned by spin-dependent scattering at the interfaces ofnonmagnetic/ferromagnetic layers [18]. This scattering ishighly sensitive to the atomic arrangement and crystallinityof each layer, and the emergence of fcc Cu has been reportedto reduce magnetoresistance ratios [7]. Our results suggestthat MBE-grown films are well suited if the thickness of thenonmagnetic Cu layer is around 1 nm, where the bcc struc-ture is preserved.Collectively, these results provide a comprehensive pictureof the growth behavior of Cu ultrathin films on FeCo(100).At the initial stage (1 nm), the Cu layer adopts a bcc structuretemplated by the FeCo substrate, where a sharp 1 × 1 LEEDpattern confirms the formation of an ordered surface. Oncethe Cu thickness exceeds ~3 nm, the film transforms into thefcc phase, accompanied by surface roughness and interfacialdislocations at the Cu/FeCo interface, while surface flatnessrecovers at larger thicknesses (≥30 nm). The crystallographicrelationship above the intermediate regime (≥4 nm) is estab-lished as MgO[110] || FeCo[100] || fcc-Cu[110]. AlthoughMBE-grown films undergo the bcc-to-fcc transition at small-er thicknesses than their sputter-deposited counterparts, MBEuniquely provides clean and well-ordered surfaces in theultrathin regime (<1 nm), enabling advanced in-situ surface-sensitive characterization.IV. CONCLUSIONSIn this study, Cu/FeCo(100) thin films were successfullygrown by MBE, and their thickness-dependent structuralevolution was systematically investigated. Ultrathin Culayers initially exhibited a metastable bcc structure inheritedfrom the structure of the FeCo template. As the Cu thicknessincreased to several nanometers, the Cu layer transformedinto a stable fcc structure. The surface structural order wastransiently disrupted at intermediate film thicknesses. Ourfindings clarify the growth behavior of Cu/FeCo(100), anddemonstrate that MBE-grown ultrathin films, though lessstable than their sputter-deposited counterparts, uniquelyprovide atomically clean surfaces in the ultrathin regime,thereby enabling advanced in-situ surface-sensitive charac-terization.AcknowledgmentsThe authors thank Mr. Fumiya Arai for technical supports, andalso thank Dr. Shunsuke Tsuda and Dr. Fumio Komori for discus-sions. This work was particularly supported by JST CREST, Japan(Grant Numbers: JPMJCR21O1, JPMJCR21O4), JST PRESTO,Japan (Grant Numbers: JPMJPR17NB, JPMJPR20T7), JSPS/MEXT KAKENHI, Japan (Grant Numbers: 25K01628,24K01243, 24K01352, 21H01638, 21H04696), the Innovative Sci-ence and Technology Initiative for Security, ATLA, Japan (GrantNumber: JPJ004596), and Advanced Research Infrastructure forMaterials and Nanotechnology in Japan (ARIM), of MEXT (GrantNumbers: JPMXP1225NM5366, JPMXP1224NM5290), and JSTSPRING (Grant Number: JPMJSP2151). This research was fundedin part by the Austrian Science Fund (FWF) under doi:10.55776/F8100, Taming Complexity in Materials Modeling (TACO).Figure 4: Cross-sectional HAADF-STEM images of 4 nm-Cu,along the [110]bcc-FeCo direction. (a) Low-magnification image andits EDS elemental mapping. (b) High-magnification image. The ⊥-shape markers indicate the positions of dislocations.Regular Papere-J. Surf. Sci. Nanotechnol. 24, 21–25 (2026) | DOI: 10.1380/ejssnt.2026-004 24https://doi.org/10.55776/F8100https://doi.org/10.55776/F8100https://doi.org/10.1380/ejssnt.2026-004AppendixThe I(V) spectra from all comparable LEED spots and an addi-tional HAADF-STEM image are available in Supplementary Mate-rial at https://doi.org/10.1380/ejssnt.2026-004.References[1] N. Kaiser, Appl. Opt. 41, 3053 (2002).[2] H. Li, D. Tian, J. Quinn, Y. S. Li, F. Jona, and P. M. Marcus,Phys. Rev. B 43, 6342 (1991).[3] Q. Guo and G. B. Thompson, Acta Mater. 148, 63 (2018).[4] Z. Q. Wang, S. H. Lu, Y. S. Li, F. 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