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Shohei Miyagawa, [Yuta Ishii](https://orcid.org/0000-0002-8957-5833), Masato Anada, Kazuki Nagai, Miho Kitamura, Hiroshi Kumigashira, Yusuke Wakabayashi

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[Non-destructive measurement of the charge transfer across LaMnO3/SrTiO3 interfaces](https://mdr.nims.go.jp/datasets/e74d599f-17f0-489a-8bb1-af4f98319b28)

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Non-destructive Measurement of the Charge Transfer AcrossLaMnO3/SrTiO3 InterfacesShohei Miyagawa1, Yuta Ishii2, Masato Anada1,3, Kazuki Nagai1,3,Miho Kitamura4, Hiroshi Kumigashira5, and Yusuke Wakabayashi1a1 Department of Physics,Graduate School of Science,Tohoku University, Sendai 980-8578, Japan2 National Institute for Materials Science,Center for Basic Research on Materials (CBRM),1-2-1 Sengen, Tsukuba,Ibaraki 305-0003, Japan3Division of Materials Physics,Graduate School of Engineering Science,Osaka University, Toyonaka 560-8531, Japan4National Institutes for Quantum Science and Technology (QST),Sendai, 980-8579, Japan5Institute of Multidisciplinary Research for Advanced Materials,Tohoku University, Sendai 980-8577, Japan(Dated: February 26, 2025)a wakabayashi@tohoku.ac.jp1AbstractLocal polarization and charge transfer at the interface between 7-unit cell-thick LaMnO3 (LMO)ultrathin films and substrates composed of Nb-doped (0.05 wt%) and undoped SrTiO3 (STO) areexamined by resonant surface x-ray diffraction. Notably, this photon-in photon-out technique al-lows us to simultaneously examine the valence distribution and local polarization with minimalradiation damage. Consistent with previous reports, Mn2+ is observed at the interface. Further-more, the degree of charge transfer is nearly unchanged by the 0.05 wt% of Nb doping. In the middleof the LMO films, the valence of Mn is 3+. Local polarization estimated from the cation/aniondisplacements shows that the electric field points outward in the LMO films, and polarization issuppressed in the STO substrate region.I. INTRODUCTIONSpontaneous charge transfer across the interfaces of metal oxides often plays a decisive rolein their functionality[1–6]. Thus, the valence distribution around the interface has previouslybeen investigated by applying cross-sectional transmission electron microscopy (TEM) incombination with electron energy-loss spectroscopy (EELS)[2, 7]. However, sample damagecaused by sample preparation or electron beam irradiation may cause some uncertainty inthe TEM-EELS measurements. For example, Mn2+ found at the interface between LaMnO3(LMO) and LaNiO3 (LNO) was attributed to sample damage in ref.2, whereas the Mn2+found by x-ray absorption spectroscopy (XAS) at the interface between LMO and SrTiO3(STO)[8] or at the surface[9] was regarded as intrinsic. Photon-in photon-out methods,such as XAS, are less invasive than those using electron beams, making x-ray spectroscopicprobes more suitable for observing valence states. Nevertheless, it remains difficult to achieveatomic spatial resolution for XAS. The interfacial effects on the valence distribution can belocalized to the interface in a few angstroms[7], and therefore, a less invasive probe that alsohas an Å-scale spatial resolution is desired for examining the valence condition.Notably, surface x-ray diffraction provides detailed information on the surface/interfacestructures[10–12], and its spatial resolution allows us to observe local electric polarization[13,14]. When we consider the absorption edge, this technique reveals not only the elementalarrangement[15, 16] but also the valence of ions[17–19]. By leveraging this technique, weexamined the self-doping effect proposed at LMO/STO interfaces.2The charge, spin, and orbital (or lattice) degrees of freedom are intertwined in perovskiteMn oxides[20]. Furthermore, the magnetic properties of thick LMO/STO films have beenexamined as a function of the c-lattice spacing[8, 21–24]. Based on these studies, LMO/STOsamples with a c-lattice spacing shorter than 3.94 Å (= cFM) exhibit ferromagnetic insulatorbehavior, whereas those with spacing longer than 3.97 Å exhibit bulk-like antiferromagneticinsulator behavior, and the lattice spacing depends on the annealing conditions. In ultrathinfilms, LMO/STO displays thickness-dependent magnetism[4], which may be determined bythe electron transfer from the surface to the interface[4, 25–28]. It was also suggested thatlattice strain plays a significant role in charge transfer at the interface[27]. Therefore, atomicresolution of the valence state is required to examine charge transfer phenomena at oxideinterfaces.In this study, we experimentally observed the valence arrangement across the LMO/STOinterface, as well as the local polarization, ion concentration, and fluctuation of atomic posi-tions. We examined two types of samples, namely ultrathin LMO films grown on STO andNb-doped STO substrates. The samples were evaluated by employing the crystal truncationrod (CTR) scattering technique, which is a form of surface x-ray diffraction, utilizing theanomalous dispersion effect around the Mn K-absorption edge. We found that the struc-tures of the two samples are nearly identical, and Mn2+, which is not usually found in bulkMn oxides, was found only at the interfaces.II. EXPERIMENT AND ANALYSIS METHODSeven-unit cell-thick epitaxial LMO films were fabricated on atomically flat TiO2-terminated STO and 0.05 wt% Nb:STO (001) substrates by pulsed laser deposition (PLD),denoted as (LMO)7/STO and (LMO)7/Nb:STO, respectively. First, the substrates wereannealed at 1050◦C under an oxygen pressure of 1×10−7 Torr to obtain atomically flat andchemically clean surfaces. During LMO deposition, the substrate was kept at 600◦C, andthe oxygen pressure was maintained at 1×10−3 Torr. A Nd-doped yttrium-aluminum-garnetlaser in the frequency-triplet mode (λ = 355 nm) was used for ablation. The repetition rateand laser power were set to 1 Hz and 60-75 mJ, respectively. The thickness of the LMOlayers was precisely controlled during growth by monitoring the intensity oscillation of thespecular spot obtained under reflection high-energy electron diffraction. The LMO films3were subsequently annealed at 400◦C for 45 min under an atmospheric pressure of oxygento fill residual oxygen vacancies.CTR scattering measurements were performed using the BL-3A beamline at the PhotonFactory in Japan. The incident beam was linearly polarized perpendicular to the scatteringplane. The scattered x-ray intensity was measured using a two-dimensional pixel arraydetector (XPAD-S70, imXpad, France). Samples were placed in a 10−5 Torr vacuum chamberto protect them from potential radiation damage, and all the measurements were performedat room temperature. Using 12 keV x-rays, a non-resonant condition, several hkζ rods weremeasured up to ζ = 4.1. Measurements along the 00ζ rod were also performed using the MnK-absorption edge to discriminate the Mn valences. For measurements close to Mn K-edge,the polarization vector was parallel to [100] of the substrate.After the measurements, illumination area correction and Lorentz factor correction weremade by following the procedure described in ref. 29. The hkζ intensity profiles along therods were measured under both non-resonant and resonant conditions and simultaneouslyanalyzed using exchange Monte-Carlo (MC) sampling software based on the Bayesian in-ference, CTR-structure code[30, 31]. In our analysis, the in-plane atomic positions werefixed to the simple cubic STO structure. Other parameters (i.e., atomic displacement in thesurface normal direction (dz) with respect to the substrate lattice, atomic occupancy (occ),and isotropic atomic displacement parameter (B)) were refined. The total atomic occupancyfor each site was set to unity, except for the surface region. The parameter uncertainty wasestimated by MC sampling, accounting for the effective number of observations[31].The spatial distribution of the Mn valence was derived from the (00ζ) CTR profilesmeasured at various x-ray energies. The energy spectrum obtained at a certain scatteringvector depends on the non-resonant scattering amplitude, as well as the spatial arrangementof the Mn valence. Although the non-resonant amplitude was calculated based on the resultof the interfacial structure analysis described above, it inevitably involves some error. Theerror was corrected by adding a small complex value as a function of the scattering vector,C(Q⃗), to the non-resonant scattering amplitude; this correction was also used in ref. 32. Theanomalous scattering factors f ′ and f ′′ were calculated as functions of E for Mn3+ usingFDMNES code[33] and the bulk LaMnO3 structure[34]. The anomalous scattering factorsfor Mn2+ and Mn4+ were obtained by applying chemical shifts of −3 eV and 4 eV to thoseobtained for Mn3+[35, 36]. The anomalous scattering factors are also sensitive to Jahn-40 1 2 3 4 510010310610910121015101810211024102710300 1 210110410710101013Intensity (arb. units)z (r.l.u.)12 keV00ζ10ζ20ζ30ζ11ζ21ζ31ζ22ζ32ζ(LMO)7/STOz (r.l.u.)6.530keV6.540keV6.545keV0 1 2 3 41001031061091012101510180 1 210110410710101013Intensity (arb. units)z (r.l.u.)12 keV00ζ10ζ20ζ11ζ21ζ(LMO)7/Nb:STOz (r.l.u.)6.300keV6.500keV6.548keV(a) (b)FIG. 1. CTR scattering intensity profiles for (a) (LMO)7/STO and (b) (LMO)7/Nb:STO measuredat (left) 12 keV and the (right) Mn K-edge. Red curves show the fitting results.Teller mode distortion[32] or shift of the Mn ion from the center of oxygen octahedron; wedid not take such effects into account as the anisotropic effect is in-plane averaged when wethink about 00ζ-rod. The analysis was also performed considering the Bayesian inference,assuming that the magnitude of the noise was 10 % of the intensity. The error bars ofthe Mn4+ concentration were estimated by MC sampling, whereas the effective number ofobservations for the energy spectra was assumed to be the same as the number of datapoints.III. RESULTS AND DISCUSSIONThe non-resonant and resonant CTR profiles for (LMO)7/STO and (LMO)7/Nb:STOfilms are presented in Fig. 1. Here, R (≡∑ζ,E[|(|Fexpt|− |Fcalc|)|]/[∑ζ,E |Fexpt|], where Fexptand Fcalc denote the square root of the measured and calculated intensity) was 0.11 and 0.13for (LMO)7/STO and (LMO)7/Nb:STO, respectively, which are ordinary values for CTRanalysis[19, 37]. The resulting structural parameters for each site are shown in Fig. 2 asfunctions of the depth, z. The position of z = 0 corresponds to the interface, and the surfaceis located at z ≃ 7 for the two samples.Figure 2 shows that there is little difference in structure between the (LMO)7/STO and(LMO)7/Nb:STO samples, especially in the depth dependence of the elemental occupancy5-0.2-0.10.00.10.2 Asite Bsite O1site O2sitedz (Å)(a) (LMO)7/STO-0.30.00.20.40.60.81.0-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9012345(f)(g)3.903.954.00z (STO u.c.)(h)-0.30.00.20.40.60.81.0-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9012345(b)3.903.954.00c-spacing (Å) Sr La Ti Mnocc A,O1 B,O2B (Å2 )z (STO u.c.)(c)(d)-0.2-0.10.00.10.2 (e) (LMO)7/Nb:STOFIG. 2. Depth dependence of the obtained structure parameters for (a-d) (LMO)7/STO and (e-h)(LMO)7/Nb:STO. The origin of the horizontal axis is set as the interface. (a,e) Atomic displacementwith respect to the substrate lattice, (b, f) c-lattice spacing defined by the inter-A-site distance,(c,g) occupancy, and (d,h) isotropic atomic displacement parameter B. Atomic occupancies andthe B parameters of the oxygen atoms are assumed to be the same as those of the cation in thesame z layer. The shaded areas indicate the regions with small occupancy parameters, where thereliability of all other parameters is poor. The horizontal dashed lines in panels (b) and (f) showthe cFM.presented in panels (c) and (g). This demonstrates the reproducibility of the film growth.The transient regions of the occupancy at the interfaces are thicker than those at the surfaces,meaning that the LMO/STO interface contains a significant amount of atomic interdiffu-sion. Conversely, the transient regions at the LNO/LaAlO3 (LAO) interface have similarthicknesses to those at their surfaces [37]. This discrepancy may be attributed to the largediffusion constant of Sr[38] or the polar discontinuity. Another possible explanation for thethicker transient region in the present samples is that the PLD process produces a flattersurface than the substrate. The atomic displacement parameters B are ∼0.5 Å2 in the sub-strate lattice, ∼ 1.5 Å2 at the interface, and less than 2 Å2 in most of the film, as shownin panels (d) and (h). These values are similar to LNO/LAO[37] and LNO/LMO bilayerson STO substrates[19]. Slightly enhanced B values in the film region appear to be common6characteristics in epitaxial oxide films.The local polarization can be examined based on the atomic displacements presented inFig. 2(a) and (e). In this study, the atomic displacement along the c direction is examined,thus the c component of the polarization can be discussed. The cations are always locatedabove oxygen atoms, meaning that the electric field in this material points outwards, whichis consistent with the theoretical expectation for lattices containing few vacancies[8, 9].The polarization distribution in LMO/STO suggests moderate charge distribution in LMOregion. Charges are distributed mainly in STO side and on the surface. The polarizationdirection does not change at the interface, which exhibits a contrast with the polarizationdirection change at LNO/LMO interface[19]. Such a local polarization suggests that electricflux diverges from the space charge layer in the STO, and converges into the negativelycharged contamination outside of the film. The polarization distribution is also differentfrom 2D-electron gas system LAO/STO[14], in which the electric flux diverges from the spacecharge layer in the STO and converges to the interface. Since LAO/STO and LMO/STOare both polar/nonpolar interfaces, the difference in the polarization distribution is causedby the different band gap and band alignment between the two interfaces.The c-lattice spacing is defined by the distance between the A-site ions, and thus thevalue represents the available space for the central Mn ion. The depth profiles are presentedin Fig. 2(b) and (f). Deep inside the substrate, the value is the same as that of the bulk STOlattice parameter, 3.905 Å. The c-lattice spacings of the films were 3.91±0.01Å in the z < 5region, which are shorter than cFM. The spacing is increased at the surface; surface expansionis also observed in La1−xSrxMnO3 films on STO[38, 39], whereas no such characteristic wasreported for LNO/LAO[37, 40], LNO/STO[41], or LAO/STO[13, 14, 42, 43]. Herein, thec-lattice spacings expanded to 3.95±0.01Å near the surface, in the range of 5 ≤ z < 7.Therefore, most of the LMO film is expected to be ferromagnetic, and only the surfaceregion can be antiferromagnetic. The c-lattice spacing can also be used as a measure of theMn valence. The c-lattice spacing and the hole concentration (x) dependence of the c-latticespacing for LaMn3+O3 grown on STO were estimated as c = 3.973 Å and dc/dx=−0.34 Å[19],respectively. Using these values, the valences of Mn ions at c = 3.90 Å and 3.94 Å (= cFM)are estimated to be 3.21+ and 3.10+. This estimation can be used only in the middle of theLMO layer, where the occupancy of the La, Mn, and O are supposed to be 1.To examine the valence arrangement at the interface, many energy spectra were measured70500100015000500100015002000050001000015000050010006.53 6.54 6.55 6.56 6.57 6.58050010006.53 6.54 6.55 6.56 6.57 6.580500010000z=0.7 z=0.8Intensity (arb. units)z=0.9 z=1.2Energy (keV)z=1.6 (LMO)7/STOEnergy (keV)z=1.8 (LMO)7/STO0200400050010000500010000050010006.53 6.54 6.55 6.56 6.57 6.58050010006.53 6.54 6.55 6.56 6.57 6.5801000200030004000ζ=0.7 ζ=0.8ζ=0.9 ζ=1.2ζ=1.6 ζ=1.8Intensity (arb. units.)Energy (keV)(LMO)7/Nb:STOEnergy (keV)(LMO)7/Nb:STO(a) (b)(LMO)7/Nb:STO0.5 1.0 1.5 2.0z (r.l.u.)10-11011031050.5 1.0 1.5 2.0z (r.l.u.)|C(Q)|2, I calc(Q) (arb. units)6.52 6.54 6.56 6.58-10-505Scattering factor  f ', f '' (2+)  f ', f '' (3+)  f ', f '' (4+)Energy (keV)f 'f ''(c) (d)(LMO)7/STOIcalc|C(Q)|2FIG. 3. Energy spectra at several (00ζ) positions around the Mn K-edge for (a) (LMO)7/STOand (b) (LMO)7/Nb:STO. Red curves show the fitting results. (c) Anomalous scattering factorsfor Mn2+, Mn3+, and Mn4+. (d) Calculated intensity and modulus square of the correction termC(ζ).over the Mn K-edge on the 00ζ-rod, as shown in Fig. 3. The energy spectra measurementsslightly differed for the two samples. For (LMO)7/STO, we measured numerous (00ζ) profilesat various x-ray energies, and for (LMO)7/Nb:STO, we measured energy spectra at several(00ζ) positions, resulting in a different number of data points for the ζ profiles (denser in(LMO)7/STO) and energy spectra (denser in (LMO)7/Nb:STO). The red plots in Fig. 3(d)show the calculated intensity Icalc(Q⃗), and the black plots show the modulus square of thecorrection term C(Q⃗). The analysis shows that Icalc is always greater than |C(Q⃗)|2, and inmost cases, the correction term is found to be minor, showing that the error in our structuremodel is minimal.The derived valence distributions of Mn ions for the two samples are shown in Fig. 4. Mostof the Mn ions are close to Mn3+, as expected. Panels (a) and (c) show the occupancy ofdivalent, trivalent, and tetravalent Mn derived from our MC analysis. The only information80.00.51.02.53.03.5-5 0 5 100.00.51.0-5 0 5 102.53.03.5occ(LMO)7/STOAverage Mn valence(LMO)7/STO Mn Mn²⁺ Mn³⁺ Mn⁴⁺z (STO u.c.)(LMO)7/Nb:STOz (STO u.c.)(a) (b)(d)(c) (LMO)7/Nb:STOFIG. 4. Depth profiles of (a) the occupancy parameters for Mn, Mn2+, Mn3+ and Mn4+ and (b)the average Mn valence for (LMO)7/STO. (c), (d) show those for (LMO)7/Nb:STO.that affects the energy spectra of the intensity is the averaged anomalous scattering factorf =∑n[f′(n+) + if ′′(n+)]occ(n+) for each Mn site, and f is only sensitive to the averageMn valence. The average valences of Mn at z = 4.5, where the occupancy of Mn is unity,for the two samples are 3.04+ and 3.12+, as shown in panels (b) and (d). These values aresimilar to those estimated from the c-lattice spacing. At the surface, the average valenceof Mn was 3.04(3)+ for (LMO)7/STO and 3.68(13)+ for(LMO)7/Nb:STO, but it is unclearwhether the surface valence has been affected by contamination, given that the samples werehandled in air. At the interface, both samples contain Mn2+. Note that Mn2+ is found wherethe occupancy of Mn is significantly reduced from 1. Although Sr diffusion should causeMn4+ accumulation at the interface, Mn2+ is found, which is consistent with the electronicpotentials calculated for the ideal interfacial structure[8, 9].IV. CONCLUSIONWe performed surface/interface structure analyses based on surface x-ray diffraction andobtained energy spectra of the surface diffraction intensity for LMO/STO and LMO/Nb:STOinterfaces. The structural information was derived from the non-resonant scattering inten-9sity, and the electric field in the LMO region was oriented outward, consistent with a previousreport[8]. Using the structural information and the energy spectra, we derived the Mn va-lence distribution. Mn2+ ions were only found at the interface, and the spatial distributionof Mn2+ was insensitive to the Nb doping of the STO substrate. Overall, the resonant CTRmethod is useful for simultaneously deriving the valence information and local polarizationon the same sample with minimal sample damage.ACKNOWLEDGMENTSThis work was supported by a Grant-in-Aid for Scientific Research [Japan Society forthe Promotion of Science (JSPS) KAKENHI, Grant Nos. JP22H02024 and JP23K23292].The synchrotron radiation experiments at the Photon Factory were performed with theapproval of the Photon Factory Program Advisory Committee (Proposal No. 2018G533 andNo. 2020G526). 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