15214095, 2022, 36, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202203071 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article www.advmat.de Top-Layer Engineering Reshapes Charge Transfer at Polar Oxide Interfaces Gabriele De Luca,* Jonathan Spring, Moloud Kaviani, Simon Jöhr, Marco Campanini, Anna Zakharova, Charles Guillemard, Javier Herrero-Martin, Rolf Erni, Cinthia Piamonteze, Marta D. Rossell, Ulrich Aschauer, and Marta Gibert* the emergence of a 2D electron liquid at Charge-transfer phenomena at heterointerfaces are a promising pathway the interface between LaAlO3 (LAO) and to engineer functionalities absent in bulk materials but can also lead to SrTiO3 (STO),[1] but other investigations degraded properties in ultrathin films. Mitigating such undesired effects with involving transition metal cations with an interlayer reshapes the interface architecture, restricting its operability. partially filled d-bands revealed that inter- facial ferromagnetism,[2] exchange bias,[3] Therefore, developing less-invasive methods to control charge transfer will and orbital reconstructions[4,5] can be tar- be beneficial. Here, an appropriate top-interface design allows for remote geted with proper material design.[6] manipulation of the charge configuration of the buried interface and con- Interfacial charge-transfer phenomena, current restoration of the ferromagnetic trait of the whole film. Double- however, are also associated with detri- perovskite insulating ferromagnetic La2NiMnO6 (LNMO) thin films grown on mental effects on the desired function- perovskite oxide substrates are investigated as a model system. An oxygen- alities, especially in the ultrathin limit.[7,8] A favorable strategy to control the extent vacancy-assisted electronic reconstruction takes place initially at the LNMO of charge transfer consists in inserting polar interfaces. As a result, the magnetic properties of 2–5 unit cell LNMO a specific interlayer that modulates the films are affected beyond dimensionality effects. The introduction of a top electrostatic screening at the interface.[9] electron-acceptor layer redistributes the electron excess and restores the Unfortunately, this approach cannot be ferromagnetic properties of the ultrathin LNMO films. Such a strategy can be used if one would like to retain certain properties of the original interface (for extended to other interfaces and provides an advanced approach to fine-tune instance, octahedral connectivity,[10,11] or the electronic features of complex multilayered heterostructures. substrate-driven field effect[12,13]). There- fore, alternative methods to influence the electronic configuration of a buried oxide 1. Introduction interface are very intriguing.[14,15] In this work, we demonstrate that the heterointerface In the last two decades, a growing interest in charge-transfer between a polar ferromagnetic insulating double perovskite phenomena in oxide heterostructures resulted in the dis- thin film (La2NiMnO6, LNMO) and a non-polar single perov- covery of many interfacial properties that are not exhibited by skite substrate (STO) is a prototypical example of an interfa- the bulk counterparts. The most famous example concerns cial charge transfer triggered by a polarity mismatch.[16] The G. De Luca,[+] J. Spring, S. Jöhr, M. Gibert[++] M. Kaviani, U. Aschauer Department of Physics Department of Chemistry University of Zurich Biochemistry and Pharmaceutical Sciences Winterthurerstrasse 190, Zurich 8057, Switzerland University of Bern E-mail: gabriele.deluca@icn2.cat; marta.gibert@ifp.tuwien.ac.at Freiestrasse 3, Bern 3012, Switzerland M. Campanini, R. Erni, M. D. Rossell The ORCID identification number(s) for the author(s) of this article Electron Microscopy Center can be found under https://doi.org/10.1002/adma.202203071. Empa © 2022 The Authors. Advanced Materials published by Wiley-VCH Swiss Federal Laboratories for Materials Science and Technology GmbH. This is an open access article under the terms of the Creative Überlandstrasse 129, Dübendorf 8600, Switzerland Commons Attribution-NonCommercial License, which permits use, dis- A. Zakharova, C. Piamonteze tribution and reproduction in any medium, provided the original work is Swiss Light Source properly cited and is not used for commercial purposes. Paul Scherrer Institut Villigen 5232, Switzerland [+]Presentaddress: Catalan Institute of Nanoscience and Nanotechnology C. Guillemard, J. Herrero-Martin (ICN2), Campus UAB, 08193, Bellaterra, Barcelona, Spain ALBA Synchrotron Light Source [++]Present address: Institute of Solid State Physics, TU Wien, Wiedner Carrer de la Llum 2-26, Cerdanyola del Vallès 08290, Spain Hauptstrasse 8-10, 1040, Vienna, Austria DOI: 10.1002/adma.202203071 Adv. Mater. 2022, 34, 2203071 2203071 (1 of 9) © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2022, 36, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202203071 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de resulting electronic reconstructions strongly affect the mag- superexchange between the long-range-ordered Ni2+ (3d8) and netic properties of the heterostructure, adding to dimension- Mn4+ (3d3) cations in a rock-salt-type structure.[17–19] To eval- ality effects. To restore the optimal electronic configuration uate dimensionality and interface effects on LNMO magnetic of the buried interface, we design a top-interface engineering and electronic properties, we grew a set of films of different approach through a LaNiO3 (LNO) overlayer. The presence of thicknesses, ranging from 90 to 2 pseudocubic unit cell (pc LNO is fundamental in displacing the interfacial charge and uc), on prototypical STO substrates (tensile strain = +0.7%). allows reestablishing the ferromagnetism in LNMO. This Structural characterization indicates epitaxial growth and high method can be extended to other systems and certifies that crystalline and surface quality in all the films (Figure S1, Sup- top-layer engineering can be successfully exploited to remotely porting Information).[19] As shown in Figure 1a, films thicker control the electronic configuration of buried complex oxide than 30 uc (≈12 nm) exhibit bulk-like ferromagnetic properties, interfaces. that is, TC ≈ 270 K and saturation magnetization larger than 4 µB/formula units.[19] However, the Curie temperature and the magnetization decrease as films are grown thinner. Still, 15 and 2. Results and Discussion 5 uc-thick films display TC ≈ 250 K and TC ≈ 165 K, respectively. This magnetic trend is further confirmed by X-ray mag- 2.1. Magnetic Properties of La2NiMnO6//SrTiO3 (LNMO//STO) netic circular dichroism (XMCD) measurements. The reduc- Heterostructures tion of both Mn and Ni L3,2 integrals in normalized XMCD spectra is indicative of a decrease in the magnetic moment of LNMO is a ferromagnetic insulator with a bulk Curie tem- both the Mn and Ni sublattices in the ultrathin limit (L3-edges perature (TC) around 270 K, resulting from strong positive shown in Figure 1b-c, the full L3,2-edges displayed in Figure S2, Figure 1. a) Magnetization versus temperature measured in a magnetic field of 0.5 T for various LNMO film thicknesses. b) Mn L3-edges XMCD spectra measured at 20 K and 5 T. c) Same as (b) for Ni. d) Integral of the Ni XMCD L3-edge as a function of the sample temperature for selected film thicknesses. Scaled data from a SQUID is shown for comparison (dashed and dotted lines). In the right panel, the behavior of the ultrathin 2 uc film is highlighted. Here, the line is a guide to the eye. e) XMCD asymmetry as a function of the magnetic field calculated at both Mn and Ni L3-edges for the 90 uc (top, 20 K), 3 uc (middle, 20 K), and 2 uc (bottom, 5 K) LNMO films. The lines are guides to the eye. Adv. Mater. 2022, 34, 2203071 2203071 (2 of 9) © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2022, 36, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202203071 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Supporting Information). In Figure 1d, we plot the evolution of hysteresis whereas the Mn sublattice is severely affected by the the Ni XMCD L3-integral as a function of the temperature as thickness reduction, nearly losing its ferromagnetic character. an alternative way to determine the Curie temperature. The ele- mental sensitivity of XMCD compared to SQUID magnetom- etry allows films as thin as 2 uc (≈0.8 nm) to be characterized. 2.2. Charge Transfer Induced by Interfacial Polarity Mismatch We find that 2 uc films have TC ≈ 10 K. To gain insight into the origin of this thickness-dependent Since the electronic configuration of Ni and Mn is key to the degradation of the magnetic properties, it is instructive to ferromagnetic behavior of the LNMO system,[17] we investigated observe the difference between the Mn and Ni XMCD asym- the valence state of the cations using X-ray absorption spectro­ metry measured as a function of the external magnetic field for scopy (XAS). In Figure 2a, we show the L3,2-edges of Mn as a different film thicknesses, as displayed in Figure 1e. For a bulk- function of the LNMO thickness. The XAS data of the 90 uc like 90-uc-thick film, both the Mn and Ni asymmetry saturate sample is consistent with previous observations on bulk spec- at similar values of the magnetic field. For a 3-uc-thick film, imens and certifies the valence state of Mn to be prevalently we still observe a similar behavior between Mn and Ni despite 4+.[20] As the film thickness is reduced, a pronounced shoulder asymmetries saturating at larger fields. For the 2-uc-thick film, in the low-energy range and an increased spectral weight the Ni sublattice behavior is still reminiscent of a ferromagnetic around 641 eV emerge. These features can be associated with Figure 2. a) XAS of Mn L3,2-edges for different LNMO thicknesses (top). Data is collected at 20 K. Reference XAS spectra for various electronic configu- rations of Mn[22] (bottom). b) Same as (a) for Ni L2-edge.[23] The Ni L3-edge overlaps with the La M4-edge and it is not shown. In the inset, a zoom on the Ni L2-edge maxima highlights the progressive shift to lower energies for decreasing LNMO thickness. c) Sketch of interfacial stacking sequence of LNMO//STO. The nominal ionic charge of each layer is indicated on the left. d) pDOS for the Ni0.5Mn0.5O2 layer adjacent to the LNMO//STO interface. Mn states at the Fermi level indicate the presence of charge transfer. e) pDOS for the interfacial Ni0.5Mn0.5O2 layer in presence of oxygen vacancies. In this case, both majority spin Mn and minority spin Ni states are populated by the excess electrons. The overlap between Mn and Ni states is due to orbital hybridization. Adv. Mater. 2022, 34, 2203071 2203071 (3 of 9) © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2022, 36, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202203071 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de an increase of Mn3+ content located at the interface between interfacial oxygen vacancies is not epitaxial strain[21] but rather LNMO and STO. However, we cannot exclude the presence of the polar discontinuity itself. additional Mn3+ states present also in the top surface layer, as To contemplate this possibility, we performed additional DFT we previously detected in 30-uc-thick LNMO//STO heterostruc- calculations introducing oxygen vacancies in our heterostruc- tures.[19] We also observe a small alteration of the Ni valence ture (Figure S5, Supporting Information). We find it is most state (Figure 2b) to which we will come back. favorable for the system to form oxygen vacancies when these From previous structural characterizations, we know that are located at the film surface, followed by positions in the our relatively thick LNMO films grow with a prevalent rock- Ni0.5Mn0.5O2 interfacial slab. Breaking substrate TiO bonds, salt ordering of Ni2+ and Mn4+ cations and display bulk-like instead, leads to higher formation energies. In this scenario, TC and saturation magnetization.[19] Therefore, we can exclude due to extra electron doping from the oxygen vacancies and a sub-optimal growth conditions. It is known that in perovskite defect-state-induced band gap reduction, an LNMO thickness oxide films the formation energy of bulk oxygen vacancies is smaller than 3.65 uc is enough to trigger the electronic recon- reduced with increasing tensile strain.[21] A systematic XAS struction, in line with our XAS observations. These calcula- study on 5 uc-thick LNMO films grown on a variety of sub- tions further reveal that a few of the oxygen-vacancies-induced strates (imposing epitaxial strains ranging from −2.1% to doping electrons are now also populating the minority spin Ni +1.8%[19]) reveals that the LNMO film with the largest Mn3+ states in the Ni0.5Mn0.5O2 layer adjacent to the substrate, besides content is the one grown on STO, followed by the one grown Mn (Figure 2e and Figure S5b, Supporting Information). This on LSAT (Figure S3, Supporting Information). As none of these indicates the appearance of Ni(2−x)+ states at the LNMO//STO two substrates exerts the largest tensile strain on the films, the interface modeled with oxygen vacancies, in contrast with the strain-triggered oxygen-vacancy-formation scenario cannot be simple ionic picture calculations previously discussed. the dominant mechanism involved. Experimentally, when decreasing the LNMO thickness on Instead, we should recall that the ABO3 perovskite crystal STO substrates, we observe a shift to lower energies of the Ni structure can be thought of as a sequence of AO and BO2 L2-edge spectral weight (Figure 2b) suggesting a small reduc- planes that carry a nominal ionic charge depending on the spe- tion of the Ni valence state at the LNMO interfacial layers. cific A and B cations. This applies to the LNMO//STO interface Thus, both the calculations and the experimental observations which can be pictured as a sequence of uncharged (SrO)0 and indicate that a complex polarity-induced charge transfer medi- (TiO2)0 layers followed by charged (LaO)+ and (Ni0.5Mn0.5O2)− ated by oxygen vacancies is taking place within the LNMO films stacks along the [001]pc orientation (Figure 2c). Hence, the polar to compensate their polar discontinuity with the substrate. This discontinuity at the interface builds up a diverging potential, scenario intuitively explains the presence of Mn3+ also observed generally referred to as “polar catastrophe” that can be relieved at the (less) polar LNMO//LSAT interface (Figure S3, Sup- by an electronic reconstruction.[16] porting Information).[19,28] Notice that all other investigated sub- To investigate the implications of this, we performed density strates constitute nonpolar interfaces with LNMO and therefore functional theory (DFT) calculations. Layer-resolved projected in these heterostructures we always observe a reduced amount density of states (pDOS) for the stoichiometric LNMO//STO of Mn3+ compared to the STO and LSAT ones. heterostructure without oxygen vacancies reveals the appear- ance of Mn electronic states located at the Fermi level in the Ni0.5Mn0.5O2 layer adjacent to the TiO2-interface layer as a 2.3. Atomically Resolved Cation and Electron Distribution mechanism to counteract the polar discontinuity (Figure 2d). Interestingly, this electron redistribution is not accompanied To further verify our observations, we performed energy-disper- by a concomitant change in the Ni electronic states. Equivalent sive X-ray (EDX) and electron energy-loss spectroscopy (EELS) calculations for an LNMO//LaGaO3 (LGO) heterostructure, analyses across the LNMO//STO heterostructures in a scan- which is characterized by a similar tensile strain (= +0.4%) ning transmission electron microscope (STEM). By imaging but without polar discontinuity, show no trace of extra states the films along the [110]LNMO pc zone axis we get insight into the (Figure S4, Supporting Information). These results support NiMn cationic order.[19,29] The high-angle annular dark-field the emergence of an electronic reconstruction at the LNMO// (HAADF) STEM image of a representative 13 uc LNMO//STO STO interface with the interfacial electrons preferentially local- film is presented in Figure 3a together with the combined ized at the Mn sites, similar to what was already observed in EDX elemental maps obtained for Mn, Ni, and Ti (Figure 3b). LaMnO3//STO.[24,25] The individual elemental maps are shown in Figure S6, Sup- From the DFT calculations, the built-in electric field within porting Information. We observe epitaxial coherent growth with the LNMO film is estimated to be about 0.118 V Å−1. Using the absence of defects such as dislocations or precipitates through computed LNMO bandgap (≈1.66 eV) to estimate the critical the entire surveyed area. From the EDX map, we deduce that LNMO thickness for the polar-discontinuity-induced electronic the first layer is partially intermixed with Ti, as often realized reconstruction, we obtain ≈3.65 uc (≈14 Å). Experimentally, at the interface between a polar transition metal oxide and however, we observe an electron redistribution in films as thin STO.[30,31] Note that this tendency of the transition metal cation as 2 uc (Figure 2a,b), suggesting that the ionic picture alone and Ti to swap is generally not enough to screen the interfacial is not sufficient to explain the electronic configuration of our polarity.[32] Away from the interface, we see that the film grows ultrathin LNMO films. It has been argued that a discontinuity in a perfectly ordered manner,[19] as evidenced by the checker- in the interface polarity can also be a driving force for the for- board patterns observed in the EDX maps. Closer to the inter- mation of oxygen vacancies.[26,27] Note that the origin of these face, we can appreciate the existence of a transition region of Adv. Mater. 2022, 34, 2203071 2203071 (4 of 9) © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2022, 36, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202203071 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 3. a) HAADF image of a 13 uc LNMO//STO film. b) STEM-EDX map showing the distribution of Mn (green), Ni (red), and Ti (blue) in the LNMO//STO heterostructure, and schematic of the cation distribution in the octahedral sites. c) EELS of the O K and Mn L3,2 edges across the LNMO// STO heterostructure obtained at the positions indicated by the colored dots in (a). The intensity is normalized to the O Kb main peak and the Mn L3 line. Scale bars: 1 nm. around 5–6 uc with only partial rock-salt ordering of the LNMO Experimental Section, we obtain an average valence state of the film. DFT calculations considering various NiO6 and MnO6 interfacial manganese atoms of +3.4, while a value of +3.1 is octahedra double-perovskite configurations, namely rock salt, retrieved for the surface manganese atoms. At the interface, the layered and columnar, show that rock-salt ordering is the most fractional contributions of Mn3+ and Mn4+ are determined as stable configuration for LNMO. However, the presence of an 0.6 and 0.4, respectively. additional electron, such as the one transferred to the interface due to the polar discontinuity, turns the columnar ordering into the most favorable structure (Figure S7, Supporting Infor- 2.4. Charge-Transfer Engineering via Top-Interface Design mation). Hence, in the proximity of the Mn3+ ions located at the LNMO//STO heterointerface an increased tendency to The electronic configuration obtained from both experimental B/B′-site disorder is expected, in agreement with the EDX ele- and theoretical investigations of the LNMO//STO heterostruc- mental maps shown in Figure 3b and Figure S6, Supporting ture is depicted in Figure 4a. Electron-doped states triggered Information. by the polar-discontinuity-induced oxygen vacancies are pre- Further, our STEM-EELS data confirm the formation of sent at both interfaces. As these states are detrimental to the oxygen vacancies both in the Ni0.5Mn0.5O2 interfacial layers and Mn4+–Ni2+-related LNMO ferromagnetism,[17,35] we want to at the film surface. In Figure 3c, we show the O K and Mn L3,2 control their extent. To this aim, we capped the LNMO layers edges acquired across the LNMO film thickness at the layer with LNO. This compound is characterized by bulk paramag- positions indicated with the colored dots in Figure 3a. The EEL netic metallic behavior over the whole temperature range[36] spectra acquired at the middle Ni0.5Mn0.5O2 layers (layers 3 and its Ni3+ cation is a well-known electron acceptor.[2,3,5,37,38] to 12 from the interface) certify the valence state of Mn to be Due to the strong electronegativity difference between Mn3+ prevalently 4+. However, at the Ni0.5Mn0.5O2 interfacial layers, and Ni3+,[17,38] in the LNO/LNMO//STO heterostructure, the the Mn L3 edge shifts to lower energies (from ≈642 to ≈641 eV), effects of the polar discontinuity present at the LNMO bottom and the O Ka pre-edge peak shifts to higher energies (from ≈528 interface can potentially be relieved by an electronic recon- to ≈529 eV). An even larger shift of the Mn L3 edge of ≈2 eV struction taking place at the LNMO top interface, toward the is observed at the film surface, while the O Ka pre-edge peak LNO side (Figure 4b). We verified the feasibility of this sce- almost disappears. These observations are associated with an nario by aligning the DFT density of states of LNMO and LNO increase of Mn3+ content and oxygen vacancies located at the via their work functions, as detailed in Figure S8, Supporting LNMO//STO interface and at the LNMO film surface,[33,34] Information. which is in excellent agreement with the DFT calculations. We The striking effect of LNO on the Mn/Ni electronic configu- can also use the EELS data to quantify the local Mn valence ration of our LNMO//STO heterointerfaces is observed in the state of each Ni0.5Mn0.5O2 layer. The obtained results are shown Mn and Ni L3,2-edge XAS spectra (Figure 4c,d). The Mn3+ con- as vertical dashed lines in the right panel of Figure S6b, Sup- tent present in a bare electron-doped 2 uc-thick LNMO is mark- porting Information. Using the procedure described in the edly reduced upon capping it with a 2 uc-thick LNO overlayer. Adv. Mater. 2022, 34, 2203071 2203071 (5 of 9) © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2022, 36, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202203071 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 4. a) Schematic of the charge configuration of the LNMO//STO heterostructure in presence of oxygen vacancies. Electrons transferred to the interfaces are depicted in yellow. b) New charge configuration after capping the LNMO//STO heterostructure with LNO. The doping electrons are now populating the first few LNO layers. In both (a) and (b), the colored squares highlight the location within the heterostructures of the indicated electronic configuration. c) XAS at Mn L3 edge of a 2 uc-thick LNMO film with and without top LNO. 10 uc-thick LNMO shown for comparison. Spectra shifted for better visibility. d) Same as (c) for the Ni L2 edge. The dashed line indicates the energy of the Ni L2-edge maximum of the reference LNMO sample. e) Mn XMCD asymmetry measured at 20 K for LNO/LNMO//STO (green circles) and LNMO//STO (red squares) heterostructures. Lines are a guide to the eye. f) Measured Curie temperature as a function of the LNMO thickness (log scale) for engineered (green circles) and bare (red squares) heterostructures. The data are fitted using a linear function and a power law for the capped heterostructures and a power law for the uncapped heterostructures.[39] The inset shows the low-thickness range using a linear scale. The same effect is also obtained on a 5 uc-thick LNMO film heterostructures with XAS (Figure S9b, Supporting Infor- (Figure S9a, Supporting Information). Such a drastic change mation). The observation of the same functional behavior on in the Mn electronic configuration is not observed instead another polar interface further corroborates the ability of LNO when the LNMO films are capped with LAO, as shown in to attract the electrons that are doping the opposite interface Figure S9a, Supporting Information, and as predicted by our due to a polar discontinuity between the film and the substrate. DFT calculations (Figure S8, Supporting Information). We can As a result of this optimized electronic configuration, the therefore exclude that in our LNO/LNMO//STO heterostruc- magnetic properties of the LNO/LNMO//STO heterostructures ture, La vacancies (instead of Ni) are playing the role of elec- are drastically enhanced. As shown in Figure 4e through the tron acceptors.[40] Indeed, when the LNO overlayer is only field-dependence of the Mn XMCD asymmetry measured at 2 uc thick, the overall Ni L2-edge shape suggests that the Ni in 20 K, the ferromagnetic behavior of the Mn sublattice in a 2-uc- LNO adopts a 2+ configuration (blue square and blue line in thick LNMO film is recovered upon LNO capping. The remark- Figures 4b and 4d, respectively). A transition to more conven- able thickness-dependent evolution of the Curie temperature is tional Ni3+ (green square in Figure 4b) is observed instead when summarized in Figure 4f. We observe a systematic boost of TC the LNO thickness is increased (i.e., the case of 8 uc LNO cap- in the top-interface-engineered heterostructures that are 5-uc- ping shown by the green line in Figure 4d). The fact that the thick or less. For example, an LNO-capped 2(5)-uc-thick LNMO charge transfer is not observed when LAO is used as capping is characterized by a TC ≈ 80 (180) K as determined by combined layer (the Al cation has no d electrons) highlights the pivotal role XMCD and SQUID magnetometry (Figure S10, Supporting of Ni3+ in engineering such a functionality. We also verified the Information). This is to be compared with TC ≈ 10 (165) K for generality of this approach by characterizing LNO/LNMO//LSAT a bare 2(5)-uc-thick LNMO film (Figure 1d). For thicker LNMO Adv. Mater. 2022, 34, 2203071 2203071 (6 of 9) © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2022, 36, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202203071 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de films, the improvement is less evident given the reduced frac- as the sum of spectra measured with vertical and horizontal linear tion that the interfacial layers contribute to the total film thick- polarization and were normalized to the peak of the respective L3,2-edge. ness and due to the increased distance between top and bottom The reference spectra were obtained from MnCl2 (Mn2+), Mn2O3 (Mn3+), SrMnO3 (Mn4+), Nd0.8Sr0.2NiO2+δ (Ni1.5+), NiO (Ni2+), and NdNiO3 interface that can inhibit the interlayer charge transfer.[41] (Ni3+) and were normalized to the peak of the respective L3-edge. The Furthermore, it should be noted that in the bare LNMO// Nd0.8Sr0.2NiO2+δ sample was obtained using a topotactic reduction.[45] STO heterostructures, a conventional power law reduction Only for this specific case, the effective Ni valence was inferred by of TC is observed in the whole thickness range. For the LNO- comparison with charge transfer multiplet theory calculations.[46] The engineered heterostructures, we can rather appreciate the XMCD was defined as the difference between spectra measured with presence of two different regimes for TC as a function of the left and right circular polarization and normalized to the L3 maximum from the XAS. The XMCD asymmetry was defined as (L − R ) / (L + R ) with LNMO thickness. The power-law reduction at larger thickness L(R) obtained as the difference between the L3-resonant and off-resonant is followed by a linear behavior in the ultrathin domain (inset energies for left (right) circular polarization.[19] The XMCD Ni L3 integral of Figure 4f). This linear behavior is caused by the renormali- values as a function of the temperature presented in Figure 1d have been zation of the long-range spin-spin interaction due to finite-size obtained in an external magnetic field of 0.5 T for the 5 and 3 uc-thick effects.[39] By extrapolating the linear behavior to 0 uc LNMO, LNMO films, and in an external magnetic field of 5 T for the 2 uc-thick we obtain a TC value compatible with 0 K suggesting that the film. This difference in magnetic field intensity causes an increase in TC of around 10 K.[19] This information has been used for a more realistic reduced dimensionality of the LNMO films is the only domi- determination of TC in the thinner LNMO sample. nant factor determining the reduction of TC in the interface- Density Functional Theory (DFT): DFT calculations were performed engineered LNO/LNMO//STO heterostructures. using the PBEsol exchange-correlation functional as implemented in the VASP package.[47] Electron–core interactions were described via the projector augmented wave (PAW) method[48] with La(5s, 5p, 5d, 6s), Mn(3p, 3d, 4s), Ni(3p, 3d, 4s), Sr(4s, 4p, 5s), Ti(3s, 3p, 3d, 4s), 3. Conclusion and Ga(3d, 4s, 4p). Valence shells and wave functions were expanded in a plane waves basis with 500 eV kinetic energy cutoff. A Hubbard U We have shown that ultrathin LNMO//STO heterostructures correction[49] was applied to the d states of Mn (3.0 eV), Ni (6.0 eV), experience a degradation of their magnetic properties and and Ti (4.0 eV). The interface was modeled as cell with 2 × 2 in-plane poorer Ni/Mn cation ordering as a consequence of an oxygen- dimension (5.454 × 5.454 Å) with respect to the 5-atom cubic cell. In the vacancy-assisted electronic reconstruction taking place at out-of-plane direction, 4 uc layers of STO and 6 uc layers of LNMO were LNMO polar interfaces. Introducing an LNO overlayer in these used together with a vacuum of 12 Å. Reciprocal space was sampled heterostructures redistributes the electron excess and restores using 6 × 6 × 1 mesh. A ferromagnetic spin configuration was initialized for Mn and Ni atoms in the thin film. Structures were relaxed until forces the LNMO ferromagnetic properties. The LNO-induced elec- converged below 0.01 eV Å−1. tron migration is very general and can be adapted to other func- Scanning Transmission Electron Microscopy (STEM): Electron tional interfaces, being also obtained on heterostructures grown transparent cross-sectional samples for transmission electron on LSAT, where a polarity mismatch takes place too. Such a top- microscopy were prepared by means of an FEI Helios NanoLab 600i layer engineering approach to manipulate the electronic state focused ion beam operated at accelerating voltages of 30 and 5 kV. of a buried interface gets us closer to reaching a systematic and High-angle annular dark-field STEM (HAADF-STEM), EDX spectroscopy, and EELS were carried out using a probe-corrected FEI Titan Themis exact control of the desired functionalities in complex oxide microscope equipped with ChemiSTEM technology and a CEOS heterostructures. Energy-Filtering and Imaging Device (CEFID) in combination with a direct electron detector (ELA, Dectris). The microscope was operated at an accelerating voltage of 300 kV. For the HAADF and EDX data 4. Experimental Section acquisition, a probe convergence semiangle of 18 mrad was set and the annular semidetection range of the annular dark-field detector was Sample Preparation and Characterization: LNO/LNMO//STO calibrated at 84–200 mrad. The elemental maps were calculated from heterostructures were grown using off-axis radio-frequency magnetron the EDX spectrum image using the Sr Kα, Ti Kα, La Lα, Mn Kα, and Ni sputtering in a flowing gas mixture of argon and oxygen stabilizing a Kα lines. Note that even though the EDX spectrum image in Figure 3b pressure of 0.18 mbar. LNMO was grown at 720 °C while LNO was was performed with a low beam current of 80 pA, the two topmost grown at 510 °C. The same conditions were used also on different Ni0.5Mn0.5O2 layers were sputtered away during the 6.6 min long substrates.[19] After the deposition, the heterostructures were cooled acquisition. to room temperature in the same gas environment. Film thickness The EELS data were obtained by setting the convergence and was calibrated via X-ray diffraction finite-size fringes[42] measured with collection semiangles to 26 and 35 mrad, respectively, yielding an a Rigaku Smartlab diffractometer operating in line focus mode with a effective collection semiangle of about 29.5 mrad for the used energy- monochromated Cu Kα1 radiation. Magnetic measurements were carried loss range. An energy dispersion of 0.19 eV per channel was chosen to out in a Quantum Design superconducting quantum interference device simultaneously record both the O K and Mn L3,2 edges. A 390 × 150 pixel (SQUID) magnetometer operating in vibrating sample mode (VSM) with EEL spectrum image (see Figure S6, Supporting Information) was the magnetic field applied parallel to the plane of the sample. A residual acquired with a pixel dwell time of 0.5 ms, resulting in a total acquisition magnetic moment from the STO substrates has been subtracted. Curie time of 30 s. Note that the electron probe was focused on the film for temperatures were determined from SQUID magnetometry taking the only about half of the time, while the rest of the time it was focused minimum of the derivative of the magnetic moment. on the substrate. The low beam current used (80 pA) in combination X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular with the extremely short acquisition time prevented film damage from Dichroism (XMCD): Measurements were performed at both the X-Treme electron beam irradiation. This was possible thanks to the large CEFID beamline[43] at the Swiss Light Source (SLS) and BOREAS beamline[44] at entrance aperture used along with the high sensitivity of the ELA the ALBA Synchrotron. The spectra were acquired by measuring the total detector. All spectra were background subtracted by fitting a decaying electron yield with an incidence angle (the angle between the X-ray beam power-law function to an energy window just in front of the core-loss direction and the sample normal) of 60°. The XAS curves were obtained edge onsets. The EEL spectra shown in Figure 3c were calculated by Adv. Mater. 2022, 34, 2203071 2203071 (7 of 9) © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2022, 36, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202203071 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de summing all the spectra at each individual Ni0.5Mn0.5O2 slab. The local Received: April 5, 2022 Mn valence state of each Ni0.5Mn0.5O2 layer was quantified from the Mn Revised: July 12, 2022 L3,2 white-line intensity ratio after background subtraction using scaled Published online: August 7, 2022 Hartree-Slater cross-sections.[33,50,51] For the determination of the Mn oxidation state, reference spectra for Mn4+ and Mn3+ were acquired from CaMnO3 and YMnO3, respectively. The valence state of the manganese atoms was estimated by fitting a linear regression to the experimental [1] A. Ohtomo, H. Y. Hwang, Nature 2004, 427, 423. Mn L3/L2 intensity ratio of the two reference spectra over the manganese [2] A. J. Grutter, H. Yang, B. J. Kirby, M. R. 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Vullum, M. Moreau, J. E. Boschker, S. M. Selbach, This research was supported by the Swiss National Science Foundation R. Holmestad, T. Tybell, Appl. Phys. Lett. 2015, 106, 041604. (SNSF) under Project No. PP00P2_170564 and R’Equip Project No. [9] H. Boschker, J. Verbeeck, R. Egoavil, S. Bals, G. van Tendeloo, 206021_150784 ASKUZI. S.J. and M.G. acknowledge the financial support M. Huijben, E. P. Houwman, G. Koster, D. H. A. a Blank, of Agility plus grant from MARVEL NCCR (SNSF Project No. 51NF40- G. Rijnders, Adv. Funct. Mater. 2012, 22, 2235. 182892). M.K. and U.A. were supported by the SNSF, Project No. [10] D. Kan, R. Aso, R. Sato, M. Haruta, H. Kurata, Y. Shimakawa, Nat. 200021_178791. M.C, R.E., and M.D.R. acknowledge the financial support Mater. 2016, 15, 432. of the SNSF, R’Equip Project No. 206021_189625. A.Z. acknowledges the [11] Z. Liao, M. Huijben, Z. Zhong, N. Gauquelin, S. Macke, R. J. Green, financial support of the SNSF, Project No. 200021_169467. G.D.L., J.S., S. 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