Hard x-ray standing-wave photoemission study of the interfaces in a BiFeO$_3$/La$_{0.7}$Sr$_{0.3}$MnO$_3$ superlattice
H. P. Martins, S. A. Khan, G. Conti, A. A. Greer, A. Y. Saw, G. K. Palsson, M. Huijben, K. Kobayashi, S. Ueda, C. M. Schneider, I. M. Vishik, J. Minár, A. X. Gr… 展开作者
H. P. Martins, S. A. Khan, G. Conti, A. A. Greer, A. Y. Saw, G. K. Palsson, M. Huijben, K. Kobayashi, S. Ueda, C. M. Schneider, I. M. Vishik, J. Minár, A. X. Gray, C. S. Fadley, S. Nemšák
Hybrid multiferroics such as BiFeO$_3$ (BFO) and La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) heterostructures are highly interesting functional systems due to their complex electronic and magnetic properties. One of the key parameters influencing the emergent properties is the quality of interfaces, where varying interdiffusion lengths can give rise to different chemistry and distinctive electronic states. Here we report high-resolution depth resolved chemical and electronic investigation of BFO/LSMO superlattice using standing-wave hard X-ray photoemission spectroscopy in the first-order Bragg as well as near-total-reflection geometry. Our results show that the interfaces of BFO on top of LSMO are atomically abrupt, while the LSMO on top of BFO interfaces show an interdiffusion length of around 1.2 unit cells. The two interfaces also exhibit different chemical gradients, with the BFO/LSMO interface being Sr-terminated by a spectroscopically distinctive high binding energy component in Sr 2p core-level spectra, which is spatially contained within 1 unit cell from the interface. From the electronic point of view, unique valence band features were observed for bulk-BFO, bulk-LSMO and their interfaces. Our X-ray optical analysis revealed a unique electronic signature at the BFO/LSMO interface, which we attribute to the coupling between those respective layers. Valence band decomposition based on the Bragg-reflection standing-wave measurement also revealed the band alignment between BFO and LSMO layers. Our work demonstrates that standing-wave hard x-ray photoemission is a reliable non-destructive technique for probing depth-resolved electronic structure of buried layers and interfaces with sub-unit-cell resolution. Equivalent investigations can be successfully applied to a broad class of material such as perovskite complex oxides with emergent interfacial phenomena.
First-principles techniques are used to investigate the behavior of BiFeO$_{3}$/LaFeO$_{3}$ perovskite oxide superlattices epitaxially grown on a (001)-SrTiO$_3$ substrate. The calculations show that 1/1 superlattices exhibit a $Pmc2_1$ ground state combining a trilinear coupling of one polar and two oxygen rotational lattice modes, and weak ferromagnetism. The microscopic mechanism allowing one to manipulate the magnetization with an electric field in such systems is presented and its dependence on strain and chemical substitution is discussed. BiFeO$_{3}$/LaFeO$_{3}$ artificial superlattices…
Epitaxial BiFeO3/SrRuO3 superlattices have been grown by pulsed laser deposition on a (001) oriented LaAlO3 substrate and probed by X-ray diffraction and Raman spectroscopy. To investigate the structural competition between rhombohedral BiFeO3 and orthorhombic SrRuO3 the total thickness of all SLs was kept constant and the bilayer thickness (period) Λ was varied. The interlayer strain effects are therefore tuned from large strain effects (short Λ period) to quasi-relaxed structure (large Λ). A complementary investigation using X-ray diffraction and phonon dynamics hints to change from a rhombo…
(BiFeO3)(1-x)Λ/(LaFeO3)xΛ superlattices (SLs) with varying x have been grown by pulsed laser deposition on (111) oriented SrTiO3 substrates. In order to obtain good epitaxy and flat samples a conducting SrRuO3 buffer has been deposited prior to the superlattices to screen the polar mismatch for such (111) SrTiO3 orientation. X-ray diffraction reciprocal space mapping on different family of planes were collected and evidenced a room temperature structural change at x=0.5 from a rhombohedral/monoclinic structure for rich BiFeO3 to an orthorhombic symmetry for rich LaFeO3. This symmetry change ha…
In ABO3 perovskites, oxygen octahedron rotations are common structural distortions that can promote large ferroelectricity in BiFeO3 with an R3c structure [1], but suppress ferroelectricity in CaTiO3 with a Pbnm symmetry [2]. For many CaTiO3-like perovskites, the BiFeO3 structure is a metastable phase. Here, we report the stabilization of the highly-polar BiFeO3-like phase of CaTiO3 in a BaTiO3/CaTiO3 superlattice grown on a SrTiO3 substrate. The stabilization is realized by a reconstruction of oxygen octahedron rotations at the interface from the pattern of nonpolar bulk CaTiO3 to a different…
We report the observation of field-induced magnetization of BiFeO3 (BFO) in an ultrathin BFO/La0.7Sr0.3MnO3 (LSMO) superlattice using polarized neutron reflectivity (PNR). Our PNR results indicate parallel alignment of magnetization across BFO/LSMO interfaces. The study showed an increase in average magnetization on increasing applied magnetic field at 10K. We observed a saturation magnetization of 110 \pm 15 kA/m (~0.8 μB/Fe) for ultrathin BFO layer (~2 unit cell) sandwiched between ultrathin LSMO layers (~ 2 unit cell), which is much higher than the canted moment (0.03 μB/Fe) in the bulk BFO…
We studied the charge-orbital ordering in the superlattice of charge-ordered insulating Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ and ferromagnetic metallic La$_{0.5}$Sr$_{0.5}$MnO$_3$ by resonant soft x-ray diffraction. A temperature-dependent incommensurability is found in the orbital order. In addition, a large hysteresis is observed that is caused by phase competition between insulating charge ordered and metallic ferromagnetic states. No magnetic phase transitions are observed in contrast to bulk, confirming the unique character of the superlattice. The deviation from the commensurate orbital order can…
Superlattices composed of ferromagnetics, namely La$_{0.7}$Ca$_{0.3}$MnO$_3$ (LCMO), and ferroelectrics, namely, BaTiO$_3$(BTO) were grown on SrTiO$_3$ at 720$^o$C by pulsed laser deposition process. While the out-of-plane lattice parameters of the superlattices, as extracted from the X-ray diffraction studies, were found to be dependent on the BTO layer thickness, the in-plane lattice parameter is almost constant. The evolution of the strains, their nature, and their distribution in the samples, were examined by the conventional sin$^2ψ$ method. The effects of structural variation on the phys…
We studied the magnetic properties of La$_{0.7}$Ca$_{0.3}$MnO$_3$ / YBa$_2$Cu$_3$O$_{7-δ}$ superlattices. Magnetometry showed that with increasing YBa$_2$Cu$_3$O$_{7-δ}$ layer thickness the saturation magnetization per La$_{0.7}$Ca$_{0.3}$MnO$_3$ layer decreases. From polarized neutron reflectometry we determined that this magnetization reduction is due to an inhomogenous magnetization depth profile arising from the suppression of magnetization near the La$_{0.7}$Ca$_{0.3}$MnO$_3$ / YBa$_2$Cu$_3$O$_{7-δ}$ interface. Electron energy loss spectroscopy indicates an increased 3d band occupation of…
We present a study of interlayer coupling and proximity effects in a La$_{0.66}$Ca$_{0.33}$MnO$_3$(10 nm)/YBa$_2$Cu$_3$O$_7$(10 nm) superlattice. Using element-sensitive x-ray probes, the magnetic state of Mn can be probed without seeing the strong diamagnetism of the superconductor, which makes this approach ideal to study changes in the magnetic properties across the superconducting transition. By a combined experiment using {\it in situ} transport measurements during polarized soft x-ray measurements, we were able to see no noticeable influence of the superconducting state on the magnetic p…
Interface magnetic coupling and magnetization dynamic of La$_{2/3}$Sr$_{1/3}$MnO$_3$ single layer and (La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrRuO$_3$)$_n$ (n = 1, 5) superlattice on SrTiO$_3$(001) substrate
We present a systematic approach to modeling the electrical and structural properties of charge-mismatched superlattices from first principles. Our strategy is based on bulk calculations of the parent compounds, which we perform as a function of in-plane strain and out-of-plane electric displacement field. The resulting two-dimensional phase diagrams allow us to accurately predict, without performing further calculations, the behavior of a layered heterostructure where the aforementioned building blocks are electrostatically and elastically coupled, with an arbitrary choice of the interface ch…
Epitaxial La1.85Sr0.15CuO4/La2/3Ca1/3MnO3 superlattices on (001)-oriented LaSrAlO4 substrates have been grown with pulsed laser deposition technique. Their structural, magnetic and superconducting properties have been determined with in-situ reflection high energy electron diffraction, x-ray diffraction, specular neutron reflectometry, scanning transmission electron microscopy, electric transport, and magnetization measurements. We find that despite the large mismatch between the in-plane lattice parameters of LSCO and LCMO these superlattices can be grown epitaxially and with a high crystalli…
We study the magnetic structure of the (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ superlattices from density-functional calculations. In agreement with the experiments, we find that the magnetism changes with the layer thickness `n'. The reason for the different magnetic structures is shown to be the varying potential barrier across the interface, which controls the leakage of the Mn-e$_g$ electrons from the LMO side to the SMO side. This in turn affects the interfacial magnetism via the carrier-mediated Zener double exchange. For n=1 superlattice, the Mn-e$_g$ electrons are more or less spread over t…
The electron-phonon interaction is of central importance for the electrical and thermal properties of solids, and its influence on superconductivity, colossal magnetoresistance, and other many-body phenomena in correlated-electron materials is currently the subject of intense research. However, the non-local nature of the interactions between valence electrons and lattice ions, often compounded by a plethora of vibrational modes, present formidable challenges for attempts to experimentally control and theoretically describe the physical properties of complex materials. Here we report a Raman s…
We present a novel approach to reduce the thermal conductivity ($κ$) in thermoelectric composite materials using acoustic impedance mismatch and the Debye model. Also, the correlation between interface thermal resistance (R$_{int}$) and the particle size of the dispersed phase on the k of the composite is discussed. In particular, the $κ$ of an oxide composite which consists of a natural superlattice Aurivillius phase (SrBi$_4$Ti$_4$O$_{15}$) as a matrix and perovskite (La$_{0.7}$Sr$_{0.3}$MnO$_3$) as a dispersed phase is investigated. A significant reduction in the $κ$ of composite, even lowe…
We investigate the structural and magnetic properties of La$_{0.7}$Sr$_{0.3}$CrO$_3$ (LSCO)/La$_{0.7}$Sr$_{0.3}$MnO$_3$(LSMO) heterostructures grown on (001)-oriented MgO by molecular beam epitaxy. Due to the large film-substrate lattice mismatch, strain relaxation is found to occur within the first 2-3 unit cells (uc) of the film as evidenced by reflection high energy electron diffraction and high-resolution synchrotron X-ray reciprocal space mapping. We find that the presence of the LSCO spacer and capping layers leads to ferromagnetism in ultra-thin LSMO layers with thicknesses on the order…
The structural and magnetic properties of a series of superlattices consisting of two ferromagnetic metals La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) and SrRuO$_3$ (SRO) grown on (001) oriented SrTiO$_3$ are studied. Superlattices with a fixed LSMO layer thickness of 20 unit cells (u.c.) and varying SRO layer thickness show a sudden drop in magnetization on cooling through temperature where both LSMO and SRO layers are ferromagnetic. This behavior suggests an antiferromagnetic coupling between the layers. In addition, the samples having thinner SRO layers (n \TEXTsymbol{<} 6) exhibit enhanced saturati…
We perform density functional theory plus dynamical mean field theory calculations to investi- gate internal charge transfer in an artificial superlattice composed of alternating layers of vanadate and manganite perovskite and Ruddlesden-Popper structure materials. We show that the elec- tronegativity difference between vanadium and manganese causes moderate charge transfer from VO2 to MnO2 layers in both perovskite and Ruddlesden-Popper based superlattices, leading to hole doping of the VO2 layer and electron doping of the MnO2 layer. Comparison of the perovskite and Ruddlesden-Popper based h…
Control of polarization and polar chiral textures in BiFeO$_3$ by epitaxial strain and interfacial chemistry
Laboratoire Albert Fert, CNRS, Thales, Université Paris–Saclay（阿尔贝·费尔特实验室，法国国家科学研究中心，泰雷兹，巴黎-萨克雷大学）；Department of Materials, ETH Zurich（苏黎世联邦理工学院材料系）；Université Grenoble Alpes, CNRS, Institut Néel（格勒诺布尔阿尔卑斯大学，法国国家科学研究中心，内尔研究所）；Electron Microscopy Center, Empa（电子显微镜中心，瑞士联邦材料科技与测试研究院）
Artificially engineered superlattices were designed and fabricated to induce different growth mechanisms and structural characteristics. DC sputtering was used to grow ferromagnetic (La$_{0.8}$Ba$_{0.2}$MnO$_3$) / ferroelectric (Ba$_{0.25}$Sr$_{0.75}$TiO$_3$ or BaTiO$_3$) superlattices. We systematically modified the thickness of the ferromagnetic layer to analyze dimensional and structural effects on the superlattices with different structural characteristics. The crystalline structure was characterized by X-Ray diffraction and transmission electron microscopy. The magnetic and electronic pro…
Given the paucity of single phase multiferroic materials (with large ferromagnetic moment), composite systems seem an attractive solution in the quest to realize magnetoelectric cou-pling between ferromagnetic and ferroelectric order parameters. Despite having antiferro-magnetic order, BiFeO3 (BFO) has nevertheless been a key material in this quest due to excel-lent ferroelectric properties at room temperature. We studied a superlattice composed of 8 repetitions of 6 unit cells of La0.7Sr0.3MnO3 (LSMO) grown on 5 unit cells of BFO. Significant net uncompensated magnetization in BFO is demonstr…
Epitaxial oxide interfaces with broken translational symmetry have emerged as a central paradigm behind the novel behaviors of oxide superlattices. Here, we use scanning transmission electron microscopy to demonstrate a direct, quantitative unit-cell-by-unit-cell mapping of lattice parameters and oxygen octahedral rotations across the BiFeO3-La0.7Sr0.3MnO3 interface to elucidate how the change of crystal symmetry is accommodated. Combined with low-loss electron energy loss spectroscopy imaging, we demonstrate a mesoscopic antiferrodistortive phase transition and elucidate associated changes in…
We present a resonant x-ray scattering study of the octahedral tilt order between 50K and 310K in La_7/8Sr_1/8MnO_3. At the La L_II-edge the resonant (300) reflection probes cooperative tilts of the MnO_6-octahedra in this material, as verified by a model caclulation as well as a LDA+U study. The investigation of the octahedral tilts as a function of temperature and the comparison to the lattice parameters, the magnetization and the superlattice reflections related to charge and/or orbital order reveal an intimate coupling between electronic and tilt degrees of freedom in La_7/8Sr_1/8MnO_3.
We investigate the possibility of controlling the magnetic phase transition of the heterointerface between a half-doped manganite La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ and a multiferroic BiFeO$_{3}$ through magnetoelectric coupling. Using macroscopic magnetometry and element-selective x-ray magnetic circular dichroism at the Mn and Fe \emph{L}-edges, we discover that the ferroelectric polarization of BFO controls simultaneously the magnetization of BFO and LCMO. X-ray absorption spectra at oxygen \emph{K}-edge and linear dichroism at Mn \emph{L}-edge suggest that the interfacial coupling is mainly der…
