Microscopic origins for stabilizing room-temperature ferromagnetism in ultrathin manganite layers

Room-temperature sub-100 nm Néel-type skyrmions in non-stoichiometric van der Waals ferromagnet Fe3-xGaTe2 with ultrafast laser writability

Realizing room-temperature magnetic skyrmions in two-dimensional van der Waals ferromagnets offers unparalleled prospects for future spintronic applications. However, due to the intrinsic spin fluctuations that suppress atomic long-range magnetic order and the inherent inversion crystal symmetry that excludes the presence of the Dzyaloshinskii-Moriya interaction, achieving room-temperature skyrmions in 2D magnets remains a formidable challenge. In this study, we target room-temperature 2D magnet Fe3GaTe2 and unveil that the introduction of iron-deficient into this compound enables spatial inversion symmetry breaking, thus inducing a significant Dzyaloshinskii-Moriya interaction that brings about room-temperature Néel-type skyrmions with unprecedentedly small size. To further enhance the practical applications of this finding, we employ a homemade in-situ optical Lorentz transmission electron microscopy to demonstrate ultrafast writing of skyrmions in Fe3-xGaTe2 using a single femtosecond laser pulse. Our results manifest the Fe3-xGaTe2 as a promising building block for realizing skyrmion-based magneto-optical functionalities.

Resolving the polar interface of infinite-layer nickelate thin films

Nickel-based superconductors provide a long-awaited experimental platform to explore possible cuprate-like superconductivity. Despite similar crystal structure and d electron filling, however, superconductivity in nickelates has thus far only been stabilized in thin-film geometry, raising questions about the polar interface between substrate and thin film. Here we conduct a detailed experimental and theoretical study of the prototypical interface between Nd_1-xSr_xNiO₂ and SrTiO₃. Atomic-resolution electron energy loss spectroscopy in the scanning transmission electron microscope reveals the formation of a single intermediate Nd(Ti,Ni)O₃ layer. Density functional theory calculations with a Hubbard U term show how the observed structure alleviates the polar discontinuity. We explore the effects of oxygen occupancy, hole doping and cation structure to disentangle the contributions of each for reducing interface charge density. Resolving the non-trivial interface structure will be instructive for future synthesis of nickelate films on other substrates and in vertical heterostructures.

Substrate engineering for wafer-scale two-dimensional material growth: strategies, mechanisms, and perspectives

The fabrication of wafer-scale two-dimensional (2D) materials is a prerequisite and important step for their industrial applications. Chemical vapor deposition (CVD) is the most promising approach to produce high-quality films in a scalable way. Recent breakthroughs in the epitaxy of wafer-scale single-crystalline graphene, hexagonal boron nitride, and transition-metal dichalcogenides highlight the pivotal roles of substrate engineering by lattice orientation, surface steps, and energy considerations. This review focuses on the existing strategies and underlying mechanisms, and discusses future directions in epitaxial substrate engineering to deliver wafer-scale 2D materials for integrated electronics and photonics.

Significant Reduction of the Dead Layers by the Strain Release in La0.7Sr0.3MnO3 Heterostructures

Great efforts have been devoted to exploring the emergent phenomena occurring in heterostructures of correlated oxides. However, the presence of both magnetic and electrical dead layers in functional oxide films generally obstructs the device functionalization and miniaturization. Here, we demonstrate an effective strategy to significantly reduce the thickness of dead layers in a prototypical correlated oxide system, La_0.7Sr_0.3MnO₃ (LSMO) grown on LaAlO₃ (LAO) substrates, via strain engineering by inserting a Sr₃Al₂O₆ buffer layer with a different thickness at heterointerfaces. In this way, the thicknesses of the magnetic and electrical dead layers of LSMO films on the LAO substrates notably decrease from 8 to 4 unit cells and from 13 to 9 unit cells, respectively. Our results provide a convenient method to minimize or even eliminate the dead layers of correlated oxides through the interfacial strain engineering, which has potential applications in nanoscale oxide spintronic devices.

Controllable Preparation of 2D Vertical van der Waals Heterostructures and Superlattices for Functional Applications

2D van der Waals heterostructures (vdWHs) and superlattices (SLs) with exotic physical properties and applications for new devices have attracted immense interest. Compared to conventionally bonded heterostructures, the dangling-bond-free surface of 2D layered materials allows for the feasible integration of various materials to produce vdWHs without the requirements of lattice matching and processing compatibility. The quality of interfaces in artificially stacked vdWHs/vdWSLs and scalability of production remain among the major challenges in the field of 2D materials. Fortunately, bottom-up methods exhibit relatively high controllability and flexibility. The growth parameters, such as the temperature, precursors, substrate, and carrier gas, can be carefully and comprehensively controlled to produce high-quality interfaces and wafer-scale products of vdWHs/vdWSLs. This review focuses on three types of bottom-up methods for the assembly of vdWHs and vdWSLs with atomically clean and electronically sharp interfaces: chemical/physical vapor deposition, metal-organic chemical vapor deposition, and ultrahigh vacuum growth. These methods can intuitively illustrate the great flexibility and controllability of bottom-up methods for the preparation of vdWHs/vdWSLs. The latest progress in vdWHs and vdWSLs, related physical phenomena, and (opto)electronic devices are summarized. Finally, the authors discuss current challenges and future perspectives in the synthesis and application of vdWHs and vdWSLs.

Evidence of Mn-Ion Structural Displacements Correlated with Oxygen Vacancies in La0.7Sr0.3MnO3 Interfacial Dead Layers

The properties of half-metallic manganite thin films depend on the composition and structure in the atomic scale, and consequently, their potential functional behavior can only be based on fine structure characterization. By combining advanced transmission electron microscopy, electron energy loss spectroscopy, density functional theory calculations, and multislice image simulations, we obtained evidence of a 7 nm-thick interface layer in La_0.7Sr_0.3MnO₃ (LSMO) thin films, compatible with the formation of well-known dead layers in manganites, with an elongated out-of-plane lattice parameter and structural and electronic properties well distinguished from the bulk of the film. We observed, for the first time, a structural shift of Mn ions coupled with oxygen vacancies and a reduced Mn valence state within such layer. Understanding the correlation between oxygen vacancies, the Mn oxidation state, and Mn-ion displacements is a prerequisite to engineer the magnetotransport properties of LSMO thin films.

Creating Ferromagnetic Insulating La0.9Ba0.1MnO3 Thin Films by Tuning Lateral Coherence Length

In this work, heteroepitaxial vertically aligned nanocomposite (VAN) La_0.9Ba_0.1MnO₃ (LBMO)-CeO₂ films are engineered to produce ferromagnetic insulating (FMI) films. From combined X-ray photoelectron spectroscopy, X-ray diffraction, and electron microscopy, the elimination of the insulator-metal (I-M) transition is shown to result from the creation of very small lateral coherence lengths (with the corresponding lateral size ∼ 3 nm (∼7 u.c.)) in the LBMO matrix, achieved by engineering a high density of CeO₂ nanocolumns in the matrix. The small lateral coherence length leads to a shift in the valence band maximum and reduction of the double exchange (DE) coupling. There is no "dead layer" effect at the smallest achieved lateral coherence length of ∼3 nm. The FMI behavior obtained by lateral dimensional tuning is independent of substrate interactions, thus intrinsic to the film itself and hence not related to film thickness. The unique properties of VAN films give the possibility for multilayer spintronic devices that can be made without interface degradation effects between the layers.

Strong Ferromagnetism Achieved via Breathing Lattices in Atomically Thin Cobaltites

Low-dimensional quantum materials that remain strongly ferromagnetic down to monolayer thickness are highly desired for spintronic applications. Although oxide materials are important candidates for the next generation of spintronics, ferromagnetism decays severely when the thickness is scaled to the nanometer regime, leading to deterioration of device performance. Here, a methodology is reported for maintaining strong ferromagnetism in insulating LaCoO₃ (LCO) layers down to the thickness of a single unit cell. It is found that the magnetic and electronic states of LCO are linked intimately to the structural parameters of adjacent "breathing lattice" SrCuO₂ (SCO). As the dimensionality of SCO is reduced, the lattice constant elongates over 10% along the growth direction, leading to a significant distortion of the CoO₆ octahedra, and promoting a higher spin state and long-range spin ordering. For atomically thin LCO layers, surprisingly large magnetic moment (0.5 μ_B /Co) and Curie temperature (75 K), values larger than previously reported for any monolayer oxides are observed. The results demonstrate a strategy for creating ultrathin ferromagnetic oxides by exploiting atomic heterointerface engineering, confinement-driven structural transformation, and spin-lattice entanglement in strongly correlated materials.

Probing Charge Accumulation at SrMnO3/SrTiO3 Heterointerfaces via Advanced Electron Microscopy and Spectroscopy

The last three decades have seen a growing trend toward studying the interfacial phenomena in complex oxide heterostructures. Of particular concern is the charge distribution at interfaces, which is a crucial factor in controlling the interface transport behavior. However, the study of the charge distribution is very challenging due to its small length scale and the intricate structure and chemistry at interfaces. Furthermore, the underlying origin of the interfacial charge distribution has been rarely studied in-depth and is still poorly understood. Here, by a combination of aberration-corrected scanning transmission electron microscopy (STEM) and spectroscopy techniques, we identify the charge accumulation in the SrMnO₃ (SMO) side of SrMnO₃/SrTiO₃ heterointerfaces and find that the charge density attains the maximum of 0.13 ± 0.07 e^-/unit cell (uc) at the first SMO monolayer. Based on quantitative atomic-scale STEM analyses and first-principle calculations, we explore the origin of interfacial charge accumulation in terms of epitaxial strain-favored oxygen vacancies, cationic interdiffusion, interfacial charge transfer, and space-charge effects. This study, therefore, provides a comprehensive description of the charge distribution and related mechanisms at the SMO/STO heterointerfaces, which is beneficial for the functionality manipulation via charge engineering at interfaces.

Free-Standing CoO-POM Janus-like Ultrathin Nanosheets

A single-step solution-based strategy is used to obtain 2D Janus-like free-standing ultrathin nanosheets build from two structurally unrelated species, that is, polyoxomolybdate (POM) and CoO. A controlled 2D-to-1D morphological transition was achieved by judiciously adjusting the solvent choice. These POM-CoO heterostructures can behave as an ideal catalyst for the epoxidation of styrene. Benefiting from their amphiphilic nature, these 2D POM-CoO nanosheets have also been used as surfactant to emulsify immiscible solvents. It is anticipated that structurally diverse polyoxometalates will offer promise as design elements for variety of structurally and compositionally tunable van der Waals integrated heteromaterials having a broad range applications.

Emerging Electron Microscopy Techniques for Probing Functional Interfaces in Energy Materials

Interfaces play a fundamental role in many areas of chemistry. However, their localized nature requires characterization techniques with high spatial resolution in order to fully understand their structure and properties. State-of-the-art atomic resolution or in situ scanning transmission electron microscopy and electron energy-loss spectroscopy are indispensable tools for characterizing the local structure and chemistry of materials with single-atom resolution, but they are not able to measure many properties that dictate function, such as vibrational modes or charge transfer, and are limited to room-temperature samples containing no liquids. Here, we outline emerging electron microscopy techniques that are allowing these limitations to be overcome and highlight several recent studies that were enabled by these techniques. We then provide a vision for how these techniques can be paired with each other and with in situ methods to deliver new insights into the static and dynamic behavior of functional interfaces.

Unusual exchange bias in Sr2FeIrO6/La0.67Sr0.33MnO3 multilayer

Here, we study interface induced magnetic properties in a 3d-5d based multilayer made of La_0.67Sr_0.33MnO₃ and double perovskite Sr₂FeIrO₆, respectively. Bulk La_0.67Sr_0.33MnO₃ is metallic and shows ferromagnetic (FM) ordering above room temperature. In contrast, bulk Sr₂FeIrO₆, is an antiferromagnet (AFM) with a Néel temperature around 45 K ([Formula: see text]) and exhibits an insulating behavior. Two set of multilayers have been grown on SrTiO₃ (1 0 0) crystal with varying thickness of FM layer. A multilayer with equal thickness of La_0.67Sr_0.33MnO₃ and Sr₂FeIrO₆ (∼10 nm) shows exchange bias (EB) effect both in conventionally field cooled (FC) as well as in zero field cooled (ZFC) magnetic hysteresis measurements which is rather unusual. The ZFC EB effect is weakened both with increasing maximum field during initial magnetization process at low temperature and with increasing temperature. Interestingly, a multilayer with reduced thickness of La_0.67Sr_0.33MnO₃ (∼5 nm) does not exhibit ZFC EB phenomenon, however, the FC EB effect is strengthened showing much higher value. We believe that an AFM type exchange coupling at the interface and its evolution during initial application of magnetic field causes this unusual EB in present multilayers.

Interfacial Engineering of Ferromagnetism in Epitaxial Manganite/Ruthenate Superlattices via Interlayer Chemical Doping

Interfacial charge transfer and structural proximity effects are the two essential routes to trigger and tune numerous functionalities of perovskite oxide heterostructures. However, the cooperation and competition of these two interfacial effects in one epitaxial system have not been fully understood. Herein, we fabricate a series of La_0.67Ca_0.33MnO₃/CaRuO₃ superlattices and introduce various chemical doping in the nonmagnetic CaRuO₃ interlayers. We found that Ti, Sr, and La doping in the CaRuO₃ layer can effectively tune the interfacial charge transfer and octahedral rotation, thus modulating the ferromagnetism of the superlattices. Specifically, the B-site Ti doping depletes the Ru 4d band and suppresses the interfacial charge transfer, leading to a decay of ferromagnetic Curie temperature ( T_C). In contrast, the A-site Sr doping maintains a sizable charge transfer and meanwhile suppresses the octahedral rotation, which facilitates ferromagnetism and significantly enhances the T_C up to 291 K. The La doping turns out to localize the itinerant electrons in the CaRuO₃ layer, which suppresses both the interfacial charge transfer and ferromagnetism. The observed intriguing interfacial engineering of magnetism would pave a new way to understand the collective effects of interfacial charge transfer and structural proximity on the physical properties of oxide heterostructures.

Metal-to-Insulator Transition in Ultrathin Manganite Heterostructures

Thickness-driven phase transitions have been widely observed in many correlated transition metal oxides materials. One of the important topics is the thickness-driven metal to insulator transition in half-metal La2/3Sr1/3MnO3 (LSMO) thin films, which has attracted great attention in the past few decades. In this article, we review research on the nature of the metal-to-insulator (MIT) transition in LSMO ultrathin films. We discuss in detail the proposed mechanisms, the progress made up to date, and the key issues existing in understanding the related MIT. We also discuss MIT in other correlated oxide materials as a comparison that also has some implications for understanding the origin of MIT.

Metal Oxide Nanocomposites: A Perspective from Strain, Defect, and Interface

Vertically aligned nanocomposite thin films with ordered two phases, grown epitaxially on substrates, have attracted tremendous interest in the past decade. These unique nanostructured composite thin films with large vertical interfacial area, controllable vertical lattice strain, and defects provide an intriguing playground, allowing for the manipulation of a variety of functional properties of the materials via the interplay among strain, defect, and interface. This field has evolved from basic growth and characterization to functionality tuning as well as potential applications in energy conversion and information technology. Here, the remarkable progress achieved in vertically aligned nanocomposite thin films from a perspective of tuning functionalities through control of strain, defect, and interface is summarized.

Data-driven approach for the prediction and interpretation of core-electron loss spectroscopy

Spectroscopy is indispensable for determining atomic configurations, chemical bondings, and vibrational behaviours, which are crucial information for materials development. Despite their importance, the interpretation of spectra using "human-driven" methods, such as the manual comparison of experimental spectra with reference/simulated spectra, is difficult due to the explosive increase in the number of experimental spectra to be observed. To overcome the limitations of the "human-driven" approach, we develop a new "data-driven" approach based on machine learning techniques by combining the layer clustering and decision tree methods. The proposed method is applied to the 46 oxygen-K edges of the ELNES/XANES spectra of oxide compounds. With this method, the spectra can be interpreted in accordance with the material information. Furthermore, we demonstrate that our method can predict spectral features from the material information. Our approach has the potential to provide information about a material that cannot be determined manually as well as predict a plausible spectrum from the geometric information alone.

Engineered superlattices with crossover from decoupled to synthetic ferromagnetic behavior

The extent of interfacial charge transfer and the resulting impact on magnetic interactions were investigated as a function of sublayer thickness in La_0.7Sr_0.3MnO₃/La_0.7Sr_0.3CoO₃ ferromagnetic superlattices. Element-specific soft x-ray magnetic spectroscopy reveals that the electronic structure is altered within 5-6 unit cells of the chemical interface, and can lead to a synthetic ferromagnet with strong magnetic coupling between the sublayers. The saturation magnetization and coercivity depends sensitively on the sublayer thickness due to the length scale of this interfacial effect. For larger sublayer thicknesses, the La_0.7Sr_0.3MnO₃ and La_0.7Sr_0.3CoO₃ sublayers are magnetically decoupled, displaying two independent magnetic transitions with little sublayer thickness dependence. These results demonstrate how interfacial phenomena at perovskite oxide interfaces can be used to tailor their functional properties at the atomic scale.

Engineering magnetism at functional oxides interfaces: manganites and beyond

The family of transition metal oxides (TMOs) is a large class of magnetic materials that has been intensively studied due to the rich physics involved as well as the promising potential applications in next generation electronic devices. In TMOs, the spin, charge, orbital and lattice are strongly coupled, and significant advances have been achieved to engineer the magnetism by different routes that manipulate these degrees of freedom. The family of manganites is a model system of strongly correlated magnetic TMOs. In this review, using manganites thin films and the heterostructures in conjunction with other TMOs as model systems, we review the recent progress of engineering magnetism in TMOs. We first discuss the role of the lattice that includes the epitaxial strain and the interface structural coupling. Then we look into the role of charge, focusing on the interface charge modulation. Having demonstrated the static effects, we continue to review the research on dynamical control of magnetism by electric field. Next, we review recent advances in heterostructures comprised of high T _c cuprate superconductors and manganites. Following that, we discuss the emergent magnetic phenomena at interfaces between 3d TMOs and 5d TMOs with strong spin-orbit coupling. Finally, we provide our outlook for prospective future directions.

Electron Accumulation and Emergent Magnetism in LaMnO_{3}/SrTiO_{3} Heterostructures

Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched LaMnO_{3}/SrTiO_{3} (001) heterostructures. Using a combination of element-specific x-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation, and ferromagnetism have been observed within the polar, antiferromagnetic insulator LaMnO_{3}. Our results show that the critical thickness for the onset of electron accumulation is as thin as 2 unit cells (UC), significantly thinner than the observed critical thickness for ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely induced by electron overaccumulation. In turn, by controlling the doping of the LaMnO_{3}, we are able to neutralize the excessive electrons from the polar mismatch in ultrathin LaMnO_{3} films and thus enable ferromagnetism in films as thin as 3 UC, extending the limits of our ability to synthesize and tailor emergent phenomena at interfaces and demonstrating manipulation of the electronic and magnetic structures of materials at the shortest length scales.

Hidden peculiar magnetic anisotropy at the interface in a ferromagnetic perovskite-oxide heterostructure

Understanding and controlling the interfacial magnetic properties of ferromagnetic thin films are crucial for spintronic device applications. However, using conventional magnetometry, it is difficult to detect them separately from the bulk properties. Here, by utilizing tunneling anisotropic magnetoresistance in a single-barrier heterostructure composed of La0.6Sr0.4MnO3 (LSMO)/LaAlO3 (LAO)/Nb-doped SrTiO3 (001), we reveal the presence of a peculiar strong two-fold magnetic anisotropy (MA) along the [110]c direction at the LSMO/LAO interface, which is not observed in bulk LSMO. This MA shows unknown behavior that the easy magnetization axis rotates by 90° at an energy of 0.2 eV below the Fermi level in LSMO. We attribute this phenomenon to the transition between the e g and t 2g bands at the LSMO interface. Our finding and approach to understanding the energy dependence of the MA demonstrate a new possibility of efficient control of the interfacial magnetic properties by controlling the band structures of oxide heterostructures.

Quantifying the critical thickness of electron hybridization in spintronics materials

In the rapidly growing field of spintronics, simultaneous control of electronic and magnetic properties is essential, and the perspective of building novel phases is directly linked to the control of tuning parameters, for example, thickness and doping. Looking at the relevant effects in interface-driven spintronics, the reduced symmetry at a surface and interface corresponds to a severe modification of the overlap of electron orbitals, that is, to a change of electron hybridization. Here we report a chemically and magnetically sensitive depth-dependent analysis of two paradigmatic systems, namely La_1−xSr_xMnO₃ and (Ga,Mn)As. Supported by cluster calculations, we find a crossover between surface and bulk in the electron hybridization/correlation and we identify a spectroscopic fingerprint of bulk metallic character and ferromagnetism versus depth. The critical thickness and the gradient of hybridization are measured, setting an intrinsic limit of 3 and 10 unit cells from the surface, respectively, for (Ga,Mn)As and La_1−xSr_xMnO₃, for fully restoring bulk properties.

Surface versus bulk effects in electronic structure of spintronics materials are crucial to their applications but are yet well understood. Here the authors experimentally determine the critical thickness that defines the crossover of electron hybridization between surface and bulk for two prototype spintronics materials.

Hidden Interface Driven Exchange Coupling in Oxide Heterostructures

A variety of emergent phenomena have been enabled by interface engineering in complex oxides. The existence of an intrinsic interfacial layer has often been found at oxide heterointerfaces. However, the role of such an interlayerin controlling functionalities is not fully explored. Here, we report the control of the exchange bias (EB) in single-phase manganite thin films with nominallyuniform chemical composition across the interfaces. The sign of EB depends on the magnitude of the cooling field. A pinned layer, confirmed by polarized neutron reflectometry, provides the source of unidirectional anisotropy. The origin of the exchange bias coupling is discussed in terms of magnetic interactions between the interfacial ferromagnetically reduced layer and the bulk ferromagnetic region. The sign of EB is related to the frustration of antiferromagnetic coupling between the ferromagnetic region and the pinned layer. Our results shed new light on using oxide interfaces to design functional spintronic devices.

Impact of interfacial coupling of oxygen octahedra on ferromagnetic order in La0.7Sr0.3MnO3/SrTiO3 heterostructures

La_0.7Sr_0.3MnO₃, a half-metallic ferromagnet with full spin polarization, is generally used as a standard spin injector in heterostructures. However, the magnetism of La_0.7Sr_0.3MnO₃ is strongly modified near interfaces, which was addressed as "dead-layer" phenomenon whose origin is still controversial. Here, both magnetic and structural properties of La_0.7Sr_0.3MnO₃/SrTiO₃ heterostructures were investigated, with emphasis on the quantitative analysis of oxygen octahedral rotation (OOR) across interfaces using annular-bright-field imaging. OOR was found to be significantly altered near interface for both La_0.7Sr_0.3MnO₃ and SrTiO₃, as linked to the magnetism deterioration. Especially in La_0.7Sr_0.3MnO₃/SrTiO₃ superlattices, the almost complete suppression of OOR in 4 unit-cell-thick La_0.7Sr_0.3MnO₃ results in a canted ferromagnetism. Detailed comparisons between strain and OOR relaxation and especially the observation of an unexpected La_0.7Sr_0.3MnO₃ lattice c expansion near interfaces, prove the relevance of OOR for the magnetic properties. These results indicate the capability of tuning the magnetism by engineering OOR at the atomic scale.

Interfacial Control of Ferromagnetism in Ultrathin La0.67Ca0.33MnO3 Sandwiched between CaRu1-xTixO3 (x = 0-0.8) Epilayers

Controlling functionalities in oxide heterostructures remains challenging for the rather complex interfacial interactions. Here, by modifying the interface properties with chemical doping, we achieve a nontrivial control over the ferromagnetism in ultrathin La_0.67Ca_0.33MnO₃ (LCMO) layer sandwiched between CaRu_1-xTi_xO₃ [CRTO(x)] epilayers. The Ti doping suppresses the interfacial electron transfer from CRTO(x) to LCMO side; as a result, a steadily decreased Curie temperature with increasing x, from 262 K at x = 0 to 186 K at x = 0.8, is observed for the structures with LCMO fixed at 3.2 nm. Moreover, for more insulating CRTO(x ≥ 0.5), the electron confinement induces an interfacial Mn-e_g(x²-y²) orbital order in LCMO which further attenuates the ferromagnetism. Also, in order to characterize the heterointerfaces, for the first time the doping- and thickness-dependent metal-insulator transitions in CRTO(x) films are examined. Our results demonstrate that the LCMO/CRTO(x) heterostructure could be a model system for investigating the interfacial multiple interactions in correlated oxides.

Synthesis of freestanding single-crystal perovskite films and heterostructures by etching of sacrificial water-soluble layers

The ability to create and manipulate materials in two-dimensional (2D) form has repeatedly had transformative impact on science and technology. In parallel with the exfoliation and stacking of intrinsically layered crystals, atomic-scale thin film growth of complex materials has enabled the creation of artificial 2D heterostructures with novel functionality and emergent phenomena, as seen in perovskite heterostructures. However, separation of these layers from the growth substrate has proved challenging, limiting the manipulation capabilities of these heterostructures with respect to exfoliated materials. Here we present a general method to create freestanding perovskite membranes. The key is the epitaxial growth of water-soluble Sr ₃Al ₂O ₆ on perovskite substrates, followed by in situ growth of films and heterostructures. Millimetre-size single-crystalline membranes are produced by etching the Sr ₃Al ₂O ₆ layer in water, providing the opportunity to transfer them to arbitrary substrates and integrate them with heterostructures of semiconductors and layered compounds.

Interfacial magnetism in complex oxide heterostructures probed by neutrons and x-rays

Magnetic complex-oxide heterostructures are of keen interest because a wealth of phenomena at the interface of dissimilar materials can give rise to fundamentally new physics and potentially valuable functionalities. Altered magnetization, novel magnetic coupling and emergent interfacial magnetism at the epitaxial layered-oxide interfaces are under intensive investigation, which shapes our understanding on how to utilize those materials, particularly for spintronics. Neutron and x-ray based techniques have played a decisive role in characterizing interfacial magnetic structures and clarifying the underlying physics in this rapidly developing field. Here we review some recent experimental results, with an emphasis on those studied via polarized neutron reflectometery and polarized x-ray absorption spectroscopy. We conclude with some perspectives.

Surface Control of Epitaxial Manganite Films via Oxygen Pressure

The trend to reduce device dimensions demands increasing attention to atomic-scale details of structure of thin films as well as to pathways to control it. This is of special importance in the systems with multiple competing interactions. We have used in situ scanning tunneling microscopy to image surfaces of La5/8Ca3/8MnO3 films grown by pulsed laser deposition. The atomically resolved imaging was combined with in situ angle-resolved X-ray photoelectron spectroscopy. We find a strong effect of the background oxygen pressure during deposition on structural and chemical features of the film surface. Deposition at 50 mTorr of O2 leads to mixed-terminated film surfaces, with B-site (MnO2) termination being structurally imperfect at the atomic scale. A relatively small reduction of the oxygen pressure to 20 mTorr results in a dramatic change of the surface structure leading to a nearly perfectly ordered B-site terminated surface with only a small fraction of A-site (La,Ca)O termination. This is accompanied, however, by surface roughening at a mesoscopic length scale. The results suggest that oxygen has a strong link to the adatom mobility during growth. The effect of the oxygen pressure on dopant surface segregation is also pronounced: Ca surface segregation is decreased with oxygen pressure reduction.

Intrinsic interfacial phenomena in manganite heterostructures

We review recent advances in our understanding of interfacial phenomena that emerge when dissimilar materials are brought together at atomically sharp and coherent interfaces. In particular, we focus on phenomena that are intrinsic to the interface and review recent work carried out on perovskite manganites interfaces, a class of complex oxides whose rich electronic properties have proven to be a useful playground for the discovery and prediction of novel phenomena.

Observation of the strain induced magnetic phase segregation in manganite thin films

Epitaxial strain alters the physical properties of thin films grown on single crystal substrates. Thin film oxides are particularly apt for strain engineering new functionalities in ferroic materials. In the case of La(2/3)Ca(1/3)MnO(3) (LCMO) thin films, here we show the first experimental images obtained by electron holography demonstrating that epitaxial strain induces the segregation of a flat and uniform nonferromagnetic layer with antiferromagnetic (AFM) character at the top surface of a ferromagnetic (FM) layer, the whole film being chemical and structurally homogeneous at room temperature. For different substrates and growth conditions the tetragonality of LCMO at room temperature, defined as τ = |c - a|/a, is the driving force for a phase coexistence above an approximate critical value of τC ≈ 0.024. Theoretical calculations prove that the increased tetragonality changes the energy balance of the FM and AFM ground states in strained LCMO, enabling the formation of magnetically inhomogeneous states. This work gives the key evidence that opens a new route to synthesize strain-induced exchanged-biased FM-AFM bilayers in single thin films, which could serve as building blocks of future spintronic devices.

Magneto-tunable photocurrent in manganite-based heterojunctions

Correlated electron oxide heterojunctions and their photovoltaic effect have attracted increasing attention from the viewpoints of both possible application to novel devices and basic science. In such junctions, correlated electron physics has to be taken into account in addition to conventional semiconductor modelling to explain distinctively emerging features. However, extracting novel functionalities has not been easy because it is not possible to predict their interfacial properties solely from their bulk characteristics. Here we describe a magneto-tunable photocurrent in a pn junction based on a correlated electron oxide La0.7Sr0.3MnO3 combined with a semiconducting SrTiO3 substrate. On applying an epitaxial strain, the photocurrent is enhanced threefold, which is increased 30% further by a magnetic field. Such a magneto-tunable effect is possible for only a narrow window of the correlated gap, which is itself adjusted by bandwidth and temperature. These results provide a guideline for utilization of correlated phenomena into the novel electronic devices.

Correlating interfacial octahedral rotations with magnetism in (LaMnO3+δ)N/(SrTiO3)N superlattices

Lattice distortion due to oxygen octahedral rotations have a significant role in mediating the magnetism in oxides, and recently attracts a lot of interests in the study of complex oxides interface. However, the direct experimental evidence for the interrelation between octahedral rotation and magnetism at interface is scarce. Here we demonstrate that interfacial octahedral rotation are closely linked to the strongly modified ferromagnetism in (LaMnO3+δ)N/(SrTiO3)N superlattices. The maximized ferromagnetic moment in the N=6 superlattice is accompanied by a metastable structure (space group Imcm) featuring minimal octahedral rotations (a(-)a(-)c(-), α~4.2°, γ~0.5°). Quenched ferromagnetism for N<4 superlattices is correlated to a substantially enhanced c axis octahedral rotation (a(-)a(-)c(-), α~3.8°, γ~8° for N=2). Monte-Carlo simulation based on double-exchange model qualitatively reproduces the experimental observation, confirming the correlation between octahedral rotation and magnetism. Our study demonstrates that engineering superlattices with controllable interfacial structures can be a feasible new route in realizing functional magnetic materials.

Strain engineering induced interfacial self-assembly and intrinsic exchange bias in a manganite perovskite film

The control of complex oxide heterostructures at atomic level generates a rich spectrum of exotic properties and unexpected states at the interface between two separately prepared materials. The frustration of magnetization and conductivity of manganite perovskite at surface/interface which is inimical to their device applications, could also flourish in tailored functionalities in return. Here we prove that the exchange bias (EB) effect can unexpectedly emerge in a (La,Sr)MnO₃ (LSMO) “single” film when large compressive stress imposed through a lattice mismatched substrate. The intrinsic EB behavior is directly demonstrated to be originating from the exchange coupling between ferromagnetic LSMO and an unprecedented LaSrMnO₄-based spin glass, formed under a large interfacial strain and subsequent self-assembly. The present results not only provide a strategy for producing a new class of delicately functional interface by strain engineering, but also shed promising light on fabricating the EB part of spintronic devices in a single step.

Effect of interfacial octahedral behavior in ultrathin manganite films

We investigate structural coupling of the MnO6 octahedra across a film/substrate interface and the resultant changes of the physical properties of ultrathin La2/3Sr1/3MnO3 (LSMO) films. In order to isolate the effect of interfacial MnO6 octahedral behavior from that of epitaxial strain, LSMO films are grown on substrates with different symmetry and similar lattice parameters. Ultrathin LSMO films show an increased magnetization and electrical conductivity on cubic (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) compared to those grown on orthorhombic NdGaO3 (NGO) substrates, an effect that subsides as the thickness of the films is increased. This study demonstrates that interfacial structural coupling can play a critical role in the functional properties of oxide heterostructures.

Visualizing the interfacial evolution from charge compensation to metallic screening across the manganite metal-insulator transition

Electronic changes at polar interfaces between transition metal oxides offer the tantalizing possibility to stabilize novel ground states yet can also cause unintended reconstructions in devices. The nature of these interfacial reconstructions should be qualitatively different for metallic and insulating films as the electrostatic boundary conditions and compensation mechanisms are distinct. Here we directly quantify with atomic-resolution the charge distribution for manganite-titanate interfaces traversing the metal-insulator transition. By measuring the concentration and valence of the cations, we find an intrinsic interfacial electronic reconstruction in the insulating films. The total charge observed for the insulating manganite films quantitatively agrees with that needed to cancel the polar catastrophe. As the manganite becomes metallic with increased hole doping, the total charge build-up and its spatial range drop substantially. Direct quantification of the intrinsic charge transfer and spatial width should lay the framework for devices harnessing these unique electronic phases.

Hot electron transport in a strongly correlated transition-metal oxide

Oxide heterointerfaces are ideal for investigating strong correlation effects to electron transport, relevant for oxide-electronics. Using hot-electrons, we probe electron transport perpendicular to the La_0.7Sr_0.3MnO₃ (LSMO)- Nb-doped SrTiO₃ (Nb:STO) interface and find the characteristic hot-electron attenuation length in LSMO to be 1.48 ± 0.10 unit cells (u.c.) at −1.9 V, increasing to 2.02 ± 0.16 u.c. at −1.3 V at room temperature. Theoretical analysis of this energy dispersion reveals the dominance of electron-electron and polaron scattering. Direct visualization of the local electron transport shows different transmission at the terraces and at the step-edges.

Electric field control of magnetism in multiferroic heterostructures

We review the recent developments in the electric field control of magnetism in multiferroic heterostructures, which consist of heterogeneous materials systems where a magnetoelectric coupling is engineered between magnetic and ferroelectric components. The magnetoelectric coupling in these composite systems is interfacial in origin, and can arise from elastic strain, charge, and exchange bias interactions, with different characteristic responses and functionalities. Moreover, charge transport phenomena in multiferroic heterostructures, where both magnetic and ferroelectric order parameters are used to control charge transport, suggest new possibilities to control the conduction paths of the electron spin, with potential for device applications.

Stabilization of ferromagnetic order in La(0.7)Sr(0.3)MnO3-SrRuO3 Superlattices

The study of spatially confined complex oxides is of wide interest, since correlated electrons at interfaces might form exotic phases. Here La(0.7)Sr(0.3)MnO(3)/SrRuO(3) superlattices with coherently grown interfaces were studied by structural techniques, magnetization, and magnetotransport measurements. Magnetization measurements showed that ferromagnetic order in ultrathin La(0.7)Sr(0.3)MnO(3) layers is stabilized in the superlattices down to layer thicknesses of at least two unit cells. This stabilization is destroyed, if the ferromagnetic layers are separated by two unit cell thick SrTiO(3) layers. The resistivity of the superlattices showed metallic behavior and was dominated by the conducting SrRuO(3) layers, the off-diagonal resistivity showed an anomalous Hall effect from both SrRuO(3) and La(0.7)Sr(0.3)MnO(3) layers. This shows that the La(0.7)Sr(0.3)MnO(3) layers are not only ferromagnetic but also highly conducting; probably a conducting hole gas is induced at the interfaces that stabilizes the ferromagnetic order. This result opens up an alternative route for the fabrication of two-dimensional systems with long-range ferromagnetic order.

Exchange coupling and exchange bias in La0.7Sr0.3MnO3-SrRuO3 superlattices

La(0.7)Sr(0.3)MnO(3)-SrRuO(3) superlattices with and without nanometrically thin SrTiO(3), BaTiO(3) and Ba(0.7)Sr(0.3)TiO(3) interlayers were grown by pulsed laser deposition. Transmission electron microscopy studies showed coherent growth of La(0.7)Sr(0.3)MnO(3), SrRuO(3) and SrTiO(3) layers with atomically sharp interfaces, even if individual layers were as thin as one or two unit cells. In contrast, misfit dislocations and unit cell high interfacial steps were observed at the interfaces between BaTiO(3) and one of the ferromagnetic layers. The presence of the interlayers as well as these extended defects had a significant influence on the magnetic properties of the superlattices, especially on the antiferromagnetic interlayer exchange coupling between the La(0.7)Sr(0.3)MnO(3) and SrRuO(3) layers and the exchange biasing. Surprisingly, exchange biasing was found to increase with decreasing strength of the antiferromagnetic interlayer exchange coupling. This was explained by different magnetization reversal mechanisms acting in the regimes of strong and weak interlayer exchange coupling.