Intrinsic antiferromagnetic/insulating phase at manganite surfaces and interfaces

The visible magneto-optical response of RE1-xAxMnO3manganites: relationship with the charge component of the material

Magnetic circular dichroism (MCD) spectroscopy for manganite films of various compositions and morphologies has been studied in the range of 1.2-3.7 eV. The primary focus was on the temperature behavior of the MCD spectra, as well as the magnetization and resistivity of the films. The data obtained were analyzed in comparison with magneto-optical spectroscopy of the Kerr rotation (KR) on both single crystal and thin film of manganites. It has been established that the MCD response at 2.3 eV is typical for manganites transitioning into a conducting state. Consequently, it reflects a change in the band structure of the material. This response is also observed in the KR spectrum of manganites in the range 2.3-2.6 eV below the metal-insulator transition temperature. These findings complement the understanding of the electronic structure of manganites in general. Moreover, they also provide a basis for the search for new functional materials.

Strain-induced charge ordering above room temperature in rare-earth manganites

Most known mixed manganites containing rare-earth elements demonstrate a pronounced charge ordering (CO) state below room temperature. The behavior of the magnetic susceptibility and electronic magnetic resonance of polycrystalline Pr1-xSrxMnO3/YSZ (x = 0.2 and x = 0.4) films without a pronounced texture indicates the formation of the CO phase in the samples at temperatures close to and above room temperature. Moreover, this phase manifests itself with a typical sign of martensitic transformation. The same phenomenon has been traced for textured polycrystalline La0.7Sr0.3MnO3/YSZ films. Electron microscope data indicate the presence of internal strain within the films, which is probably responsible for the formation of the CO phase. It is assumed that the reasons for the appearance of such strain include the crystallite size and the boundary between them. The results obtained provide the basis for the development of new research and technological tasks for the generation of the high-temperature CO state in various polycrystalline rare-earth manganites, since this state contributes to the manifestation of interesting magnetocaloric, magnetoelectric and multiferroic properties. In addition, recent data has opened up new opportunities for studying the strain-induced phenomena in such materials.

Nanoscale magnetic and charge anisotropies at manganite interfaces

Strong correlated manganites are still under intense research owing to their complex phase diagrams in terms of Sr-doping and their sensitivity to intrinsic and extrinsic structural deformations. Here, we performed X-ray absorption spectroscopy measurements of manganite bilayers to explore the effects that a local Sr-doping gradient produce on the charge and antiferromagnetic anisotropies. In order to gradually tune the Sr-doping level along the axis perpendicular to the samples we have grown a series of bilayers with different thicknesses of low-doped manganites (from 0 nm to 6 nm) deposited over a La_0.7Sr_0.3MnO₃ metallic layer. This strategy permitted us to resolve with high accuracy the thickness region where the charge and spin anisotropies vary and the critical thickness t_c over which the out of plane orbital asymmetry does not have any further modifications. We found that the antiferromagnetic spin axis points preferentially out of the sample plane regardless the capping layer thickness. However, it tilts partially into the sample plane far from this critical thickness, owing to the combined contributions of the external structural strain and electron doping. Furthermore, we found that the doping level of the capping layer strongly affects the critical thickness, giving clear evidence of the influence exerted by the electron doping on the orbital and magnetic configurations. These anisotropic changes induce subtle modifications on the domain reorientation of La_0.7Sr_0.3MnO₃, as evidenced from the magnetic hysteresis cycles.

Nanoscale variation of antiferromagnetic and charge anisotropies has been found at manganite interfaces with an artificially created Sr-doping.

Polaronic Emergent Phases in Manganite-based Heterostructures

Transition metal functional oxides, e.g., perovskite manganites, with strong electron, spin and lattice correlations, are well-known for different phase transitions and field-induced colossal effects at the phase transition. Recently, the interfaces between dissimilar perovskites were shown to be a promising concept for the search of emerging phases with novel functionalities. We demonstrate that the properties of manganite films are effectively controlled by low dimensional emerging phases at intrinsic and extrinsic interfaces and appeared as a result of symmetry breaking. The examples include correlated Jahn–Teller polarons in the phase-separated (La1−yPry)0.7Ca0.3MnO3, electron-rich Jahn–Teller-distorted surface or “dead” layer in La0.7Sr0.3MnO3, electric-field-induced healing of “dead” layer as an origin of resistance switching effect, and high-TC ferromagnetic emerging phase at the SrMnO3/LaMnO3 interface in superlattices. These 2D polaronic phases with short-range electron, spin, and lattice reconstructions could be extremely sensitive to external fields, thus, providing a rational explanation of colossal effects in perovskite manganites.

Layer-by-Layer Resistive Switching: Multistate Functionality due to Electric-Field-Induced Healing of Dead Layers

Materials exhibiting reversible resistive switching in electrical fields are highly demanded for functional elements in oxide electronics. In particular, multilevel switching effects allow for advanced applications like neuromorphic circuits. Here, we report a structurally driven switching mechanism involving the so-called "dead" layers of perovskite manganite surfaces. Forming a tunnel barrier whose thickness can be changed in monolayer steps by electrical fields, the switching effect exhibits well-defined and robust resistive states.

Electrostatic potential and valence modulation in La0.7Sr0.3MnO3 thin films

The Mn valence in thin film La_0.7Sr_0.3MnO₃ was studied as a function of film thickness in the range of 1–16 unit cells with a combination of non-destructive bulk and surface sensitive X-ray absorption spectroscopy techniques. Using a layer-by-layer valence model, it was found that while the bulk averaged valence hovers around its expected value of 3.3, a significant deviation occurs within several unit cells of the surface and interface. These results were supported by first principles calculations. The surface valence increases to up to Mn^3.7+, whereas the interface valence reduces down to Mn^2.5+. The change in valence from the expected bulk value is consistent with charge redistribution due to the polar discontinuity at the film-substrate interface. The comparison with theory employed here illustrates how this layer-by-layer valence evolves with film thickness and allows for a deeper understanding of the microscopic mechanisms at play in this effect. These results offer insight on how the two-dimensional electron gas is created in thin film oxide alloys and how the magnetic ordering is reduced with dimensionality.

Atomically Resolved Electronic States and Correlated Magnetic Order at Termination Engineered Complex Oxide Heterointerfaces

We map electronic states, band gaps, and interface-bound charges at termination-engineered BiFeO₃/La_0.7Sr_0.3MnO₃ interfaces using atomically resolved cross-sectional scanning tunneling microscopy. We identify a delicate interplay of different correlated physical effects and relate these to the ferroelectric and magnetic interface properties tuned by engineering the atomic layer stacking sequence at the interfaces. This study highlights the importance of a direct atomically resolved access to electronic interface states for understanding the intriguing interface properties in complex oxides.

Surface-Driven Magnetotransport in Perovskite Nanocrystals

Unique insights into magnetotransport in 20 nm ligand-free La_0.67 Sr_0.33 MnO₃ perovskite nanocrystals of nearly perfect crystalline quality reveal a chemically altered 0.8 nm thick surface layer that triggers exceptionally large magnetoresistance at low temperature, independently of the spin polarization of the ferromagnetic core. This discovery shows how the nanoscale impacts magnetotransport in a material widely spread as electrode in hybrid 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 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.

Macroscopic evidence of nanoscale resistive switching in La2/3Sr1/3MnO3 micro-fabricated bridges

In this work we report on a combined macro, micro and nanoscale investigation where electronic transport properties through La⅔Sr⅓MnO3 (LSMO) microfabricated bridges, in which nano-sized resistive states are induced by using a conducting scanning probe microscope (C-SPM), are analyzed. The strategy intentionally avoids the standard capacitor-like geometry, thus allowing the study of the electronic transport properties of the locally modified region, and approaches the integration of functional oxides in low dimensional devices while providing macroscopic evidence of nanoscale resistive switching (RS). The metallic and ferromagnetic LSMO is locally modified from its low resistance state (LRS) to a high resistance state (HRS) when a bias voltage is applied on its surface through the conducting tip, which acts as a mobile electrode. Starting from a metallic oxide the electroforming process is not required, thus avoiding one of the major drawbacks for the implementation of memory devices based on RS phenomena. The application of a bias voltage generates an electric field that promotes charge depletion, leading to a strong increase of the resistance, i.e. to the HRS. This effect is not only confined to the outermost surface layer, its spatial extension and final HRS condition can be modulated by the magnitude and duration of the potential applied, opening the door to the implementation of multilevel devices. In addition, the half-metallic character, i.e. total spin polarization, of LSMO might allow the implementation of memory elements and active spintronic devices in the very same material. The stability of the HRS and LRS as a function of temperature, magnetic field and compliance current is also analyzed, allowing the characterization of the nature of the switching process and the active material.