Pavlo Zubko

Exchange bias in LaNiO3-LaMnO3 superlattices.

The wide spectrum of exotic properties exhibited by transition-metal oxides stems from the complex competition between several quantum interactions. The capacity to select the emergence of specific phases at will is nowadays extensively recognized as key for the design of diverse new devices with tailored functionalities. In this context, interface engineering in complex oxide heterostructures has developed into a flourishing field, enabling not only further tuning of the exceptional properties of these materials, but also giving access to hidden phases and emergent physical phenomena. Here we demonstrate how interfacial interactions can induce a complex magnetic structure in a non-magnetic material. We specifically show that exchange bias can unexpectedly emerge in heterostructures consisting of paramagnetic LaNiO3 (LNO) and ferromagnetic LaMnO3 (LMO). The observation of exchange bias in (111)-oriented LNO-LMO superlattices, manifested as a shift of the magnetization-field loop, not only implies the development of interface-induced magnetism in the paramagnetic LNO layers, but also provides us with a very subtle tool for probing the interfacial coupling between the LNO and LMO layers. First-principles calculations indicate that this interfacial interaction may give rise to an unusual spin order, resembling a spin-density wave, within the LNO layers.

Electric-Field Control of the Metal-Insulator Transition in Ultrathin NdNiO3 Films

Field-effect transistors (FETs) are ubiquitous in our everyday life. Applying the fi eld-effect technique to new materials can lead not only to a modulation of their conductivity, but also to electrostatically driven phase transitions. [ 1–3 ] This method is particularly appealing for complex oxides, which exhibit a wide range of functional properties including metal-insulator (MI) transitions, colossal magnetoresistance and highT c superconductivity, all very sensitive to the level of electronic doping. The electric fi eld-effect approach seems thus to be a natural tool to try controlling and modifying reversibly the ground states of these materials. However, the required changes in carrier densities, which typically exceed 10 14 cm − 2 , [ 1 , 4 ] are diffi cult to induce with standard dielectrics. Recently, electric double layer transistors (EDLT), in which an ionic liquid or an electrolyte is used as a gate dielectric, have been the subject of intense research and rapidly increasing interest owing to the considerable amount of charge which can be provided by this technique. [ 5–7 ]

Metal-insulator transition in ultrathin LaNiO3 films.

Transport in ultrathin films of LaNiO(3) evolves from a metallic to a strongly localized character as the films thickness is reduced and the sheet resistance reaches a value close to h/e(2), the quantum of resistance in two dimensions. In the intermediate regime, quantum corrections to the Drude low-temperature conductivity are observed; they are accurately described by weak localization theory. Remarkably, the negative magnetoresistance in this regime is isotropic, which points to magnetic scattering associated with the proximity of the system to either a spin-glass state or the charge ordered antiferromagnetic state observed in other rare earth nickelates.

Electric-field tuning of the metal-insulator transition in ultrathin films of LaNiO3

Epitaxial ultrathin films of the metallic perovskite LaNiO3 were grown on (001) SrTiO3 substrates using off-axis rf magnetron sputtering. The film structure was characterized and their electrical properties investigated. Films thinner than 8 unit cells display a metal-insulator transition at a thickness dependent characteristic temperature. Hall measurements revealed p-type conduction, which was confirmed by electric field-effect experiments. Large changes in the transport properties and the metal-insulator transition temperature were observed for the thinnest LaNiO3 films as the carrier density was electrostatically tuned.

Ultrafast Strain Engineering in Complex Oxide Heterostructures

We report on ultrafast optical experiments in which femtosecond midinfrared radiation is used to excite the lattice of complex oxide heterostructures. By tuning the excitation energy to a vibrational mode of the substrate, a long-lived five-order-of-magnitude increase of the electrical conductivity of NdNiO(3) epitaxial thin films is observed as a structural distortion propagates across the interface. Vibrational excitation, extended here to a wide class of heterostructures and interfaces, may be conducive to new strategies for electronic phase control at THz repetition rates.

Electrostatic Coupling and Local Structural Distortions at Interfaces in Ferroelectric/Paraelectric Superlattices

The performance of ferroelectric devices is intimately entwined with the structure and dynamics of ferroelectric domains. In ultrathin ferroelectrics, ordered nanodomains arise naturally in response to the presence of a depolarizing field and give rise to highly inhomogeneous polarization and structural profiles. Ferroelectric superlattices offer a unique way of engineering the desired nanodomain structure by modifying the strength of the electrostatic interactions between different ferroelectric layers. Through a combination of X-ray diffraction, transmission electron microscopy, and first-principles calculations, the electrostatic coupling between ferroelectric layers is studied, revealing the existence of interfacial layers of reduced tetragonality attributed to inhomogeneous strain and polarization profiles associated with the domain structure.

Negative capacitance in multidomain ferroelectric superlattices

The stability of spontaneous electrical polarization in ferroelectrics is fundamental to many of their current applications, which range from the simple electric cigarette lighter to non-volatile random access memories. Research on nanoscale ferroelectrics reveals that their behaviour is profoundly different from that in bulk ferroelectrics, which could lead to new phenomena with potential for future devices. As ferroelectrics become thinner, maintaining a stable polarization becomes increasingly challenging. On the other hand, intentionally destabilizing this polarization can cause the effective electric permittivity of a ferroelectric to become negative, enabling it to behave as a negative capacitance when integrated in a heterostructure. Negative capacitance has been proposed as a way of overcoming fundamental limitations on the power consumption of field-effect transistors. However, experimental demonstrations of this phenomenon remain contentious. The prevalent interpretations based on homogeneous polarization models are difficult to reconcile with the expected strong tendency for domain formation, but the effect of domains on negative capacitance has received little attention. Here we report negative capacitance in a model system of multidomain ferroelectric–dielectric superlattices across a wide range of temperatures, in both the ferroelectric and paraelectric phases. Using a phenomenological model, we show that domain-wall motion not only gives rise to negative permittivity, but can also enhance, rather than limit, its temperature range. Our first-principles-based atomistic simulations provide detailed microscopic insight into the origin of this phenomenon, identifying the dominant contribution of near-interface layers and paving the way for its future exploitation.

Spatially resolved ultrafast magnetic dynamics initiated at a complex oxide heterointerface

Static strain in complex oxide heterostructures has been extensively used to engineer electronic and magnetic properties at equilibrium. In the same spirit, deformations of the crystal lattice with light may be used to achieve functional control across heterointerfaces dynamically. Here, by exciting large-amplitude infrared-active vibrations in a LaAlO3 substrate we induce magnetic order melting in a NdNiO3 film across a heterointerface. Femtosecond resonant soft X-ray diffraction is used to determine the spatiotemporal evolution of the magnetic disordering. We observe a magnetic melt front that propagates from the substrate interface into the film, at a speed that suggests electronically driven motion. Light control and ultrafast phase front propagation at heterointerfaces may lead to new opportunities in optomagnetism, for example by driving domain wall motion to transport information across suitably designed devices.

Tuning of the Depolarization Field and Nanodomain Structure in Ferroelectric Thin Films

The screening efficiency of a metal-ferroelectric interface plays a critical role in determining the polarization stability and hence the functional properties of ferroelectric thin films. Imperfect screening leads to strong depolarization fields that reduce the spontaneous polarization or drive the formation of ferroelectric domains. We demonstrate that by modifying the screening at the metal-ferroelectric interface through insertion of ultrathin dielectric spacers, the strength of the depolarization field can be tuned and thus used to control the formation of nanoscale domains. Using piezoresponse force microscopy, we follow the evolution of the domain configurations as well as polarization stability as a function of depolarization field strength.

Interfacial Control of Magnetic Properties at LaMnO3/LaNiO3 Interfaces.

The functional properties of oxide heterostructures ultimately rely on how the electronic and structural mismatches occurring at interfaces are accommodated by the chosen materials combination. We discuss here LaMnO3/LaNiO3 heterostructures, which display an intrinsic interface structural asymmetry depending on the growth sequence. Using a variety of synchrotron-based techniques, we show that the degree of intermixing at the monolayer scale allows interface-driven properties such as charge transfer and the induced magnetic moment in the nickelate layer to be controlled. Further, our results demonstrate that the magnetic state of strained LaMnO3 thin films dramatically depends on interface reconstructions.