Heterostructure engineering provides a versatile route for tailoring emergent functionalities that are often difficult to realize in single-phase materials. In this work, the focus is on superlattices composed of the insulating and ferromagnetic double perovskites La2NiMnO6 and Sm2NiMnO6, which constitute an intriguing model system. These layered structures are predicted to feature unequal antipolar displacements of the La and Sm ions; when combined with odd periodicity stacking, this structural motif is expected to give rise to polar behavior. The respective superlattices are grown with atomic precision and display robust ferromagnetism, as confirmed by in-house magnetometry and synchrotron measurements. Scanning transmission electron microscopy combined with first-principles calculations confirms the presence of the predicted unequal antipolar displacements, paving the way for the realization of hybrid improper ferroelectricity in such oxide heterostructures.

The properties of functional oxide heterostructures are strongly influenced by the physics governing their interfaces. Modern deposition techniques allow us to accurately engineer interface physics through the growth of atomically precise heterostructures. This enables minute control over the electronic, magnetic, and structural characteristics, which in turn allows for the tuning of the properties of the heterostructures and can even lead to the emergence of properties not present in the individual heterostructure components. Here, we investigate the magnetic properties of tailor-made superlattices employing the ferromagnetic and insulating double perovskites RE2NiMnO6 (RE = La, Nd), featuring distinct Curie temperatures. Adjusting the superlattice periodicity at the unit cell level allows us to engineer the magnetic phase diagram. Large periodicity superlattices conserve the individual para- to ferromagnetic transitions of the La2NiMnO6 and Nd2NiMnO6 parent compounds. As the superlattice periodicity is reduced, the Curie temperatures of the superlattice constituents converge and, finally, collapse into one single transition for the lowest period samples, illustrating that low-periodicity superlattices behave as a unique material. This is a consequence of the magnetic order parameter propagating across the superlattice interfaces, as supported by a minimal Landau theory model. Further, we find that the Nd-Ni-Mn exchange interaction can be enhanced by the superlattice interfaces. This leads to a field-induced reversal of the Nd magnetic moments, as confirmed by synchrotron X-ray magnetic circular dichroism measurements and supported by first-principles calculations. Our work demonstrates how superlattice engineering can be employed to fine-tune the magnetic properties in oxide heterostructures and broadens our understanding of magnetic interfacial effects.

We report on the effect of magnetic impurities on the microscopic superconducting (SC) properties of the kagome-lattice superconductor La(Ru1-xFex)3Si2 using muon spin relaxation/rotation. A strong suppression of the superconducting critical temperature Tc, the SC volume fraction, and the superfluid density was observed. We further find a correlation between the superfluid density and Tc which is considered a hallmark feature of unconventional superconductivity. Most remarkably, measurements of the temperature-dependent magnetic penetration depth lambda reveal a change in the low-temperature behavior from exponential saturation to a linear increase, which indicates that Fe doping introduces nodes in the superconducting gap structure at concentrations as low as x = 0.015. Our results point to a rare example of unconventional superconductivity in the correlated kagome lattice and accessible tunability of the superconducting gap structure, offering new insights into the microscopic mechanisms involved in superconducting order.

Abstract The discovery of superconductivity in doped infinite‐layer nickelate thin films has brought increased attention to the behavior of the doped perovskite phase. Despite this interest, the majority of existing studies pertain to hole‐doped perovskite rare‐earth nickelate thin films, while most electron‐doping studies have been performed on bulk materials so far. To tackle this imbalance, a detailed study that addresses doping of NdNiO3 thin films using A‐site substitution is presented, using Pb as a dopant and taking advantage of its valence‐skipping nature. Through a combination of complementary techniques including X‐ray diffraction, transport measurements, X‐ray absorption spectroscopy, electron energy‐loss spectroscopy and scanning transmission electron microscopy, the valence of Pb in the Nd1−xPbxNiO3 structure is confirmed to be 4+, and the behavior of the doped thin films is found to be controlled by a competition between carrier injection and structural distortions, which respectively reduce and increase the metal‐to‐insulator transition temperature. This work provides a systematic study of electron doping in NdNiO3, demonstrating that A‐site substitution with Pb is an appropriate method for such doping in perovskite rare‐earth nickelate systems.

We investigate ferromagnetic and insulating thin films of the B-site ordered double perovskite Nd2NiMnO6 (NNMO) grown by radio frequency off-axis magnetron sputtering. The films grow epitaxially strained on a selection of substrates and display a strain-independent and bulklike T-C of 200 K at a thickness of 30 unit cells. We explore the thickness dependence of the NNMO/SrTiO3(001) system and find ferromagnetism down to ultralow thicknesses of only 3 unit cells (similar to 1.2 nm). Below 10 unit cells, the magnetic properties deteriorate due to an interfacial charge transfer caused by the polar discontinuity at the NNMO/SrTiO3 interface. A detailed xray magnetic circular dichroism study allows us to separate the magnetic components into a robust ferromagnetic Ni/Mn sublattice and a paramagnetic Nd sublattice.

We report on the effect of magnetic impurities on the microscopic superconducting (SC) properties of the kagome-lattice superconductor La(Ru$_{1-x}$Fe$_{x}$)$_{3}$Si$_{2}$ using muon spin relaxation/rotation. A strong suppression of the superconducting critical temperature $T_{\rm c}$, the SC volume fraction, and the superfluid density was observed. We further find a correlation between the superfluid density and $T_{\rm c}$ which is considered a hallmark feature of unconventional superconductivity. Most remarkably, measurements of the temperature-dependent magnetic penetration depth ${\lambda}$ reveal a change in the low-temperature behavior from exponential saturation to a linear increase, which indicates that Fe doping introduces nodes in the superconducting gap structure at concentrations as low as $x=$~0.015. Our results point to a rare example of unconventional superconductivity in the correlated kagome lattice and accessible tunability of the superconducting gap structure, offering new insights into the microscopic mechanisms involved in superconducting order.

We report muon spin rotation (µSR) experiments on the microscopic properties of superconductivity and magnetism in the kagome superconductor CeRu2withTc≃5 K. From the measurements of the temperature-dependent magnetic penetration depthλ, the superconducting order parameter exhibits nodeless pairing, which fits best to an anisotropics-wave gap symmetry. We further show that theTc/λ-2ratio is comparable to that of unconventional superconductors. Furthermore, the powerful combination of zero-field (ZF)-µSR and high-fieldµSR has been used to uncover magnetic responses across three characteristic temperatures, identified asT1∗≃110 K,T2∗≃65 K, andT3∗≃40 K. Our experiments classify CeRu2as an exceedingly rare nodeless magnetic kagome superconductor.

Charge-transfer phenomena at heterointerfaces are a promising pathway to engineer functionalities absent in bulk materials but can also lead to degraded properties in ultrathin films. Mitigating such undesired effects with an interlayer reshapes the interface architecture, restricting its operability. Therefore, developing less-invasive methods to control charge transfer will be beneficial. Here, an appropriate top-interface design allows for remote manipulation of the charge configuration of the buried interface and concurrent restoration of the ferromagnetic trait of the whole film. Double-perovskite insulating ferromagnetic La2 NiMnO6 (LNMO) thin films grown on perovskite oxide substrates are investigated as a model system. An oxygen-vacancy-assisted electronic reconstruction takes place initially at the LNMO polar interfaces. As a result, the magnetic properties of 2-5 unit cell LNMO films are affected beyond dimensionality effects. The introduction of a top electron-acceptor layer redistributes the electron excess and restores the ferromagnetic properties of the ultrathin LNMO films. Such a strategy can be extended to other interfaces and provides an advanced approach to fine-tune the electronic features of complex multilayered heterostructures.

Tuning of electronic density-of-states singularities is a common route to unconventional metal physics. Conceptually, van Hove singularities are realized only in clean two-dimensional systems. Little attention has therefore been given to the disordered (dirty) limit. Here, we provide a magnetotransport study of the dirty metamagnetic system calcium-doped strontium ruthenate. Fermi liquid properties persist across the metamagnetic transition, but with an unusually strong variation of the Kadowaki-Woods ratio. This is revealed by a strong decoupling of inelastic electron scattering and electronic mass inferred from density-of-state probes. We discuss this Fermi liquid behavior in terms of a magnetic field tunable van Hove singularity in the presence of disorder. More generally, we show how dimensionality and disorder control the fate of transport properties across metamagnetic transitions. Strongly correlated materials can exhibit deviations from Fermi-liquid behavior partly due to anomalies in the density of states at the Fermi level, such as van Hove singularities. Here, the authors investigate the unusual Fermi liquid behavior of calcium-doped strontium ruthenate and find an unusual variation of the Kadowaki-Woods ratio which may originate from disorder.

The field of oxide spintronics can strongly benefit from the establishment of ferromagnetic insulators with near room-temperature Curie temperature. Here, we investigate the structural, electronic, and magnetic properties of epitaxially strained thin films of the double perovskite La2NiMnO6 (LNMO) grown by off-axis radio-frequency magnetron sputtering. We find that the films retain insulating behavior and a bulk-like Curie temperature in the order of 280 K independently of the epitaxial strain conditions. These results suggest a prospective implementation of LNMO films in multi-layer device architectures where a high-temperature ferromagnetic insulating state is a prerequisite.

Memristive devices are among the most prominent candidates for future computer memory storage and neuromorphic computing. Though promising, the major hurdle for their industrial fabrication is their device-to-device and cycle-to-cycle variability. These occur due to the random nature of nanoionic conductive filaments, whose rupture and formation govern device operation. Changes in filament location, shape, and chemical composition cause cycle-to-cycle variability. This challenge is tackled by spatially confining conductive filaments with Ni nanoparticles. Ni nanoparticles are integrated on the bottom La(0.2)Sr(0.7)Ti(0.9)Ni(0.1)O(3-)(delta)electrode by an exsolution method, in which, at high temperatures under reducing conditions, Ni cations migrate to the perovskite surface, generating metallic nanoparticles. This fabrication method offers fine control over particle size and density and ensures strong particle anchorage in the bottom electrode, preventing movement and agglomeration. In devices based on amorphous SrTiO3, it is demonstrated that as the exsolved Ni nanoparticle diameter increases up to approximate to 50 nm, the ratio between the ON and OFF resistance states increases from single units to 180 and the variability of the low resistance state reaches values below 5%. Exsolution is applied for the first time to engineer solid-solid interfaces extending its realm of application to electronic devices.

Memristor devices are promising potential hardware components for computer memory and neural network computing. Advances over the last years on understanding and implementing memristor technology had positioned them as a major candidate to overcome current bottlenecks in current electronic-based transistors in terms of downscaling capabilities and energy consumption. In particular, current challenges preventing a widespread implementation of oxygen-based memristors in today’s integrated circuits include the need to address cycle-to-cycle and device-to-device variabilities. These occur due to the random nature of nanoionic conductive filaments, whose rupture and formation govern the device operation. Changes in the filament location, shape and chemical composition during resistance switching are responsible for the cycle-to-cycle variability. Here we tackle the variability challenge by employing a double stabilization mechanism, where we spatially confine the conductive filaments with Ni nanoparticles that are capable of oxygen exchange between the set and reset cycles. Ni nanoparticles are fabricated on the bottom La0.2Sr0.7Ti0.9Ni0.1O3-δ thin film electrode by an exsolution process, during which Ni dopant metal cations exsolve from the perovskite backbone at high temperatures and reducing conditions. This method of nanoparticle fabrication offers great control over particle size and density tuned by the exsolution conditions. Particles with average diameters ranging from 10 to 60 nm were fabricated at 900 and 1000°C under a 5% H2 atmosphere. Exsolution also insures a good particle anchorage into the bottom electrode preventing particle agglomeration and movement. SrTiO3 is deposited on top of the Ni-decorated bottom electrode as the switching material followed by and the device is toped with a platinum top electrode. Cyclic voltammetry shows, that the ratio between the ON and OFF resistance states increases from single units to 180 as the particle diameter increases. Additionally, the variability of the low resistance state decreases below 5% as the average particle diameter increases to 60 nm. These findings offer a straightforward strategy on improving cycle-to-cycle variability in memristive devices, which is a major challenge for commercial application of the technology.

To incorporate LNMO films in realistic spintronic devices, it is important to retain such magnetic properties down to low thickness (few unit cells). We recently verified that our LNMO films are indeed ferromagnetic down to 1nm, though they exhibit both reduced magnetization and Curie temperature compared to thicker films (Fig. 2). We associate this behavior to the presence of Mn at the interface with the SrTiO3 substrate.

Oxide-based valence-change memristors are promising nonvolatile memories for future electronics that operate on valence-change reactions to modulate their electrical resistance. The memristance is associated with the movement of oxygen ionic carriers through oxygen vacancies at high electric field strength via structural defect modifications that are still poorly understood. This study employs a Ce1-xGdxO2-y solid solution model to probe the role of oxygen vacancies either set as "free" or as "immobile and clustered" for the resistive switching performance. The experiments, together with the defect chemical model, show that when the vacancies are set as "free", a maximum in memristance is found for 20 mol % of GdO1.5 doping, which clearly coincides with the maximum in ionic conductivity. In contrast, for higher gadolinia concentration, the oxide exhibits only minor memristance, which originates from the decrease in structural symmetry, leading to the formation of "immobile" oxygen defect clusters, thereby reducing the density of mobile ionic carriers available for resistive switching. The research demonstrates guidelines for engineering of the oxide's solid solution series to set the configuration of its oxygen vacancy defects and their mobility to tune the resistive switching for nonvolatile memory and logic applications.

The miniaturization of standard silicon-based memory technologies reaches physical limits in terms of size and power dissipation. Novel memory and computing architectures based on resistive switches are promising for future electronics beyond Von Neumann computing architecture. In oxide-based resistive switches, the memristance is associated with the movement of ionic carriers through oxygen vacancies in an oxide film at high electric field strength via structural defect modifications that are still poorly understood. Here, we newly employ an oxide solid solution by systematic extrinsic doping to probe the role of oxygen vacancy configuration either set as free, immobile or clustered to directly answer the fundamental question how the mobility of oxygen vacancies affects the switching characteristics of the devices at high electric fields. Till date strategies to tailor systematically oxygen vacancy concentration and configuration for oxide-based resistive switches are still to be explored. For this, we design resistive switching devices of Ce1-xGdxO2-y as a novel resistive switching oxide constituent. The active oxide layer of the devices, consisting of a 500 nm thick CeO2 film with Gd3+ doping concentrations of 3 to 30 mol% deposited by Pulsed Laser Deposition (PLD), is sandwiched between micro-fabricated Pt-crosspoint electrodes. Cyclovoltammetry measurements show a strong dependence of the ROFF/RON -resistance ratio on the effective gadolinia concentration per bit. At low gadolinia doping concentration < 10 mol%, a purely capacitive behaviour prevails. By increasing up to 20 mol% a significant hystereis in the current-voltage profile is measured with resistance ROFF/RON -ratios up to 100. Turning to high gadolinia concentrations from 20 to 30 mol%, decreases the hysteretic current-voltage profile. To analyse the structural changes related to the oxygen vacancy concentration and configuration, changes opposed by the extrinsic gadolinia dopant in the ceria lattice of the resistive switching devices, we turn to Raman Spectroscopy. From these results we conclude that for moderate gadolinium concentrations up to 20 mol% the concentration of mobile oxygen vacancies steadily increases and is available as "free" carriers for the resistive switches; in this state the oxide is in a fluorite structure type. Further increase in doping, for which we see a reduction of switching characteristics, leads to the formation of oxygen vacancy clusters in the structure, which are simply as a defect configuration "immobile". For this state we confirm a phase change to a more bixbyite structure. Unequivocally the experiments together with the defect chemical model, show that when the vacancies are set as "free", a maximum in memristance is found for 20 mol% Gd3+ doping, which clearly coincides with the maximum in ionic conductivity. In contrast, for higher gadolinia concentration the oxide exhibits only minor memristance, which originates from the decrease in structural symmetry leading to the formation of "immobile" oxygen defect clusters, thereby reducing the density of mobile ionic carriers available for resistive switching. A clear correlation between memristance and oxygen vacancies’ mobility was seen in this defect chemical model experiment on gadolinium doped ceria device structures. Extrinsic doping allows to tune the configuration, concentration and mobility of the oxygen vacancies in a much more systematic and controlled manner than what can be achieved through oxygen-deficient deposition or annealing steps as being the state-of-the-art in resistive switches. The research demonstrates guidelines for engineering of the oxide`s solid solution series to set the configuration of its oxygen vacancy defects and their mobility to tune the memristance for future nonvolatile memory and logic applications.