Raoul Scherwitzl

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.

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.

Ground-state oxygen holes and the metal–insulator transition in the negative charge-transfer rare-earth nickelates

The metal–insulator transition and the intriguing physical properties of rare-earth perovskite nickelates have attracted considerable attention in recent years. Nonetheless, a complete understanding of these materials remains elusive. Here we combine X-ray absorption and resonant inelastic X-ray scattering (RIXS) spectroscopies to resolve important aspects of the complex electronic structure of rare-earth nickelates, taking NdNiO3 thin film as representative example. The unusual coexistence of bound and continuum excitations observed in the RIXS spectra provides strong evidence for abundant oxygen holes in the ground state of these materials. Using cluster calculations and Anderson impurity model interpretation, we show that distinct spectral signatures arise from a Ni 3d8 configuration along with holes in the oxygen 2p valence band, confirming suggestions that these materials do not obey a conventional positive charge-transfer picture, but instead exhibit a negative charge-transfer energy in line with recent models interpreting the metal–insulator transition in terms of bond disproportionation.

Electronic transitions in strained SmNiO3 thin films

Nickelates are known for their metal to insulator transition (MIT) and an unusual magnetic ordering, occurring at T = TNeel. Here, we investigate thin films of SmNiO3 subjected to different levels of epitaxial strain. We find that the original bulk behavior (TNeel < TMI) is strongly affected by applying compressive strain to the films. For small compressive strains, a regime where TNeel = TMI is achieved, the paramagnetic insulating phase characteristic of the bulk compound is suppressed and the MIT becomes 1st order. Further increasing the in-plane compression of the SmNiO3 lattice leads to the stabilization of a single metallic paramagnetic phase.

Tailoring the electronic transitions of NdNiO3 films through (111)pc oriented interfaces

Bulk NdNiO3 and thin films grown along the pseudocubic (001)pc axis display a 1st order metal to insulator transition (MIT) together with a Neel transition at T = 200 K. Here, we show that for NdNiO3 films deposited on (111)pc NdGaO3, the MIT occurs at T = 335 K and the Neel transition at T = 230 K. By comparing transport and magnetic properties of layers grown on substrates with different symmetries and lattice parameters, we demonstrate a particularly large tuning when the epitaxy is realized on (111)pc surfaces. We attribute this effect to the specific lattice matching conditions imposed along this direction when using orthorhombic substrates.

Unusual temperature dependence of the spectral weight near the Fermi level of NdNiO 3 thin films

We investigate the behavior of the spectral weight near the Fermi level of NdNiO