Fundamentals of oxide heterostructures

Resume : Transition metal oxides such as LiCoO2 are used in Li-ion batteries as active positive electrode (cathode) materials. Ion transfer and reactivity of cathode materials have increasingly come into focus due to the development of high voltage- and solid state devices for future energy storage. Related charge transfer properties depend on double layer properties and presence of surface layers. For electrodes in a semiconducting state, diffuse double layers (space charge layers) are encountered, expected to affect the composition in the subsurface region of the electrode material. It is known that over-delithiation of LiCoO2 electrodes leads to the formation of Co-oxide at the surface, i.e. the presence of Co2+. In this contribution, we present data obtained by photoelectron spectroscopy (XPS) on fully lithiated LiCoO2 thin film electrode surfaces after gas phase adsorption (e.g. DEC, H2O) and thin film deposition (LiPON). Next to substrate induced changes in the condensed layer, we observe different degrees of space charge layer formation and presence of Co2+ species in the subsurface region of the substrate, depending on type of interface. Causes and interrelation of the observed phenomena are discussed.

Resume : The search for suitable cathode materials for intermediate temperature solid oxide fuel cells has led to the study of complex oxide perovskite materials such as LaCoO3 doped with Sr and Fe. LaCoO3 has a complicated magnetic structure, with several transitions occurring as the temperature is increased. Building a coherent model of this material has proved challeging for theorists. We survey a range of computational modelling approaches to study the electronic and magnetic structure of LaCoO3 to determine the optimum method. We study how defects affect the compound's properties, showing how the magnetic structure is highly sensitive to the local defect structure. Our results are in good agreement with relevant experiment.

Resume : Spintronics constantly seeks to expand its borders to encompass new materials and devices that offers tunability for different device functionalities. Complex oxides are a new addition where new physical phenomena and novel functionalities emerge due to the interplay between strain, charge-transfer, orbital-reconstruction etc. Devices based on oxide semiconductors such as Nb doped SrTiO_3, allow additional functionalities derived from temperature and electric field dependence of their dielectric permittivity, allowing manipulation by electric and magnetic field and tuning by spin-orbit relativistic effects. In this talk, I will discuss emerging functionalities that have been observed at oxide interfaces a) between Nb doped SrTiO_3 and SrRuO_3- a pervoskite metal, where ferromagnetism and metallicity evolve with decreasing temperature and increasing thickness, influencing electronic transport at the interface b) between carefully tuned spin injection contacts and Nb doped SrTiO_3, exhibiting an unusal bias dependence of the spin lifetime and features that demonstrate a strong influence of the interface electric field on spin accumulation and charge transport, unlike that found in conventional semiconductors and c) influence of dielectric screening on charge and spin transport when complex oxides are integrated with graphene. Our findings provides new pathways for the field of oxide electronics and spintronics.

Resume : Half-doped manganites La(1-x)A(x)MnO3 (where A is a divalent ion and x=0.5) recently attracted a renewed attention because their ground state (ferromagnetic or antiferromagnetic, metallic or insulator) can be easily modified by engineering the bandwidth or modifying the carrier density. The aim of this study is to explore how epitaxial strain affects the magnetic and electric ground state in films of La0.5Sr0.5MnO3 (LSMO-05) by using 55Mn NMR resonance as probe to local magnetism and charge localization. Various oxide substrates with a different lattice mismatch to LSMO-05 were used in order to induce different strain effect (tensile or compressive strain). 55Mn NMR spin echo experiments have been carried out at 4.2 K on films with thickness of 20 and 35 nm. The NMR spectra consist of the Mn4+ line and the resonance line corresponding to Mn in a mixed valence state revealing the presence of hole localized and hole itinerant states, respectively. The analysis of NMR data leads to the following conclusions: (1) Strain induces a separation into antiferromagnetic and ferromagnetic phases rather than reducing an overall magnetization of a film, which could be suggested by the macroscopic magnetization measurements. (2) A degree of phase separation depends on the film thickness and on magnetic history of the film. (3) 55Mn NMR frequency (55Mn hyperfine field) observed at manganese ions participating in the double exchange interaction is surprisingly high compared to that expected for this composition (x=0.5) and similar to the frequency observed for the optimally doped composition (x=0.3). This observation strongly suggests that besides phase separation also the electronic charge redistribution takes place between the ferromagnetic and the antiferromagnetic phases.

Resume : Using ultrafast resonant soft X-ray diffraction, we demonstrate optical melting of antiferromagnetic order in the correlated electron insulator NdNiO3. Time-dependent analysis of the resonant spectra allows us to follow the temporal evolution of the charge imbalance between adjacent Ni sites. A direct correlation between the melting of magnetic order and charge rebalancing is found. Furthermore, we report on experiments in which femtosecond mid-infrared radiation is used to excite the lattice of complex oxide heterostructures. By tuning the excitation energy to a vibrational mode of the substrate, magnetic melting is observed in an epitaxial NdNiO3 thin film. The propagation of a melting front across the interface is measured by femtosecond x-ray scattering at a free-electron laser. 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.

Resume : Magnetic semiconductors entwine two of the most successful concepts in both fundamental physics and industrial applications where ferromagnetic materials have played an undismissable role. Recently antiferromagnets have been proposed as alternative material systems[1,2]. Antiferromagnetic spintronics have been demonstrated by the fabrication of tunnel devices[3-5], and atomic-size proof-of concepts. Here we survey the progress in antiferromagnetic spintronics and we present the control of the electrical conductivity of an antiferromagnetic semiconductor (Sr2IrO4) by manipulating the magnetic state of a contiguous ferromagnetic layer acting. We present an oxide-based fully epitaxial heterostructure, its structural characterization and the electrical measurements showing a direct link between state of the ferromagnetic gate and ohmic resistance of the semiconductor, even displaying distinct remnant resistance states[6]. We will also show that distinct remnant states can also been obtained at room temperature without requirement of the spin-gating element, promising potential applicability[7]. [1] S. Shick et al., Phys. Rev. B 81, 212409 (2010) [2] T. Jungwirth et al., Phys. Rev. B 83, 035321 (2011) [3] B.G. Park et al., Nat. Mat. 10, 347-351 (2011) [4] X. Marti et al., Phys. Rev. Lett. 108, 017201 (2012) [5] D. Petti, I. Fina, et al., Appl. Phys. Lett. 102, 192404 (2013). [6] I. Fina, et al., Nat Commun 5, 4671 (2014). [7] X. Marti, I. Fina, et al., Nat. Mat. 13, 367 (2014)

Resume : The coupling of spin and valley physics is nowadays regarded as a promising route toward next-generation spintronic and valleytronic devices. In the aim of engineering functional properties for valleytronic applications, we focus on the (111) ferroelectric oxide-based heterostructure BiAlO3/BiIrO3. There we show, by means of first-principles simulations, that the complex interplay among trigonal crystal field, layer degrees of freedom and spin-orbit coupling (SOC) mediates a strong spin-valley coupling. Furthermore, we show that ferroelectricity provides a non-volatile handle to manipulate and switch the emerging valley-contrasting spin polarization. Our theoretical findings therefore suggest that the realization of spin-valley physics in transition-metal-oxides is indeed possible, allowing in principle for larger effects (due to atomic SOC, which is typically large in 4d or 5d transition-metal ions), increased tunability and for the integration of additional functionalities, such as ferroelectricity, which could be exploited in advanced next-generation electronic devices.

Resume : Nanoparticles of transition-metal spinel ferrites exhibit fundamental magnetic properties such as superparamagnetism, spin-canting and magnetic anisotropy. These properties lead to a wide range of applications including biomedical imaging, hyperthermia cancer therapy, magneto-optical or magnetic recording devices. The “hard magnetic” feature of cobalt ferrite (CoFe2O4) coupled to its great physical and chemical stability make the CoFe2O4 nanoparticles suitable for applications. In order to determine the spin and orbit magnetic moments, the magnetic couplings and the exchange magnetic anisotropies of the CoFe2O4 nanospinels, X-ray magnetism circular dichroism (XMCD) measurements were performed at Co and Fe L2,3 edges on the DEIMOS beamline at Synchrotron SOLEIL. CoFe2O4 nanoparticles are obtained from two different synthesis routes: co-precipitation process and polyol process synthesis pathways. From XMCD, the relation between the particle sizes and the chemical order was determined. The stoichiometry as well as the magnetic anisotropy of the CoFe2O4 nanospinels were measured from XMCD magnetization curves. All these provide information on the site symmetry of Co(II) and Fe(III), i.e. octahedral vs tetrahedral sites. The comparison between the magnetization curves measured at either Co or Fe edges sitting either on the tetrahedral sites or on the octahedral sites provides a clear picture of the complex magnetic structure of these highly anisotropic nanospinels.

Resume : Single magnetic domain nanoparticles are very promising for many advanced applications such as hyperthermia or magnetic storage media. Nevertheless, the miniaturization of devices which is correlated to the size reduction of nanoparticles usually results in the decrease of the magnetocrystalline anisotropy and in unblocked domains at room temperature, e.g. superparamagnetism. An alternative and very promising approach is heterostructures such as core-shell nanoparticles featured by exchange bias coupling between F(i)M and AFM phases. Although exchange bias has been well investigated during last years, the large panel of parameters which affect it still need to be investigated. We report on a systematic study which consists in the modulation of the shell structure and of the AFM/FiM interface in order to study their influence on the exchange bias coupling. Fe3-δO4@CoO core-shell nanoparticles have been synthesized by a one-pot seed-mediated growth method based on the thermal decomposition of metal complexes at high temperature.[1] The shell structure and AFM/FiM interface are demonstrated to be strongly modulated by the synthetic conditions.[2] While exchange bias coupling is strongly influenced by the shell thickness, the AFM/FiM interface were intermixing of Co and Fe atoms takes place also enhances the magnetic properties. Furthermore, the exchange bias coupling is also investigated as function of dipolar interactions between nanoaprticles which strength strongly affect the magnetic properties of nanoparticles.[3] [1] W. Baaziz et al., J. Phys. Chem. C, 2013, 117, 11436 [2] X. Liu et al, J. Amer. Chem. Soc., 2015, submitted [3] Y. Liu et al., to be submitted

Resume : This study reports the facile fabrication of 1D ZnO/α-Fe2O3 semiconductor nanoheterostructures (SNHs), and we investigate the strong interfacial interactions at the heterojunction, resulting in novel multifunctionality in the hybrid structure. ZnO-coated α-Fe2O3 nanowires (NWs) have been prepared by combining electrodeposition and wet chemical methods. Significant improvement in electrical conductivity, photoluminescence, and room temperature magnetic properties have been observed for the ZnO/α-Fe2O3 SNHs over the pristine α-Fe2O3 NWs because of the contribution of the ZnO nanolayer. The increase in electrical conductivity in ZnO/α-Fe2O3 SNHs is because of the increase in free electrons in the conduction band of the SNHs due to the formation of type-II n-n band configuration at the heterojunction. The SNHs are found to exhibit enhanced visible green photoluminescence along with the UV emission at room temperature. The band-gap emission of the α-Fe2O3 NWs coupled to the defect emissions of the ZnO in SNHs can be attributed to the profound enhancement of the visible green luminescence. Ferromagnetism of the SNHs is found to be increased nearly five times in magnitude over the primeval α-Fe2O3 NWs, which can be ascribed to the exchange coupling of the interfacial spin at ZnO/α-Fe2O3 interface, the surface spin of ZnO nanolayer, along with the structural defects like the cation vacancies (VZn) and the singly ionized oxygen vacancies (Vo•) present in SNHs.

Resume : Interfaces of oxide heterostructures can host novel quantum phases, with the high-mobility 2D electron system (2DES) in LaAlO3/SrTiO3 (LAO/STO) as prominent example. Despite extensive research the origin of the 2DES and its unusual properties are still a matter of intense debate. Photoelectron spectroscopy with soft (SX-ARPES) and hard (HAXPES) x-rays is a powerful method to provide detailed insights into the electronic structure of these hetero-interfaces. Here I will discuss our recent photoemission results on LAO/STO(100) and (111) heterostructures as well as on the related system -Al2O3/STO(100), which hosts a 2DES of even higher mobility. These data provide information on the orbital character of the 2DES carriers, on interfacial band alignment and bending, and even on k-resolved band dispersions and Fermi surface topology. Moreover, from resonant photoemission at the Ti L-edge we conclude on the coexistence of localized and itinerant Ti 3d electrons, which can be controlled by in-situ variation of the oxygen stoichiometry. By comparison to free STO surfaces it can be demonstrated that the interface 2DES in LAO/STO contains an intrinsic component, independent of O vacancies. Work in collaboration with J. Mannhart (MPI-FKF, Stuttgart), N. Pryds (TU Denmark), G. Rijnders (U Twente), S. Suga (U Osaka), M. Gorgoi (BESSY), W. Drube (DESY Photon Science), V.N. Strocov (Swiss Light Source), J.D. Denlinger (Advanced Light Source) and T.-L. Lee (Diamond Light Source).

Resume : Interface effects in epitaxial structures can lead to a significant improvement of the existing functional properties of the constituting materials or even to novel exciting phenomena. Here, instead of conventional homogeneous doping, we study the possibility of enhancing the hole concentration in lanthanum cuprate (to obtain high Tc-superconductivity) through the insertion of ad hoc interfaces. Such structures are achieved by substituting single atomic layers of LaO with SrO during the MBE growth of La2CuO4 (heterogeneous doping). Despite the insulating character of the La2CuO4 matrix, heterogenously doped La2CuO4 exhibits high-temperature superconductivity with Tc up to ≈40 K. Complementary experimental techniques, such as aberration corrected TEM (including EELS and EDX analyses), were used to determine the charge concentration profiles. These reveal the occurrence of two different situations: the forward interfaces (oriented toward the growing direction) exhibit a cationic redistribution resembling a homogeneous doping case, while the backward interfaces exhibit holes accumulation, which is decoupled from the presence of Sr. The contribution of the backward interfaces to the overall superconductivity is supported by further complementary tests. This study represents a novel approach in interface engineering by a nominal two-dimensional confinement of the dopant at the atomic-layer level and sheds light onto the complex phenomena occurring at oxide heterointerfaces.

Resume : We discuss the effect of ionic defects on the properties of the conducting interface between the two wide-band-gap insulators LaAlO3 (LAO) and SrTiO3 (STO). Using established thermodynamic models as well as growth and transport studies, we will give an overview about the decisive defect species dominating charge compensation and electron scattering in the space charge layer at the LAO/STO interface [1, 2]. While established defect chemistry models of the LAO/STO interface were so far constricted to local charge-neutrality [1] and thus neglected gradual defect concentration profiles, we included local space charges in a simple model considering the electro-chemical potential not only for electrons and electron holes (band bending), but also for all other charged defect species. Applying this electrostatic and thermodynamic model, we will show that the electrical potential confining the electron gas at the interface is expected to modify the equilibrium defect concentrations in the STO region adjacent to the interface. We find for the n-conducting interface that positively charged oxygen vacancies are depleted within the space charge region, while the negatively charged strontium vacancies accumulate. In agreement with established models, we find a change from dominant electronic charge compensation to dominant ionic charge compensation depending on ambient pO2. [1] F. Gunkel et al., Nanoscale 7 (3) (2015) [2] F. Gunkel et al., Appl. Phys. Lett. 100, 052103 (2011)

Resume : The new functionality at oxide interface has spurred research over the past years. Due to their distinct topology, interfaces with (111) crystallographic orientation promise to host even more exotic electronic states than the much studied (001) systems. Based on density functional theory calculations with an on-site Coulomb repulsion term, we explore the role of confinement, symmetry breaking, polarity mismatch and strain. The results illuminate a rich set of competing electronic states in polar (LaAlO3)_N/(SrTiO3)_M(111) [1] and non-polar (LaNiO3)_N/ (LaAlO3)_M(111) [2] superlattices, ranging from spin-polarized, Dirac-point Fermi surfaces protected by lattice symmetry to charge-ordered Mott or Peierls insulating phases. Analogous to the (001) counterparts [3,4], orbital reconstructions and metal-to-insulator transitions depend critically on the thickness of the quantum well N and in-plane strain, thus opening avenues to engineer properties at the nanoscale. Research supported by the DFG, SFB/TR80. [1] D. Doennig, W. E. Pickett, and R. Pentcheva, Phys. Rev. Lett. 111, 126804 (2013). [2] D. Doennig, W. E. Pickett, and R. Pentcheva, Phys. Rev. B 89, 121110 (R) (2014). [3] D. Doennig and R. Pentcheva, Sci. Rep 5 07907 (2015). [4] A. Blanca Romero and R. Pentcheva, Phys. Rev. B 84 195450 (2011).

Resume : Domain wall motion governs the speed of ferroelectric switching, thus being of strong funda-mental as well as practical interest. Nucleation and domain wall motion in thin film ferroelec-trics have been studied using piezoresponse force microscopy (PFM), revealing a decisive role of crystallographic defects and electrode interfaces as nucleation and pinning centers. On the other hand, elastic strain of thin epitaxial films was recently utilized to control the remanent polarization, stable domain patterns or even the ferroic phase. Based on the strong coupling of strain and ferroelectricity, one expects substantial influence of strain on domain dynamics, which was hard to investigate experimentally because of the difficulty to vary the elastic strain in a single sample with fixed microstructure. We contribute in filling this gap by introducing a reversibly strainable thin film substrate to force microscopy. Using this ap-proach, lateral velocity of non-ferroelastic domain walls and domain retention, e. g. in epitaxial PbZr0.2Ti0.8O3 films [1], have been found to grow with the in-plane lattice parameter, a finding discussed in the framework of present understanding. [1] E.-J. Guo et al., Adv. Mat., DOI: 10.1002/adma.201405205

Resume : Ferroelectric oxides integrated on semiconductor substrates are of particular interest for various silicon-based electronic and photonic devices. Among them, the perovskite BaTiO3 is an attractive candidate for integrated photonics and low power logic devices. The control of the crystalline orientation of the ferroelectric tetragonal cell (c- versus a-axis orientation) as a function of the processing parameters is a key issue. In order to support the MBE growth strategy on SrTiO3-buffered Si of epitaxial BaTiO3 thin films with the desired orientation, high-resolution transmission electron microscopy (HRTEM) and high angle annular dark-field (HAADF) atomic structure images of various films were acquired and treated quantitatively using geometric phase analysis. Different deposition conditions were investigated (temperature, oxygen pressure, annealing). In this work, maps of the strain in the BaTiO3 films with respect to the Si substrate are determined at the nanometric scale (1-2nm) to evidence the local tetragonality (orientation and c/a ratio) of the BaTiO3 films. Correlations with the local cation composition are also proposed on the basis of electron energy loss spectroscopy (EELS) performed at the atomic scale. HRTEM work is performed on an image corrected Hitachi HF3300S microscope (I2TEM-Toulouse) and HAADF-EELS on a FEI Titan Low-Base 60-300 (Zaragoza).

Resume : In hexagonal YMnO3, each Mn3+ ion is surrounded by three in-plane and two apical oxygen ions, which leads to the formation of the MnO5 bipyramid with a symmetry of D3h. The crystal field of D3h splits the Mn 3d orbitals into two doublets, e1g (xz, yz) and e2g (xy, x2-y2), and one singlet, a1g (3z2-r2), in order of energy. The MnO5 bipyramid plays a key role in the occurrence of the multiferroicity. We have investigated the temperature dependence of the intra-atomic Mn3+ photoluminescence (PL) in a multiferroic YMnO3 epitaxial film grown by pulsed laser deposition. The prominent finding is that the temperature dependence of the Mn3+ PL intensities highly correlates with that of the magnetic spin order originating from the antiferromagnetic transition. In contrast, the decay times of the Mn3+ PL bands are independent of temperature; namely, the transition probabilities are not affected by the antiferromagnetic transition.[2]. The above results suggest that the electron transfer process to the Mn3+ excited state from the conduction band is dominated by the magnetic spin order, which should be the origin of the photo-induced current in YMnO3 films. In this paper, we discuss the relationship between the electronic structure and the switchable photo-induced current in YMnO3 epitaxial films. The effect of the antimagnetic ordering is also discussed. [1] S. Y. Yang et al, Nat. Nanotechnol., 451 (2010) 143, [2] M. Nakayama et al., Appl. Phys. Express, 7 (2014) 023002

Resume : Epitaxial growth of CeO2 layers on Si substrates has been studied for the application to microelectronics. We have found that orientation selective epitaxial (OSE) growth of (100) and (110) oriented epitaxial CeO2 layers on Si(100) is capable by controlling surface potential distribution. We are studying the hybrid orientation structure of the CeO2(100) and (110) regions on Si(100) substrates using electron beam-induced OSE growth by reactive magnetron sputtering. Two separate areas of growth are seen, with CeO2(100) layers found to grow in areas irradiated by electrons during the growth process, and the CeO2(110) layers growing in the areas without irradiation. The lateral orientation mapping reveals the existence of transition regions between these two orientation areas. The width of the transition region is found to decrease proportionally as the logarithm of the underlying Si substrate resistivity. To make a breakthrough in the limitation in reduction of the transition region width, we propose a new method of OSE growth on silicon on insulator (SOI) substrates with lithographically formed trenches. The trenches are expected to prevent spread of the potential distribution to the neighboring Si island. Here, we report the experimental results showing perfect isolation of hybrid OSE growth regions, optimizing the Si layer thickness of SOI and the geometry of the trenches.

Resume : We report the preparation of pure zinc oxide (ZnO) and nickel-doped zinc oxide thin films by radio-frequency magnetron sputtering on glass substrates using powder target and their characterization in the perspective of transparent conducting oxide (TCO). XRD analysis revealed that all films consist of single phase ZnO and were well crystallised in würtzite phase with the crystallites preferentially oriented towards (002) direction parallel to c-axis. XRD analysis revealed that all films consist of single phase ZnO and were well crystallised in würtzite phase with the crystallites preferentially oriented towards (002) direction parallel to c-axis. The average transmittance of all the films is higher than 80% in the visible wavelength region. Doping by nickel resulted in a slight decrease in the optical band gap energy of the films and a noticeably change in optical constants. Room temperature PL spectra depict violet, blue and green emission in deposited films.

Resume : At present, there are no data about the operating characteristics stability of photoconverters (PCs) based on layered III-VI crystals including their radiation resistance, reliability and reproducibility. The aim of this work was to research the electrical and photoelectric properties of intrinsic oxide-p-InSe PCs measured in 3 years after γ-irradiation. The heterostructures were irradiated bremsstrahlung γ-quanta with energies ranging from 0 to 34 MeV and fluences 10^12-10^14 cm^-2. For just irradiated PCs it was detected an improvement of their parameters after irradiation. The intrinsic oxide films on InSe surfaces appeared to be insensitive to γ-irradiation in the all range of fluences. After 3 years keeping of the samples under standard conditions (t=15-25°C, humidity 50-65%) they were measured repeatedly. The rectification factor values become nearly by an order of magnitude. The non-ideality factor of the I-V characteristics remains the same. For the PCs only a good tendency of increasing the UOC and JSC by 2-3% took place. A comparison of the photoresponce spectra for the just irradiated and aged samples shows that there are no changes in the energy position of the spectra or their shape. The origination of these effects is explained as due to a structure-defect ordering at the interfaces and as well as by “inevitable” thermal annealing of radiation-induced defects even at room temperature in layered crystals. It is caused by a low Debye temperature for InSe crystals.

Resume : This work presents results of group theory analysis of symmetry and crystallographic orientation of secondary magnetic spinel phases in dilute magnetic semiconductors (DMS) with wurtzite crystal structure. As an example of such DMS system, we considered wurtzite ZnO doped with Co, which is magnetic impurity substituting Zn atoms in the host ZnO matrix. At Co concentration higher than 10% formation of clusters of ZnCo2O4 with cubic spinel structure is expected. This structure is described by spatial symmetry group Fd3m. Primitive cell of these crystals contains 14 atoms, and crystallographic cell - 56 atoms. It is shown, that perpendicular basis vectors of the crystallographic cell of the spinel cluster are agreed with every three parallel to them directions of the 6-th order axis of the wurtzite structure. Diperiodic (thin layer) and one-periodic (nanorod) symmetry groups of the spinel ZnCo2O4 structure are considered. Symmetry of phonons and selection rules for optical transitions are determined for one-, two- and three-period structures for spinel ZnCo2O4 phases. Presented group theory analysis of symmetry and orientation of magnetic spinel phases, and selection rules for optical transitions in DMs of different periodicity is necessary in order to predict their ferromagnetic or antiferromagnetic properties and can be used for development of components of nanospintronic devices. This work was supported by NATO SfP Grant 984735.

Resume : In oxide heterostructures, the occurrence of interface phenomena is often related to a unique redistribution of the mobile charge carriers. In this contribution, we present the study of the interface between lanthanum cuprate (La2CuO4) and lanthanum strontium nickelate (La2-xSrxNiO4). Despite none of the constituents taken singularly exhibits superconducting properties, high-temperature superconductivity (Tc up to ≈40 K) has been found for epitaxial superlattices grown by atomic-layer-by-layer oxide MBE as a result of local holes enrichment in La2CuO4. By taking advantage of different characterization techniques (e.g. XRD, high resolution aberration corrected TEM), systematic investigations have been performed, which disclosed the relation between several structural parameters (e.g. layers thickness, dopant distribution across the interface, lattice constant) with the resulting superconducting properties. We discuss the experimental findings and consider the possible scenarios which may contribute to the surprising functionalities of this system, with particular emphasis on the local charge carrier chemistry and interface effects.

Resume : Yttrium iron garnet (YIG) is attracting a high interest for applications in optical communications and spintronics. The deposition of YIG thin films on gadolinium gallium garnet (GGG) crystals is usually a desirable requirement. Yet, in spite of the outstanding structural compatibility between YIG and GGG, the strong paramagnetic susceptibility of the latter is a serious shortcoming when it comes to the magnetic characterization, as strong magnetic signals from GGG substrates obscure severely the much weaker response of YIG thin films. Here we show that magneto-optics is a way around to circumvent this difficulty. The magneto-optic activity results from the interaction of polarized light with magnetic materials and the spectral response is extremely sensitive to the electronic structure. With this in mind, we have measured the Kerr rotation and ellipticity of both Ce-doped YIG thin films grown on GGG and bare GGG substrates. We used cerium as a dopant as it is known to enhance further the magneto-optic activity of YIG. Our extensive spectroscopic study in the visible helped to identify the particular wavelengths for which the rotation and ellipticity of GGG vanished, while the magneto-optic activity of Ce-YIG remained large. That allowed us to access the individual magnetic properties of the Ce-YIG films that were otherwise unreachable using conventional magnetometry and was instrumental to achieve thin films with optimal magnetic properties.

Resume : The in-plane magnetotransport properties of the artificial manganite superlattice (SL) [(NdMnO3)n /(SrMnO3)n /(LaMnO3)n]m are presented. It has been previously shown that the ferroelectric and magnetic properties of those SLs can be tuned by an external magnetic field as well as by varying the layer thickness n.[1, 2] Here, we advance these studies by investigating the evolution of the in-plane magnetotransport with the geometric control parameter n. In particular we show that the conduction type as well as the magnetoresistance magnitude and sign vary with layer thickness n. Considering that the charge transport in these heterostructures is confined to the interfaces and occurs via variable range hopping (VRH), the tuning of the interfacial separation modulates the interplay between spin and charge degree of freedom resulting in transport regimes dominated either by spin or charge correlations between carriers of neighboring interfaces. Our results demonstrate the ability to geometrically adjust the electrical transport between regimes dominated by either charge or spin correlations. [1] K. Rogdakis et al., Nature Comm. 3,1064 (2012). [2] J. W. Seo et al., Phys. Rev. B. 82, 140405 (R) (2010).

Resume : The spherical aberration correction in the electron microscope has dramatically changed our view of oxide structures. If oxides alone show intriguing phenomena, such as colossal magnetoresistance, high Tc superconductivity or multiferroicity, when combined they present new and unexpected phenomena, being the two-dimensional electron gas in the interface between two oxide band-insulators a paradigmatic example. Understanding the physics underlying such phenomena, specially in low dimensional systems relies on the availability of techniques capable of looking at these systems in real space and with atomic resolution. For perovskite oxides, ABO3, the system properties depend on slight structural distortions in the oxide lattice at the interfaces or around point defects. Such distortions can typically be cation deformations, rotations or displacements within BO6 octahedra. However, an atomic-scale comprehension of interfaces and defects is, in many cases, still missing because this requires precise measurements of the positions of both cations and oxygen. This talk will briefly review some state-of-the-art applications to oxide heterostructures using annular bright-field (ABF) combined with high angle annular dark field (HAADF) imaging modes in the aberration-corrected transmission electron microscope (STEM), which are perfectly capable to visualize both cation and oxygen atomic positions in oxide heterointerfaces such as LaAlO3/SrTiO3 and high Tc superconducting nanocomposites.

Resume : Metal supported ultrathin oxide films constitute a novel class of materials with unusual properties due to the presence of metal/oxide interface. One of the intriguing features of such materials is a possibility to charge neutral atoms adsorbed on a film, where charge originates from the metal/oxide interface. The mechanism of this process is still under debates. Here we present our density functional theory study of ScN, MgO and NaF thin films supported with Mo (ScN/Mo, MgO/Mo and NaF/Mo) and Cu atom adsorbed on them. We have investigated the role of surface and interface deformations in charge transfer from the metal/film interface to the Cu adatom. The weak interaction between the film and the metal is found to be auspicious for adatom charging. The detailed study of Cu/NaF/Mo and NaF/Mo shows that Mo support significantly enhances the flexibility of the NaF film. Also, distortion of film caused by Cu adsorption is found to be strongly anharmonic, which we attribute to the coupling with the charge redistribution in the system and adatom charging.

Resume : Spintronic devices currently rely on magnetic switching or controlled motion of magnetic domain walls by an external magnetic field or spin-polarized current. Achieving the same degree of magnetic controllability using an electric field has potential advantages including low power consumption. Here, an approach to electrically control local magnetic properties will be discussed [1-4]. The method is based on recurrent strain transfer from regular ferroelastic stripe domains in a ferroelectric BaTiO3 substrate to magnetostrictive films (e.g. CoFe, CoFeB, and Fe). Dominance of the strain-induced magnetoelastic anisotropy in these heterostructures causes full imprinting of ferroelectric domain patterns into ferromagnetic films and strong pinning of magnetic domain walls onto ferroelectric boundaries. Optical polarization microscopy measurements of the ferromagnetic and ferroelectric domain structures indicate that domain correlations and strong inter-ferroic domain wall pinning are maintained in an applied electric field. As a result, deterministic electric-field control over the formation and erasure of ferromagnetic domains and reversible motion of magnetic domain walls is obtained, opening up new routes towards electric-field driven spintronics. [1] T.H.E. Lahtinen et al., Adv. Mater. 23, 3187 (2011) [2] T.H.E. Lahtinen et al., Sci. Rep. 2, 258 (2012) [3] K. J. A. Franke et al., Phys. Rev. B 85, 094423 (2012) [4] K. J. A. Franke et al., Phys. Rev. Lett. 112, 017201 (2014)

Resume : BiFeO3 epitaxial thin films were deposited on piezoelectric 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 (PMN-PT) substrates with a conductive buffer layer (La0.7Sr0.3MnO3 or SrRuO3) using pulsed laser deposition. The calibration of the strain values induced by the applied voltage on the piezoelectric PMN-PT substrates was realised using X-Ray diffraction measurements. Raman spectra monitoring as a function of the applied voltage (and hence strain) was performed in resonant conditions, using the 442 nm line of a HeCd laser. The piezoelectric induced strain in the BiFeO3 films causes shifts in the phonon position. The method of piezoelectrically induced strain allows to obtain a quantitative correlation between strain and the shift of the Raman-active phonons, ruling out the influence of extrinsic factors, such as growth conditions, crystalline quality of substrates, or film thickness. Using the Poisson number previously reported for BiFeO3 films [1], one can determine the volume change induced by strain, and therefore the Grüneisen parameters for specific phonon modes. [1] M.D. Biegalski, K. Dörr, D.H. Kim, and H.M. Christen, Appl. Phys. Lett. 96, 151905 (2010).

Resume : Controlling magnetism by electric fields is a key issue for the future development of low-power spintronics. We will present a new approach for the electrical control of magnetic and spintronic properties based on the combination of ferroelectric materials with magnetic transition-metal alloys. Experimental results based on X-ray diffraction and magnetometry techniques will be presented, demonstrating a giant, low-voltage control of magnetism, just above room temperature. The data will be interpreted in the light of first-principles in terms of both strain and field-effect. The corresponding magnetoelectric coupling is larger than previous reports by at least one order of magnitude and open new perspectives for the use of ferroelectrics in magnetic storage and spintronics [1]. Acknowledgments: This work received financial support from the ERC Advanced Grant FEMMES (contract no. 267579). [1] R. O. Cherifi et al, Nature Materials 13, 345 (2014)