D. Imhoff

Strain relaxation in the epitaxy of La2/3Sr1/3MnO3 grown by pulsed-laser deposition on SrTiO3(001)

With a Curie point at 370 K, the half-metal (La0.7Sr0.3)MnO3 (LSMO) is one of the most interesting candidates for electronic devices based on tunnel magnetoresistance. SrTiO3 (STO) is up to now the best substrate for the epitaxy of suitable thin films of LSMO. The pseudocubic unit cell of rhombohedral LSMO has a parameter a LSMO such that (a STO − a LSMO)/a LSMO = + 0.83% (where a STO is the parameter of cubic STO) and an angle of 90.26°. As strained growth is tetragonal, relaxation implies recovery of both the pseudocubic parameter and of the original angle. In the LSMO layers that we prepare by pulsed-laser deposition, we show that these two processes are quite independent. The angular distortion is partially recovered by twinning in films 25 nm thick, while recovery of the parameter never occurs in the thickness range that we explored (up to 432 nm). A relaxation, however, takes place above a thickness of 100 nm, associated with a transition from two-dimensional to three-dimensional columnar growth. It is accompanied by chemical fluctuations. Our magnetic measurements exhibit Curie temperatures and magnetic moments very close to the bulk values in those layers where the crystal parameter is strained but the angle partially relaxed.

Electron energy loss spectroscopy determination of Ti oxidation state at the (001) LaAIO3/SrTiO3 interface as a function of LaAIO3 growth conditions

After the epitaxy of LaAlO3 on a TiO2-terminated {100} surface of SrTiO3, a high-mobility electron gas may appear, which has been the object of numerous works over the last four years. Its origin is a subject of debate between the interface polarity and unintended doping. Here we use electron energy loss spectrum images, recorded in cross-section in a scanning transmission electron microscope, to analyse the Ti3+ ratio, characteristic of extra electrons. We find an interface concentration of Ti3+ that depends on growth conditions.

Interfaces in {100} epitaxial heterostructures of perovskite oxides

The exact perovskite structure has simple cubic symmetry and composition ABO3, where A is a relatively large cation and B a smaller one. The choices of A and B cations, and the substitutions possible on either site, generate a large variety of materials sharing the same base, with relatively small distortions of the size and shape of the cube. In addition, these oxides often allow oxygen non-stoichiometry with or without order to the amount of several percent. They form a vast set of technologically important materials due to their conducting, insulating, ferroelectric, magnetic, superconducting, etc., properties. Heteroepitaxy of perovskite oxides allows one to construct atomically sharp interfaces between these materials and therefore to envisage a set of useful heterojunctions. The epitaxy has side effects that may also prove useful: (1) it forces a chemical neighbouring that would not occur naturally, creating a two-dimensional third material, and (2) it imposes a lateral strain. Both of these effects allow one to explore novel, sometimes unforeseen, properties with a strong two-dimensional character. This paper first reviews some of the knowledge that has been accumulated on {100} surfaces and interfaces of perovskites, with an emphasis on properties that could be used in future all-oxide microelectronics. It then exposes the case of the interface between the half-metal La2/3Sr1/3MnO3 and the insulator SrTiO3, which plays a key role in the magnetoresistance of magnetic tunnel junctions. It particularly presents thorough electron energy loss spectroscopy measurements that uncover the atomic scale structural and electronic properties of these objects.

Changes in Cu–silica interfacial chemistry with oxygen chemical potential

Abstract Interfacial chemistry and equilibrium morphology of SiO 2 glass precipitates in a solid copper matrix are studied as a function of the oxygen chemical potential. Spherical SiO 2 –glass precipitates are formed within copper by internal oxidation of (Cu,Si) single crystals at different oxygen activities. The metal–glass interfaces are rough at an atomic level. Atomic and electronic structures of the different interfaces are analyzed by high spatial resolution EELS. At high oxygen activity, two interfacial zones with specific electronic states are distinguished: on the metal-side of the interface, the correlated modifications in the O-1s and Cu-2p edges indicate O-2p–Cu-3d hybridization; on the glass-side of the interface, a modification of the Cu-2p edge is observed, which is interpreted in terms of a weak interaction of highly perturbed metallic copper with the constituents of the adjacent glass network. At intermediate oxygen activity, only the latter modification is observed at the interface. Oxygen adsorption and desorption to the silica–copper interface are modeled by a continuum approach; model results are compared to those obtained by experiments.

Interfacial chemistry in internally oxidized (Cu,Mg)-alloys

Abstract (Cu,Mg) alloys are internally oxidized at different oxygen chemical potential at 900°C. Oxidation scale microstructure is studied by SEM and TEM. MgO forms as large magnesia agglomerates without any special orientation relationship and isolated cubo-octahedral topotaxial MgO precipitates, the shape of which varies with decreasing oxygen activity from octahedral to cubic. The interfaces of the cubo-octahedral precipitates are studied in detail by CTEM, HREM and EELS. At the highest oxygen activity, important rigid-body contraction/expansion across the interface is found together with a strong modification in the interfacial electronic structure (compared to the adjacent bulk phases) indicating important hybridization of O 2 p and Cu 3 d states. Both suggest oxide bonding. At lower oxygen activity, interfaces show increasing structural disorder in the copper phase and microfaceting or terracing of the interfacial plane; the intensity of interfacial ELNES features associated to the O 2 p and Cu 3 d hybridization diminishes and finally disappears with decreasing oxygen activity. Changes with oxygen chemical potential in precipitate morphology, interface atomic and electronic structure are explained by Gibbs’ adsorption/desorption of excess oxygen to the interface. Adsorption isotherms are modeled for various configurations and compared to the experimental results.

Multi-scale analysis of the nanostructured granular solid CoO–Ag by TEM and EELS

Abstract Nanostructured granular solid consisting of antiferromagnetic CoO particles embedded in a metallic medium (Ag) has been investigated by both transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) on a multiple scale. Quantitative composition determination was carried out by energy dispersive X-ray spectroscopy (EDXS) for Ag and Co, and by EELS for oxygen and Co quantification. Because of the complex morphology of the indicated CoO–Ag granular system, only a detailed analysis of the spatial variation of the EELS signals can provide information on the particle size and distribution. A mean size of 5 nm for CoO particles has been evaluated. The oxidation state of Co has also been studied using fine structures on EELS spectra. CoO phase is clearly identified as the oxide but appears mixed with small metallic Co area.

Analysis of electromechanical behaviour of (1-x)PMN-xPT (with x≤0.1) bulk ceramics

Abstract The electromechanical behaviour of the (1– x )PMN– x PT ( x ⩽0.1) bulk ceramics is studied in particular through the sensitivity of its nanostructure to the electric field, stress and temperature. At first, it was shown by deviation to the Curie–Weiss type behaviour that a local polarisation appears at a T d temperature (around 200°C) i.e. largely above the temperature of the maximum of permittivity ( T m , respectively −13°C and +36°C for x =0 and 0.1), which is consistent with the nucleation of polar clusters within a paraelectric matrix. Moreover, the dielectric relaxation observed for 0.9PbMg 1/3 Nb 2/3 O 3 –0.1PbTiO 3 –0.12MgO, in a large frequency range (100 Hz–15 MHz), and corresponding to a multi-Debye process with broadening of the relaxation time distribution as the temperature decreases, has been correlated to a nucleation and growth mechanism of polar clusters with decreasing temperature below T d , which might result from the successive transitions of different compositions. This hypothesis has been confirmed by nanoanalysis thanks to EDXS and EELS techniques: in fact large fluctuations of the local composition around the nominal one have been revealed, lead and magnesium deficient areas enriched in niobium coexisting with nanodomains (around 10 nm) strongly enriched in lead and slightly in magnesium. Consequently, due to such heterogeneities, the material remains mainly paraelectric up to very low temperatures. This effect can be balanced by the application of a high electric field which induces the growth of the polar clusters up to a macroscopic ferroelectric transition for some conditions of temperature and electric field. The specific electromechanical characteristics of 0.9PMN–0.1PT are reported, in particular the strong dependence of their elastic compliance with the electric field, the prestress applied to the material and the temperature. This behaviour is attributed to the growth of compliant polar clusters induced by decreasing temperature or increasing electric field and inhibited by the application of a uniaxial stress. Finally, as 0.9PMN–0.1PT bulk ceramics are characterised by a tunable compliance, their potential interest as active vibration control is presented and its electromechanical behaviour as actuator, under a sinusoidal electrical signal added to a 0.6 kV/mm DC electric field, is characterised.

Nanoscale analysis of a Co-SrTiO3 interface in a Magnetic tunnel junction

We use High Resolution Electron Microscopy together with Electron Energy Loss Spectroscopy to analyze the crystallography and the chemical configuration of a Co/SrTiO 3 interface in a Co/SrTiO 3 /La 2/3 Sr 1/3 MnO 3 magnetic tunnel junction. PACS: 75.47.-m, 75.70.Cn, 68.37.Lp, 79.20.Uv