Ll. Abad

Surface behavior of La2∕3Ca1∕3MnO3 epitaxial thin films

The role of the surface layers in La2∕3Ca1∕3MnO3 magnetic oxide epitaxial thin films is analyzed. We show that the topmost layers do play a very relevant role on the transport properties acting as an insulating barrier. Atomic force microscopy (AFM) measurements in the current sensing mode exhibit typical features of tunneling conduction. The analysis of the I-V curves by using the Simmons model give values of barrier thickness in good agreement with nonmagnetic layer thickness estimates from magnetic measurements. Ex situ annealing in air at high temperature clearly improve the magnetotransport properties of the films reducing the surface insulating barrier.

Cationic diffusion in La2∕3Ca1∕3MnO3 thin films grown on LaAlO3 (001) substrates

Microstructural features of La2∕3Ca1∕3MnO3 layers of various thicknesses grown on top of (001) LaAlO3 substrates are studied by using transmission electron microscopy and electron energy loss spectroscopy. Films are of high microstructural quality but exhibit some structural relaxation and mosaicity both when increasing thickness or after annealing processes. The existence of a cationic segregation process of La atoms toward free surface has been detected, as well as a Mn oxidation state variation through layer thickness. La diffusion would lead to a Mn valence change and, in turn, to reduced magnetization.

X-ray magnetic circular dichroism in reflection geometry: A tool for investigating surface magnetism in thin films

Surface and interface magnetisms in oxide thin films and heterostructures have been a recurrent topic during the past years due to their relevance in the implementation of magnetoelectronic devices. Magneto-optical techniques, such as x-ray magnetic circular dichroism, turn out to be a very efficient tool to study surface magnetism due to their sensitivity to magnetic and chemical variations across the sample depth. Nevertheless, the application of the sum rules for the determination of the spin magnetic moment might lead to uncertainties as large as 40%. To overcome this problem we present an alternative approach consisting in using x-ray magnetic circular dichroism in reflection geometry. Data analysis by using a computer code based on a 4×4 matrix formalism indicates that surface and interface roughnessas are of major relevance for a proper description of the experimental data and a correct interpretation of the results. By using such an approach, we discuss the presence of a narrow surface region with...

Surface degradation of magnetic properties in manganite thin films proved with magneto-optical techniques in reflection geometry

Magneto-optical techniques in reflection geometry turn out to be a very efficient tool to study the surface magnetism due to their sensitivity to magnetic and chemical variations across the sample depth. The existence of a surface layer of about one to five unit cells with strongly depressed magnetic properties, when compared with the rest of the film, in La2∕3Ca1∕3MnO3 thin films is shown. These results strongly support previous theoretical predictions as well as recent findings showing the insulating nature of the topmost layers in these materials.

Mn valence instability in La 2 ∕ 3 Ca 1 ∕ 3 Mn O 3 thin films

A Mn valence instability on La2/3Ca1/3MnO3 thin films, grown on LaAlO3 (001)substrates is observed by x-ray absorption spectroscopy at the Mn L-edge and O K-edge. As-grown samples, in situ annealed at 800 C in oxygen, exhibit a Curie temperature well below that of the bulk material. Upon air exposure a reduction of the saturation magnetization, MS, of the films is detected. Simultaneously a Mn2+ spectral signature develops, in addition to the expected Mn3+ and Mn4+ contributions, which increases with time. The similarity of the spectral results obtained by total electron yield and fluorescence yield spectroscopy indicates that the location of the Mn valence anomalies is not confined to a narrow surface region of the film, but can extend throughout the whole thickness of the sample. High temperature annealing at 1000 C in air, immediately after growth, improves the magnetic and transport properties of such films towards the bulk values and the Mn2+ signature in the spectra does not appear. The Mn valence is then stable even to prolonged air exposure. We propose a mechanism for the Mn2+ ions formation and discuss the importance of these observations with respect to previous findings and production of thin films devices.

Mn valence instability in La2/3Ca1/3Mn03 thin films

A Mn valence instability on La2/3Ca1/3MnO3 thin films, grown on LaAlO3 (001)substrates is observed by x-ray absorption spectroscopy at the Mn L-edge and O K-edge. As-grown samples, in situ annealed at 800 C in oxygen, exhibit a Curie temperature well below that of the bulk material. Upon air exposure a reduction of the saturation magnetization, MS, of the films is detected. Simultaneously a Mn2+ spectral signature develops, in addition to the expected Mn3+ and Mn4+ contributions, which increases with time. The similarity of the spectral results obtained by total electron yield and fluorescence yield spectroscopy indicates that the location of the Mn valence anomalies is not confined to a narrow surface region of the film, but can extend throughout the whole thickness of the sample. High temperature annealing at 1000 C in air, immediately after growth, improves the magnetic and transport properties of such films towards the bulk values and the Mn2+ signature in the spectra does not appear. The Mn valence is then stable even to prolonged air exposure. We propose a mechanism for the Mn2+ ions formation and discuss the importance of these observations with respect to previous findings and production of thin films devices.

Transport properties across the La2/3Ca1/3MnO3/SrTiO3 heterointerface

The transport properties across La2∕3Ca1∕3MnO3∕SrTiO3 (LCMO/STO) heterostructures with different thicknesses of the STO insulating barrier have been studied by using atomic force microscopy measurements in the current sensing (CS) mode. To avoid intrinsic problems of the CS method we have developed a nanostructured contact geometry of Au dots. The conduction process across the LCMO/STO interface exhibits the typical features of a tunneling process. The analysis of I(V) curves by using the Simmons model allows us to determine the barrier height (φ0≈0.6eV) of STO barriers.

Soft x-ray absorption spectroscopy of strained epitaxial manganite thin films

The O K-edge pre-edge structure in the x-ray absorption spectra of high quality La2∕3Ca1∕3MnO3 epitaxial thin films grown on top of (001) SrTiO3 and NdGaO3 substrates is analyzed as a function of structural strain and temperature. A clear correlation between temperature dependent spectral changes and the structural strain, induced by the film/substrate mismatch, is found.

Impact of pore anisotropy on the thermal conductivity of porous Si nanowires

Porous materials display enhanced scattering mechanisms that greatly influence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individual porous Si nanowires (NWs) prepared from MACE, with values as low as 0.87 W·m−1·K−1 for 90 nm diameter wires with 35–40% porosity. Despite the strong suppression of long mean free path phonons in porous materials, we find a linear correlation of κ with the NW diameter. We ascribe this dependence to the anisotropic porous structure that arises during chemical etching and modifies the phonon percolation pathway in the center and outer regions of the nanowire. The inner microstructure of the NWs is visualized by means of electron tomography. In addition, we have used molecular dynamics simulations to provide guidance for how a porosity gradient influences phonon transport along the axis of the NW. Our findings are important towards the rational design of porous materials with tailored thermal and electronic properties for improved thermoelectric devices.

Author Correction: Impact of pore anisotropy on the thermal conductivity of porous Si nanowires

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