V. Da Costa

Broad distribution effects in sums of lognormal random variables

Abstract:The lognormal distribution describing, e.g., exponentials of Gaussian random variables is one of the most common statistical distributions in physics. It can exhibit features of broad distributions that imply qualitative departure from the usual statistical scaling associated to narrow distributions. Approximate formulae are derived for the typical sums of lognormal random variables. The validity of these formulae is numerically checked and the physical consequences, e.g., for the current flowing through small tunnel junctions, are pointed out.

Epitaxial MgO layer for low-resistance and coupling-free magnetic tunnel junctions

Epitaxially grown magnetic tunnel junctions MgO(100)/Fe/MgO/Fe/Co/Pd have been elaborated by molecular beam epitaxy, with insulating layer thickness down to 0.8 nm. The continuity of this layer was checked at different spatial scales by means of morphological (high resolution transmission electronic microscopy), electric (local impedance), and magnetic (magnetoresistance and hysteresis loop) measurements. These junctions show a low resistance (4 kΩ μm2), tunnel magnetoresistance up to 17%, and a very small interlayer magnetic coupling.

Nanometric cartography of tunnel current in metal–oxide junctions

Recently, ferromagnet/insulator/ferromagnet trilayer junctions were shown to exhibit large magnetoresistance effects. However, these effects proved to be poorly reproducible from sample to sample. To get a nanoscopic insight on the origin of these fluctuations, we have used a combined atomic force microscope/scanning tunneling microscope setup to map the tunnel current that flows through metal/oxide stacks. The current histogram, which extends over 1–2 orders of magnitude, is found to be well described by a simple model which takes into account tiny spatial fluctuations of tunnel barrier thickness (typically, 0.1 nm). Moreover, our analysis shows that the total conductance of imperfect metal/oxide junctions tends to be dominated by very few sites. This result allows us to relate the sample-to-sample conductance fluctuations to slight local variations of the tunnel barrier parameters.

Effects of swift heavy ion bombardment on magnetic tunnel junction functional properties

In this article, we report on recent irradiation experiments on magnetic tunnel junctions (MTJ) using either light (C, O) or heavy (Ni) ion beams with energies in the range of 10 MeV/A. For all ions, albeit with different fluence thresholds, the tunnel magnetoresistance decreases irreversibly with increasing ion fluence, with conversely little or no impact on overall resistance. This can only be explained by a modification of the alumina barrier stoechiometry, rather than by interdiffusion. Measurements on irradiated so-called “spin-mirrors” sheet films show that AlOx/metal interface is altered by radiations. Finally, it is shown that MTJs are not a fortiori insensible to swift heavy ion bombardment.

Far‐infrared reflection–absorption spectroscopy of thin polyethylene oxide films

We report the temperature dependent far‐infrared spectrum of ultra‐thin films of polyethylene oxide (PEO). Using the orientational specificity of infrared and far‐infrared reflection–absorption spectroscopy and in‐situ recrystallization, we find that during spin coating the PEO helices are initially in the plane of the film, but on crystallization reorient to be normal to the substrate. A splitting of the C–O torsional mode near 109 cm−1 is identified as arising from a distortion of the normal helical structure of PEO. Comparison with transmission spectra of cast films demonstrates the value of far‐infrared reflection–absorption spectroscopy (FIRRAS) in the study of crystalline polymers in the far infrared.

Quantum coherent transport versus diode-like effect in semiconductor-free metal/insulator structure

Quantum coherent transport in double barrier tunnel junctions has been exploited for building micrometric-size semiconductor-free diodes. At room temperature, we observe strongly asymmetric current–voltage characteristics with an asymmetry ratio increasing with the bias voltage, reaching a maximum of 20 at 1 V. Our experimental data can be perfectly explained using a theoretical model involving resonance-assisted tunneling. The coherent/resonant tunneling regime is achieved using metallic 3 nm diameter monodisperse Cu clusters, sandwiched between two Al2O3 barriers.

Wavelength dependence of photoinduced deformation in BiFeO3

In electrically polar solids optomechanical effects result from the combination of two main processes, electric field-induced strain and photon-induced voltages. Whereas the former depends on the electrostrictive ability of the sample to convert electric energy into mechanical energy, the latter is caused by the capacity of photons with appropriate energy to generate charges and, therefore, can depend on wavelength.We report here on mechanical deformation of BiFeO3 and its response time to discrete wavelengths of incident light ranging from 365 to 940 nm. The mechanical response of BiFeO3 is found to have two maxima in near-UV and green spectral wavelength regions.

First glimpse of the soft x-ray induced excited spin-state trapping effect dynamics on spin cross-over molecules

The dynamics of the soft x-ray induced excited spin state trapping (SOXIESST) effect of Fe(phen)2(NCS)2 (Fe-phen) powder have been investigated by x-ray absorption spectroscopy (XAS) using the total electron yield method, in a wide temperature range. The low-spin (LS) state is excited into the metastable high-spin (HS) state at a rate that depends on the intensity of the x-ray illumination it receives, and both the temperature and the intensity of the x-ray illumination will affect the maximum HS proportion that is reached. We find that the SOXIESST HS spin state transforms back to the LS state at a rate that is similar to that found for the light induced excited spin state trapping (LIESST) effect. We show that it is possible to use the SOXIESST effect in combination with the LIESST effect to investigate the influence of cooperative behavior on the dynamics of both effects. To investigate the impact of molecular cooperativity, we compare our results on Fe-phen with those obtained for Fe{[Me2Pyrz]3BH}2 (Fe-pyrz) powder, which exhibits a similar thermal transition temperature but with a hysteresis. We find that, while the time constant of the dynamic is identical for both molecules, the SOXIESST effect is less efficient at exciting the HS state in Fe-pyrz than in Fe-phen.

High quality SrTiO3 tunnel barrier obtained by pulsed laser deposition

The quality of a SrTiO3 tunnel barrier deposited by pulsed laser deposition on SrTiO3 (001)‖Sr2FeMoO6 is investigated. Epitaxy and two-dimensional growth are obtained and the root-mean-square roughness is 0.3nm. The distribution of the local current measured by conductive atomic force microscopy indicates that hot spots are almost absent. The standard deviation of the barrier thickness distribution is lower than 0.05nm. Current-voltage characteristics of patterned magnetic tunnel junctions provide evidence of the high quality of the barrier. The electrical properties of the SrTiO3 barrier are at least as good as the ones obtained by sputtering or e-beam evaporation.

Local investigation of thin insulating barriers incorporated in magnetic tunnel junctions

In this work we study properties of very thin insulating Al oxide films, used as barriers in magnetic tunnel junctions. For the small barrier thicknesses required for technological applications (∼10 A), the presence of pinholes (direct contact between the ferromagnetic metals through the barrier), or oxidation inhomogeneities, are the major factors for vanishing of the tunnel magnetoresistance effect. We have produced and characterized very thin, pinhole-free Al oxide layers, incorporated in magnetic tunnel junctions. The transport properties of the different barriers were analyzed by barrier impedance scanning microscopy and were correlated with the magnetotransport properties of the patterned microsized junctions.