Electromigration in thin tunnel junctions with ferromagnetic/nonmagnetic: nanoconstrictions, local heating, and direct and wind forces

Current Induced Resistance Switching (CIS) was recently observed in thin tunnel junctions with ferromagnetic (FM) electrodes \emph{i.e} FM/I/FM. This effect was attributed to electromigration of metallic atoms in nanoconstrictions in the insulating barrier (I). Here we study how the CIS effect is influenced by a thin non-magnetic (NM) Ta layer, deposited just below the AlO$_x$ insulating barrier in tunnel junctions of the type FM/NM/I/FM (FM=CoFe). Enhanced resistance switching occurs with increasing maximum applied current (\Imax), until a plateau of constant CIS is reached for \Imax\sim65 mA (CIS$\sim$60%) and above. However, such high electrical currents also lead to a large ($\sim$9%) irreversible resistance decrease, indicating barrier degradation. Anomalous voltage-current characteristics with negative derivative were also observed near \pm\Imax and this effect is here attributed to heating in the tunnel junction. One observes that the current direction for which resistance switches in FM/NM/I/FM (clockwise) is opposite to that of FM/I/FM tunnel junctions (anti-clockwise). This effect will be discussed in terms of a competition between the electromigration contributions due to the so called direct and wind forces. It will be shown that the direct force is likely to dominate electromigration in the Ta (NM) layers, while the wind contribution likely dominates in the CoFe (FM) layers

Limb bone scaling in hopping diprotodonts and quadrupedal artiodactyls

Bone adaptation is modulated by the timing, direction, rate, and magnitude of mechanical loads. To investigate whether frequent slow, or infrequent fast, gaits could dominate bone adaptation to load, we compared scaling of the limb bones from two mammalian herbivore clades that use radically different high-speed gaits, bipedal hopping and quadrupedal galloping. Forelimb and hindlimb bones were collected from 20 artiodactyl and 15 diprotodont species (body mass M 1.05 - 1536 kg) and scanned in clinical computed tomography or X-ray microtomography. Second moment of area (Imax) and bone length (l) were measured. Scaling relations (y = axb) were calculated for l vs M for each bone and for Imax vs M and Imax vs l for every 5% of length. Imax vs M scaling relationships were broadly similar between clades despite the diprotodont forelimb being nearly unloaded, and the hindlimb highly loaded, during bipedal hopping. Imax vs l and l vs M scaling were related to locomotor and behavioural specialisations. Low-intensity loads may be sufficient to maintain bone mass across a wide range of species. Occasional high-intensity gaits might not break through the load sensitivity saturation engendered by frequent low-intensity gaits

IMAX camera (12-IML-1)

The IMAX camera system is used to record on-orbit activities of interest to the public. Because of the extremely high resolution of the IMAX camera, projector, and audio systems, the audience is afforded a motion picture experience unlike any other. IMAX and OMNIMAX motion picture systems were designed to create motion picture images of superior quality and audience impact. The IMAX camera is a 65 mm, single lens, reflex viewing design with a 15 perforation per frame horizontal pull across. The frame size is 2.06 x 2.77 inches. Film travels through the camera at a rate of 336 feet per minute when the camera is running at the standard 24 frames/sec

The Imaging Magnetograph eXperiment (IMaX) for the Sunrise balloon-borne solar observatory

The Imaging Magnetograph eXperiment (IMaX) is a spectropolarimeter built by four institutions in Spain that flew on board the Sunrise balloon-borne telesocope in June 2009 for almost six days over the Arctic Circle. As a polarimeter IMaX uses fast polarization modulation (based on the use of two liquid crystal retarders), real-time image accumulation, and dual beam polarimetry to reach polarization sensitivities of 0.1%. As a spectrograph, the instrument uses a LiNbO3 etalon in double pass and a narrow band pre-filter to achieve a spectral resolution of 85 mAA. IMaX uses the high Zeeman sensitive line of Fe I at 5250.2 AA and observes all four Stokes parameters at various points inside the spectral line. This allows vector magnetograms, Dopplergrams, and intensity frames to be produced that, after reconstruction, reach spatial resolutions in the 0.15-0.18 arcsec range over a 50x50 arcsec FOV. Time cadences vary between ten and 33 seconds, although the shortest one only includes longitudinal polarimetry. The spectral line is sampled in various ways depending on the applied observing mode, from just two points inside the line to 11 of them. All observing modes include one extra wavelength point in the nearby continuum. Gauss equivalent sensitivities are four Gauss for longitudinal fields and 80 Gauss for transverse fields per wavelength sample. The LOS velocities are estimated with statistical errors of the order of 5-40 m/s. The design, calibration and integration phases of the instrument, together with the implemented data reduction scheme are described in some detail.Comment: 17 figure

The power spectrum of solar convection flows from high-resolution observations and 3D simulations

We compare Fourier spectra of photospheric velocity fields from very high resolution IMaX observations to those from recent 3D numerical magnetoconvection models. We carry out a proper comparison by synthesizing spectral lines from the numerical models and then applying to them the adequate residual instrumental degradation that affects the observational data. Also, the validity of the usual observational proxies is tested by obtaining synthetic observations from the numerical boxes and comparing the velocity proxies to the actual velocity values from the numerical grid. For the observations, data from the SUNRISE/IMaX instrument with about 120 km spatial resolution are used, thus allowing the calculation of observational Fourier spectra well into the subgranular range. For the simulations, we use four series of runs obtained with the STAGGER code and synthesize the IMaX spectral line (FeI 5250.2 A) from them. Proxies for the velocity field are obtained via Dopplergrams (vertical component) and local correlation tracking (horizontal component). A very good match between observational and simulated Fourier power spectra is obtained for the vertical velocity data for scales between 200 km and 6 Mm. Instead, a clear vertical shift is obtained when the synthetic observations are not degraded. The match for the horizontal velocity data is much less impressive because of the inaccuracies of the LCT procedure. Concerning the internal comparison of the direct velocity values of the numerical boxes with those from the synthetic observations, a high correlation (0.96) is obtained for the vertical component when using the velocity values on the log($tau_{500}$) = -1 surface in the box. The corresponding Fourier spectra are near each other. A lower maximum correlation (0.5) is reached (at $tau_{500}$ = 1) for the horizontal velocities as a result of the coarseness of the LCT procedure.Comment: 12 pages, 9 figures, accepted in A&

Where the granular flows bend

Based on IMaX/Sunrise data, we report on a previously undetected phenomenon in solar granulation. We show that in a very narrow region separating granules and intergranular lanes the spectral line width of the Fe I 5250.2 A line becomes extremely small. We offer an explanation of this observation with the help of magneto-convection simulations. These regions with extremely small line widths correspond to the places where the granular flows bend from mainly upflow in granules to downflow in intergranular lanes. We show that the resolution and image stability achieved by IMaX/Sunrise are important requisites to detect this interesting phenomenon.Comment: Accepted for the Sunrise Special Issue of ApJ

Fully resolved quiet-Sun magnetic flux tube observed with the Sunrise IMaX instrument

Until today, the small size of magnetic elements in quiet Sun areas has required the application of indirect methods, such as the line-ratio technique or multi-component inversions, to infer their physical properties. A consistent match to the observed Stokes profiles could only be obtained by introducing a magnetic filling factor that specifies the fraction of the observed pixel filled with magnetic field. Here, we investigate the properties of a small magnetic patch in the quiet Sun observed with the IMaX magnetograph on board the balloon-borne telescope Sunrise with unprecedented spatial resolution and low instrumental stray light. We apply an inversion technique based on the numerical solution of the radiative transfer equation to retrieve the temperature stratification and the field strength in the magnetic patch. The observations can be well reproduced with a one-component, fully magnetized atmosphere with a field strength exceeding 1 kG and a significantly enhanced temperature in the mid- to upper photosphere with respect to its surroundings, consistent with semi-empirical flux tube models for plage regions. We therefore conclude that, within the framework of a simple atmospheric model, the IMaX measurements resolve the observed quiet-Sun flux tube.Comment: Accepted for publication in The Astrophysical Journal Letters on Aug 11 201

Measurement of 0.25-3.2 GeV antiprotons in the cosmic radiation

The balloon-borne Isotope Matter-Antimatter Experiment (IMAX) was flown from Lynn Lake, Manitoba, Canada on 16–17 July 1992. Using velocity and magnetic rigidity to determine mass, we have directly measured the abundances of cosmic ray antiprotons and protons in the energy range from 0.25 to 3.2 GeV. Both the absolute flux of antiprotons and the antiproton/proton ratio are consistent with recent theoretical work in which antiprotons are produced as secondary products of cosmic ray interactions with the interstellar medium. This consistency implies a lower limit to the antiproton lifetime of ∼10 to the 7th yr

The formation and disintegration of magnetic bright points observed by Sunrise/IMaX

The evolution of the physical parameters of magnetic bright points (MBPs) located in the quiet Sun (mainly in the interwork) during their lifetime is studied. First we concentrate on the detailed description of the magnetic field evolution of three MBPs. This reveals that individual features follow different, generally complex, and rather dynamic scenarios of evolution. Next we apply statistical methods on roughly 200 observed MBP evolutionary tracks. MBPs are found to be formed by the strengthening of an equipartition field patch, which initially exhibits a moderate downflow. During the evolution, strong downdrafts with an average velocity of 2.4 km/s set in. These flows, taken together with the concurrent strengthening of the field, suggest that we are witnessing the occurrence of convective collapses in these features, although only 30% of them reach kG field strengths. This fraction might turn out to be larger when the new 4 m class solar telescopes are operational as observations of MBPs with current state of the art instrumentation could still be suffering from resolution limitations. Finally, when the bright point disappears (although the magnetic field often continues to exist) the magnetic field strength has dropped to the equipartition level and is generally somewhat weaker than at the beginning of the MBP's evolution. Noteworthy is that in about 10% of the cases we observe in the vicinity of the downflows small-scale strong (exceeding 2 km/s) intergranular upflows related spatially and temporally to these downflows.Comment: 19 pages, 13 figures; final version published in "The Astrophysical Journal