318 research outputs found
Analysis of breakdown in ferromagnetic tunnel junctions
Due to their very thin tunnel barrier layer, magnetic tunnel junctions show dielectric breakdown at voltages of the order of 1 V. At the moment of breakdown, a highly conductive short is formed in the barrier and is visible as a hot spot. The breakdown effect is investigated by means of voltage ramp experiments on a series of nominally identical Co/Al2O3/Co tunnel junctions. The results are described in terms of a voltage dependent breakdown probability, and are further analyzed within the framework of a general model for the breakdown probability in dielectric materials, within which it is assumed that at any time the breakdown probability is independent of the (possibly time-dependent) voltage that has been previously applied. The experimental data can be described by several specific forms of the voltage breakdown probability function. A comparison with the models commonly used for describing thin film SiO2 breakdown is given, as well as suggestions for future experiments
Perpendicular-current Studies of Electron Transport Across Metal/Metal Interfaces
We review what we have learned about the scattering of electrons by the
interfaces between two different metals (M1/M2) in the
current-perpendicular-to-plane (CPP) geometry. In this geometry, the intrinsic
quantity is the specific resistance, AR, the product of the area through which
the CPP current flows times the CPP resistance. We describe results for both
non-magnetic/non-magnetic (N1/N2) and ferromagnetic/non-magnetic (F/N) pairs.
We focus especially upon cases where M1/M2 are lattice matched (i.e., have the
same crystal structure and the same lattice parameters to within ~ 1%), because
in these cases no-free-parameter calculations of 2AR agree surprisingly well
with measured values. But we also list and briefly discuss cases where M1/M2
are not lattice matched, either having different crystal structures, or lattice
parameters that differ by several percent. The published calculations of 2AR in
these latter cases do not agree so well with measured values.Comment: 6 pages, 2 figures, 2 tables. In Press: Applied Surface Scienc
CO adsorption on neutral iridium clusters
The adsorption of carbon monoxide on neutral iridium clusters in the size
range of n = 3 to 21 atoms is investigated with infrared multiple photon
dissociation spectroscopy. For each cluster size only a single v(CO) band is
present with frequencies in the range between 1962 cm-1 (n = 8) and 1985 cm-1
(n = 18) which can be attributed to an atop binding geometry. This behaviour is
compared to the CO binding geometries on clusters of other group 9 and 10
transition metals as well as to that on extended surfaces. The preference of Ir
for atop binding is rationalized by relativistic effects on the electronic
structure of the later 5d metals
Observation of band structure and density of states effects in Co-based magnetic tunnel junctions
Utilizing Co/AlO/Co magnetic tunnel junctions (MTJs) with Co
electrodes of different crystalline phases, a clear relationship between
electrode structure and junction transport properties is presented. For
junctions with one fcc(111) textured and one polycrystalline (poly-phase and
poly-directional) Co electrode, a strong asymmetry is observed in the
magnetotransport properties, while when both electrodes are polycrystalline the
magnetotransport is essentially symmetric. These observations are successfully
explained within a model based on ballistic tunneling between the calculated
band structures (DOS) of fcc-Co and hcp-Co.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Secondary structure of Ac-Ala-LysH polyalanine peptides (=5,10,15) in vacuo: Helical or not?
The polyalanine-based peptide series Ac-Ala_n-LysH+ (n=5-20) is a prime
example that a secondary structure motif which is well-known from the solution
phase (here: helices) can be formed in vacuo. We here revisit this conclusion
for n=5,10,15, using density-functional theory (van der Waals corrected
generalized gradient approximation), and gas-phase infrared vibrational
spectroscopy. For the longer molecules (n=10,15) \alpha-helical models provide
good qualitative agreement (theory vs. experiment) already in the harmonic
approximation. For n=5, the lowest energy conformer is not a simple helix, but
competes closely with \alpha-helical motifs at 300K. Close agreement between
infrared spectra from experiment and ab initio molecular dynamics (including
anharmonic effects) supports our findings.Comment: 4 pages, 4 figures, Submitted to JPC Letter
Optical creation of vibrational intrinsic localized modes in anharmonic lattices with realistic interatomic potentials
Using an efficient optimal control scheme to determine the exciting fields,
we theoretically demonstrate the optical creation of vibrational intrinsic
localized modes (ILMs) in anharmonic perfect lattices with realistic
interatomic potentials. For systems with finite size, we show that ILMs can be
excited directly by applying a sequence of femtosecond visible laser pulses at
THz repetition rates. For periodic lattices, ILMs can be created indirectly via
decay of an unstable extended lattice mode which is excited optically either by
a sequence of pulses as described above or by a single picosecond far-infrared
laser pulse with linearly chirped frequency. In light of recent advances in
experimental laser pulse shaping capabilities, the approach is experimentally
promising.Comment: 20 pages, 7 eps figures. Accepted, Phys. Rev.
Infrared Multiple Photon Dissociation Action Spectroscopy and Theoretical Studies of Diethyl Phosphate Complexes: Effects of Protonation and Sodium Cationization on Structure
The gas-phase structures of deprotonated, protonated, and sodium-cationized complexes of diethyl phosphate (DEP) including [DEP − H]−, [DEP + H]+, [DEP + Na]+, and [DEP − H + 2Na]+ are examined via infrared multiple photon dissociation (IRMPD) action spectroscopy using tunable IR radiation generated by a free electron laser, a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) with an electrospray ionization (ESI) source, and theoretical electronic structure calculations. Measured IRMPD spectra are compared to linear IR spectra calculated at the B3LYP/6-31G(d,p) level of theory to identify the structures accessed in the experimental studies. For comparison, theoretical studies of neutral complexes are also performed. These experiments and calculations suggest that specific geometric changes occur upon the binding of protons and/or sodium cations, including changes correlating to nucleic acid backbone geometry, specifically P–O bond lengths and ∠OPO bond angles. Information from these observations may be used to gain insight into the structures of more complex systems, such as nucleotides and solvated nucleic acids
Reconfigurable superconducting vortex pinning potential for magnetic disks in hybrid structures
High resolution scanning Hall probe microscopy has been used to directly visualise the superconducting vortex behavior in hybrid structures consisting of a square array of micrometer-sized Py ferromagnetic disks covered by a superconducting Nb thin film. At remanence the disks exist in almost fully flux-closed magnetic vortex states, but the observed cloverleaf-like stray fields indicate the presence of weak in-plane anisotropy. Micromagnetic simulations suggest that the most likely origin is an unintentional shape anisotropy. We have studied the pinning of added free superconducting vortices as a function of the magnetisation state of the disks, and identified a range of different phenomena arising from competing energy contributions. We have also observed clear differences in the pinning landscape when the superconductor and the ferromagnet are electron ically coupled or insulated by a thin dielectric layer, with an indication of non-trivial vortex-vortex interactions. We demonstrate a complete reconfiguration of the vortex pinning potential when the magnetisation of the disks evolves from the vortex-like state to an onion-like one under an in-plane magnetic field. Our results are in good qualitative agreement with theoretical predictions and could form the basis of novel superconducting devices based on reconfigurable vortex pinning sites
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