The role of symmetry on interface states in magnetic tunnel junctions

When an electron tunnels from a metal into the barrier in a magnetic tunnel junction it has to cross the interface. Deep in the metal the eigenstates for the electron can be labelled by the point symmetry group of the bulk but around the interface this symmetry is reduced and one has to use linear combinations of the bulk states to form the eigenstates labelled by the irreducible representations of the point symmetry group of the interface. In this way there can be states localized at the interface which control tunneling. The conclusions as to which are the dominant tunneling states are different from that conventionally found.Comment: 14 pages, 5 figures, accepted in PRB, v2: reference 3 complete

Fully relativistic calculation of magnetic properties of Fe, Co and Ni adclusters on Ag(100)

We present first principles calculations of the magnetic moments and magnetic anisotropy energies of small Fe, Co and Ni clusters on top of a Ag(100) surface as well as the exchange-coupling energy between two single adatoms of Fe or Co on Ag(100). The calculations are performed fully relativistically using the embedding technique within the Korringa-Kohn-Rostoker method. The magnetic anisotropy and the exchange-coupling energies are calculated by means of the force theorem. In the case of adatoms and dimers of iron and cobalt we obtain enhanced spin moments and, especially, unusually large orbital moments, while for nickel our calculations predict a complete absence of magnetism. For larger clusters, the magnitudes of the local moments of the atoms in the center of the cluster are very close to those calculated for the corresponding monolayers. Similar to the orbital moments, the contributions of the individual atoms to the magnetic anisotropy energy strongly depend on the position, hence, on the local environment of a particular atom within a given cluster. We find strong ferromagnetic coupling between two neighboring Fe or Co atoms and a rapid, oscillatory decay of the exchange-coupling energy with increasing distance between these two adatoms.Comment: 8 pages, ReVTeX + 4 figures (Encapsulated Postscript), submitted to PR

Lattice relaxation driven reorientation transition

The magnetic anisotropy energy of Ni n ͞Cu͑100͒ is calculated in terms of the spin-polarized fully relativistic Korringa-Kohn-Rostoker method including surface relaxation by using 2D structure constants originally described for low-energy electron diffraction calculations. Investigating different relaxations, an explanation for the reorientation transition from in-plane to perpendicular can be given. For a relaxation of 25.5% (c͞a 0.945) this reorientation occurs at about seven layers of Ni and yields second order terms to the magnetic anisotropy energy that are in excellent agreement with experiment. [ S0031-9007(98)08322-7] PACS numbers: 75.30. Gw, 75.70.Ak, 75.70.Cn Thin films of Ni on Cu(100) show an unexpected behavior of magnetic phase transitions [2] and references therein) can phenomenologically be described by where K 2 refers to the second order term of the magnetic anisotropy energy (MAE) and u denotes the angles of M with respect to the surface normal. As indicated in Eq. In the present paper the fully relativistic spin-polarized screened Korringa-Kohn-Rostoker (KKR) method The magnetic anisotropy energy DE a , defined as the energy difference between a uniform in-plane (perpendicular to the surface normal in all planes of atoms) and a uniform perpendicular (along the surface normal in all planes of atoms) orientation of the magnetization of the system was obtained In order to evaluate DE b 990 k k points in the ISBZ were used, guaranteeing well converged quantities. 0031-9007͞99͞82(6)͞1289(4)$15.0

Attosecond imaging of molecular electronic wavepackets

International audienceA strong laser field may tunnel ionize a molecule from several orbitals simultaneously, forming an attosecond electron–hole wavepacket. Both temporal and spatial information on this wavepacket can be obtained through the coherent soft X-ray emission resulting from the laser-driven recollision of the liberated electron with the core. By characterizing the emission from aligned N 2 molecules, we demonstrate the attosecond contributions of the two highest occupied molecular orbitals. We determine conditions where they are disentangled in the real and imaginary parts of the emission dipole moment. This allows us to carry out a tomographic reconstruction of both orbitals with angstrom spatial resolution. Their coherent superposition provides experimental images of the attosecond wavepacket created in the ionization process. Our results open the prospect of imaging ultrafast intramolecular dynamics combining attosecond and angstrom resolutions

Attosecond pulse characterization

In this work we propose a novel procedure for the characterization of attosecond pulses. The method relies on the conversion of the attosecond pulse into electron wave-packets through photoionization of atoms in the presence of a weak IR field. It allows for the unique determination of the spectral phase making up the pulses by accurately taking into account the atomic physics of the photoionization process. The phases are evaluated by optimizing the fit of a perturbation theory calculation to the experimental result. The method has been called iPROOF (improved Phase Retrieval by Omega Oscillation Filtering) as it bears a similarity to the PROOF technique [Chini et al. Opt. Express 18, 13006 (2010)]. The procedure has been demonstrated for the characterization of an attosecond pulse train composed of odd and even harmonics. We observe a large phase shift between consecutive odd and even harmonics. The resulting attosecond pulse train has a complex structure not resembling a single attosecond pulse once per IR period, which is the case for zero phase. Finally, the retrieval procedure can be applied to the characterization of single attosecond pulses as well