70 research outputs found
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
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
Effect of interface bonding on spin-dependent tunneling from the oxidized Co surface
We demonstrate that the factorization of the tunneling transmission into the
product of two surface transmission functions and a vacuum decay factor allows
one to generalize Julliere's formula and explain the meaning of the ``tunneling
density of states'' in some limiting cases. Using this factorization we
calculate spin-dependent tunneling from clean and oxidized fcc Co surfaces
through vacuum into Al using the principal-layer Green's function approach. We
demonstrate that a monolayer of oxygen on the Co (111) surface creates a
spin-filter effect due to the Co-O bonding which produces an additional
tunneling barrier in the minority-spin channel. This changes the minority-spin
dominated conductance for the clean Co surface into a majority spin dominated
conductance for the oxidized Co surface.Comment: 7 pages, revtex4, 4 embedded eps figure
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
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
Atomic, electronic, and magnetic properties of magnetic tunnel junctions
We report results of first-principles density-functional studies of the atomic and electronic structure of Co/Al2O3/Co and Co/SrTiO3/Co magnetic tunnel junctions (MTJs). The atomic structure has been studied for different interface terminations and the interfacial energetics was quantified based on the work of separation. Based on energetics of cohesion, the O-terminated and TiO2-terminated interfaces were identified as the most stable structures for Co/Al2O3/Co and Co/SrTiO3/Co MTJs, respectively. The electronic structure was analyzed using local densities of states. We found that the electronic structure of the O-terminated Co/Al2O3/Co tunnel junction exhibits negative spin polarization at the Fermi energy within the first few monolayers of alumina but it eventually becomes positive for distances beyond 10 Å. Calculated electronic structure of the TiO2-terminated Co/SrTiO3/Co MTJ shows an exchange coupling between the interface Co and Ti atoms mediated by oxygen. This coupling induces a magnetic moment of 0.25 µB on the interface Ti atom, which is aligned antiparallel to the magnetic moment of the Co layer. This is a possible cause of inversion of the spin polarization of tunneling across the SrTiO3 barrier that was found in recent experiments
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