65 research outputs found
Inherent temperature effects in magnetic tunnel junctions
Theoretical studies of the temperature dependence of the tunneling magnetoresistance ratio (TMR) are presented. A successful elastic tunneling model has been extended to handle temperature dependence. It treats Fermi smearing and applies Stoner-like behavior to the exchange split band structure in the electrodes to calculate TMR(T). As expected, the effects of Fermi smearing are small, but small changes in the magnetic band structure produce large changes in TMR. For a Co/I/Co junction produced by LeClair et al. [Phys. Rev. Lett. 84, 2933 (2000)], calculations using bulk magnetization predicted 33% of the experimental loss of TMR from 0 to 300 K with only a 1.5% change in magnetization. A mere 3.2% change in magnetization produced 100% of the observed drop in TMR. These results imply larger than imagined intrinsic temperature dependence for TMR. ©2001 American Institute of Physics
Inherent temperature effects in magnetic tunnel junctions
Theoretical studies of the temperature dependence of the tunneling magnetoresistance ratio (TMR) are presented. A successful elastic tunneling model has been extended to handle temperature dependence. It treats Fermi smearing and applies Stoner-like behavior to the exchange split band structure in the electrodes to calculate TMR(T). As expected, the effects of Fermi smearing are small, but small changes in the magnetic band structure produce large changes in TMR. For a Co/I/Co junction produced by LeClair et al. [Phys. Rev. Lett. 84, 2933 (2000)], calculations using bulk magnetization predicted 33% of the experimental loss of TMR from 0 to 300 K with only a 1.5% change in magnetization. A mere 3.2% change in magnetization produced 100% of the observed drop in TMR. These results imply larger than imagined intrinsic temperature dependence for TMR. ©2001 American Institute of Physics
Maghemite-like regions at crossing of two antiphase boundaries in doped BiFeO3
We report the observation of a novel structure at the point where two antiphase boundaries cross in a doped bismuth ferrite of composition (Bi0.85Nd0.15)(Fe0.9Ti0.1)O0.3. The structure was investigated using a combination of high angle annular dark field imaging and electron energy loss spectroscopy spectrum imaging in the scanning transmission electron microscope. A three-dimensional model was constructed by combining the position and chemistry data with previous results and assuming octahedral coordination of all Fe and Ti atoms. The resulting structure shows some novel L shaped arrangements of iron columns, which are coordinated in a similar manner to FeO6 octahedra in maghemite. It is suggested that this may lead to local ferromagnetic orderings similar to those in maghemite
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
Induced four fold anisotropy and bias in compensated NiFe/FeMn double layers
A vector spin model is used to show how frustrations within a multisublattice
antiferromagnet such as FeMn can lead to four-fold magnetic anisotropies acting
on an exchange coupled ferromagnetic film. Possibilities for the existence of
exchange bias are examined and shown to exist for the case of weak chemical
disorder at the interface in an otherwise perfect structure. A sensitive
dependence on interlayer exchange is found for anisotropies acting on the
ferromagnet through the exchange coupling, and we show that a wide range of
anisotropies can appear even for a perfect crystalline structure with an
ideally flat interface.Comment: 7 pages, 7 figure
Quantum oscillation of magnetoresistance in tunneling junctions with a nonmagnetic spacer
We make a theoretical study of the quantum oscillations of the tunneling
magnetoresistance (TMR) as a function of the spacer layer thickness. Such
oscillations were recently observed in tunneling junctions with a nonmagnetic
metallic spacer at the barrier-electrode interface. It is shown that momentum
selection due to the insulating barrier and conduction via quantum well states
in the spacer, mediated by diffusive scattering caused by disorder, are
essential features required to explain the observed period of oscillation in
the TMR ratio and its asymptotic value for thick nonmagnetic spacer.Comment: 4 pages, 5 figures, two column, REVTex4 styl
Structural and magnetic properties of Fe/ZnSe(001) interfaces
We have performed first principles electronic structure calculations to
investigate the structural and magnetic properties of Fe/ZnSe(001) interfaces.
Calculations involving full geometry optimizations have been carried out for a
broad range of thickness of Fe layers(0.5 monolayer to 10 monolayers) on top of
a ZnSe(001) substrate. Both Zn and Se terminated interfaces have been explored.
Total energy calculations show that Se segregates at the surface which is in
agreement with recent experiments.
For both Zn and Se terminations, the interface Fe magnetic moments are higher
than the bulk bcc Fe moment.
We have also investigated the effect of adding Fe atoms on top of a
reconstructed ZnSe surface to explore the role of reconstruction of
semiconductor surfaces in determining properties of metal-semiconductor
interfaces. Fe breaks the Se dimer bond formed for a Se-rich (2x1)
reconstructed surface. Finally, we looked at the reverse growth i.e. growth of
Zn and Se atoms on a bcc Fe(001) substrate to investigate the properties of the
second interface of a magnetotunnel junction. The results are in good agreement
with the theoretical and experimental results, wherever available.Comment: 7 pages, 8 figures, accepted for publication in PR
Nonmonotonic inelastic tunneling spectra due to surface spin excitations in ferromagnetic junctions
The paper addresses inelastic spin-flip tunneling accompanied by surface spin
excitations (magnons) in ferromagnetic junctions. The inelastic tunneling
current is proportional to the magnon density of states which is
energy-independent for the surface waves and, for this reason, cannot account
for the bias-voltage dependence of the observed inelastic tunneling spectra.
This paper shows that the bias-voltage dependence of the tunneling spectra can
arise from the tunneling matrix elements of the electron-magnon interaction.
These matrix elements are derived from the Coulomb exchange interaction using
the itinerant-electron model of magnon-assisted tunneling. The results for the
inelastic tunneling spectra, based on the nonequilibrium Green's function
calculations, are presented for both parallel and antiparallel magnetizations
in the ferromagnetic leads.Comment: 9 pages, 4 figures, version as publishe
Ideal Spin Filters: Theoretical Study of Electron Transmission Through Ordered and Disordered Interfaces Between Ferromagnetic Metals and Semiconductors
It is predicted that certain atomically ordered interfaces between some
ferromagnetic metals (F) and semiconductors (S) should act as ideal spin
filters that transmit electrons only from the majority spin bands or only from
the minority spin bands of the F to the S at the Fermi energy, even for F with
both majority and minority bands at the Fermi level. Criteria for determining
which combinations of F, S and interface should be ideal spin filters are
formulated. The criteria depend only on the bulk band structures of the S and F
and on the translational symmetries of the S, F and interface. Several examples
of systems that meet these criteria to a high degree of precision are
identified. Disordered interfaces between F and S are also studied and it is
found that intermixing between the S and F can result in interfaces with spin
anti-filtering properties, the transmitted electrons being much less spin
polarized than those in the ferromagnetic metal at the Fermi energy. A patent
application based on this work has been commenced by Simon Fraser University.Comment: RevTeX, 12 pages, 5 figure
The T309G MDM2 gene polymorphism is a novel risk factor for proliferative vitreoretinopathy
Proliferative vitreoretinopathy (PVR) is still the major cause of failure in retinal detachment (RD) surgery. It is believed that down-regulation in the p53 pathway could be an important key in PVR pathogenesis. The purpose was to evaluate the impact of T309G MDM2 polymorphism (rs2279744) in PVR. Distribution of T309G MDM2 genotypes among European subjects undergoing RD surgery was evaluated. Proportions of genotypes between subsamples from different countries were analyzed. Also, a genetic interaction between rs2279744 in MDM2 and rs1042522 in p53 gene was analyzed. Significant differences were observed comparing MDM2 genotype frequencies at position 309 of intron 1 between cases (GG: 21.6%, TG: 54.5%, TT: 23.8%) and controls (GG: 7.3%, TG: 43.9%, TT: 48.7%). The proportions of genotypes between sub-samples from different countries showed a significant difference. Distribution of GG genotype revealed differences in Spain (35.1-53.0)/(22.6-32.9), Portugal (39.0-74.4)/(21.4-38.9), Netherlands (40.6-66.3)/(25.3-38.8) and UK (37.5-62.4)/(23.3-34.2). The OR of G carriers in the global sample was 5.9 (95% CI: 3.2 to 11.2). The OR of G carriers from Spain and Portugal was 5.4 (95% CI: 2.2-12.7), whereas in the UK and the Netherlands was 7.3 (95% CI: 2.8-19.1). Results indicate that the G allele of rs2279744 is associated with a higher risk of developing PVR in patients undergoing a RD surgery. Further studies are necessary to understand the role of this SNP in the development of PVR. Copyright
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