416 research outputs found
Pseudo-half-metalicity in the double perovskite SrCrReO from density-functional calculations
The electronic structure of the spintronic material SrCrReO is
studied by means of full-potential linear muffin-tin orbital method. Scalar
relativistic calculations predict SrCrReO to be half-metallic with a
magnetic moment of 1 . When spin-orbit coupling is included, the
half-metallic gap closes into a pseudo-gap, and an unquenched rhenium orbital
moment appears, resulting in a significant increase of the total magnetic
moment to 1.28 . This moment is significantly larger than the
experimental moment of 0.9 . A possible explanation of this discrepancy
is that the anti-site disorder in SrCrReO is significantly larger than
hitherto assumed.Comment: 3 Pages, 1 figure, 1 Tabl
Transport in magnetically ordered Pt nanocontacts
Pt nanocontacts, like those formed in mechanically controlled break
junctions, are shown to develop spontaneous local magnetic order. Our density
functional calculations predict that a robust local magnetic order exists in
the atoms presenting low coordination, i. e., those forming the atom-sized
neck. In contrast to previous work, we thus find that the electronic transport
can be spin-polarized, although the net value of the conductance still agrees
with available experimental information. Experimental implications of the
formation of this new type of nanomagnet are discussed.Comment: 4 pages, 3 figure
Effect of electron correlations in Pd, Ni, and Co monowires
We investigated the effect of mean-field electron correlations on the band
electronic structure of Co, Ni, and Pd ultra-thin monatomic nanowires, at the
breaking point, by means of density-functional calculations in the
self-interaction corrected LDA approach (LDA+SIC) and alternatively by the
LDA+ scheme. We find that adding static electron correlations increases the
magnetic moment in Pd monowires, but has negligible effect on the magnetic
moment in Co and Ni. Furthermore, the number of -dominated conductance
channels decreases somewhat compared to the LDA value, but the number of
-dominated channels is unaffected, and remains equal to one per spin.Comment: to appear in PR
Magnetism in Atomic-Sized Palladium Contacts and Nanowires
We have investigated Pd nanowires theoretically, and found that, unlike
either metallic or free atomic Pd, they exhibit Hund's rule magnetism. In long,
monoatomic wires, we find a spin moment of 0.7 Bohr magnetons per atom, whereas
for short, monoatomic wires between bulk leads, the predicted moment is about
0.3 Bohr magnetons per wire atom. In contrast, a coaxial (6,1) wire was found
to be nonmagnetic. The origin of the wire magnetism is analyzed.Comment: 6 pages, including 4 figure
Anisotropic magnetoresistance in nanocontacts
We present ab initio calculations of the evolution of anisotropic
magnetoresistance (AMR) in Ni nanocontacts from the ballistic to the tunnel
regime. We find an extraordinary enhancement of AMR, compared to bulk, in two
scenarios. In systems without localized states, like chemically pure break
junctions, large AMR only occurs if the orbital polarization of the current is
large, regardless of the anisotropy of the density of states. In systems that
display localized states close to the Fermi energy, like a single electron
transistor with ferromagnetic electrodes, large AMR is related to the variation
of the Fermi energy as a function of the magnetization direction.Comment: 7 pages, 4 figures; revised for publication, new figures in greyscal
A Study of Undercooling Behavior Of Immiscible Metal Alloys in the Absence of Crucible-Induced Nucleation
The purpose of this study is to investigate the question: Would eliminating the crucible eliminate the wall-induced nucleation of one of the liquid phases in an immiscible alloy and result in undercooling of the liquid into the metastable region thereby producing significant differences in the separation process and the microstructure upon solidification. Another primary objective of this research is to study systems with a metastable miscibility gap and to directly determine the metastable liquid miscibility gap by undercooling experiments. Nucleation and growth of droplets in these undercooled metallic liquid-liquid mixtures is also being studied. Results of this investigation indicate that containerless processing of immiscibles may not promote the undercooling of the single-phase liquid into the metastable region. Although no recalescence event was observed for this liquid-liquid transition, undercooling did occur across the miscibility gap for the solidification of the Ti phase that eventually separated
Orbital contribution to the magnetic properties of iron as a function of dimensionality
The orbital contribution to the magnetic properties of Fe in systems of
decreasing dimensionality (bulk, surfaces, wire and free clusters) is
investigated using a tight-binding hamiltonian in an and atomic
orbital basis set including spin-orbit coupling and intra-atomic electronic
interactions in the full Hartree-Fock (HF) scheme, i.e., involving all the
matrix elements of the Coulomb interaction with their exact orbital dependence.
Spin and orbital magnetic moments and the magnetocrystalline anisotropy energy
(MAE) are calculated for several orientations of the magnetization. The results
are systematically compared with those of simplified hamiltonians which give
results close to those obtained from the local spin density approximation. The
full HF decoupling leads to much larger orbital moments and MAE which can reach
values as large as 1 and several tens of meV, respectively, in the
monatomic wire at the equilibrium distance. The reliability of the results
obtained by adding the so-called Orbital Polarization Ansatz (OPA) to the
simplified hamiltonians is also discussed. It is found that when the spin
magnetization is saturated the OPA results for the orbital moment are in
qualitative agreement with those of the full HF model. However there are large
discrepancies for the MAE, especially in clusters. Thus the full HF scheme must
be used to investigate the orbital magnetism and MAE of low dimensional
systems
Exploiting Self-Organization in Bioengineered Systems: A Computational Approach
The productivity of bioengineered cell factories is limited by inefficiencies in nutrient delivery and waste and product removal. Current solution approaches explore changes in the physical configurations of the bioreactors. This work investigates the possibilities of exploiting self-organizing vascular networks to support producer cells within the factory. A computational model simulates de novo vascular development of endothelial-like cells and the resultant network functioning to deliver nutrients and extract product and waste from the cell culture. Microbial factories with vascular networks are evaluated for their scalability, robustness, and productivity compared to the cell factories without a vascular network. Initial studies demonstrate that at least an order of magnitude increase in production is possible, the system can be scaled up, and the self-organization of an efficient vascular network is robust. The work suggests that bioengineered multicellularity may offer efficiency improvements difficult to achieve with physical engineering approaches
Magnetic moments of W 5d in Ca2CrWO6 and Sr2CrWO6 double perovskites
We have investigated the magnetic moment of the W ion in the ferrimagnetic
double perovskites Sr2CrWO6 and Ca2CrWO6 by X-ray magnetic circular dichroism
(XMCD) at the W L(2,3) edges. In both compounds a finite negative spin and
positive orbital magnetic moment was detected. The experimental results are in
good agreement with band-structure calculations for (Sr/Ca)2CrWO6 using the
full-potential linear muffin-tin orbital method. It is remarkable, that the
magnetic ordering temperature, TC, is correlated with the magnetic moment at
the 'non-magnetic' W atom.Comment: accepted for publicatio
- …