621 research outputs found
Magnetic Phase Control in Monolayer Films by Substrate Tuning
We propose to tailor exchange interactions in magnetic monolayer films by
tuning the adjacent non-magnetic substrate. As an example, we demonstrate a
ferromagnetic-antiferromagnetic phase transition for one monolayer Fe on a
Ta(x)W(1-x)(001) surface as a function of the Ta concentration. At the critical
Ta concentration, the nearest-neighbor exchange interaction is small and the
magnetic phase space is dramatically broadened. Complex magnetic order such as
spin-spirals, multiple-Q, or even disordered local moment states can occur,
offering the possibility to store information in terms of ferromagnetic dots in
an otherwise zero-magnetization state matrix.Comment: after minor changes, 5 pages, 5 figures, revtex
Magnetism of mixed quaternary Heusler alloys: (Ni,T)MnSn (T=Cu,Pd) as a case study
The electronic properties, exchange interactions, finite-temperature
magnetism, and transport properties of random quaternary Heusler NiMnSn
alloys doped with Cu- and Pd-atoms are studied theoretically by means of {\it
ab initio} calculations over the entire range of dopant concentrations. While
the magnetic moments are only weakly dependent on the alloy composition, the
Curie temperatures exhibit strongly non-linear behavior with respect to
Cu-doping in contrast with an almost linear concentration dependence in the
case of Pd-doping. The present parameter-free theory agrees qualitatively and
also reasonably well quantitatively with the available experimental results. An
analysis of exchange interactions is provided for a deeper understanding of the
problem. The dopant atoms perturb electronic structure close to the Fermi
energy only weakly and the residual resistivity thus obeys a simple Nordheim
rule. The dominating contribution to the temperature-dependent resistivity is
due to thermodynamical fluctuations originating from the spin-disorder, which,
according to our calculations, can be described successfully via the disordered
local moments model. Results based on this model agree fairly well with the
measured values of spin-disorder induced resistivity.Comment: 13 pages, 13 figure
Magnetism of 3d transition metal atoms on W(001): submonolayer films
We have investigated random submonolayer films of 3d transition metals on
W(001). The tight-binding linear muffin-tin orbital method combined with the
coherent potential approximation was employed to calculate the electronic
structure of the films. We have estimated local magnetic moments and the
stability of different magnetic structures, namely the ferromagnetic order, the
disordered local moments and the non-magnetic state, by comparing the total
energies of the corresponding systems. It has been found that the magnetic
moments of V and Cr decrease and eventually disappear with decreasing coverage.
On the other hand, Fe retains approximately the same magnetic moment throughout
the whole concentration range from a single impurity to the monolayer coverage.
Mn is an intermediate case between Cr and Fe since it is non-magnetic at very
low coverages and ferromagnetic otherwise.Comment: 5 pages, 3 figures in 6 files; presented at ICN&T 2006, Basel,
Switzerlan
Landauer theory of ballistic torkances in non-collinear spin valves
We present a theory of voltage-induced spin-transfer torques in ballistic
non-collinear spin valves. The torkance on one ferromagnetic layer is expressed
in terms of scattering coefficients of the whole spin valve, in analogy to the
Landauer conductance formula. The theory is applied to Co/Cu/Ni(001)-based
systems where long-range oscillations of the Ni-torkance as a function of Ni
thickness are predicted. The oscillations represent a novel quantum size effect
due to the non-collinear magnetic structure. The oscillatory behavior of the
torkance contrasts a thickness-independent trend of the conductance.Comment: Version 3: 6 pages, 3 figures. Corrected version that passed the
peer-review proces
Approximations of singular vertex couplings in quantum graphs
We discuss approximations of the vertex coupling on a star-shaped quantum
graph of edges in the singular case when the wave functions are not
continuous at the vertex and no edge-permutation symmetry is present. It is
shown that the Cheon-Shigehara technique using interactions with
nonlinearly scaled couplings yields a -parameter family of boundary
conditions in the sense of norm resolvent topology. Moreover, using graphs with
additional edges one can approximate the -parameter family of
all time-reversal invariant couplings.Comment: LaTeX source file, 33 pages, with 3 eps figure
Conductance calculations for quantum wires and interfaces: mode matching and Green functions
Landauer's formula relates the conductance of a quantum wire or interface to
transmission probabilities. Total transmission probabilities are frequently
calculated using Green function techniques and an expression first derived by
Caroli. Alternatively, partial transmission probabilities can be calculated
from the scattering wave functions that are obtained by matching the wave
functions in the scattering region to the Bloch modes of ideal bulk leads. An
elegant technique for doing this, formulated originally by Ando, is here
generalized to any Hamiltonian that can be represented in tight-binding form. A
more compact expression for the transmission matrix elements is derived and it
is shown how all the Green function results can be derived from the mode
matching technique. We illustrate this for a simple model which can be studied
analytically, and for an Fe|vacuum|Fe tunnel junction which we study using
first-principles calculations.Comment: 14 pages, 5 figure
Spin-transfer torques in anti-ferromagnetic metals from first principles
In spite of the absence of a macroscopic magnetic moment, an anti-ferromagnet
is spin-polarized on an atomic scale. The electric current passing through a
conducting anti-ferromagnet is polarized as well, leading to spin-transfer
torques when the order parameter is textured, such as in anti-ferromagnetic
non-collinear spin valves and domain walls. We report a first principles study
on the electronic transport properties of anti-ferromagnetic systems. The
current-induced spin torques acting on the magnetic moments are comparable with
those in conventional ferromagnetic materials, leading to measurable angular
resistances and current-induced magnetization dynamics. In contrast to
ferromagnets, spin torques in anti-ferromagnets are very nonlocal. The torques
acting far away from the center of an anti-ferromagnetic domain wall should
facilitate current-induced domain wall motion.Comment: The paper has substantially been rewritten, 4 pages, 5 figure
Transport properties of single atoms
We present a systematic study of the ballistic electron conductance through
sp and 3d transition metal atoms attached to copper and palladium crystalline
electrodes. We employ the 'ab initio' screened Korringa-Kohn-Rostoker Green's
function method to calculate the electronic structure of nanocontacts while the
ballistic transmission and conductance eigenchannels were obtained by means of
the Kubo approach as formulated by Baranger and Stone. We demonstrate that the
conductance of the systems is mainly determined by the electronic properties of
the atom bridging the macroscopic leads. We classify the conducting
eigenchannels according to the atomic orbitals of the contact atom and the
irreducible representations of the symmetry point group of the system that
leads to the microscopic understanding of the conductance. We show that if
impurity resonances in the density of states of the contact atom appear at the
Fermi energy, additional channels of appropriate symmetry could open. On the
other hand the transmission of the existing channels could be blocked by
impurity scattering.Comment: RevTEX4, 9 pages, 9 figure
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