862 research outputs found
Substrate-induced antiferromagnetism of an Fe monolayer on the Ir(001) surface
We present detailed ab initio study of structural and magnetic stability of a
Fe-monolayer on the fcc(001) surface of iridium. The Fe-monolayer has a strong
tendency to order antiferromagnetically for the true relaxed geometry. On the
contrary an unrelaxed Fe/Ir(001) sample has a ferromagnetic ground state. The
antiferromagnetism is thus stabilized by the decreased Fe-Ir layer spacing in
striking contrast to the recently experimentally observed antiferromagnetism of
the Fe/W(001) system which exists also for an ideal bulk-truncated, unrelaxed
geometry. The calculated layer relaxations for Fe/Ir(001) agree reasonably well
with recent experimental LEED data. The present study centers around the
evaluation of pair exchange interactions between Fe-atoms in the Fe-overlayer
as a function of the Fe/Ir interlayer distance which allows for a detailed
understanding of the antiferromagnetism of a Fe/Ir(001) overlayer. Furthermore,
our calculations indicate that the nature of the true ground state could be
more complex and display a spin spiral-like rather than a
c(2x2)-antiferromagnetic order. Finally, the magnetic stability of the Fe
monolayer on the Ir(001) surface is compared to the closely related Fe/Rh(001)
system.Comment: 8 pages, 4 figure
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
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 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
Relation of Curie temperature and conductivity: (Ga,Mn)As alloy as a case study
Experimental investigations of diluted magnetic semiconductors indicate a
strong relation between Curie temperature and conductivity. Both quantities
depend non trivially on the concentration of magnetic impurities, the carrier
density, and the presence of compensating defects. We calculate both Curie
temperature and conductivity of (Ga,Mn)As alloys in a selfconsistent manner
based on the same first principles Hamiltonian in which the presence of
compensating defects is taken into account. The effect of As-antisites and
Mn-interstitials is determined separately and a good agreement between theory
and experiment exists only in the case where the dominating mechanism of is due
to the Mn-interstitials.Comment: The manuscript is accepted for publication in AP
Shot noise in magnetic tunnel junctions from first principles
We compute the shot noise in ballistic and disordered Fe/MgO/Fe tunnel
junctions by a wave function-matching method. For tunnel barriers with no more
than 5 atomic layers we find a suppression of the Fano factor as a function of
the magnetic configuration. For thicker MgO barriers the shot noise is
suppressed up to a threshold bias indicating the onset of resonant tunneling.
We find excellent agreement with recent experiments when interface disorder is
taken into accountComment: 5 pages,5 figure
Correlated Doping in Semiconductors: The Role of Donors in III-V Diluted Magnetic Semiconductors
We investigate the compositional dependence of the total energy of the mixed
crystals (Ga,Mn)As co-doped with As, Sn, and Zn. Using the ab initio LMTO-CPA
method we find a correlation between the incorporation of acceptors (Mn, Zn)
and donors (Sn, antisite As). In particular, the formation energy of As_Ga is
reduced by approx. 0.1 eV in the presence of Mn, and vice versa. This leads to
the self-compensating behavior of (Ga,Mn)As.Comment: 8 pages, 2 figures, presented at the XXXI Int. School of
Semiconducting Compounds, Jaszowiec 2002, Polan
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