48 research outputs found
Thickness-dependent magnetic structure of ultrathin Fe/Ir(001) films: from spin-spiral states towards ferromagnetic order
We present a detailed study of the ground-state magnetic structure of
ultrathin Fe films on the surface of fcc Ir(001). We use the spin-cluster
expansion technique in combination with the relativistic disordered local
moment scheme to obtain parameters of spin models and then determine the
favored magnetic structure of the system by means of a mean field approach and
atomistic spin dynamics simulations. For the case of a single monolayer of Fe
we find that layer relaxations very strongly influence the ground-state spin
configurations, whereas Dzyaloshinskii-Moriya (DM) interactions and biquadratic
couplings also have remarkable effects. To characterize the latter effect we
introduce and analyze spin collinearity maps of the system. While for two
monolayers of Fe we find a single-q spin spiral as ground state due to DM
interactions, for the case of four monolayers the system shows a noncollinear
spin structure with nonzero net magnetization. These findings are consistent
with experimental measurements indicating ferromagnetic order in films of four
monolayers and thicker.Comment: 9 pages, 7 figure
Magnetic properties of single atoms of Fe and Co on Ir(111) and Pt(111)
In using the fully relativistic versions of the Embedded Cluster and Screened
Korringa-Kohn-Rostoker methods for semi-infinite systems the magnetic
properties of single adatoms of Fe and Co on Ir(111) and Pt(111) are studied.
It is found that for Pt(111) Fe and Co adatoms are strongly perpendicularly
oriented, while on Ir(111) the orientation of the magnetization is only
out-of-plane for a Co adatom, for an Fe adatom it is in-plane. For comparison
also the so-called band energy parts of the anisotropy energy of a single layer
of Fe and Co on these two substrates are shown. The obtained results are
compared to recent experimental studies using e.g. the spin-polarized STM
technique
Tailoring exchange interactions in engineered nanostructures: Ab initio study
We present a novel approach to spin manipulation in atomic-scale
nanostructures. Our ab initio calculations clearly demonstrate that it is
possible to tune magnetic properties of sub-nanometer structures by adjusting
the geometry of the system. By the example of two surface-based systems we
demonstrate that (i) the magnetic moment of a single adatom coupled to a buried
magnetic Co layer can be stabilized in either a ferromagnetic or an
antiferromagnetic configuration depending on the spacer thickness. It is found
that a buried Co layer has a profound effect on the exchange interaction
between two magnetic impurities on the surface. (ii) The exchange interaction
between magnetic adatoms can be manipulated by introducing artificial
nonmagnetic Cu chains to link them.Comment: 4 pages, submitted to PR
Reversible enhancement of the magnetism of ultrathin Co films by H adsorption
By means of ab initio calculations, we have investigated the effect of H
adsorption in the structural, electronic and magnetic properties of ultrathin
Co films on Ru(0001). Our calculations predict that H occupies hollow sites
preserving the two-dimensional 3-fold symmetry. The formation of a complete H
overlayer leads to a very stable surface with strong H-Co bonds. H tends to
suppress surface features, in particular, the enhancement of the magnetic
moments of the bare film. The H-induced effects are mostly confined to the Co
atoms bonded to H, independent of the H coverage or of the thickness and the
structure of the Co film. However, for partial H coverages a significant
increase occurs in the magnetic moment for the surface Co atoms not bonded to
H, leading to a net enhancement of surface magnetism.Comment: 6 pages, 4 figures, 3 table
Atomistic spin-model based on a new spin-cluster expansion technique: Application to the IrMn3/Co interface
In order to derive tensorial exchange interactions and local magnetic
anisotropies in itinerant magnetic systems, an approach combining the
Spin-Cluster Expansion with the Relativistic Disordered Local Moment scheme is
introduced. The theoretical background and computational aspects of the method
are described in detail. The exchange interactions and site resolved anisotropy
contributions for the IrMn3/Co(111) interface, a prototype for an exchange bias
system, are calculated including a large number of magnetic sites from both the
antiferromagnet and ferromagnet. Our calculations reveal that the coupling
between the two subsystems is fairly limited to the vicinity of the interface.
The magnetic anisotropy of the interface system is discussed, including effects
of the Dzyaloshinskii-Moriya interactions that appear due to symmetry breaking
at the interface.Comment: 10 pages, 6 figure
Spin-polarized surface states close to adatoms on Cu(111)
We present a theoretical study of surface states close to 3d transition metal
adatoms (Cr, Mn, Fe, Co, Ni and Cu) on a Cu(111) surface in terms of an
embedding technique using the fully relativistic Korringa-Kohn-Rostoker method.
For each of the adatoms we found resonances in the s-like states to be
attributed to a localization of the surface states in the presence of an
impurity. We studied the change of the s-like densities of states in the
vicinity of the surface state band-edge due to scattering effects mediated via
the adatom's d-orbitals. The obtained results show that a magnetic impurity
causes spin-polarization of the surface states. In particular, the long-range
oscillations of the spin-polarized s-like density of states around an Fe adatom
are demonstrated.Comment: 5 pages, 5 figures, submitted to PR
Chiral asymmetry of the spin-wave spectra in ultrathin magnetic films
We raise the possibility that the chiral degeneracy of the magnons in
ultrathin films can be lifted due to the presence of Dzyaloshinskii-Moriya
interactions. By using simple symmetry arguments, we discuss under which
conditions such a chiral asymmetry occurs. We then perform relativistic first
principles calculations for an Fe monolayer on W(110) and explicitly reveal the
asymmetry of the spin-wave spectrum in case of wave-vectors parallel to the
(001) direction. Furthermore, we quantitatively interpret our results in terms
of a simplified spin-model by using calculated Dzyaloshinskii-Moriya vectors.
Our theoretical prediction should inspire experiments to explore the asymmetry
of spin-waves, with a particular emphasis on the possibility to measure the
Dzyaloshinskii-Moriya interactions in ultrathin films.Comment: 4 pages, 5 figure
Size-dependent Surface States on Strained Cobalt Nanoislands on Cu(111)
Low-temperature scanning tunneling spectroscopy over Co nanoislands on
Cu(111) showed that the surface states of the islands vary with their size.
Occupied states exhibit a sizeable downward energy shift as the island size
decreases. The position of the occupied states also significantly changes
across the islands. Atomic-scale simulations and ab inito calculations
demonstrate that the driving force for the observed shift is related to
size-dependent mesoscopic relaxations in the nanoislands.Comment: 4 pages, 4 figure
Theory of anisotropic Rashba splitting of surface states
We investigate the surface Rashba effect for a surface of reduced in-plane
symmetry. Formulating a k.p perturbation theory, we show that the Rashba
splitting is anisotropic, in agreement with symmetry-based considerations. We
show that the anisotropic Rashba splitting is due to the admixture of bulk
states of different symmetry to the surface state, and it cannot be explained
within the standard theoretical picture supposing just a normal-to-surface
variation of the crystal potential. Performing relativistic ab initio
calculations we find a remarkably large Rashba anisotropy for an
unreconstructed Au(110) surface that is in the experimentally accessible range.Comment: 4 pages, 5 figure