Surface Science 602 (2008) 2789–2795
Contents lists available at ScienceDirect
Surface Science
journal homepage: www.elsevier.com/locate/susc
Theoretical investigation of Mn adsorbates aside self-organised Bi nanolines
on hydrogenated Si(0 0 1) surface
A.Z. AlZahrani a,*, G.P. Srivastava a, R.H. Miwa b
a
b
School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
Instituto de Fisica, Universidade Federal de Uberlândia, CP 593, CEP 38400-902, Uberlândia, MG, Brazil
a r t i c l e
i n f o
Article history:
Received 4 March 2008
Accepted for publication 7 July 2008
Available online 22 July 2008
Keywords:
Si(0 0 1) surface
Bi nanolines
Mn adsorption
Density functional theory
Local density approximation
Pseudopotential method
a b s t r a c t
We have theoretically investigated the atomic structure, magnetic behaviour, and electronic properties of
Mn adsorbates on hydrogen passivated self-organised Bi nanolines on the Si(0 0 1)surface. It is found that
the most stable geometry for 16 monolayer (ML) coverage of Mn is just underneath the first Si(0 0 1) surface
layer. The Mn atoms in the optimised configuration are seven-fold coordinated with their neighbouring Si
atoms. Total energy calculations suggest that the Mn adsorbates form a degenerate state of ferromagnetic
and anti-ferromagnetic lines parallel and adjacent to the self-assembled Bi lines. The density functional
band structure calculation within the local-spin density approximation shows that the ferromagnetic system behaves like a metal in both spin channels. On the other side, the anti-ferromagnetic phase exhibits a
half-metallic phenomenon with semiconducting character for the majority spin channel and semi-metallic character for the minority spin channel.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
Dilute magnetic semiconductor (DMS) materials have been
known for a long time as the gate for great industrial applications
and semiconductor technologies. Recently, the magnetoelectronics
or spin electronics (spintronics) has opened the possibility for fabricating up-to-date unique type of multi-functional devices [1–3].
Use of both the charge and spin of the electron is expected to provide an opportunity of creating a remarkable new generation of
microelectronic devices which might be used for processing and
information storage. Technologically, it is most desirable to use
transition metals and silicon surfaces in building up heterostructures [4]. Experimentally, this can be done by either using a ferromagnetic thin film as a coating layer or depositing ferromagnetic
metals on the silicon surface. It is found that the latter is more
energetically favoured due to the strong coupling between the silicon atoms and the transition metals.
Several works have examined the possibility of developing
magnetoelectronics using silicide films involving transition metals
such as Fe, Co, and Ni [5]. However, such silicides are known to be
either weakly magnetic or non-magnetic, and thus not expected to
be useful for spintronics applications. On the other hand, it is found
that Mn silicide displays a considerably better magnetic behaviour
[2,3]. In fact, it has also been reported that the ferromagnetic phase
with a Curie temperature above room temperature can be created
* Corresponding author. Tel.: +44 1392 264198; fax: +44 1392 264111.
E-mail address: [email protected] (A.Z. AlZahrani).
0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2008.07.009
by Mn ion implantation into Si [6]. Various experimental and theoretical studies have been devoted to investigate progressive
changes in the atomic geometry and electronic structure of the
Si(0 0 1) surface upon Mn adsorption. Detailed calculations of the
electronic properties and magnetic moments of Mn adatoms have
indicated the possibility of achieving ferromagnetic semiconductor
Si(0 0 1) surfaces [7–9]. Experimental studies have demonstrated
that the deposition of Mn atoms on clean Si(0 0 1) surfaces leads
to the possibility of growing super doped Si:Mn thin films [10].
Very recently, Liu and Reinke [11] have studied the formation of
Mn nanostructures on the Si(0 0 1)-(2 1) surface using scanning
tunnelling microscope (STM) as a function of Mn coverage. They
have concluded that Mn wire formation dominates at low coverages. It has been also reported that some Heusler alloys, such as
Co2MnX (X = Si, Ge or Sn) are ferromagnets [12] and would have
the capability of providing spin polarised carriers for spintronics
usage [13]. In Heusler half metals, the density of states near Fermi
level is finite only for one spin channel. This is in marked contrast
to usual ferromagnetic metals for which both spin channels contribute at the Fermi level. The polarisation-dependent behaviour
of carriers in Heusler alloys has led to studies of the adsorption
of Mn atoms on the Si(1 0 0 ) surface [8].
Self-assembled atomic arrangements on semiconductor surfaces have been the subject of intense experimental and theoretical
investigations. Bi nanolines on the Si(0 0 1) surface, in particular,
have received a great deal of attention. Long, straight, kink- and defect-free Bi nanolines can be formed by annealing the Bi-covered
Si(0 0 1) surface at around the Bi desorption temperature (at about
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A.Z. AlZahrani et al. / Surface Science 602 (2008) 2789–2795
580 °C). Up to date, there have been three different proposals for
the geometry of the Bi nanolines on the Si(0 0 1) surfaces. From
careful investigations of STM images, Miki et al. [14] reported that
two parallel Bi dimers form the a line with a missing dimer row between them (known as the Miki model). Naitoh et al. [15] (Naitoh
model), on the other hand, suggested that the Bi lines are formed
by two parallel and adjacent Bi dimers substituting for four Si dimers, with a missing dimer row next to each Bi dimer. From a detailed theoretical study, Owen et al. [16,17] proposed the Haiku
model, demonstrating that the Bi dimers are separated by a missing Si dimer line and the Si substrate below the Bi lines is heavily
reconstructed, forming fivefold and sevenfold Si rings. In this model the Bi nanolines are 1.5 nm wide. Total energy calculations indicate that the Haiku model represents the energetically most
favourable configuration for Bi nanolines on the Si(0 0 1) clean surface [16,17] as well as on the hydrogen passivated Si(0 0 1) surface
[18,19].
Although Bi nanolines on Si(0 0 1) do not exhibit any useful electronic or optical properties, these provide an excellent template for
generating more interesting and useful nanostructures. Several
experimental and theoretical studies have been reported on the
formation of self-organised species on the Bi nanoline template.
The adsorption of group-III elements (such as In, Al, etc.) and nobel
metals (like Au, Ag, etc.) provide examples of such a system. In particular, it has recently been found that the formation of In-line
structure or In-cluster structure can be fabricated by adsorption
of In adatoms on the Bi nanoline template [20,21]. In these investigations the formation of In cluster in the vacancy site between
the Bi dimers is energetically more preferable configuration. Moreover, Owen and Miki [22] and Koga et al. [23] have reported on the
fabrication of a line structure of Ag atoms on the Bi nanolines. The
adsorption and growth of group-IV elements (such as Ge) along the
Bi nanolines template has also been theoretically investigated [24].
In that study, it was concluded that Ge atoms form mixed dimers
with the Si atoms aside the Bi nanolines. In a more recent work,
Orellana et al. [25,26] have presented a theoretical study of the
adsorption of various sub-monolayer coverages of Fe atoms along
the self-organised Bi nanolines on a hydrogenated Si(0 0 1) surface.
They addressed the equilibrium adsorption sites and bonding of Fe
adatoms with the Si substrate and the Bi nanolines. They reported
that for 1/4 and 1/2 monolayer (ML) coverages neighbouring Fe
adatoms are coupled together anti-ferromagnetically or non-magnetically with equal binding energy. For these low coverages, Fe
was concluded to form line structures. Further work is desirable
for examining structural formation, electronic structure, and development of magnetic properties with adsorption of other transition
metal atoms on the Bi nanoline template.
In the present work, we attempt to report an ab initio theoretical investigation on the electronic and magnetic properties of the
Bi nanoline template upon the adsorption of 1/6 ML of Mn atoms.
Information about the energetic stability and equilibrium geometry are detailed within the local-spin density approximation of
the density functional theory. The magnetic properties of the system is determined in terms of the magnetic moment. Furthermore,
the electronic properties have been investigated by examining the
electronic band structure and charge density. We discuss similarity
and dissimilarity of the results for Mn adsorption with Fe
adsorption.
2. Theoretical method
We have modelled the system by using a repeated supercell
structure [27]. Each unit cell is based on a (2 6) reconstructed
Si(0 0 1) surface mesh and contains ten silicon layers, bismuth
layer, and a vacuum region equivalent to about four substrate lay-
ers in thickness. This corresponds to the Mn surface coverage of 1/6
monolayers (ML). The back surface of the slab was saturated with
two pseudo-hydrogen atoms per Si atom. The dangling bonds of
the upper Si atoms were also saturated by hydrogen atoms. In this
work we consider the Haiku model [16–18] for the Bi nanolines on
the hydrogenated Si(0 0 1) surface. This requires considering a large
reconstructed surface mesh.
Experimental investigations (see e.g., [11]) suggest that the
deposition of Mn on Si(0 0 1) can result in the formation of different
types of structures, such as line, islands, and clusters. These observations, as well as observations made on the adsorption of atoms of
non-transition metals [20–22], indicate that adsorption of transition metal adsorbates on the Bi nanoline template may lead to
alloying between the metal adsorbate and the Bi atoms. However,
this is less likely to happen for the sub-monolayer coverage (1/
6 ML) of Mn considered in this work.
Previous theoretical investigations, based on ab initio calculations, [3] have concluded that, for a given coverage, transition metals such as Mn, Fe, Co, and Ni all occupy similar adsorption sites on
the Si(0 0 1) surface. This work also concludes that for a sub-monolayer coverage of 0.5 ML, transition metal adsorbates occupy the
interstitial site below the top Si atomic layer with a lower formation energy than above the top surface layer. Guided by that work,
as well as more recent works [25,26], we have examined several
trial adsorption sites for the 1/6 ML coverage of Mn adatoms, and
the results are discussed for the most stable geometry.
The results of all calculations presented in this paper are obtained by using the density functional theory [28] in its local-spin
density approximation (DFT–LSDA). The Ceperley-Alder exchangecorrelation scheme [29] is considered in the form parametrised by
Perdew and Zunger [30]. The electron-ion interactions were treated by using the norm-conserving [32] and fully separable pseudopotentials [31]. The single-particle Kohn-Sham [33] wave
functions were expanded in the framework of a plane wave basis
set with a kinetic energy cutoff of 25 Ryd. Throughout the calculations we use the calculated Si equilibrium lattice constant of
5.39 Å, which is slightly smaller than the measured value of
5.43 Å. Self-consistent solutions of the Kohn-Sham equations were
obtained by employing a 4 2 1 k-points (viz four special kpoints) Monkhorst-Pack set [34] within the surface Brillouin zone.
The Hellmann-Feynmann forces on ions were calculated and minimised to obtain the relaxed atomic geometry. The equilibrium
atomic positions were determined by relaxing all atoms in the unit
cell except the bottom Si-layer which was frozen into its bulk
position.
Energy convergence with respect to cell size, energy cut-off, and
k-points was tested for bulk Mn (fcc structure) as well as for one of
the Mn adsorption sites on the surface. Calculations for bulk Mn
produce reasonably well converged results for the equilibrium lattice constant and magnetic moment for 25 Ryd energy cut-off. The
(2 6) reconstructed Si(0 0 1) surface mesh was found to be the
minimum acceptable size from energetics point of view. Increasing
the cell size to (2 7) and (2 8) decreases the total energy,
respectively, by 0.01 and 0.02 eV/(2 1) unit cell. Similarly, the
choice of four special k-points for self-consistent calculation of
charge density is also found to be quite acceptable, as consideration of eight special k-points only decreases the total energy by
0.08 eV/(2 6) cell. Increasing the energy cut-off from 25 Ryd to
30 Ryd decreases the total energy by only 0.01 eV/(2 6) cell. It
is well documented in several previous publications that energy
differences between two competing structures converge much faster than do the total energies for individual structures. The FM and
AFM states were theoretically prepared by assigning suitable initial
magnetic moment values at the Mn sites. For the FM case, we assigned magnetic moments of +0.7 lB at each of the two Mn sites
within the unit cell. For the AFM case, we assigned magnetic
A.Z. AlZahrani et al. / Surface Science 602 (2008) 2789–2795
moments of +0.7 lB at one Mn site and 0.7 lB at the other Mn site
within the unit cell. Changing the initial value of 0.7 lB to 0.5 lB (or
to 1.0 lB) does not alter the final result.
3. Results and discussion
Following the normal procedure, we considered various initial
adsorption sites for the pair of Mn atoms and relaxed the geometries. Based on the conclusion drawn in a previous work [26], we
considered four different adsorption sites: Mn atoms at the bridge
sites inside the Bi nanoline, Mn atoms at the bridge sites aside the
Bi nanoline, Mn atoms inside the six-fold Si cage between the first
and second layer Si surface atoms underneath the Bi nanoline, and
Mn atoms inside the five-fold Si cage between the first and second
layer Si surface atoms aside the Bi nanoline. A summary of the total
energies and magnetic moments for all studied sites is presented in
Table 1
The total energies of the studied geometries with respect to the most stable
configuration. The local spin moments per Mn atom are also indicated in units
of lB.
Configuration
a
b
c
d
Total energy (eV/1 1)
Magnetic moment (lB/Mn)
0.000
2.280
0.312
3.690
0.687
2.490
0.756
2.520
2791
Table 1. In the most stable configuration, shown in Fig. 1a, Mn adatoms form a line parallel to, but aside, the Bi nanoline, and are
chemisorbed interstitially underneath the top-layer Si surface
layer. We have modelled the non-magnetic (NM) phase of the Mn
line by making calculations within the LDA scheme (i.e., with
spin-degenerate charge density, or with zero magnetic moment
for each Mn atom). The ferromagnetic (FM) and anti-ferromagnetic
(AFM) phases of the Mn line were investigated within the LSDA
scheme (i.e., within spin-polarised scheme) with, respectively, parallel and antiparallel electron spin orientations for the two Mn
atoms within a unit cell. For the NM phase we compared the total
energies for the (2 6) unit cell with one and two Mn adatoms in
the five-fold Si cage. It was found that the binding energy increased
by 0.04 eV/(1 1) unit cell when the second Mn atom was adsorbed. This indicates an attractive interaction between neighbouring atoms along the Mn line. Furthermore, we performed additional
calculations for interstitial adsorption of Mn with 1/6 ML coverage
on the clean surface. It is found that there is an increased binding
energy of 0.143 eV/(1 1) unit cell when the H-terminated surface
is considered. The relative stabilities of different Mn coverages are
not expected to change (i.e., would be almost the same for both
clean and H-terminated surfaces). It is of particular interest to note
that, for the most plausible configuration, the total energy of the
system with Mn adatoms adsorbed along the Bi nanoline surface
is reduced by 0.51 eV/Mn atom (0.72 eV/Mn atom) comparing with
that of Mn along the hydrogenated surface (bare surface). This
a
b
A
A’
B
B’
c
d
H atoms
Bi atoms
Mn atoms
1st layer Si atoms
2nd layer Si atoms
Other layers Si atoms
Fig. 1. Schematic side and top views of a few possible geometries for the Mn adsorbates on Bi/H/Si(0 0 1)-(2 6) surface. The most stable configuration is shown in panel (a).
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A.Z. AlZahrani et al. / Surface Science 602 (2008) 2789–2795
structure with NM and AFM phases [25]. We find that Mn lines
present quite a different scenario for the magnetic phases compared with the ones obtained previously for the Fe line structure.
Our calculations suggest that the NM phase is at least 0.11 eV/
Mn less stable than the magnetic phases. The FM and AFM phases
are almost equally stable configurations. The magnetic moments
for the FM and AFM are 2.280 lB and 0.865 lB per Mn atom,
respectively. It is found that the magnetic moment of the Mn atom
is dependent on its adsorption site. For example, when Mn is adsorbed interstitially between the nanoline, its magnetic moment
increases to 3.69 lB. On the other hand, the magnetic moment is
2.5 lB when Mn is adsorbed at bridge sites. The lowest magnetic
moment for the interstitial sites aside the nanoline results from a
strong bond hybridisation between the Mn and neighbouring Si
atoms. This value of the magnetic moment for the Mn line formation on the Bi nanoline template is lower than the value for the ferromagnetic Mn fcc crystal, 4.0 lB [35] and 3.68 lB from our
calculations. This is a consequence of the hybridisation of Mn d
reduction indicates that the Bi nanoline fully occupies the uppermost dangling bonds of Si atoms.
As shown in Fig. 1a, the Mn atoms have seven Si neighbours
with interatomic distances ranging between 2.10 and 2.47 Å. The
average Mn–Si bond length of 2.28 Å is in good agreement with
Dalpian et al. [8]. The Mn atoms are located 0.42 Å below the Si dimer, whose length increases by 1.7–2.38 Å. This value (viz the Si–Si
bond length) agrees well with the value obtained by Hortamani et
al. [4]. The Bi–Bi dimer length is also increased due to Mn adsorption, from 3.07 Å for the clean Haiku model of the nanoline to
3.17 Å. The lateral distance between the Bi dimer lines is 6.20 Å.
The average Si-Bi bond length is approximately 2.75 Å, very close
to the value obtained for the Bi nanoline system [18]. This average
length is reasonably smaller than the sum of their atomic radii
(about 3.1 Å), indicating a stronger bonding between the Bi and
Si atoms.
In a recent work it was found that the formation of a line structure of Fe adatoms on the Bi nanoline template forms a degenerate
a
1.5
1
Energy (eV)
J
0.5
J’
0
J
J’
Energy (eV)
b
c
1
1
0.5
0.5
Cd2
Cu2
0
Vu1
Cd1
0
Cu1
Vd1
Vu2
Vd2
Vu3
J
Vd3
J’
J
Energy (eV)
d
e
1
1
0.5
0.5
Cd1’
Cu1’
Vu1’
0
Vd1’
0
Vu2’
Vd2’
Vu3’
Vu4’
J
J’
Vd3’
J’
J
J’
Fig. 2. (a) Band structure plot for the clean Bi lines on the H/Si(0 0 1)-(2 6) surface. Electronic band structure for (b) majority spin, and (c) minority spin of the ferromagnetic
Mn adsorbates on the Bi/H/Si(0 0 1)-(2 6) surface. Electronic band structure for (d) majority spin, and (e) minority spin of the antiferromagnetic Mn adsorbates on the Bi/H/
Si(0 0 1)-(2 6) surface. The Fermi level is set to be at the zero point. The surface Brillouin zone is indicated.
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A.Z. AlZahrani et al. / Surface Science 602 (2008) 2789–2795
orbitals with neighbouring Si s and p orbitals within the Bi nanoline system.
It is interesting to note that the adsorption and surface environment dependence of the magnetic moment of Mn has also been
investigated in a previous work. Dalpian et al. [8] reported that,
for the bare Si(0 0 1) surface, the magnetic moment of the Mn
atoms for the most stable structure (interstitial site) is 0.88 lB. This
value increases to 2.26 lB for the pedestal site which is less favourable than the former. For the hydrogenated surface, on the other
hand, they determined that the magnetic moment of the Mn atom
in the most stable configuration (bridge site) has a value of 3.25 lB.
This value dramatically decreases to 0.90 lB if the Mn is adsorbed
on an interstitial site.
Turning our attention to the electronic properties of the system,
we performed calculations for both FM and AFM structures of the
most stable configuration. However, in order to understand the
changes occurring in these structures, we have started with studying the electronic band structure of the host template, that is the
clean Bi nanolines growth on hydrogenated Si(0 0 1) surface. We
performed electronic band calculations for the clean Bi nanolines
surface along the high symmetry directions: perpendicular to the
Bi lines denoted by JC and parallel to the Bi lines denoted by CJ0 .
(a) FM
250
200
DOS (states/eV)
150
P
Bi/H/Si(001)
Total
100
S
50
0
0
2.5
Energy (eV)
(b) AFM
DOS (states/eV)
250
P
200
Bi/H/Si(001)
150
Total
100
S
50
0
0
2.5
Energy (eV)
Fig. 3. DOS diagrams for the majority and minority spin orientations of the (a) ferromagnetic and (b) antiferromagnetic Mn adsorbates on the Bi/H/Si(0 0 1)-(2 6) surface.
The dashed curves represent the clean Bi lines on H/Si(0 0 1). The Fermi level is set at the energy zero.
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A.Z. AlZahrani et al. / Surface Science 602 (2008) 2789–2795
Fig. 2a shows that the clean surface is semiconducting with the
LDA band gap of 0.98 eV. This result is in good agreement with
the previous study by Orellana et al. [25]. Within the band gap region, there are no surface states originating from Bi atoms or Si
atoms. The clarity of the gap region can be easily explained. On
the one hand, the filled states related to the Bi atoms lie below
the valence band maximum (VBM). On the other hand, there is
no any sp contribution from the Si dangling bonds because they
are totally saturated by the hydrogen atoms.
The electronic band structures of the FM phase of Mn line on the
Bi nanoline template for the majority and minority spin channels
0.4
Charge Density (e/au)
3
a
are shown in Fig. 2b and c, respectively. The corresponding density
of states (DOS) are shown in Fig. 3a. For majority-spin channel of
the FM system, shown in Fig. 2b, we clearly identify two empty
states, cu1 and cu2, and three filled states, vu1, vu2 and vu3 within
the gap region of the nanoline template. Comparing these band
dispersions with those performed for the nanoline template, we
conclude that the origins of these surface states are the Mn (3d)
orbitals. Along the CJ direction the bands within the gap region
are very flat, indicating that there is no interaction between the
Mn lines in two neighbouring unit cells. In addition, the flatness
of those energy bands for wave vectors perpendicular to the Bi
0.3
0.2
0.1
0
Si
Charge Density (e/au) 3
b
Mn
0.08
0.06
0.04
0.02
0
Si
Si
c
d
2.38
2.35
2.09
Mn1
2.24
2.35
2.35
2.40
2.24
2.10
2.39
2.35
Mn2
2.38
Fig. 4. Total charge density along (a) the line connecting Mn atom with its neighbouring Si atom and (b) the Si–Si dimer just above the Mn atom. Total charge density
contours on a plane (c) parallel to the Bi line and passing through the Mn atoms (the vertical plane containing line BB0 in Fig. 1a) and (d) perpendicular to the Bi line and
passing through the Si–Si dimers just above the Mn atoms (the horizontal plane containing line AA0 in Fig. 1a).
A.Z. AlZahrani et al. / Surface Science 602 (2008) 2789–2795
nanolines supports the adequacy of the supercell size used in our
calculations. On the other hand, along the CJ0 direction there is a
reasonable amount of dispersion of such bands, indicating significant interaction between the orbitals of neighbouring Mn atoms
along a Mn line. The density of states near the Fermi level is finite
for both spin channels, as seen from Fig. 3a.
In Fig. 4a, we have shown the total charge density along the
lines joining the Mn atom with its neighbouring Si atoms. There
is a clear bond formation between Mn and Si atoms. This bond
has some degree of covalency, but shows a large amount of ionic
nature. The adsorption of Mn atoms has weakened the Si–Si dimer,
the maximum charge density in which is quite small, as seen in Fig.
4b. In fact the level of covalency between the Si dimer atoms is
much lower than that in the Mn–Si bond. These observations are
confirmed from an examination of the charge density contour plots
in Fig. 4c and d, where they show the bonding between the Mn
atoms with Si chain atoms and the bonding between Si dimer
atoms, respectively.
The band structure results for the AFM system are shown in Fig.
2d and e. Similar to the FM case, the bands near the Fermi level are
very flat along CJ (i.e., normal to the Mn line) and show reasonable
amount of dispersion along CJ0 (i.e., along the Mn line). For the
majority channel, we have observed five surface states inside the
gap region of the clean Bi–Si( 0 01 ) surface. One of these, labelled
cu10 , is empty while the others, labelled vu10 , vu20 , vu30 and vu40 , are
filled states. The majority-spin channel keeps the semiconducting
character with a band gap of approximately 0.26 eV at the C point.
For the minority channel, we have identified three occupied surface states (labelled vd10 , vd20 and vd30 ) within the gap region, and
one partially occupied (labelled cd10 ). cd10 cuts through the Fermi level which makes the minority-spin channel semimetallic.
Consistent with the band structure calculations, the DOS calculation for the AFM system, shown in the lower panel of Fig. 3, indicates that the structure has an energy gap only in the spin-up state.
The semi-metallic nature of the minority spin channel for the Mnline deposited on the Bi nanoline template is similar to what was
noted for the adsorption of a much smaller coverage of Fe on the
same template [25]. It is clear that the spin-integrated DOS is finite
for both FM and AFM phases. It is also noticed that, upon Mn
adsorption, some of the peaks of the template have been shifted,
and increased in intensity throughout the whole energy range with
more around the band gap regions. Close to the Fermi level, we
have clearly identified some peaks due to the Mn-hybridisation
with the Si atoms. In particular, a shoulder ‘S’ at Ev 1.25 eV and
a peak ‘P’ at Ev + 1.62 eV, are indicated.
4. Summary and conclusion
We have theoretically investigated the structural, electronic,
and magnetic properties of a line formation of Mn adatoms on
the clean Bi nanolines assembled along a hydrogenated Si(0 0 1)(2 6) surface, with a coverage of 16 ML. It is found that the ferromagnetic (FM) and antiferromagnetic (AFM) structures are ener-
2795
getically degenerate. The Mn adatoms in the most stable
geometry are highly coordinated, forming well developed bonds
with seven neighbouring Si atoms. The FM configuration produces
metallic characteristics in both up and down spin channels. However, the AFM system produces half-metallic band structure behaviour, with semiconducting in the majority-spin channel and
semimetallic in the minority-spin channel. The magnetic moment
of the adsorbed Mn atoms is much reduced from its calculated value of 3.68 lB for the fcc crystalline phase. The bond formation between the Mn and Si atoms results in magnetic moment values of
2.280 lB and 0.865 lB per Mn atom for the FM and AFM phases,
respectively.
Acknowledgement
A. AlZahrani gratefully acknowledges financial support from
King Abdulaziz University (KAU), Saudi Arabia.
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