Ion-implanted cobalt atoms into a silver matrix with a layer thickness of about 20 nm were studied by x-ray absorption near-edge spectroscopy ͑XANES͒ at the Co K edge. Full multiple scattering ab initio calculations of Co XANES at the K edge provide a phase fingerprint to distinguish the Co structure of samples prepared at different doses and annealing temperatures. The bcc Co phase is formed for the as-prepared sample with 6 at. % and the fcc Co phase is formed at the expense of the bcc phase for the sample with 12 at. % after annealing at 400°C

Aiguo Li

Guilin Zhang

Jing Zhang

M I Abbas

R Hu

Xinbo Ni

Yeukunng Hwu

Yinsong Wang

Yongpeng Tong

Z Y Wu

CiteSeerX

PHYSICAL REVIEW B 69, 115405 ~2004!XANES investigation of the local structure of Co nanoclusters embedded in Ag
Guilin Zhang,1 Z. Y. Wu,2,3 Aiguo Li,1 Yinsong Wang,1 Jing Zhang,2 M. I. Abbas,2 R. Hu,2
Xinbo Ni,1 Yongpeng Tong,1 and Yeukunng Hwu4
1Shanghai Institute of Nuclear Research, Chinese Academy of Science, Shanghai 201800, China
2Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences,
P.O Box 918, 100039 Beijing, People’s Republic of China
3Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, Via Enrico Fermi 40, I-00044 Frascati, Italy
4Institute of Physics, Academia Sinica, Nankang Taipei 11529, Taiwan, Republic of China
~Received 29 September 2003; published 11 March 2004!
Ion-implanted cobalt atoms into a silver matrix with a layer thickness of about 20 nm were studied by x-ray
absorption near-edge spectroscopy~XANES! at the CoK edge. Full multiple scatteringab initio calculations
of Co XANES at theK edge provide a phase fingerprint to distinguish the Co structure of samples prepared at
different doses and annealing temperatures. The bcc Co phase is formed for the as-prepared sample with 6
at. % and the fcc Co phase is formed at the expense of the bcc phase for the sample with 12 at. % after
annealing at 400 °C.
DOI: 10.1103/PhysRevB.69.115405 PACS number~s!: 61.46.1w, 36.40.Mrr
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izedI. INTRODUCTION
Bulk Co has a hcp crystal structure at room temperatu
and transforms to fcc above 425 °C. In rare cases the unc
mon bcc phase can be found.1,2 Other possible forms such a
truncated octahedral, icosahedra or chainlike agglomer
have been reported.3–5 Clusters of Co embedded in Ag, how
ever, generally show a fcc structure.6 Contrary to these find-
ings, using x-ray absorption near edge structure~XANES!,
assisted with a full multiple scattering~MS! ab initio calcu-
lation of the CoK-edge spectra, we identified a bcc phase
samples prepared by ion implantation of cobalt atoms int
silver matrix with a layer thickness of about 20 nm.
Effects such as the giant magnetoresistance in Co/Cu
Co/Ag films7,8 and the increase of the magnetic moment
atom9 are known to depend dramatically on the shape,
size, and the phase of the clusters. Therefore the precise
finement of the nanoclusters in a matrix and their evolut
under a thermal treatment are crucial to the overall proper
of the granular films. However, the determination of t
structure of Co clusters formed by ion implantation is ge
erally hindered by the small quantity of Co atoms contain
in the matrix and by other special reasons such as the ne
identical hyperfine fields of Co in the hexagonal and
cubic forms.10,11 It is, for instance, not a trivial task to de
termine the phase by Mo¨ssbauer spectroscopy and /or
NMR for nonhomogeneous Co clusters. Since the coord
tion numbers of the first and second shells of Co hcp and
structures are quite similar, and since ion implantation of
in Ag produces very small clusters embedded in a shal
region, the application of an extended x-ray absorption fi
structure analysis is also very cumbersome. Therefore,
need a more accurate method to distinguish the different
sible phases. In this work we demonstrate that useful st
tural information on such nanoclusters can be extracted
XANES.12 Lacking reference spectra from rare atomic
rangements such as the bcc phase of Co, we used inste
full MS ab initio calculation to generate them. We found th
the difference between different structures is sensitively
produced and that the theoretical calculation matches the0163-1829/2004/69~11!/115405~4!/$22.50 69 1154e,
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perimental data in an excellent way, enabling us to iden
the phases and their evolution.
The embedded Co clusters were formed by ion implan
tion. Silver was chosen as a host matrix since Co is imm
cible with Ag and tends to occupy substitutional sites
aggregate in the host Ag matrix lattice. The thermal anne
ing procedure was used to study the Co cluster phase tra
tion.
II. EXPERIMENT
Four silver polycrystalline foils were used as substrat
with a thickness of 60mm and a purity of 99.99%. Before
ion implantation the samples were annealed in a hydro
atmosphere at 800 °C. The Co ions were implanted in
substrate at room temperature (20 °C) by the mass sepa
in the Shanghai Institute of Nuclear Research. The energy
the Co ions was 70 keV, corresponding to a mean range o
nm calculated by TRIM~transport and range of ions throug
matter!. The doses of 1.531016 and 331016 atoms cm22
were chosen for two pairs of samples. Their correspond
concentrations in Ag are about 6% and 12%. Two samp
with two different doses were annealed in a hydrogen atm
sphere at a pressure of 43104 Pa at 400 °C.
XANES spectra at the CoK edge were measured in th
fluorescence mode by using synchrotron radiation with
Si~111! double crystal monochromator at the beam li
4W1B of the Beijing Synchrotron Radiation Facilit
~BSRF!. The storage ring worked at a typical energy of 2
GeV with an electron current of about 100 mA. To suppre
the unwanted higher harmonics, a detuning of 30% was
formed between the two silicon crystals. The incident a
output beam intensities were monitored and recorded u
ionization chambers filled by argon gas and a 25% arg
doped nitrogen mixture. The spectra were scanned in
range of 7.5–7.9 keV with steps of 0.5 eV and an ene
resolution of 1.5 eV. Common background subtraction w
employed in the data reduction and the data were normal
and calibrated to the edge of Co metal foil.©2004 The American Physical Society05-1
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GUILIN ZHANG et al. PHYSICAL REVIEW B 69, 115405 ~2004!III. CALCULATION DETAILS
Co K edge XANES spectra have been simulated via
full MS theory.13 In dealing with the x-ray absorption pro
cess one is faced with two fundamental problems, nam
~a! a reduction of the inherent many-body problem to t
simpler scenario of one electron moving in an effective o
tical potential, and~b! a full description of all orders of the
MS events that the excited photoelectron undergoes in
way in the system, since the perturbation expansion of
MS problem might not converge. A possible solution to the
problems has been described,14 both with respect to the
choice of the optical potential and to the treatment of MS
all orders of the perturbation theory~full MS!. Actually, the
striking success of the MS approach in dealing with the a
plitude and phase problem of the fine structure oscillation
the absorption spectra15 makes one confident that this a
proach may also be extended to more complica
materials.16
The construction of the charge density and potential
low the same patterns as Ref. 13. We use the Matth
prescription17 to construct the cluster density. The Coulom
part of the potential is obtained by a superposition of neu
atomic charge densities taken from the Clementi-Ro
tables.18 For the exchange-correlation part of the poten
we have used the optical Hedin-Lundqvist potential.19 In or-
der to simulate the charge relaxation around the core hol
the photoabsorber of atomic numberZ (Co527) we select
the Z11 approximation~final state rule!,13,19 which consists
FIG. 1. XANES spectra at the CoK edge for the metallic Co
foil ~a!, as-implanted and annealed CoAg samples@~b!-~e!#, the
dose (at/cm2) and annealing temperature (°C) are indicated in e
curve.11540e
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of taking the orbitals of theZ11 atom and constructing th
charge density by using the excited electronic configurat
of the photoabsorber with the core electron promoted t
valence orbital. The calculated spectra are further convolu
with a Lorentzian shaped function with a full width 2.8 e
~Ref. 20! to account for the core hole lifetime and the expe
mental resolution. We have chosen the muffin-tin radii a
cording to the Norman criterion,21 and allowed a 7% overlap
between contiguous spheres to simulate the atomic bond22
IV. RESULTS AND DISCUSSION
The near-edge fine structures of theK-edge absorption
spectra are the fingerprints of the local geometry around
absorbing atom, since it is driven by multiple correlatio
functions beyond the mere pair one. Figure 1 presents the
K-edge XANES spectra, arranged from top to bottom a
function of different preparation and condition~i.e., dose and
annealing temperature!. Also shown in Fig. 1 is the Co
K-edge spectrum of pure hcp Co metal, which is in go
agreement with other published work.2 For convenience, all
spectra are scaled so that the intensity of the main featu
the same in each spectrum. All these spectra display
main features denoted by A, B, C, and D in order of incre
ing photon energy. Several variations in these spectra
monitored, such as the relative intensity of peaks B and
and the separation between peaks B and D. All these cha
in the spectra could be interpreted in terms of the first-sh
and the next-nearest local structure around the photo
h
FIG. 2. XANES spectrum at the CoK edge for the metallic Co
foil ~b! and a simulated spectrum by the full MS theory~a!.5-2
.
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-
XANES INVESTIGATION OF THE LOCAL STRUCTURE . . . PHYSICAL REVIEW B 69, 115405 ~2004!sorber, due to the different multiple scattering processes
The experimental CoK-edge XANES spectrum of pur
hcp Co foil is compared in Fig. 2 with theab initio calcu-
lated XANES using full MS theory with a relaxed final-sta
potential for a finite large cluster which contains 195 ato
within a radius of 8 Å from the central Co atom. The he
agonally close-packed Co atom, surrounded by two nea
neighbors (616) at 2.497 and 2.507 Å, respectively, show
two transition peaks with an energy separation of about
eV between peaks B and D. The agreement between
theory using the pure Co hcp crystalline data and experim
tal data is quite good. Features A, B, C, and D can also
assigned unambiguously to one-electron excitations. Th
states are due to multiple-scattering resonances of thep pho-
toelectron in the continuum, i.e., the maxima of the lo
density of states at the Co site ofp type selected by the
dipole selection rule (D l 561).
In Fig. 3 we present the experimental data of the sam
as implanted with 1.531016atoms cm22, along with the MS
calculation using a bcc cluster containing 181 atoms withi
8-Å sphere. In this structural phase, the central atom is
rounded by two subshells, eight atoms at 2.46 Å and six
2.84 Å respectively. The spectrum is very clearly differe
from that of the standard Co hcp foil as shown in Fig.
@curve ~a!# and/or in Fig. 2. We found a different energ
separation between peaks B and D which is now about 8
smaller than in Fig. 2. However, a very good agreement
tween the experimental and theoretical spectra has b
FIG. 3. XANES spectrum at the CoK edge for the as-implanted
sample with a dose of 1.531016 at/cm2 ~b! and a simulated spec
trum by the full MS theory~a!.11540s
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at
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achieved, demonstrating that the phase transformed fro
hcp to a bcc structure.
In Fig. 4 we show the experimental XANES data of th
sample implanted with 3.031016 atoms cm22 after anneal-
ing at 400 °C. It was found that the spectral behavior sign
cantly deviated from the previous two~typical hcp and bcc
fingerprints!, not only for the relative intensity of peaks B
and C but also for the energy separation of the B and
features~about 33 eV!. A theoretical MS calculation using a
Co fcc crystalline structure~12 nearest neighbors at 2.50 Å!
with a 201-atom cluster resembles the experimental data
dicating that in this case a transformation from an hcp t
fcc-like phase takes place.
The samples prepared in the intermediate conditions
shown in Fig. 1@curves~c! and~d!#, presumably contain two
or three different phase structures. It is known that the ph
as well as its size23 of particles embedded in a matrix no
mally depend on the annealing temperature and on the
planted dose. Therefore, for the samples prepared unde
termediate conditions the phase transformation probably
not take place completely. In those samples it is possible
form a mixed phase of bcc and fcc.
For the annealed samples the fcc and hcp Co phase s
tures were also found by other researchers,24 but with differ-
ent preparation conditions, such as sputtering. Compa
with those studies, in the implanted samples a lot of vac
cies could be produced, leading to a migration enhancem
of Co atoms in Ag. Consequently the grain growth in tho
FIG. 4. XANES spectrum at the CoK edge for the sample with
a dose of 3.031016 at/cm2 after annealing at 400 °C~b! and a simu-
lated spectrum by the full MS theory~a!.5-3
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GUILIN ZHANG et al. PHYSICAL REVIEW B 69, 115405 ~2004!samples would be faster than in the ones prepared by o
methods.
For the as-prepared samples, the phase structure o
grains formed at an early stage is very difficult to be det
mined by TEM due to its small size, being less than 2 nm
6% Co in Ag.25 Therefore no satisfactory report about th
has appeared so far. It was, however, found that a bcc
phase was stabilized in rf-sputtered Co/Fe multilayers2 for a
Co layer thickness lower than 2 nm and a bcc-like crystall
structure with a distorted surface was formed in a polym
matrix.26 Combining those results with ours, we might su
gest that crystalline Co in small size could be stabilized i
bcc phase structure. For the as-implanted Co in Ag, the
grains might be subjected to strong stresses, leading to
formation of a metastable bcc phase. To our knowledge,
is the first time the bcc Co phase structure was observe
the as-implanted CoAg sample. This experimental result
repeated three times.. P
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11540er
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V. CONCLUSION
The bcc Co phase was formed in the as-implanted Ag
with 6-at. % Co and the fcc phase was formed at the expe
of the bcc phase in the sample with 12 at. % after annea
at 400 °C, based on the comparison between the experim
tal results and a theoretical simulation.
ACKNOWLEDGMENTS
G.L.Zh. acknowledges the financial support from the N
tional Natural Science Foundation of China~10075074!.
Z.Y.W. acknowledges the financial support of the100-Talent
Research Programof the Chinese Academy of Sciences a
of the Outstanding Youth Fund~10125523! of the National
Natural Science Foundation of China. The authors wo
like to thank A. Marcelli and H. Pattyn for valuable discu
sion and the staffs of BSRF for their experimental assistanli,
s.
,
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XANES investigation of the local structure of Co nanoclusters embedded in Ag. Phys

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XANES investigation of the local structure of Co nanoclusters embedded in Ag. Phys

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