Nuclear Instruments and Methods in Physics Research B 212 (2003) 530–534

www.elsevier.com/locate/nimb

Ion irradiation induced impurityredistribution in Pt/C multilayers

S. Bera a, D.K. Goswami a, K. Bhattacharjee a, B.N. Dev a,*,G. Kuri b,1, K. Nomoto c, K. Yamashita c

a Institute of Physics, Sachivalaya Marg, Bhubaneswar 751005, Indiab Hamburg Synchrotron Radiation Laboratory (HASYLAB) at DESY, Notkestar 85, D-22603 Hamburg, Germany

c Department of Physics, Nagoya University, Nagoya 464-8602, Japan

Abstract

Ion irradiation induced modifications of a periodic Pt/C multilayer system containing Fe impurity have been ana-

lyzed by X-ray techniques suitable for exploring nanometer depth scales with sub-nanometer depth resolution. The

multilayer stack with 15 Pt/C layer pairs (period 4.23 nm, total thickness 63.45 nm) was fabricated on a glass substrate.

A 2 MeV Au2þ ion beam was rastered on the sample to obtain uniformly irradiated strips with fluences from 1� 1014 to

1� 1015 ions/cm2. These strips were analyzed with X-ray standing wave and X-ray reflectivity experiments. Ion induced

atomic displacements across multilayer interfaces are known [Appl. Phys. Lett. 79 (2001) 467]. Here additionally we

identify irradiation induced redistribution of Fe impurity atoms, which cannot be explained simply by atomic dis-

placements due to ion–atom collision. With increasing ion fluences more Fe atoms migrate from C- to Pt-layers. This

behaviour has been explained in terms of radiation induced enhanced diffusion and Fe–Pt and Fe–C phase diagrams.

� 2003 Published by Elsevier B.V.

PACS: 61.80; 68.65.A; 68.49.U

Keywords: Layered synthetic microstructures; X-ray standing wave; Ion-beam induced impurity redistribution

1. Introduction

The knowledge obtained from the studies of

ion–solid interactions has been utilized in many

applications, for example, surface chemical and

structural analysis, sputtering in production of

* Corresponding author. Tel.: +91-674-2301058; fax: +91-

674-2300142.

E-mail address: bhupen@iopb.res.in (B.N. Dev).1 Present address: Paul Scherrer Institute, CH-5232 Villigen

PSI, Switzerland.

0168-583X/$ - see front matter � 2003 Published by Elsevier B.V.

doi:10.1016/S0168-583X(03)01508-8

thin films and surface cleaning, ion implantation,ion beam modification of structure and properties

etc. Currently exciting possibilities are emerging in

the area of ion beam modifications of layered

synthetic microstructures (LSM�s). LSM�s are

fabricated by depositing alternating layers of two

different materials on a substrate. The LSM�s havewide ranging applications. Properties of such

multilayers can be modified by introducing impu-rities or by other means, such as, exposure to en-

ergetic ion beams. Magnetic multilayers, e.g. Co/

Cu/Co/. . .. multilayers, have shown drastic change

in magnetic coupling and magnetoresistance in the

S S S

S

Si(Li)NaI

K

1

2

S0

1 2 3

4

A B C D E F

SA

Ge(111)

IC

DORIS

RingStorage

NaI

Ge(111)

SA

θ

θ

Fig. 1. Schematic experimental setup for XSW and XRR ex-

periments with synchrotron radiation. S0–S4: slits; IC: ioniza-

tion chamber; K: kapton foil; Ge(1 1 1): Ge double crystal

monochromator; SA: sample; NaI, Si(Li): detectors. Top view

of the irradiated sample configuration is shown in the inset.

S. Bera et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 530–534 531

presence of magnetic impurities (e.g. Ni) in the

nonmagnetic (Cu) layers [1]. Co/Pt/Co/. . . multi-

layers, when exposed to energetic ion beams, show

spin reorientation transition. Such systems aresuited for ultrahigh density recording media [2].

Ion beam is known to cause atomic displacements

across the interfaces of the multilayer system

(MLS) [3]. From these examples it is clear that

atomic distribution and ion beam induced redis-

tribution in multilayers are important aspects to be

investigated for an understanding of the multilayer

properties. However, wherever layer thicknesses ofthe order of 1 nm in a multilayer are involved there

are not many techniques available to study atomic

distribution over this depth scale. Combined X-ray

standing wave (XSW) and reflectivity (XRR)

analysis has been shown to be capable of deter-

mining this distribution [4]. In the present paper

we report the preliminary results of a combined

XSW and XRR analysis for the impurity (Fe) re-distribution in a Pt/C periodic MLS due to ion

irradiation. A Pt/C multilayer has been chosen as a

model system as the large electron density contrast

between Pt and C gives rise to strong Bragg dif-

fraction peaks from a periodic Pt/C multilayer.

This in turn generates a strong standing wave field

in the multilayer. Additionally, ion irradiated Pt/C

multilayers with Fe impurity may themselves showinteresting magnetic behaviour as there are possi-

bilities of formation of Fe–Pt magnetic alloys,

possibly as small clusters.

2. Experimental

Pt/C multilayers were fabricated on float glasssubstrates, kept at room temperature, by ion beam

sputtering. Samples were grown at a low Argon

pressure of 0.1 mbar. Fe impurity was introduced

in the multilayers during growth. The expected Fe

concentration in the C-layers was about 10 at.%.

The Pt/C multilayer sample with Fe incorporation,

used in this study, was prepared at Nagoya Uni-

versity. The sample specifications are: N ¼ 15 (thenumber of layer-pairs in the multilayer stack),

d ¼ 4:23 nm (multilayer period, i.e. the thickness

of a Pt/C layer-pair), C ¼ 0:38 (ratio of Pt layer

thickness to d). The total thickness of the multi-

layer stack is 63.45 nm. Different parts of a large

sample (30� 70) mm2 were irradiated with 2 MeV

Au2þ ions by rastering the ion beam on (30� 5)

mm2 strips at various fluences (ions/cm2) [A(vir-

gin), B(1� 1014), C(3� 1014), D(5� 1014),

F(1� 1015)] at Institute of Physics, Bhubaneswar.

The sample configuration (top view) is shownschematically in the inset of Fig. 1. The range of 2

MeV Au ions in the sample is such that the im-

planted ions are buried deep into the glass sub-

strate well past the multilayer stack. XSW and

XRR experiments were carried out at Hamburg

Synchrotron Radiation Laboratory (HASYLAB

at DESY), Hamburg, at the ROEMO-I beamline

using 14.0 keV monochromatized X-rays. Theexperimental setup is schematically shown in Fig.

1. Fe–K and Pt–L fluorescent photons were de-

tected by a Si(Li) detector and the reflectivity was

measured by a NaI(2) detector. Scattered X-rays

from a kapton foil (K) was monitored in a NaI(1)

detector for the normalization of incident X-ray

intensity. Slits S2 define the incident beam. S3 are

the antiscattering slits.

3. Results and discussion

Let us first discuss what is expected from dif-

ferent distributions of Fe in the Pt/C multilayers.

The inset in Fig. 2 shows the schematic cross-sec-

tional view of the periodic Pt/C MLS. The firstorder Bragg diffraction region from the Pt/C MLS

is shown in Fig. 2 along with some additional

theoretical plots. Standing waves of X-rays are

generated in the MLS when Bragg diffraction oc-

curs. As the angle of incidence ðhÞ of X-rays

0.54 0.64 0.740.74

0

0.2

0.4

0.6

0.8

A

B

C

D

Flu

ores

cenc

e Y

ield

(A

rbitr

ary

Uni

ts)

θ (degree)R

efle

ctiv

ity

F

Fig. 3. Experimental reflectivity and FY: the Bragg peak (––)

for the virgin sample (A) and the Fe–Ka FY (. . .) for the virgin

(A) and the irradiated samples (B–F). The FY curves have been

shifted vertically for clarity.

0.54 0.64 0.740

0.5

1

1.5

2

2.5

3

0

0.2

0.4

0.6

0.8

1

Glass

PtC

Flu

ores

cenc

e Y

ield

(N

orm

aliz

ed)

Ref

lect

ivity

(degree)θ

Fig. 2. Theoretical plots showing the first order Bragg peak and

Fe FY from a Pt/C multilayer for three cases. Reflectivity (––),

Fe in uniform depth distribution only in the C-layers (– – –), Fe

in uniform distribution in both Pt- and C-layers (. . .), Fe in

uniform distribution only in the Pt-layers (– - –). The cross-

sectional view of the multilayer is shown in the inset.

532 S. Bera et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 530–534

advances, in passing the Bragg peak, the periodic

antinodal planes move from the centre of the C-

layers to the centre of the adjacent lower Pt-layers

(through half the period distance d) over the wholedepth of the multilayer. Thus the impurity atoms(as well as host atoms) are exposed to a variation

of X-ray intensity as h changes. As the intensity of

emission of fluorescent photons from an atom in

the sample would be proportional to the field in-

tensity on that atom, the fluorescence yield (FY)

also would have a h-dependence. The h-depen-dence of the Fe FY would depend on the depth

distribution of Fe in the MLS. Some theoreticalplots of Fe FY as a function of h for specific Fe

distributions, following [4], are shown in Fig. 2.

The reflectivity and the field intensities (or FYs)

have been calculated for pure Pt- and C-layers.

These are used only as a guide to understand the

Fe FY variations in Fig. 3.

We notice from Fig. 2 that if Fe is uniformly

distributed only in the C-layers, Fe FY would peakon the rising edge of the Bragg peak. A uniform

depth distribution of Fe within both Pt- and C-

layers would still give rise to a Fe FY peak on the

rising edge of reflectivity, although with a lower

intensity. On the other hand, a uniform distribution

of Fe only in the Pt-layers would give rise to a Fe

FY peak at the falling edge of the Bragg peak. Thus

a simultaneous measurement of reflectivity and Fe

FY on samples irradiated with different ion fluences

would give information on Fe redistribution in the

MLS. XRR measurements were performed for the

h-range: 0–2.9� and simultaneous reflectivity and

FY (Fe–Ka;b, Pt-La;b;c) measurements were per-formed over the first order Bragg peak region. A

full quantitative analysis of data will be presented

elsewhere. Here we present a qualitative analysis

which is adequate to show impurity redistribution.

We may define a parameter a as the ratio of FY

peak intensity at the low-angle side of the Bragg

peak to that at the high-angle side. We notice from

Fig. 2 that a � 1, a > 1 and a < 1 represent thethree situations discussed above (Fe in C-layers, Fe

in both Pt- and C-layers, Fe in Pt-layers), respec-

tively. As more Fe migrates from C- to Pt-layers,

the Fe FY peak on the rising edge of the Bragg peak

gradually decreases and the FY peak on the falling

edge gradually increases. This corresponds to a

gradual decrease of the value of a. So we can now

discuss redistribution of Fe in terms of a.

S. Bera et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 530–534 533

The results of measurements of reflectivity over

the first order Bragg peak region and the corre-

sponding Fe–Ka FY for the strips A, B, C, D and

F are shown in Fig. 3. (The FY curves have beengiven the required corrections as discussed in [4].)

The first order Bragg reflection from the virgin

sample (A) is shown. The angular position of the

Bragg peak for irradiated samples are usually

somewhat different [3]. The details along with full

XSW and XRR analysis to extract various mul-

tilayer parameters will be presented elsewhere.

The Fe–Ka FY curves, shown in Fig. 3, do notexactly correspond to the angles shown in the

abscissa. They represent the FY variation with

respect to their corresponding Bragg peaks. We

notice for the virgin sample (A) that the Fe–Ka

FY peak at the low-angle side of the Bragg peak

is higher. The FY curve shows similarity to the

theoretical curve (Fig. 2) for uniform distribution

of Fe in both Pt-and C-layers. (The detailed dis-tribution may be somewhat different.) This cor-

responds to a > 1. From the fabrication of the

multilayer, although the Fe impurity was ex-

pected predominantly in the C-layers, it is clear

from the Fe–Ka FY curve A that there is already

a higher concentration of Fe in Pt-layers than in

C-layers in the virgin sample. FY data marked B–

F are for increasing ion fluences. (We have notmade measurements on the strip E.) In B, Fe FY

intensities at both edges of the Bragg peak are

approximately equal ða ’ 1Þ. For higher ion flu-

ences (C–F), the intensity is always higher on the

high-angle edge ða < 1Þ, corresponding to in-

creasing Fe concentration in the Pt-layers. This

indicates that on passing from A to F, Fe in the

Pt/C MLS has been considerably redistributedwith higher ion fluences, generating a higher

concentration of Fe in the Pt layers. In fact the

Fe FY shapes for the samples D and F nearly

correspond to the Pt–L FY shapes (not shown

here). Pt–La yield curves for all cases correspond

to a < 1. (The detailed distributions of Fe and

other parameters of the Pt/C multilayers, can be

obtained from a complete combined XSW andXRR analysis [3,4].)

Reflectivity at the Bragg peak depends on the

electron density contrast between the two compo-

nents of a bilayer in a MLS and on interface

roughness. When C-layers are depleted of Fe, the

electron density contrast between the Pt- and the

C-layers is expected to increase. This would in turn

give rise to a more intense Bragg peak. Indeed thereflectivity at the Bragg peak for the sample D (not

shown here) is 0.69 and that for the virgin sample

(A) is 0.65 (shown in Fig. 3). For the virgin sample

the typical surface and interface roughnesses are

0.3–0.4 nm [3,4]. For higher ion fluences, intro-

duction of increasing interface roughness and re-

duction of electron density contrast, due to the

displaced atoms of one type into the other layerfrom ion induced collision cascades, lead to a

lower peak intensity. The peak reflectivity ob-

served for the sample F is 0.57.

Let us try to understand how and why Fe mi-

grates preferentially into the Pt-layers from the C-

layers. Energetic ion beams are known to cause

atomic displacements. A simulation of 2 MeV Au

ion bombardment of a Pt/C multilayer has earliershown displacements of C and Pt across the in-

terfaces producing a concentration of Pt in C-

layers and a concentration of C in Pt-layers [3]. In

the ion bombardment process Fe also would be

displaced. However, preferential Fe migration into

Pt-layers cannot be explained simply by the ion–

atom collision process. However, an enhanced

diffusion can set in due to ion irradiation causingFe migration. The preferential Fe migration per-

haps can be understood from the Fe–Pt [5] and

Fe–C [6] phase diagrams. FexPt1�x can exist

practically for any value of x. However for

FexC1�x, x can assume only large values. There are

no known phases of FexC1�x for x < 0:75. The

most carbon-rich phase is Fe3C. Thus with a small

quantity of Fe being present in the multilayer, Featoms would prefer to be in the neighbouring Pt

layers provided they have mobility to migrate from

the C-layers. In the Pt layers, a small amount of

Fe can form Pt-rich phases, e.g. FePt3. Fe in

Pt may exist in several possible phases or even as

Fe clusters. These aspects can be investigated by

exploring the local environment of Fe in extended

X-ray absorption fine structure experiments bytuning the antinodes of the standing wave field in

the Pt-layers. We have performed such experi-

ments. However, these aspects are beyond the

scope of this paper.

534 S. Bera et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 530–534

Formation of Fe clusters or Fe–Pt clusters

could be very interesting as there could be a drastic

variation of magnetic moments for clusters. For

example, the magnetic moment of a Rh atom is3lB/atom while bulk Rh is nonmagnetic. Rh

clusters containing between 9 and 100 atoms have

been found to be magnetic. Small Rh clusters such

as dimers (2lB/atom) and trimers (1lB/atom) carry

substantial moments [7]. In view of this and our

observation of ion beam induced Fe migration

into Pt layers in Pt/C(Fe) multilayers, we predict

that Pt/C(Fe) multilayers and ion irradiated Pt/C(Fe) multilayers would show very interesting

magnetic behaviour.

4. Conclusions

We have carried out irradiation of Pt/C multi-

layers, containing a small amount of Fe impurity,

with 2 MeV Au2þ ions at different ion fluences.

The samples were characterized by a combined

XSW and XRR analysis. Ion beam induced Fe

redistribution was observed. Higher ion fluenceslead to increasing Fe concentrations in the Pt-

layers due to migration of Fe from C-layers to Pt-

layers. This phenomenon cannot be explained

simply by the ion–atom collision process. The re-

sults have been explained in terms of radiation

enhanced Fe diffusion and Fe–Pt and Fe–C phase

diagrams.

Acknowledgements

We thank the technical staff of the Ion Beam

Laboratory of IOP, Bhubaneswar. BND ac-

knowledges the local hospitality from HASYLAB

during the experiments with synchrotron radia-

tion. The work was partly supported by ONR

Grant no. N00014-95-1-0130.

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