Nuclear Instruments and Methods in Physics Research B 216 (2004) 245–250

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Structural and magnetic properties of Fe–Al silicacomposites prepared by sequential ion implantation

C. de Juli�an Fern�andez a,*, M.A. Tagliente b, G. Mattei a, C. Sada a,V. Bello a, C. Maurizio c, G. Battaglin d, C. Sangregorio e,

D. Gatteschi e, L. Tapfer b, P. Mazzoldi a

a Dip. Fisica, INFM Univ. Padova, via Marzolo 8, 54124 Padova, Italyb ENEA-CR Brindisi, SS.7 Appia km. 714, 72100 Brindisi, Italy

c INFM – GILDA, ESRF, Rue J. Horowitz, BP 200 Grenoble, Franced INFM Dip. Chimica-Fisica, Univ. Venezia, Dorsoduoro 2137, 30170 Mestre, Italy

e LAMM – Dip. Chimica, Univ. Firenze, via della Lastruccia 3, 50019 Sesto Fiorentino, Italy

Abstract

The nanostructural and magnetic properties of Fe–Al/SiO2 granular solids prepared by ion implantation have been

investigated. A strong effect of the implantation order of the Fe and Al ions has been evidenced. By implanting first the

Al ions and later Fe ions, 5–40 nm core–shell nanoparticles are formed with a magnetic behavior similar to that of Fe.

The lattice parameter of the nanoparticles is consistent with that of the a-Fe. By changing the implantation order, 10–15nm core–shell nanoparticles of a bcc Fe-based phase with a lattice 2.5% smaller than that of a-Fe are formed. Thetemperature dependence of the magnetization indicates a superparamagnetic behavior.

� 2003 Elsevier B.V. All rights reserved.

PACS: 36.40.C; 75.50; 61.46; 68.55.L

Keywords: Magnetic nanoparticles; Silica composites; Fe–Al alloys; Ion implantation; Superparamagnetism

1. Introduction

Composite materials made of metal particles

with nanometric dimensions dispersed in dielectric

or metallic matrices have received an increasing

interest due to their peculiar properties which can

* Corresponding author. Tel.: +39-049-827-7040; fax: +39-

049-827-7003.

E-mail address: dejulian@padova.infm.it (C. de Juli�an

Fern�andez).

0168-583X/$ - see front matter � 2003 Elsevier B.V. All rights reser

doi:10.1016/j.nimb.2003.11.041

be exploited for magnetic, transport, optoelec-

tronic, photonic and catalytical applications. Inparticular, bimetallic nanoparticles are very

promising candidates for technological purposes

because the composition is a further freedom de-

gree to tailor their properties for specific purposes.

Ion-implantation is a suitable technique to obtain

bimetallic nanoparticles in dielectric matrix [1–3].

In this way, it has been obtained composite sys-

tems consisting of dielectric matrices with Au–Cu,Au–Ag, Au–Pd clusters with interesting optical

properties [1,3,4], with Co–Ni, Co–Cu, Fe–Pt,

ved.

246 C. de Juli�an Fern�andez et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 245–250

Co–Pt clusters with peculiar magnetic properties

[5–7] and with GaN, GaAs, ZnSn clusters for

luminescence applications [2,3,8].

In this work, we present a study of the nano-structural and magnetic properties of composites

prepared by sequentially implanting Fe and Al

ions in silica matrix in order to form Fe–Al

nanoparticles. In the bulk Fe–Al alloys it was

found that starting from pure Fe, the progressive

substitution of Fe sites by Al atoms leads to a

gradual decrease of the magnetization and for an

Al concentration of about 35% the alloy becomesnon-magnetic [9,10]. In this range of compositions

two ordered compounds are present: the ferro-

magnetic Fe3Al and the non-magnetic FeAl.

However, alloys prepared by non-equilibrium

techniques, like rapid quenching or mechanical

milling, are magnetic up to 50 at.% Al [11,12]. This

was explained considering that the composition

and the structural disorder produce the competi-tion of ferromagnetic and antiferromagnetic

interactions between the Fe atoms [12,13]. Ion

implantation technique is an out-of-equilibrium

technique, so it is expected that Fe–Al nanoparti-

cles show this effect even if it modified by the size

[13]. Moreover, in this paper the magnetic prop-

erties of these composites and the relationship with

the nanostructure and composition have beeninvestigated.

2. Experimental

The same doses of Fe and Al ions (15 · 1016/cm2) were sequentially implanted at room tem-

perature into fused silica (Heraeus Herasil), atenergies of 110 keV for Fe and 50 keV for Al, such

that the respective projected ranges were similar.

Also, single-implanted with Fe and Al samples

were prepared with the same implantation dose

and energies. Fe–Al implants were performed by

implanting first Fe ions and later Al ions whereas

in Al–Fe samples the ion implantation order was

reversed. Implants were performed at the ENEAIon Implantation laboratory in Brindisi (Italy).

Samples were not annealed after implantation.

GIXRD experiments were carried out employ-

ing a X-ray diffractometer Philips MPD PW1880

in Parallel Beam geometry, equipped with a X-ray

tube emitting CuKa radiation ðkCuKa ¼ 0:154186nm) and operated at 40 kV, 40 mA. The incident

X-ray beam was fixed at 0.5� (which correspondsto a penetration depth of 280 nm in silica) and the

detector was moved along the goniometer circle in

the 2h range between 10� and 70�. The samples fortransmission electron microscopy (TEM) were

prepared and examined at CNR-LAMEL Institute

in Bologna with a FEI TECNAI F20 Supertwin

field emission microscope, operating at 200 kV.

Rutherford backscattering spectrometry (RBS)measurements were performed using 4Heþ ions of

energy 2.2 MeV at the Van der Graaff Accelerator

of INFN Legnaro Laboratory. The magnetic

characterization was carried out using a Cryogenic

S600 SQUID magnetometer. Zero-field-cooled

(ZFC) and field-cooled (FC) measurements were

performed applying a magnetic field of 5 mT in the

plane of the glass slide.

3. Results and discussion

From RBS measurements, we observed that the

implanted ion doses in the Fe–Al and Al–Fe

samples differed from the nominal ones. In par-

ticular, in the Fe–Al sample the measured doseswere 15 · 1016Feþ/cm2 and 10 · 1016 Alþ/cm2 sothat the average composition was Fe60Al40. In the

Al–Fe sample, the respective doses were 17 · 1016Feþ/cm2 and 8 · 1016Alþ/cm2 and the average

composition was about Fe70Al30.

Figs. 1(a) and (b) show the bright-field cross-

sectional micrographs of the Al–Fe and Fe–Al

samples, respectively. In both samples, nanopar-ticles dispersed in the matrix and concentrated in a

layer with a width of about 100 nm are observed.

The morphology of the nanoparticles appears to

depend on the implantation order: in the Al–Fe

sample the nanoparticles have diameters between 3

and 40 nm whereas in the Fe–Al sample the par-

ticle size is between 10 and 15 nm. In both cases,

the smaller nanoparticles are dispersed in thedeeper side of the implanted region and a core–

shell structure is observed in the larger nanopar-

ticles. However, in the Al–Fe sample the core–shell

nanoparticles had several cores while in the Fe–Al

Fig. 1. Cross-sectional bright-field TEM micrographs of the (a) Al–Fe and (b) Fe–Al samples.

10 20 30 40 50 60 70

2θ (degrees)

Al(2

20)

Al(2

00)

Fe(1

10)

Al(1

11)

Fe(1

11)

Al-Fe

Fe-Al

Fe

Al

X-ra

y di

ffrac

ted

inte

nsity

(a.u

.)

Fig. 2. GI-XRD spectra of the Fe, Al, Fe–Al and Al–Fe

samples.

C. de Juli�an Fern�andez et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 245–250 247

one there were single-core clusters. Further studies

on the morphology are in progress.

Fig. 2 shows the GIXRD scans of the Al, Fe,

Fe–Al and Al–Fe samples. The patterns corre-sponding to the single-implanted samples Fe and

Al exhibit the diffraction peaks of the bulk bcc a-Fe and bulk fcc Al phases, respectively. The pat-

tern of the sample Al–Fe shows a broad diffraction

peak which was ascribed to the (1 1 0) diffraction

of the bcc a-Fe phase. No peak is observed thatindicates the presence of metallic aluminium.

In the Fe–Al sample pattern the Al (1 1 0) peakis observed, indicating the presence of Al crystal-

line domains. The pattern of sample Fe–Al shows

two others diffraction peaks at about 46� and 67�from an other crystalline phase not clearly identi-

fiable.

A detailed peak shape analysis was accom-

plished to these patterns. In particular, the dif-

fraction line profiles were fitted by a pseudo-Voigtfunction and it was assumed that the background

in proximity of the reflection is linear. According

to the results, the lattice parameter of Fe in sample

Al–Fe is 0.287(2) nm which is consistent with that

of the bulk value and the coherent domains size is

1.6(2) nm. In the sample Fe–Al the not-identified

bcc phase has a lattice parameter a ¼ 0:279ð2Þ nmand the average size of the coherent domains is

248 C. de Juli�an Fern�andez et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 245–250

6.5(4) nm. Such a phase could be the Fe phase

hardly stressed considering that it is contracted of

2.5% with respect to the lattice parameter mea-

sured in the Fe case. The observed pattern doesnot correspond with any Fe, Al oxides or silicate.

We observe that in both Fe–Al and Al–Fe samples

the TEM particle size is 5–10 times larger than the

XRD coherent domains dimensions, which indi-

cates that the clusters are polycrystalline. Finally,

we point out that Fe–Al bulk bcc alloys have a

larger lattice parameter than that of Fe. Therefore

the formation of the Fe–Al alloy in the nanopar-ticles of the Al–Fe and Fe–Al samples was ruled

out.

The magnetic behavior of the samples was

investigated by measuring the hysteresis loops at

3 K and applying a magnetic field up to 6 T.

The diamagnetic contribution of the silica was

subtracted from the measured magnetization.

Then the magnetic moment per Fe atom was cal-culated considering the number of atoms in the

sample from the RBS measurements. The results

are presented in Fig. 3. The magnetic moments per

Fe atom in the Fe implanted and Al–Fe samples at

6 T are similar, about 2.1 ± 0.2 lB, and slightlysmaller than the bulk value of a-Fe, 2.2 lB. Theseresults confirm the absence of Fe–Al intermetallic

compound and indicate that a little oxidationoccurred. However the magnetic moment in the

Fe–Al sample is much smaller: 1.0 ± 0.2 lB. We

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

-2

-1

0

1

2

-0.10 -0.05 0.00 0.05 0.10-2

-1

0

1

2

Mag

netic

mom

ent p

er F

e at

om (µ

B)

Applied Field (T)

Fe Al-Fe Fe-Al

Applied Field (T)

Fig. 3. Hysteresis loops measured at 3 K of the Fe, Fe–Al and

Al–Fe samples. Inset: detail at low field of the hysteresis loops

of the Fe–Al and Al–Fe samples.

attribute the reduced magnetic moment to a fea-

ture of the bcc Fe-based nanoparticles. Indeed, the

presence of oxides or free atoms dispersed in the

matrix should give, at 3 K and with high magneticfields, as in our measurements, a much larger in-

crease of the magnetization with the field. In the

inset of Fig. 3 the low field region of the hysteresis

loops has been represented. It is found that the

coercive fields of the Fe–Al and Al–Fe samples are

3 and 1.4 mT, respectively. In first instance, the

nanoparticles of all the samples must be single

domains due to their nanometric size (thisapproximation will be discussed further). Then,

considering a mechanism of coherent rotation of

the magnetization, the coercive force is propor-

tional to the magnetic anisotropy of the nano-

particles and inversely proportional to their

magnetization saturation [14,15]. Considering

the difference of magnetic moment per Fe atom in

the Fe–Al and Al–Fe samples we conclude that themagnetic anisotropy of the bcc phase, presented in

the Fe–Al sample, is similar or smaller than that of

the Fe–Al.

Fig. 4 shows the zero-field cooled (ZFC) and

field cooled (FC) magnetization measurements of

the Fe, Al–Fe and Fe–Al samples. This type of

measurements is used to evidence the superpara-

magnetic behavior, that is, in nanograins, due tothe size, the thermal effects help the magnetization

in overcoming the energy barrier that fixes this in a

0 100 200 300

Mag

netiz

atio

n (a

rb. u

nits

)

Temperature (K)

Fe Al-Fe Fe-Al

Fig. 4. Temperature dependence of the ZFC–FC magnetiza-

tions of the Fe, Fe–Al and Al–Fe samples.

C. de Juli�an Fern�andez et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 245–250 249

direction of the grain and then the magnetization

tilts freely. This energy barrier is responsible of the

hysteresis that the nanoparticles show and it is

proportional to the magnetic anisotropy and to theparticle volume. The thermal effects produce the

demagnetization of the particle: it seems as a

paramagnet with high moment. The temperature

threshold, called blocking temperature TB, abovewhich a grain of volume V and with a magneticanisotropy, Kef , shows the superparamagneticbehavior is [14,15]

TB ¼ KefVa kB

; ð1Þ

where kB is the Boltzman constant and a is aconstant equal to 25 in the case of the SQUID

measurements. The ZFC–FC curves presented in

Fig. 4 are representative of a distribution of TB,mainly associated to the broad size distribution,and being the temperature of the maximum of the

ZFC curve linked to the average TB. In additionthe magnetization does not follow a Curie law at

high temperature in these films. The temperature

at which ZFC–FC branches joints is the temper-

ature above which all the nanoparticles are su-

perparamagnetic. In the Fe and Fe–Al samples the

blocking temperature is below the room tempera-ture while for the Al–Fe sample TB is above theroom temperature. Considering the much larger

particle size of the Al–Fe than the Fe–Al sample,

the differences in the ZFC–FC curves can be dis-

cussed in terms of superparamagnetism consider-

ing the measured particle size. However we point

out that the nanoparticles of these samples have a

core–shell morphology. The magnetic anisotropywill include the contribution of the different mag-

netic surfaces (that of the core and those of the

shell) as occurs in many nanostructures [15]. The

core–shell structure promotes incoherent rotation

modes in which the magnetic anisotropy is not

related to the intrinsic properties of the material.

Moreover, as can be observed in Figs. 1(a) and (b)

nanoparticles are very densely packed so that in-terparticle interactions must be dominant giving

rise to different demagnetization processes.

Therefore, we think that the observed behavior

appearing similar to superparamagnetism is the

result of several magnetic effects and must be

investigated in order to analyse correctly the ZFC–

FC curves.

Finally we want to point out that, both fromthe point of view of the nanostructural and mag-

netic characterization, FeAl intermetallic alloy

nanoparticles were not formed in silica by using

sequential ion implantation. Several authors have

studied the mechanism of nucleation and alloying

of the nanoparticles concluding that it is driven by

competitive physical and chemical reactions be-

tween the implanted elements and those of thematrix [16,17]. For Fe and Al the respective Gibbs

energy of oxide formation are negative and very

high [18], so the formation of oxides is expected.

However, as it was shown in this work, metallic

nanoparticles were formed suggesting that com-

petitive chemical paths determine the nanostruc-

tural features as elsewhere suggested [16]. Several

studies are in progress to investigate the mecha-nism of formation of the different nanostructures

and their chemical state, the identification of the

bcc phase observed in the Fe–Al sample and the

characterization of the magnetic properties.

Acknowledgements

This work was financially supported by ENEA

in the framework of the PROTEMA project of

‘‘Intesa ENEA-MIUR’’ under project 4335/04 and

by the Italian MURST (National Projects).

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