DSpace at VNU: Magnetic study of nanocrystalline iron particles in alumina matrix

Journal of Magnetism and Magnetic Materials 262 (2003) 368–373

Magnetic study of nanocrystalline iron particles in
alumina matrix
A. Fnidikia,*, C. Doriena, F. Richommea, J. Teilleta, D. Lemarchanda, N.H. Ducb,
J. Ben Youssef c, H. Le Gallc
a

Groupe de Physique des Materiaux UMR CNRS 6634, Universit!e de Rouen, Site Universitaire du Madrillet, B.P 12,
76801 Saint-Etienne-Du-Rouvray Cedex, France
b
Cryogenic Laboratory, Faculty of Physics, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam
c
Laboratoire de Magn!etisme de Bretagne, CNRS, B.P. 809, 29285 Brest Cedex, France

Abstract
.
X-ray diffraction, Mossbauer
effect and magnetisation investigations have been performed on sputtered
Fe40(Al2O3)60 thin films. Ultrafine Fe particles of nanometer size in an amorphous Al2O3 matrix have been formed
by annealing in the temperature range from 100 C to 500 C. Their particle sizes, however, show a rather wide
.
distribution. Mossbauer
spectra are constituted of both paramagnetic and ferromagnetic contributions. The
paramagnetic contribution is associated with small grains of Fe, whereas the magnetic component is contributed by
large iron grains. This assumption is supported by the ZFC- and FC-measurements, in which the ‘‘blocking’’
temperatures of 60 and 90 K were evidenced for as-deposited and 200 C-annealed films, respectively.
r 2003 Elsevier Science B.V. All rights reserved.
PACS: 75.50.Tt, 75.75.+a
.
Keywords: Nanomagnetism; Mossbauer

spectra; Superparamagnetism

1. Introduction
Metal/insulator granular films consisting of
nanometer sized ferromagnetic metals immersed
in an insulating medium have been intensively
studied in the last decade because of their
interesting giant magnetoresistance (GMR) ef*Corresponding author. Group de Physique des MateriauxUMR CNRS 6634, Universite de Rouen, Mont-Saint-Aignam
Cedex 76821, France. Tel.: +33-35-14-67-65; fax: +33-35-1466-52.
E-mail address:
(A. Fnidiki).

fect—called tunnelling magnetoresistance (TMR)
[1,2]. These materials exhibit also other novel
phenomena such as superparamagnetism [3], giant
magnetic coercivity [3,4], etc., making them good
candidates for future technological applications.
Giant magnetic coercivity has been reported for
the ferromagnetic transition metal (T=Fe, Co)based granular films by several authors ([4,5] and
references therein). The coercivity of these granular systems is as large as about 60 mT at room
temperature as well as about 250 mT at T ¼ 2 K.
It is sensitive to grain size [3]. A surface contribution to giant magnetic coercivity was reported for

0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0304-8853(03)00064-7


A. Fnidiki et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 368–373

granular systems having uniformly nanometer

sized particles [6]. Recent magnetic studies of Fe/
Al2O3 granular films [7], however, have shown that
the presence of single domain regions is responsible for the high value of the coercivity, where
magnetisation reversal takes places only by rotation of saturation magnetisation vectors. In Ref.
[7], a so-called blocking temperature, TB ; at which
the metastable hysteretic response is lost, was
found and superparamagnetic behaviour of nanocrystalline iron particles in alumina matrix was
reported.
In this paper, we focus our attention on the
.
Mossbauer
spectroscopic evaluation of nanocrystalline iron crystallites evolved by annealing in the
sputtered Fe40(Al2O3)60 granular films. The results
are discussed in connection with structural and
magnetic data.

2. Experimental
The Fe40(Al2O3)60 thin films were deposited on a
glass substrate at 300 K using a triode RFsputtering system. The film thickness is 560 nm.
The composition was analysed using energy
dispersive X-ray spectroscopy (EDX). After depositing, samples were annealed for 1 h in a
vacuum of 5 Â 10À5 Torr in the temperature range
from 100 C to 500 C.
The microstructure of the sample was investigated using a JEOL 200 FX electron microscope
operating at 200 kV.
The structure of the samples was investigated by
X-ray diffraction using a cobalt anticathode
(lCo-Ka ¼ 0:1790 nm). The grain size was calculated from the full-width at half-maximum
(FWHM) of the principal diffraction peaks using
the Scherrer relation.

Various magnetic properties, such as magnetic
hysteresis loops, zero-field cooled (ZFC) and field
cooled (FC) magnetisation, were measured with a
SQUID magnetometer in magnetic fields up to
5 T, in the temperature range from 4.2 to 300 K.
.
The conversion electron Mossbauer
spectra
(CEMS) at room temperature were recorded using
a conventional spectrometer equipped with a
home-made helium–methane proportional coun-

369

ter. The source was a 57Co in rhodium matrix. The
films were set perpendicular to the incident
g-beam. The spectra were fitted with a least-squares
technique using a histogram method relative to
discrete distributions, constraining the linewidth of
each elementary spectrum to be the same. Isomer
shifts are given relative to BCC-Fe at 300 K.

3. Experimental results and discussion
For the Fe40(Al2O3)60 films under investigation,
the experimental XRD patterns show three characteristic peaks of (1 1 0), (2 1 1) and (2 0 0) BCCFe reflections. No Al2O3 reflections were detected
indicating the amorphous structure of the alumina
matrix. In Figs. 1(a)–(e) however, only the patterns ranging from 2y ¼ 48 to 56 , corresponding
to the (1 1 0) reflections, is displayed. For the asdeposited film, the Bragg peak centred at 2y ¼ 52
is rather broad. The average grain size of BCC-Fe
particles (dFe ) is estimated by applying the

Scherrer formula for the width of the (1 1 0)
BCC-Fe diffraction peak. It turns out that dFe
equals about 3 nm for the as-deposited sample.
The microstructure of the sputtered film was
investigated by TEM using a 60 nm thick Fe40(Al2O3)60 film deposited on a carbon thin substrate. The dark field electron micrograph in Fig. 2
reveals that the mean size of the BCC Fe grains
is 2 nm.
The annealing at TA ¼ 100 C and 200 C makes
no appreciable change in the XRD patterns. After
annealing at TA X300 C; the (1 1 0) diffraction
peak becomes narrower and its intensity increases
with the annealing temperature, see e.g. Fig. 2e for
the sample annealed at 500 C. In this case, dFe
increases up to about 6 nm. Finally, it is worthwhile to mention that, the (1 1 0) BCC-reflection
peaks shift to higher 2y angles with increasing TA :
This indicates that the annealing causes not only
the evolution of crystallites, but also the increase
of the Fe-concentration.
.
The Mossbauer
spectra recorded at room
temperature are shown in Fig. 3. The obtained
hyperfine parameters are listed in Table 1. For the
as-deposited film, the CEM spectrum consists of a
paramagnetic asymmetric doublet (with fraction


A. Fnidiki et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 368–373

370

(a)

Intensity (a.u.)

(b)
(c)

(d)

(110) Fe

(e)

48

50

52

54

56

2θ (degrees)

Fig. 1. X-ray diffraction patterns of the Fe40(Al2O3)60 thin films: (a) as-deposited film, (b) after annealing at 100 C, (c) À200 C,
(d) À300 C and (e) À500 C.

(a)

(b)
20 nm

Fig. 2. Dark field electron micrograph (a) and corresponding electron diffraction pattern of the as-deposited thin film.

Apar ¼ 51%) and a magnetic broadened sextet
(with fraction Aferro ¼ 49%:) (see Table 1). The
asymmetric doublet can be associated with the
contribution from iron grains with small sizes, and

also probably from Fe atoms diluted in the
alumina matrix. The broadened magnetic sextet
is associated with the contribution of iron crystallites with large grain sizes. It was fitted with a wide


A. Fnidiki et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 368–373

371

Velocity (mm/s)
-10

0

50

+10

40

1.02

30
20
10
0
0

10

20

30

0

10

20

30

0

10

20

30

0

10

20

30

0

10

20

30

50

1.00
1.02

40
30
20
10

1.00
1.02

P(Bhf ) (%)

Emission (%)

0
40
30
20
10
0
1.00
1.02

30
20
10
0

1.00
1.02

40
30
20
10

1.00

0

Bhf (T)
.
Fig. 3. Mossbauer
spectra and hyperfine field distributions of the Fe40(Al2O3)60 thin films: (a) as-deposited film, (b) after annealing at
100 C, (c) À200 C, (d) À300 C and (e) À500 C.

hyperfine field distribution PðBhf Þ in order to take
into account the different environments of the iron
atoms. Indeed, as can be seen from Fig. 3a, the
fitted hyperfine field exhibits a rather wide
distribution. A low fraction with Bhf ¼ 32 T
characterising the pure BCC-Fe is observed. The
main contribution, however, is centred at
Bhf ¼ 28 T. A similar behaviour was found for
the Fe/Cu granular films [6]. In Ref. [6], it was
described in terms of the transient composition
and ferromagnetism. At present, one can add to

this picture an effect of the wide distribution of Fegrain sizes. The CEM spectrum remains almost
identical for the film annealed at TA ¼ 100 C (see
Fig. 3b). After annealing at 200 C, the relative
.
BCC-Fe Mossbauer
fraction starts to increase,
while the corresponding paramagnetic contribution decreases. This process takes place strongly in
the sample annealed at TA ¼ 300 C and then is
slowing down at further heat treatments. At
TA ¼ 500 C; more than 96% of the Fe atoms are
found to be in the BCC-Fe phase. As the annealing

A. Fnidiki et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 368–373

As-deposited
TA ¼ 100 C
200 C
300 C
500 C
Accuracy

Doublet
Sextet
Doublet
Sextet
Doublet
Sextuplet
Doublet
Sextet
Doublet
Sextet

Bhf (T)

A (%)

—
18.5
—
18.3
—

20.7
—
25.3
—
28.2
0.1

51
49
51
49
24
76
9
91
4
96
2

temperature increases, the main peak observed at
28 T on the hyperfine field distribution for the asdeposited film is shifted toward the characteristic
value (33 T) of bulk BCC-Fe. This is in agreement
with the increasing size of the Fe grains observed
in XRD.
The remaining traces of the doublet in the
.
Mossbauer
spectrum indicate that the formation
of BCC-Fe was not completed. In accordance with
the XRD results, the increase of the relative

fraction of BCC-Fe phase is associated with the
increase of the Fe-grain size as well as of the Feconcentration in the grains with increasing annealing temperature, indicating that the Fe atoms
dissolved in the Al2O3 matrix diffuse to form the
BCC-Fe phase.
Fig. 4 illustrates the ZFC and FC curves
measured in a magnetic field (of 3 mT) applied in
the film-plane, for the as-deposited, 200 C- and
500 C-annealed films, respectively. It is clearly
seen that the ZFC and FC magnetisation bifurcates at different temperatures in different samples. These temperatures are regarded as the
experimentally measured blocking temperature
TB ; at which the spin are aligned or not along
the field. TB equals 60 K for the as-deposited film.
It reaches to 90 K in the 200 C-annealed films, in
accordance with the increase of the Fe grain size.
For uniaxial particles, TB can be taken as KV/
25kB, where K is the magnetic-anisotropy constant, kB is the Boltzmann constant and V is the
volume of the nanocrystallite [7]. Substituting
the values of TB ; K ¼ 50 kJ/m3 for Fe,

TA = 30 o C

M (arb. unit)

Sample

TB

0

100

200
T (K)

TB

300

TA = 200 o C

M (arb. unit)

Table 1
Hyperfine parameters for (Fe)40(Al2O3)60 granular films:
.
hyperfine field (/Bhf S) and Mossbauer
fractions (A)

0

100

200
T (K)

M (arb. unit)

372

300

TA = 500 o C

0

100

200
T (K)

300

Fig. 4. ZFC and FC curves of the (a) as-deposited film, (b)
after annealing at 200 C, (c) À500 C.

kB ¼ 1:38 Â 10À23 J into the above expression, we
find the mean diameter of the Fe particles d ¼
ð3V =4pÞ1=3 to be 8.5 and 10 nm for the asdeposited and 200 C-annealed films, respectively.
In order to obtain better results for the d
parameter, one must approach the real value of
the magnetic anisotropy constant, possibly taking
into account a surface contribution. After annealing at 500 C, a weak bifurcation is still observed at
10 K. Here, the observed ‘‘curvature’’ of the ZFC–
FC curves differs from the Langevin ‘‘curvature’’
generally observed for systems containing noninteracting particles. It indicates probably the
existence of a strong dipolar interaction between
iron grains in the samples. Indeed, in all cases, the
ratio of the remanence to the saturation magnetisation was found to be about 0.7 at T ¼ 5 K

A. Fnidiki et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 368–373

1.2
0.8
M/Ms

0.4
0.0
-0.4
-0.8
-1.2
-0.10

-0.05

(a)

0.00
B (T)

0.05

0.10

1.2

373

value of the transition temperature of about 340 K
observed on the ZFC–FC curve for the asdeposited sample (see Fig. 4). This assumption is

corroborated by the fact that the transition
temperature increases with increasing annealing
temperature (Fig. 4). Fig. 5 exhibits also a large
coercive field of the investigated sample. The
coercivity decreases with increasing annealing
temperature, i.e. with increasing Fe grain size. It
is worthwhile, however, to mention that this
behaviour is observed in a rather small grain size
region.

0.8
M/Ms

0.4

4. Concluding remarks

0.0
-0.4
-0.8
-1.2
-0.10

-0.05

(b)

0.00
B (T)

0.05

0.10

1.2
0.8
M/Ms

0.4

.
Mossbauer
spectrometry shows the existence of
a broad distribution of iron grain sizes. Ferromagnetism is formed in large sized BCC-Fe grains.
The existence of small particles of Fe leads strong
surface effects and magnetic disorder, evidenced
.
by Mossbauer
and magnetisation measurements.
A simple simulation of the hysteresis loops and the
ZFC- and FC-curves will be presented in a
forthcoming publication.

0
-0.4

Acknowledgements

-0.8

(c)

-1.2
-0.10

-0.05

0.00
B (T)

0.05

0.10

Fig. 5. Hysteresis loops measured at 5 K for the as-deposited
Fe40(Al2O3)60 thin films, the 200 C- and the 500 C-annealed
films.

(Fig. 5). The value of the saturation magnetisation
for the sample annealed at 500 C (748 kA/m)
corresponds to 43% of Fe with an average
moment of 2 mB/at, in agreement with the
.
Mossbauer
result, indicating that the nearly total
amount of Fe is in the BCC form. The smaller
values obtained for the as-deposited and the
200 C-annealed films (600 kA/m) indicate a strong
surface effect, in accordance with the small value
of the Fe grain size obtained in TEM and XRD.

These effects of the surface can explain the low

This work is partly supported by the State
Program for Natural Scientific Researches of
Vietnam, within project 420.301.

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