Granular Fe-Al2O3 films prepared by self-propagating high temperature synthesis

Article abstract of DOI:10.1016/S0304-8853(02)01081-8

Taking for example the initiation of the classical solid-state reaction between Fe₂O₃ and Al layers in films, it was shown that as a result, the granular Fe-Al₂O₃ films consisting of Fe nanoclusters embedded in an insulating Al₂O₃ matrix were made. It follows from the reaction equation that the Fe volume fraction in granular films is less than the percolation limit. This fact defines the magnetic properties of iron clusters, which are superparamagnetic. It is supposed that a nanocrystalline microstructure must be present in thin films after solid-state reaction, which occurs in self-propagating high temperature synthesis conditions.

Full text of DOI:10.1016/S0304-8853(02)01081-8

Journal of Magnetism and Magnetic Materials 258–259 (2003) 358–360
Granular Fe–Al O films prepared by self-propagating high
2
3
temperature synthesis
a,
V.G. Miagkov *, K.P. Polyakova , G.N. Bondarenko , V.V. Polyakov
a
b
a
a
Kirensky Institute of Physics SB RAS, 660036 Krasnoyarsk, Russia
b
Institute of Chemistry and Chemical Technology SB RAS, 660036 Krasnoyarsk, Russia
Abstract
Taking for example the initiation of the classical solid-state reaction between Fe O and Al layers in films, it was
2
3
2 3 2 3
shown that as a result, the granular Fe–Al O films consisting of Fe nanoclusters embedded in an insulating Al O
matrix were made. It follows from the reaction equation that the Fe volume fraction in granular films is less than the
percolation limit. This fact defines the magnetic properties of iron clusters, which are superparamagnetic. It is supposed
that a nanocrystalline microstructure must be present in thin films after solid-state reaction, which occurs in self-
propagating high temperature synthesis conditions.
r 2002 Elsevier Science B.V. All rights reserved.
Keywords: Self-propagating high temperature synthesis; Granular films; Magnetic nanoparticles
1
. Introduction
enlarge the structure characters, which define the
physical properties of nanocomposites.
Magnetic granular films consisting of ferromagnetic
nanoclusters embedded in an insulating matrix were
extensively investigated in the recent years because of
their giant magnetoresistance [1], giant extraordinary
Hall effect [2], magneto-optical features [3], quantum
size effect [4] and potential magnetic recording applica-
tions [5,6].
2. Experimental procedures
In this paper, we present the deposition method and
properties of Fe–Al O nanogranular films. The pre-
2
3
paration method was based on the use of classical solid-
state reaction:
Granular magnetic films containing nanoclusters of
2 2 3
Fe, Ni, Co and their alloys in SiO or Al O matrix are a
Fe2O3þ2Al ¼ Al2O3þ2Fe:
ð1Þ
main part of these examinations. The most widespread
granular film preparation method is the co-deposition of
metal and insulator. In this case, the nanoclusters are
distributed in matrix randomly and their size depends on
annealing and depositing conditions. However, on
successive deposition of metal and insulator, self-
organization in metal nanocluster formation can be
realized [7]. The creation of new methods of granular
film preparation is important because it allows to
It was shown [8,9] that solid-state reactions in bilayer
thin films at major heating rates are the result of self-
propagating high temperature synthesis (SHS). In thin
films, the SHS represents a wave of surface burn and is
characterized by initiating temperature T : The layers of
0
Fe O and Al O were the reagents of reaction (1). The
2
3
2
3
samples for solid-state reaction consist of two or more
layers.
The multilayer Al/Fe
successive DC-sputtering of Al and Fe
2
O
3
structures were prepared by
targets
2
O
3
without vacuum destruction. The targets were bulk
aluminum and conductive ferric oxide coat. This coat of
1 mm thickness was prepared by plasma deposition of
*
Corresponding author. Tel.: +7-3912-49-46-81; fax: +7-
3
0
912-43-89-23.
E-mail address: miagkov@iph.krasn.ru (V.G. Miagkov).
304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 0 8 1 - 8

V.G. Miagkov et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 358–360
359
ferric oxide powders on massive copper plate in
reduction atmosphere [10]. Sputtering was carried out
in (1 1 0)Fe and the feeble or complete absence of other
reflections suppose that iron nanocrystals have prefer-
ential (1 1 0) orientation. In granular materials, the
important factors defining their magnetic properties
are the distribution of nanoclusters affecting the sizes
and relative metal fraction X: From reaction (1) it
follows, that Xr ¼ 0:365: This value is less than the
percolation limit, which for many granular systems of
metal nanoclusters, embedded in dielectric matrix, is
ꢁ
2
in Ar atmosphere at a pressure 5 ꢀ 10 Pa on glass
substrates at temperature B320 K. The deposition rate
was 0.3 nm/s for Al and 0.2 nm/s for Fe O . The total
2
3
film thickness was 15–50 and 45–140 nm for Al and
Fe , respectively. The post-deposited samples were
2
O
3
heated at a rate not less than 20 K/s for initiating the
SHS wave.
0
isolated from each other. This fact is confirmed by high
.5–0.6 [11]. It is supposed that metal nanoclusters are
ꢁ
6
electrical resistivity of the obtained samples r=10
–
3
. Results and discussion
ꢁ
4
1
0
O m, that is typical for metal clusters in a non-
conductive matrix below percolation limit. The determi-
nation of iron grain size in a direction perpendicular to
the plane from broadening of diffraction peak Fe(1 1 0),
using the well-known Scherrer formula, yields an
average value of 28 nm. This value coincides with the
size at the lower end at which the particles are in single
domain [12]. Assuming that in other directions the iron
cluster size has the right value, it should be a single
domain. The sample in this case is an ensemble of single-
domain particles and its behavior is described by
Stoner–Wohlfartht theory. For example, the magnetiza-
tion curve of single-domain particles ensemble has a
The solid-state reactions of bilayer films took place in
SHS mode at the above initiating temperature
T0B800 K.
The propagating front picture was observed visually
and was typical for SHS of thin films [8,9]. The
visualization of the multilayer film SHS front was
difficult. Therefore, these samples were subjected to
annealing which included heating above the initiating
temperature to 10–20 K and an exposure at this
temperature during 5–10 min (necessary time for reac-
tion and post-annealing).
Fig. 1a shows the typical X-ray diffraction pattern of
hysteresis loop with parameters Mr ¼ 0:5MS; H
K/M [13]. Fig. 2 shows the typical magnetization curve
of Fe–Al O films received after the reaction (Eq. (1)).
C
=0.58
a (20 nm Al/40 nm Fe
reaction, which contains reflections from a-Fe
formation of a small amount of Fe O is also possible in
2
O
3
) ꢀ 4 multilayer film before
S
2
O
3
. The
2
3
3
4
From the absence of hysteresis loop, it can be supposed
that a considerable part of iron clusters is super-
paramagnetic.
initial samples. The absence of Al reflections means that
Al films are amorphous or fine crystalline. After reaction
(
Fig. 1b) the reflections from a-Fe
2
O
3
and Fe
3
O
4
3
This magnetization curve can be regarded as the
superposition of superparamagnetic and ferromagnetic
curves [14]. Coexistence of both superparamagnetic and
ferromagnetic states is possible in magnetic iron
nanoclusters embedded in a non-conductor matrix [15].
The critical cluster size, below which the iron nanoclus-
disappeared and reflections from amorphous a-Al O
2
appeared. The presence of diffractograms of reflections
α− Fe O3
2
a
Fe O4
3
8
00
00
4
α−Al O3
Fe
2
b
0
-400
-800
30
35
40
45
50
55
60
-3
-2
-1
1
3
0
H, kOe
2
2
Θ, deg.
Fig. 1. X-ray diffraction pattern of (20 nm Al/40 nm Fe
2
O
3
) ꢀ 4
Fig. 2. Magnetization curve of (20 nm Al/40 nm Fe
multilayer film after 10 min annealing at temperature 800 K.
2
O
3
) ꢀ 4
multilayer film: (a) initial sample, (b) after 10 min annealing at a
temperature of 800 K.
Magnetization is present per unit iron volume.

360
V.G. Miagkov et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 358–360
ters in matrix Al
[
2
O
3
are superparamagnetic, is B10 nm
12]. It is proposed that in the explored samples, the Fe
utilized for deriving granular mediums. So granular Fe–
Al O films were obtained after solid-phase reaction
2
3
nanocluster size distribution is in the size range of a few
tens of nanometers. In this case, it is assumed that both
superparamagnetic and ferromagnetic nanoclusters co-
exist in the obtained films. It is necessary to note that the
hysteresis loop does not occur at temperature 77 K. It
should be noticed that the blockage temperature TB of
iron nanoparticles, which are superparamagnetic, lies
below this temperature (liquid nitrogen). The formation
of reaction products occurs solely at the SHS front. The
characteristic time of reaction will be defined as t ¼
2 3
between Al and Fe O layers. The iron nanoclusters
have (1 1 0) texture. The equation of reaction defines that
the volume part of iron is lower than the percolation
threshold. It is proposed that the iron nanoclusters in
the obtained samples are in the form of superparamag-
netic and ferromagnetic states.
Acknowledgements
2
t=V ; where t is the thermal diffusivity of the bilayer
f
sample. Supposing that t is defined by the thermal
The authors would like to thank Dr. I.A. Turpanov
and Dr. S.M. Zharkov for technical assistance. Kra-
noyarsk Foundation of Science has supported this work
under grant No. 11F001C.
ꢁ
7
2
diffusivity of metal oxide t=(1–5) ꢀ 10 m /s and the
front rate of SHS initiation temperature
Vf B1 ꢀ 10ꢁ m /s, we shall obtain an estimate of the
reaction characteristic time t ¼ 1 ꢀ 10ꢁ3c: Supposing
that reaction goes in solid phase at a temperature close
to the metal’s melting point and considering diffusion
D ¼ 10ꢁ12210ꢁ14 m /s, we shall obtain an estimate of
2
2
2
1=2
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mined by X-ray diffraction and calculation on the basis
of the SHS front kinetic.
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In the present report it was shown that solid-phase
reaction, which takes place in SHS mode, could be